Introduction to Protozoa and Parasites



All living organisms live in close interdependence on other living organisms. One type of this interdependence is called symbiosis, which means "living together".

The three categories of symbiosis are:

  1. Commensualism is defined as "eating at the same table". The association is not obligatory.

  2. Mutalism involves a benefit to both organisms and the association is obligatory.

  3. Parasitism is the intimate association of two species where one species (the parasite) benefits at the expense of the other (the host). The parasite usually does not kill the host outright but profits by the advantages of living on a surviving host. A dead host is usually not of use to the parasite, although death of the intermediate host may in some cases provide a means for reaching the definitive host.

These three categories are actually intersecting sets which blur at the intersections.

Some definitions:

The "definitive" host is the host in which the parasite becomes sexually mature.
The "reservoir" host is a non-human host that maintains the infection in nature.
The "intermediate" host contains the larval or asexual stages of the parasite.
Parasites that require a host for survival are called "obligate" parasites.
 "Facultative" parasites can survive outside the host.
live inside a host and ectoparasites live on the surface of the host.

Some generalizations about parasitism:

  1. Parasites produce many progeny.

  2. Parasites usually produce some impairment in the host, which can can be subclinical or clinical. When the disease is severe, we call the organism pathogenic or virulent

  3. Parasites usually have resistant or specialized stages for moving between hosts.

  4. The life cycles are often complex, with the parasite living in several hosts in different physiological stages.

  5. Parasitism is widespread in both animals and protists. It probably evolved independently multiple times.

  6. Parasites are usually smaller than the host.

  7. Many protozoal parasites are asexual.

  8. Infectiousness is the ability of the parasite to gain entry and maintain itself within the host.

History of Parasitology and Tropical Medicine

Helminth parasites were known in ancient Greece, Rome, China and the Arab world. They were thought to arise by spontaneous generation. Malaria has been recognized as a disease of man for over 4000 years. Reference to malaria can be found in a Chinese document from 1700 BC. In ancient Greece, Hippocrates classified different fevers as quotidan, tertian and quartan. He described spleen enlargement and sickness from drinking water from marshy places. Unfortunately his teachings were lost until the mid 17th century when a physician rediscovered the different types of malarial fevers and found that an extract from a Peruvian bark (quinine) could cure the disease. Ancient human remains in Chile show clear evidence of Chagas Disease, which is caused by a protozoal parasite, Trypanosoma cruzi (see below).


Protozoal parasites could not be discovered until the invention of the microscope.

But discoveries in science require not only technology, but also a conceptual framework or paradigm, which allows predictions.

For example, Robert Hooke in 1665 developed a microscope with 30X magnification and saw "cells" in slices of cork (actually cell walls). This observation made no impact on science at all. He however did influence Antony van Leeuwenhoek to begin grinding lenses!

  {short description of image}
{short description of image} 

In the later part of the 17th century, a Dutch amateur scientist named Anton van Leeuwenhoek made some really fantastic discoveries - but they made essentially no impact on science. Using tiny single-lens microscopes of his own design,

 {short description of image} {short description of image} 

Leeuwenhoeck made the first descriptions of protozoa, bacteria, and spermatozoa which he called "animalcules" and made the first detailed descriptions of the red blood cell. He wrote letters to the Royal Society in London from 1673 to 1700, which were published. He looked at everything - rain water, canal water, his saliva, his feces. He saw an entirely new world which had never been seen before. He also discovered the first protozoal parasite - Giardia.

Let me give you one anecdote - Leeuwenhoek was proud of his white teeth - he rubbed them with salt, picked with a toothpick and rubbed again. But he still had white matter which when suspended in water had many small animals in motion=bacteria. He looked at the white matter from the teeth of a man who did not drink or smoke and an alcoholic who only rinsed his mouth with wine. The sober man had spiral-shaped bacteria whereas the alcoholic had no bacteria! The moral is obvious!

It would seem unbelievable that an amateur could see things that professional scientists of the day could not see. The reason is that he could avoid the problems of chromatic and spherical aberration by using only a single, high-power, quality lens. In the compound microscopes of the time, these errors are synergistically increased. Click here to see the Leeuwenhoek web site.

A recent study of the remaining Leeuwenhoek microscopes and several reconstructions shows their magnifications to be from 50X over 200X, with resolutions as good as 2 microns. The microscope must be held close to the eyes and is very hard to focus. It requires excellent eyesight. Plans are available for making your own reconstruction together with instructions for using the microscope!

A picture of one of Al Shinn's reconstructions.


Leeuwenhoeck was also the first to cut thin sections for microscopy. He also was quantitative - he measured sizes in comparison to sand grains and counted the objects by using a capillary tube. His sizes were very accurate. In 1980, several packets he sent to London were discovered and the material reexamined by modern microscopy by Brian Ford.

Following are figures showing these samples.

Specimens from Leeuwenhoeck attached to letters he sent to the Royal Society - rediscovered by Brian Ford in 1980

 {short description of image}
From Darnell, Lodish and Baltimore, Molecular Cell Biology, Freeman and Co., 1990

BUT - his discoveries made no impact on science. One reason for this is that the technology was not available to other scientists and the more important reason is that there was no existing theory or "paradigm" into which these observations would fit. Leeuwenhoek himself called the organisms he saw "little animals" since there was no concept yet that the cell was the basic unit of life and that some organisms were actually single cells.

An important digression

The scientific method is to make observations and then to fit these into an existing theory. This then leads to predictions of  new observations which can verify or negate the original theory. A scientific "fact" is an observation that fits into a paradigm and leads to new predictions, that is, is verifiable. Changing a paradigm is a rare event in science and creating a new paradigm is even more rare. Let me give you some examples of what are considered paradigms. The cell theory discussed below is one and I  would include in the cell theory paradigm all recent studies on molecular and cell biology. The discovery of the double helix of DNA and  the realization that this explained replication of genes created the conditions for people to reevaluate the papers of Avery and led to a new paradigm - that DNA was the genetic material. The theory of evolution was a paradigm created by Darwin and Wallace and has so thoroughly permeated modern biology that every new fact is interpreted through evolution.

Advances in microscope technology:

1823 - The achromatic lens was developed - led to a 1 µm resolution.
This led to the discovery of the cell nucleus by R. Brown in 1833, the nucleolus in plant cells by Schleiden, cells in animals by Schwann, mitochondria in 1858 by Koelliker - and finally to a conceptual change in 1839:
The Cell Theory by Schleiden and Schwann.

Another conceptual change occurred in 1858 - Virchow stated that cells come from cells and each cell is an organism.

New technology was rapidly developed:
1878 - Oil Immersion lens
Led to discovery of chromosomes by Flemming in 1879.
1886 - Apochromatic lens - resolution of 0.2 um = theoretical limit of light microscope.

Figure from Darnell, Lodish and Baltimore, Molecular Cell Biology, Freeman and Co., 1990

{short description of image}

The head of a pin at different levels of resolution (from Darnell, Lodish and Baltimore, Molecular Cell Biology, Freeman and Co., 1990)

{short description of image}

Other major conceptual advances included the proof by Pasteur that spontaneous generation did not occur and the discoveries by Koch that microorganisms caused diseases. Finally, Patrick Manson introduced the concept of vector-borne parasitic diseases and founded the field of Tropical Medicine, from which modern parasitology was derived.

Patrick Manson

{short description of image}

After receiving his MD degree in 1860, Manson went to work in Formosa and then China. In 1877 he learned of the discoveries by Bancroft of the Filarial worm in blood and tissues of a patient with elephantiasis disease. Manson found these worms in normal patients also and saw that they had a daily periodicity that was similar to that of the feeding behavior of mosquitoes. He tested his theory that mosquitoes carried the worms by allowing them to feed on an infected person; the worms then developed in the insect. This was a major advance in medicine, that a specific blood sucking insect was an intermediary in the propagation of a particular disease. In 1898 Manson was convinced by Ronald Ross that mosquitoes could possibly transmit malaria. To test this theory he actually allowed Anopheles mosquitoes which had fed on a tertian malaria patient to bite his own son and his son came down with malaria! He then went with some friends to the Roman Compagna near Ostia, which was notorious for malaria, and lived in a mosquito-proof hut from July to October; he and his friends remained in perfect health, although, I imagine, somewhat bored. These experiments established clearly that mosquitoes carried malaria.

In 1898 Manson published the Manual of Tropical Disease, which established this as a field of science, and he later founded the London School of Tropical Medicine and Hygiene. The science of parasitology emerged from the medical discipline of Tropical Medicine. It came to be limited to the study of parasitic protozoa (eukaryotic cells), helminths and arthropods, and did not include the study of bacteria, fungi or viruses, which could also be classified as parasites by our above definition. This illustrates the facts that scientific disciplines are in a basic sense arbitrary attempts to organize natural phenomena and that these attempts are molded by history and influential people.

William Leishman and Charles Donovan
and the discovery of the cause of Kala Azar

 {short description of image}  {short description of image}

Kala Azar or Black Fever probably existed for centuries in Bengal and China, but the first recognized outbreak was in 1824 in Jessore, India. It caused the deaths of 750,000 people in 3 years. It was thought to be a type of malaria with relapses, emaciation, enlargement of liver and spleen. It appeared to be communicable and spread slowly along traffic routes.

Leishman grew up in Glasgow and became a doctor in the British Army Medical Service in1887. He was sent to India where he studied kala azar and then returned to London in 1900. His first original contribution to science was the development of an easy method to stain blood for malaria parasites, which is still used today. In 1901, he stained spleen smears from a soldier dying of kala azar, and observed small bodies, 2-3 um in size, each with two stained objects - spherical and rod-shaped, and he published this in 1903. These were the amastigote forms of Leishmania donovani. Also in 1903, Donovan, who was a Professor at Madras University in India, sent a sketch of parasites he saw in the spleen of a patient suffering from spleen enlargement and fever to Ronald Ross. Ross realized that these were the same parasites which Leishman had seen and he created the genus, Leishmania, in their honor. The parasites for some time were called "Leishman-Donovan" bodies, but are now called "amastigotes".

Click here to read more about William Leishman.

Soon after this, the protozoa were cultured in a blood medium, but the parasites had flagella and resembled trypanosomes.

The search for the vector of Leishmania donovani illustrates the way science sometimes works. Donovan and others suggested that bed bugs were the vector and this led to many years down a blind alley. The evidence that sand flies were the vector came mainly from a coincidence of the range of the insect and the range of the disease. H.E. Shortt was convinced that sand flies were the vector, but definitive proof was lacking. Finally in 1939, R.O. Smith found that flies which were allowed to feed on raisins after their blood meal became clogged with parasites. The Leishmania promastigotes grew so well that they plugged the pharynx and this proved to be how they were injected into the host at the next blood meal. As quoted in Desowitz's book from a lecture by a Dr. Knowles:

"The story of the discovery of how kala azar is transmitted from man to man is one of the most amusing, also perhaps one of the sorriest in tropical medicine. It is a history of almost 20 years of wasted effort..of false starts and erroneous conclusions; of acute controversies and the flow of much ink; of wasted effort and the absence of coordinated inquiry."

David Bruce

{short description of image}

David Bruce was born in Melbourne, Australia, in 1855. He grew up in England and graduated in medicine in 1881. He joined the Army Medical Service in 1883. In 1988 he spent a leave working in Koch's laboratory. He was then sent to South Africa where he discovered that a new trypanosome (T. brucei) was the cause of a disease of domestic animals called Nagana. Bruce described the disease as follows:

"The horse stares, he has a watery discharge from his eyes and nose. Shortly afterwards a slight swelling of the belly and puffiness of the sheath may be noticed, and the animal falls off in condition. The hind extremities also tend to become swollen; and these various swellings fluctuate, one day being less marked, or having disappeared. During this time the animal is becoming more and more emaciated, he looks dull and hangs his head, his coat becoming harsh and thin in places; the mucous membranes of the eyes and gums are pale, and probably slight cloudiness of the cornea is observable. In severe stages, a horse presents a miserable appearance. He is a mere scarecrow, covered by rough hair, which falls off in places. His hind extremities and sheath may be more or less swollen, sometimes to a great extent, and he may become blind. At last he falls to the ground and dies of exhaustion. During his illness he has shown no symptoms of pain, and up to the last days has had a good appetite."

Bruce found the organism in the blood and showed that they were transmitted from antelopes to cattle by the tsetse fly, Glossina morsitans. Bruce was the first to prove that an insect carried a protozoan causing a parasitic disease. 

In 1903 he went to Uganda to investigate human Sleeping Sickness. Sleeping sickness is a  disease that was widespread in Africa. Victims have a prolonged debilitating fever and after a few months become confused and stuporous. They often may develop enlarged lymph nodes and spleen, and ultimately they relapse into a coma and die. Post-mortem examinations show pericardial and peritoneal effusion and evidence of meningitis. Dr. Aldo Castellani, another member of the commission, had found trypanosomes in the cerebrospinal fluid of sick patients. Bruce found that trypanosomes were present both in the blood and late in the infection in cerebrospinal fluid in all Sleeping Sickness patients. He also was able to infect monkeys with spinal fluid and the monkeys developed sleeping sickness.  Bruce also noticed a co-incidence of the distribution of the tsetse fly and the disease.  He organized a national tsetste fly hunt and thousands of tsetse flies were dissected to study the development of trypanosomes and their evolution. Tsetse flies were allowed to feed on animals or persons suffering from trypanosomal disease and then on other animals to see if the disease had been transmitted. Bruce published his findings in 1903. From 1908 to 1910, Bruce rejoined the Royal Societies continuing commission in Uganda, where he directed research into conditions for  transmissibility of T. gambiense by Glossina palpalis, and studied cattle and game as potential reservoirs of the parasites. The commission was closed when one of its members, an unfortunate Lieutenant Forbes Tulloch, died of the disease.

In 1911, Bruce investigated an epidemic amongst the natives of Kaviondo on the shores of Lake Nyasa, whose banks swarmed with flies.  Flies caught by the native boys (2 of whom died of the disease) were put in cages in the laboratory and then fed  on three healthy animals: a dog, a monkey and a goat, each animal receiving 180 flies. In 56 experiments using 10,000 flies, 9 monkeys, 14 dogs and 11 goats were infected – an infectivity rate of 1/500. Thus, an inhabitant had a 1/500 chance of being bitten by an infected fly.

Bruce received many honors but  his abrupt manner, blunt speech, and egotistical personality made him many enemies. A famous surgeon, Harvey Cushing,  remembered Bruce as a member of a research committee they both served on: "General Sir David Bruce sank into a divan, stretched out his highly polished boots into the middle of the room, inserted his spurs into the rug, drew his John Bull visage deep into his clothes, turtle fashion, and slept profoundly which was good for the general and also helped the meeting."

In the center is a photo of Bruce in his hunting gear. His fame led to the issuance of stamps in his honor in several countries.

Click here for a good review of Bruce's life.

Charles Laveran

{short description of image}

Charles Laveran was a French Army surgeon, who in 1880 examined the blood of a soldier suffering from malaria in Algeria detected movement of the red blood cells and found that this was due to fine filaments attached to round pigmented bodies. He published that these were parasites but the conceptual framework of the time was that malaria was caused by a bacterium from swamp water, and noone believed his theory. In 1882 Laveran visited Rome and showed his parasite to two scientists, Marchiafava and Celli. In 1884 they looked at unstained blood from malarious patients and saw a active amoeboid ring (trophopzoite) in the red blood cells. They published this finding and named it Plasmodium malariae, but did not refer to Laveran since they thought it was something different from what he showed them. Camillo Golgi in Pavia also was studying malaria and showed that the clinical features were correlated with the life cycle of these parasites. He recognized several different species with different periodicities. Finally several other Italian workers discovered P. falciparum, which had a 48 hour cycle and produced a deadly disease.

Ronald Ross

 Ross was born in India and went to Medical School in London. He joined the Madras Medical Service and became interested in malaria. Ross showed in 1895 that the parasites Laveran had seen underwent exflagellation in Anopheles mosquitoes. In 1897 he tried breeding mosquitoes in the lab and fed them on a patient with malaria. On August 20 he saw pigmented oocysts in the wall of the stomach of one of these insects which had been fed 4 days earlier. Ross then turned to the study of bird malaria. Several Italian scientists carried on the study of human malaria. Ross unfortunately had a long feud with an Italian scientist, Grassi, on who actually discovered the life cycle of Plasmodium. This, unfortunately, is also the human side of science.



Following is an interesting description of the discovery of Ross from a book by Foster:

A selection from W. D. Foster, A{short description of image} History of Parasitology, E.S. Livingston Ltd., London (1965)


….to the Director-General hoping to get seconded for special duty. After his severe labour at Otacammund he felt a " violent reaction against the microscope " and could hardly bring himself to look through it for a month. The monsoon appeared to have failed and the heat and dust seemed unbearable. He wrote:

What ails the solitude? Is this the judgment day? The sky is red as blood; The very rocks decay And crack and crumble, and There is a flame of wind Wherewith the burning sand Is ever mass'd and thin'd.

Then the thought struck him: " Why not see whether mosquitoes fed on malarial blood as before contain any of the mosquito-parasites which I had found in the Sigur Ghat? I was at full work again on 21st July, 1897 on the last lap ".

His principle of work was, as he wrote to Manson, as follows: " I am of course sticking to the flagellulae again but on a slightly modified principle. Owing to their extreme delicacy I doubt whether they can be directly followed into the insect tissues; but this difficulty can be avoided in a simple manner. The question I have set myself to answer, and which I now feel able to tackle, is " Do healthy mosquitoes fed on malarial blood, contain any parasite which similar mosquitoes fed on normal blood do not contain? ... I keep mosquitoes, after feeding on crescents, for one, two, three days and then examine, so as to allow the flagellulae time, by supposition, to mature into full-grown mosquito-parasites suspicious bodies being noted, I have only to compare with control insects . . . ".

Ross must be allowed to tell his own story: " But the weather became very hot again in August. At first I toiled comfortably, but as failure followed failure, I became exasperated and worked till I could hardly see my way home late in the afternoons. Well do I remember that dark hot little office in the hospital at Begumpett, with the necessary gleam of light coming in from under the eaves of the veranda. I did not allow the punka to be used because it blew about my dissected mosquitoes, which were partly examined without a cover glass; and the result was that swarms of ' eye-flies '-minute little insects which try to get into one's ears and eyelids-tormented me at their pleasure, while an occasional Stegomyia revenged herself on me for the death of her friends. The screws of my microscope were rusted with sweat from my forehead and hands, and its last remaining eye-piece was cracked ". Writing to his wife he said: " I have failed in finding parasites in mosquitoes fed on malaria patients, but perhaps am not using the proper kind of mosquitoes ".

But now " some Angel Fate " had put into the hands of one of his " mosquito-men " some larvae, which hatched out a different, dappled mosquito, a single specimen of which Ross had seen in Sigur Ghat. On August 16th these were fed on his malaria patient, Husein Khan. That evening he wrote to his wife: " I have found another kind of mosquito with which I am now experimenting, and hope for more satisfactory results with it ".

On the 17th he dissected two of these mosquitoes but found nothing unusual. On the 19th he killed another and found "some peculiar vacuolated cells in the stomach about 10 microns in diameter ". He thought little about them and was rather disheartened at his results with the new species. On August 2oth, a dull, hot day, Ross went to the hospital at 7 a.m., examined his patients, dealt with his correspondence and had a hurried breakfast in the mess. One of his mosquitoes had died and this he dissected without noting anything significant. He had two mosquitoes left of the batch fed on Husein Khan on the 16th and at about 1 p.m. he determined to sacrifice one. Dissecting it he scrutinized the tissues micron by micron, when suddenly, in the stomach wall he "saw a clear and almost perfectly circular outline before me of about 12 microns in diameter. The outline was much too sharp, the cell too small to be an ordinary stomach-cell of a mosquito. I looked a little further. Here was another and another exactly similar cell ". He changed the focus of his microscope and there within each of these new cells was a cluster of black pigment. He made rough drawings in his notebook, sealed his specimen, went home to tea and slept solidly for an hour. The pigment puzzled him, for the flagella contained no pigment, but the thought struck him that if the cells were really parasites they should grow in size in the last remaining mosquito during the night. He spent the night in agony lest his last remaining mosquito should die and decompose before morning. Next day he slew and dissected, with shaking hand, this remaining specimen. There were the cells again, twenty-one of them, just as before, only now much larger . . . The cells were therefore parasites, and, as they contained the characteristic malarial pigment, were almost certainly the malaria parasites growing in the mosquito's tissues. The thing was really done.


Oocysts of malaria parasites in the midgut wall of a mosquito.









Ross's notebook  Aug. 20, 1897.

Next morning, after writing to his wife, Ross scribbled in a notebook:

This day designing God
Hath put into my hand
A wonderous thing. And God
He praised. At his command
I have found thy secret deeds
Oh million-murdering Death.
I know that this little thing
A million men will save
Oh death where is thy sting?
Thy victory oh grave?

All this he reported in a letter to Manson dated August 22nd,

Ross had, still, a long way to go before the life-cycle of the malaria parasite was worked out but the great significance of his discovery on August 20th-Mosquito Day, as he ever after called it was that he had shown that the malaria parasite went through further development in the mosquito tissues, beyond the crescent-spherule flagella transformation, which occurred, at least to some degree, on a microscope slide. Immediately following these discoveries his work was held up by shortage of dappled-winged mosquitoes and on 4th September he joined his family at Bangalore for 10 days leave. He made it his first task to write a paper " On some peculiar pigmented cells found in two mosquitoes fed on malarial blood ". The article appeared in print in the British Medical journal on December 8th, 1897. Meanwhile he returned to Secunderabad and after some delay obtained a supply of mosquitoes bred from the larvae.

Note: There is a recent review of Ronald Ross in Science magazine. Worth reading!

Carlos Chagas

{short description of image}











Carlos Chagas was a Brazilian scientist who was hired to eradicate an outbreak of malaria that occurred during the construction of a railroad in Brasil in 1907.

While he was working on this, people told him about a Reduviid bug, the barbeiro, that lived in rural huts and would come out at night and bite people on the face. Chagas found large numbers of flagellates in the hind gut of these insects. He allowed them to bite lab animals and they became infected in 3-4 weeks. He showed that armadillos were the natural reservoir. The parasites could be easily cultured on blood agar. He called these Schizotryanum cruzi and today are called Trypanosoma cruzi. Then Chagas discovered the trypanosomes in the blood of patients. He identified an acute phase and a chronic phase and showed that there was a cardiac involvement.

In 1911-1913 Chagas made several expeditions to the Amazon region to investigate tropical diseases which were hindering the building of railroads. These expeditions led to increased knowledge about Chagas Disease and malaria in the Brazilian Amazon.

{short description of image}

This was a unique case of someone first discovering a parasite and its life cycle, and then discovering the disease caused by the parasite and working out all the details of the pathology and epidemiology of the disease. It was a real tour-de-force in science.

William Trager

William Trager was a Professor at Rockefeller University in New York. He worked on protozoal parasites for his whole career - beginning with termite gut protozoa and then with malarial parasites. He was fascinated by the intracellular growth of Plasmodium and decided more than 20 years ago that the ability to continuously culture malarial parasites would not only lead to understanding the reasons for this unusual habitat, but also was a prerequisite for the ultimate goal of obtaining a vaccine against malaria. For many years Trager painstakingly attempted to culture the avian malarial parasite, P. lophorae, free of red blood cells. He was able to obtain several cycles of development and learned much about the nutritional requirements of these parasites. Then in 1976, Trager and Jensen discovered a simple method to grow the pathogenic human malaria, P. falciparum, in continuous culture inside red blood cells. This breakthrough stimulated a huge amount of research and may yet lead towards the development of a vaccine.



Dr. Trager at work:


I quote from an article about his work:

In 1973, Trager wrote a journal article that in essence announced his resignation from a 30-year attempt to grow malaria in culture. It couldn't be done, he said, and he would probably turn his efforts elsewhere. Without the ability to grow malaria in the laboratory, prospects were dismal that a vaccine could be created or new drugs developed to treat the disease. Researchers instead would require ready access to malaria patients, which meant living in Africa and braving the concomitant political and physical challenges. With the grandfather of malaria research surrendering the battle and approaching retirement age, it appeared a tiny microbe had routed the scientific world.

But in 1975, with malaria cases again rising after a two-decade campaign to eradicate it initially appeared to be succeeding and then collapsed, the World Health Organization (WHO) and USAID implored Trager to try again. He relented and agreed, but this time asked for help from a young graduate student who was effectively culturing a related organism that causes disease in animals.

The student, Jim Jensen, was just finishing his doctorate at Auburn University in Alabama (after completing a BS and MS in zoology at BYU, then beginning his doctorate at Utah State University but transferring to Auburn after the death of a key Utah State professor). Jensen quickly wrapped up his dissertation and moved with his young family to New York City, where he joined Trager in his Laboratory of Parasitology at the prestigious Rockefeller University.

Within one year, Trager and Jensen had triumphed over their nemesis, finally growing malaria successfully in culture. The duo's 1976 article in the top-flight journal Science provided the formula. It also made news worldwide, including the front page of the New York Times. The two scientists had given malaria research a new life.

"Researchers don't need to go to Africa anymore," said Jensen, speaking in his large laboratory on the top floor of the Widtsoe Building. "All you need is an incubator. You can grow malaria in any laboratory."

And grown it is, in hundreds of labs around the world, including Jensen's. Today, the Science article is so classic that it is rarely cited formally. Instead, researchers simply report that they cultured the malaria parasite "according to standard procedures."


Dr. Trager died Jan. 22, 2005. Following is a Memoriam in press in J. Eukary. Microbiol.:


William Trager, Professor Emeritus at the Rockefeller University died at his home in Manhattan on January 22, 2005. Professor Trager during his long and productive career made many seminal contributions to the “how and why” of organisms living together. Now, with his passing it is fitting that we pay respect to a brilliant researcher, a thoughtful adviser and an inspirational teacher.

William Trager was born on March 20th 1910 in Newark, N.J. the son of Leon and Anna (Emilfork). Early on his parents encouraged scholarly and intellectual pursuits. He received his B.S. degree from Rutgers University in 1930 and his M.A. and PhD from Harvard University in 1931 and 1933 respectively. After completing his PhD under the direction of L.R. Cleveland where he studied the cellulase activity of termite flagellates he took a position as a Fellow of the National Research Council in the Department of Animal Pathology in the Princeton Division of the Rockefeller Institute. There, in the laboratory of R. W. Glaser—whose forte was growing axenic (bacteria-free) cultures of protozoans—Trager worked on insect physiology and pathology. This early training sowed the seeds for his talent and success in culturing parasitic protozoans. In 1934 he was appointed to the staff of the Rockefeller Institute where he remained for his entire professional life.

All of Professor Trager’s research centered on three questions:

  1. What special environmental factors does a host supply to a parasite?
  2. Can these factors be furnished in a non-living culture medium so that the parasite will go through its complete developmental and reproductive cycle?
  3. How do the products of the parasite effect appropriate modifications in the host?

Answers to these questions, he believed, would lead to new chemotherapeutic agents, protective vaccines and provide a molecular basis for understanding parasitism.

 During Professor Trager’s early period at Rockefeller he succeeded in growing mosquito larvae in axenic culture—no small feat since this was before the advent of antibiotics—and this allowed for a determination of their nutritional requirements. Another accomplishment was his ability to propagate a polyhedral virus in silkworm tissue cultures and to achieve the in vitro multiplication of Western equine encephalitis virus in mosquito tissues—the first demonstration of an arbvovirus in insect tissue culture. Trager would return to insect tissue cultures in 1958-59 and 1975 when he used tsetse fly tissue cultures for growing the insect stages of Trypanosoma vivax. Trager’s interests in exploring the host-parasite relationship led to his 1939 discovery of acquired immunity to ixodid ticks.

 Professor Trager’s earliest contributions to malaria came as a result of his World War II experiences in New Guinea as an Officer in the U.S. Army Sanitary Corps. There, along with Frederick Bang and Malcolm Ferguson, he was involved in the supervision of trials of a new antimalarial, atebrine. Upon is return from the Pacific, he began his laboratory studies at Rockefeller on the nutritional requirements of the malaria parasite using the bird malaria Plasmodium lophure, which attained very high parasitemias in white Pekin ducklings. After 4 years of painstaking effort Dr. Trager published a series of papers on the extracellular cultivation of the asexual stages of this parasite—from ring to schizont. This was a Herculean effort that defined ATP and coenzyme A, malate and pyruvate as well as high K+ as key requirements. He went on, in subsequent studies, to define the role of pantothenate, folic and folinic acid and biotin in the growth and maturation of the parasite. During the early “lophurae” period he worked with R. Barclay McGhee in describing erythrocyte receptors for the merozoite---studies later pursued by Louis Miller, Margaret Perkins, Terrence Hadlley and Geoffrey Pasvol that led to the identification of glycophorin and Duffy factor as receptors for P. falciparum and P. vivax respectively. In 1957, now at the Rockefeller Institute in New York City, Trager initiated, in collaboration with Dr. Maria Rudzinska, a cell biologist, electron microscopic studies of the intracellular development of Plasmodium and Babesia. Rudzinska and Trager discovered the parasitophorous vacuolar membrane, the large hemoglobin-containing food vacuoles, and described the mitochondrion and nucleus of P. lophurae. Although such work was later exceeded in refinement of detail by others (such as Susan Langreth, Lawrie Bannister, Masamichi Aikawa and Norbert Lanners) it is well to recognize that this pioneering work was carried out before there were better fixatives such as glutaraldehyde, and involved transmission electron microscopes with far less resolution than those in use today. In 1966, Trager in collaboration with Rudzinska and Phyllis Bradbury described the ultrastructure of P. falciparum and identified the knobs on the surface of the infected erythrocyte that contribute to sequestration. Later studies (1968) with the monkey malaria, P. coatneyi, confirmed much of this, and subsequently with Susan Langreth (1978) the fine structure of extracellularly grown parasites was described.

In 1976 Professor Trager literally invented a method for the continuous cultivation of the erythrocytic stages of P. falciparum. The work was elegant in its simplicity: a stationary layer of red cells, a medium containing human serum, a buffer to prevent pH shifts from lactic acid produced by the parasite, and a gas mixture low in oxygen and high in carbon dioxide. Initially developed by Trager as a continuous flow system it was modified through collaboration with a postdoctoral fellow, James Jensen, to one that is completely portable---the candle jar technique. The successful culture of P. falciparum spawned a renaissance of research on the immunology, cell biology and molecular biology of this parasite. Since the publication of this monumental discovery there has been an explosion of information about falciparum malaria. The availability of  “tamed” falciparum allowed this parasite to be grown anywhere in the world without sophisticated equipment, and allowed for new knowledge about the genetics of red cell susceptibility, the antigens of the parasite, how to achieve differentiation into gametocytes, the number of chromosomes, the sequencing of the genome, the development of knobless lines, the testing of parasites for drug sensitivity and much more. Trager developed well-characterized clones of P. falciparum and studied gametocyte formation in some of these clones (1984). And, his success in the extracellular development of erythrocytic merozoites of P. falciparum to the ring stage, suggested that invasion of the red cell per se need not be a trigger for differentiation.

 In his 1962  “Address of the Past President of the Society of Protozoologists”, published in the Journal of Protozoology---which he was instrumental in founding---Trager spoke not a word about malaria but confined his remarks to the “differentiation exhibited by protozoa with reference both to their own intrinsic interest and especially to their value in the study of cellular differentiation.”  This address was illustrative of his general interest in protozoa and their utility as model systems to address basic biologic problems. 

Another major research interest was the kinetoplastid protozoa. In 1952 Trager showed that the normally intracellular amastigote forms of the human parasite, Leishmania donovani could be cultivated extracellularly for several days at 370C. This work was stimulated by Trager’s close friendship with Leslie Stauber at Rutgers University who was the doyen of pathogenic Leishmania at that time. This paper provided the stimulus for the eventual development of continuous culture systems for Leishmania amastigotes in several other laboratories. In 1958, Trager spent a sabbatical year in Nigeria at the West African Institute for Trypanosomiasis Research studying T. vivax, the most important pathogenic trypanosome of cattle in West Africa. He mentioned in his Presidential Address to the American Society of Parasitologists that earlier he had been told by Dr. Robert Hegner that “if you want to see the world…become a parasitologist” and he considered this to have been sound advice. In Nigeria, Trager combined his previous expertise in insect tissue culture with his knowledge of culture conditions and established the complete life cycle of T. vivax using tsetse fly tissue cultures. He returned to Nigeria in 1973-74 and after initially obtaining negative results, probably due to strain differences of the parasites, quickly retooled and established a method to culture bloodstream forms of T. vivax using tstetse organ cultures.

In 1957 Trager developed a defined chemical medium for the continuous culture of the lizard parasite, L. tarentolae. This was the first culture of a hemoflagellate in a defined medium and allowed Trager to define several metabolic and nutritional requirements of these parasites such as the vitamin biopterin. This finding led to much recent work on selective chemotherapy of these parasites in other laboratories. In collaboration with the electron microscopist, Maria Rudzinska, Trager then showed that acriflavin-induced akinetoplastic or dyskinetoplastic  L. tarentolae cells exhibited major changes in the DNA-containing kinetoplast at the base of the flagellum. This work was the impetus for much research in many laboratories on the mitochondrial DNA of trypanosomes (including the PhD thesis research of one of the authors, LS). And finally, in 1979, Trager found a graduate student, Michael Yamin, interested in his old love, termite protozoa, and they showed that Trichomitopsis termopsidis contained cellulase activity that was not dependent on extracellular or intracellular bacteria. Defaunating and refaunating termites with these protozoa and following the effect on their longevity demonstrated the role of these protozoa in the nutrition of the termite.

 Professor Trager, the exemplar of a bench scientist, worked in his Rockefeller laboratory on a daily basis, carrying out with his own hands the preparation of reagents, setting up of cultures, carrying out unbiased analysis, and meticulously recording his observations and conclusions. The impeccable thoroughness with which he pursued his science resulted in his work having a high degree of reproducibility and reliability. Yet through it all he was always modest. At Rockefeller there was a generous exchange of ideas, of methods, and even of ducklings; there was respect for one another, and each of us was made to feel as if we were colleagues. Some of us felt uncomfortable calling our mentor “Bill” but it was encouraged by him and there was enduring warmth for each of us. That support extended from the very first day we entered his laboratory to the time of his death.  Although Professor Trager encouraged students and post-doctoral fellows to seek out independent positions he expected that we would carry with us that which we learned at Rockefeller. However, most of us felt that we never really left the Trager lab for his support and high standards were always with us. He enriched our lives beyond measure. 

During his lifetime Professor Trager received many awards and honors. He received honorary degrees from Rockefeller University (1987) and Rutgers University (1965), the Manson Medal of the Royal Society of Tropical Medicine and Hygiene (1986), the Leuckart Medal (1982), S.T. Darling Medal of the World Health Organization (1980), the Augustine Le Prince medal of the American Society for Tropical Medicine and Hygiene (1991) and Thailand’s Prince Mahidol Award (1994-1995).   In 1973 he was elected to the National Academy of Sciences (USA) and in 1973-74 was a Guggenheim Fellow. He served the Society of Protozoologists as Editor (1953-65) and President (1960-61), was President of the Society of Parasitologists (1973-74) and President of the American Society of Tropical Medicine and Hygiene (1978-79).

Professor Trager’s contributions to malaria and kinetoplastids are represented by more than 200 publications, two books (“Living Together: the Biology of Animal Parasitism”, Plenum 1986 and “Symbiosis” Rheinhold, 1970) and many honors. However, Bill Trager did much more than that; he served as the guiding light for a large number of malariologists including: Barclay McGhee, Ira Singer, Wassim Siddiqui, George Jackson, Peter Bennett, Edward Platzer, James Jensen, Susan Langreth, William Scheibel, Robert Reese, Araxie Kilejian, Ann Vezza, Phyllis Bradbury, Harold Stanley, Margaret Perkins, Milton Friedman, Norbert Lanners, Fred Brohn, Chariya Brockelman, Mary Motyl, Julie Olsen, V. Bhasin, Fred Gyang, Robert Herman, Michael Hollingdale, Phuc Nguyen Dinh, Jonathan Williams, Gokal Gill,  and Irwin Sherman. And he sparked interest in kinetoplastids by having as associates or trainees Philip D’Alesandro, Curtis Patton, Stuart Krassner, Larry Simpson, Dennis Dwyer, Andrew Balber, K.P. Chang, Alan Clarkson, Jan Keithly, Miki Rifkin, Rolf Steiger and Phyllis Straus. Others in his laboratory who worked on symbionts, parasites and insects were Earl Weidner, Joseph Peleg, Abraham Held, Dunne Fong, Wallace Fish, Dickson Despommier, Steve and Marlene Karakashian, and Michael Yamin. 

Professor Trager was a member of many scientific societies including the American Association for the Advancement of Science, New York Academy of Sciences, Entomological Society of America, American Society of Zoologists, American Society of Parasitologists, American Society of Experimental Pathology, Society of Experimental Biology and Medicine, American Society of Tropical Medicine and Hygiene, Royal Society of Tropical Medicine and Hygiene. He enjoyed summer stays at the Marine Biological Laboratory in Woods hole, MA and was a member of the Corporation. 

William “Bill” Trager was generous in providing the resources of his laboratory for training, sent his cultures to all who needed them and was unstinting in his devotion to his students and colleagues. Those of us who were fortunate enough to be friends of this very modest man will remember him with pleasure and affection. 

Irwin Sherman and Larry Simpson (2005).

Modern Parasitology

In recent years the study of parasites has had a resurgence of interest due to the development of recombinant genetics and cell and molecular biology and the ability to study parasites at these levels. There still is a close attachment of parasitology with Tropical Medicine since most of the parasitic diseases are found in the tropics and subtropics. The World Health Organization has recently stated that there are six major human tropical diseases - schistosomiasis, malaria, filariasis, African trypanosomiasis, leishmaniasis, and leprosy. The first five of these are caused by parasites.

Many parasitic diseases now also occur in temperate industrial countries in immunocompromised patients. The application of modern molecular techniques have revolutionized the study of these diseases both in terms of diagnosis, vaccine development and rational chemotherapy. In addition, it is increasingly clear that many of the protozoa parasites are among the most deeply diverged eukaryotic organisms and therefore represent model systems for the study of the evolution of eukaryotes. Finally, it has become established that symbiosis-parasitism was intimately associated with the origin of the eukaryotic cell and that our cells represent the end product of eons of interactions between different organisms and genomes.

Click here for a list of events in the history of parasitology.

The images in this Chapter are taken from several sources:
Wellcome Illustrated History of Tropical Diseases (Ed. F. Cox) Wellcome Trust Pub. Co., London (1996)
H. Scott (1939) A History of Tropical Medicine, Williams and Wilkins Co. Baltimore.

Some questions to test your understanding of this material:

1.  What is parasitism? Is it unusual in biology?

2. Who started modern Parasitology? Why does Parasitology not include the study of bacteria, fungi or viruses?

3. What was the major contribution of Leeuwenhoek? What general principle is illustrated by Leewenhoek's work?

4. Why are Leishmania amastigotes often called "Leishman-Donovan bodies"?

5. Did Ross discover that mosquitoes cause malaria? What did he discover?

6. Who first realized that an insect can be the vector for a parasitic disease?

7. What was unusual about Chagas's discoveries?

8. What was the major contribution of William Trager to Parasitology?

Hit Counter