MicrobiologyBytes: Infection & Immunity: Protozoa & Helminths Updated: August 23, 2007 Search

Protozoa and Helminths

The medically important protozoa and helminths require the invasion of a suitable host to complete all or part of their life cycle. Such organisms are therefore termed parasites and medical parasitology is the study of protozoa and helminth infections of man. The exception are some free-living amoebae that normally live and replicate in the environment but under certain circumstances can infect man. They are not therefore parasites but are included here as examples of protozoa causing human disease.

Parasite infections affect millions of people world-wide afflicting considerable human suffering and economic hardship. Far from declining, many parasite infections are increasing throughout the world. The impact of Human Immunodeficiency Virus (HIV) and AIDS has seen the emergence of "new" opportunistic parasites as well as the increased prevalence of other recognised types. Climatic changes induced through global warming has aided the spread of many parasite diseases, whilst starvation and the breakdown in sanitation that accompanies war has seen the re-emergence of others. The appearance of drug resistance has also dramatically influenced the ability to treat and control many parasite diseases. In the United Kingdom parasite infections are relatively uncommon. However, outbreaks of cryptosporidiosis associated with drinking water supplies has been of major concern, and toxoplasmosis remains a serious infection for the foetus when acquired during pregnancy.

It is beyond the scope of this brief summary to characterise all protozoal and helminth infections of man. For students wishing to obtain further information on these fascinating and largely unexplored microbes, a few reference books are given at the end of this section. World Wide Web sites also contain useful information as well as diagrams and picture images of the protozoa and helminths discussed below. Find them through key word searches using one of the several search engines available.

Medically Important Protozoa

Protozoa means "first animal" and refers to simple eukaryotic organisms composed of a single cell (e.g. amoebae). Reproduction may be through simple cell division (e.g. the amoeboflagellates) or sexual involving the fusion of gametes in part of the life cycle (e.g. the apicomplexa) as described below. Some protozoa can form a protective cyst stage capable of withstanding harsh environmental conditions.


These use pseudopodia or flagella for locomotion.

Amoebae. These are characterised by a feeding and dividing trophozoite stage that can form a temporary resistant cyst stage.

Entamoeba histolytica is the cause of amoebic dysentery producing severe infection of the intestines that can spread to the liver. The organism is characterised by a trophozoite and cyst stage. E. histolytica is an example of a true parasite in that the organism cannot multiply outside of the host. Other amoebae occur naturally in soil and water environments which is their preferred habitat for feeding and replication. These amoebae are termed "free-living" as they have no natural host in which parasitism occurs. They can infect man opportunistically producing severe and often fatal disease. Such free-living amoebae are the Acanthamoeba, Naegleria fowleri and Balamuthia mandrillaris, all of which can infect the central nervous system. In addition, Acanthamoeba species can also invade the eye.


These organisms have flagella in the trophozoite stage. Trichomonas vaginalis is a common sexually transmitted organism causing trichomoniasis infection of the vagina and urethra. Giardia lamblia causes giardiasis producing symptoms of diarrhoea and other intestinal disturbances. Infection arises from the ingestion of cysts, usually through contaminated water.

Trypanosoma brucei gambiense and T. brucei rhodesiense cause trypanosomiasis, more commonly known as African sleeping sickness. The disease is an arthropod (insect)-borne infections and is spread by the bite of the tsetse fly in which part of the trypanosome life cycle is completed. The eventual invasion of the central nervous system by the trypanosomes gives rise a comatosed state from which the common name for the disease is derived.

Trypanosoma cruzi causes Chagas' disease (American trypanosomiasis). The intermediate host in this case are triatomid bugs that feed off the blood of man. Infection results from the inoculation of the bug's faeces that contains the organism into the bite wound. Individuals who survive the acute stage of the disease are frequently left with chronic and progressive neuronal and smooth muscle lesions in the heart and gastrointestinal tract. T. cruzi has an extensive reservoir in wild and domestic mammals and therefore Chagas' disease is a zoonosis (human infections that can be caught from animals).

Leishmania species cause leishmaniasis. The disease is spread by the bite of sandflies in which part of the organism's life cycle is completed. In man, the promastigotes from the bite of the sandfly become ingested by macrophages and multiply within them as amastigotes. Cutaneous leishmaniasis occurs if the region of infection remains localised to the dermis as an open sore. In the Old World (Southern Europe, the Middle East, India, former USSR and parts of Africa) L. major, L. tropica, L. aethiopica and certain subtypes of L. infantum are responsible. In the New World (Mexico southwards and through South America) species responsible include L. braziliensis, L. mexicana and L. amazonensis. If the organism spreads, then mutocutaneous leishmaniasis can occur in which the nose, mouth and palate becomes destroyed.
Infection with members of the L. donovani-L.infantum complex produce the systematic disease of visceral leishmaniasis often known as kala-azar that occurs with a global distribution seen in Old and New World leishmaniasis. The parasites multiply within the macrophages of the liver, spleen, bone marrow and other organs. Untreated, the disease is usually fatal. As with trypanosomiasis, leishmaniasis is a zoonosis as many mammals harbour the parasite.


These possess rows of cilia around the outside of the body that aid motility. The only member of this group known to infect man is Balantidium coli. This is a cyst forming parasite that is a commensal ("table-sharing" and meaning here a non-pathogenic parasite) of domestic and wild pigs. It does, however, cause severe diarrhoea in humans.


This is a unique group because all members are parasitic. The group includes parasites causing malaria, cryptosporidiosis and toxoplasmosis. They lack any visible means of locomotion (most are intracellular) and have complex life cycles involving sexual and asexual reproduction.. The common feature of all members is the presence of an apical complex in one or more stages of the life cycle. Although the exact components of the apical complex varies among members, it contains enzymes used to penetrate host tissues.

Malaria. Plasmodium species cause malaria. The four principal species are P. falciparum, P. vivax, P. ovale and P. malariae. Malaria means "bad air" and dates from the time when the disease was thought to be spread from stagnant, foul smelling water. The disease is in fact transmitted by the female Anopheles mosquito that inhabits such environments. In the stomach of the female Anopheles male (micro-) and female (macro-) gametocytes fuse to form a zygote. This in turn forms a motile ookinete that penetrates the midgut wall and develops into an oocyst within which are many thousands of sporozoites. When mature, the sporozoites rupture the oocyst and penetrate the salivary glands. When the mosquito next feeds on man, the sporozoites are passed via the blood stream to infect parenchymal cells of the liver. Here they form pre-erythrocytic schizonts in which several thousand daughter cells, called merozoites. These merozoites enter red blood cells to start the asexual intraerythrocytic cycle and form new gametocytes. The asexual red cell stages are responsible for the pathological changes that occur in malaria (fever, chills, anaemia, liver enlargement, encephalitis renal damage and death).

Cryptosporidiosis. Cryptosporidium parvum causes diarrhoeal disease mainly in infants and small children. It is normally self-limiting but in the immunocompromised host the disease can be severe. C. parvum is enzootic in young calves and is usually passed to man in water containing oocysts of the organism. Video

Toxoplasmosis. Toxoplasma gondii causes the multi-organ infection of toxoplasmosis. The domestic cat is the definitive host for T. gondii from which man and other mammals can become infected. Infection commonly arises from the consumption of under cooked meat and in the healthy adult is usually asymptomatic. The most devastating form of toxoplasmosis is seen in congenital infection when a pregnant mother passes the organism to the foetus. This can result in severe abnormalities at birth. The life-cycle of T. gondii is complex, involving both sexual and asexual reproduction. Three main life forms of T. gondii occur: (i) the oocyst which is produced from the sexual cycle in the small intestine of the cat and contains sporozoites; (ii) the tachyzoite of the asexual invasive form found in secondary hosts which are derived from pseudocysts; and (iii) the tissue cyst that contains bradyzoites. The life cycle of T. gondii is shown in the following diagram:


The medical importance of microsporidial infections has only recently been highlighted by the frequent recognition of these obligate intracellular parasites in material from patients with HIV infection and AIDS. Examples are: Encephalitozoon species, Nosema species and Septata intestinalis. Multi-organ infections occur and S. intestinalis is found in about 2% of all AIDS patients with chronic diarrhoea.

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Medically Important Helminths

In contrast to the protozoa, helminths (worms and flukes) are multicellular with complex reproductive systems and life cycles involving intermediate hosts for the development of larval stages and a definitive host for the adult form. Adults may be dioecious with separate sexes or hermaphroditic.

Nematodes -the filariases

These are arthropod (insect)-borne infections caused by filarial worms. The classic example is elephantiasis caused by Wuchereia bancrofti. Larvae of W. bancrofti develop in the mosquito and, as in malaria, human infection results from the bite of the insect. When bitten, the larvae (as male and female forms) pass through the lymphatics and mature to thread-like adults, 4-8 cm long, in the lymphatic glands. After mating the females develop eggs and larvae that are released as microfilariae into the peripheral circulation.

The localisation of the adult filariae in the lymph glands causes obstructions in the lymphatic drainage. This then results in the grossly disfiguring condition of elephantiasis that typically involves massive swelling of the legs, scrotum and other extremities.

Other arthropod-borne microfilarial infections of man are loaiasis caused by Loa loa. The disease is spread by biting flies and the adult worms migrate along connective tissue, usually reaching the conjunctiva of the eye. Onchocerciasis ("river blindness") is caused by Onchocerca volvulus, a tissue dwelling nematode, the microfilariae of which are predominantly found in the eye and skin.

Soil-Mediated Helminthiases

Soil-transmitted helminthic infections are of two types: the hookworms, which undergo a cycle of development in the soil (the larvae being infective), and a group of nematodes that survive in the soil merely as eggs that have to be ingested in order for the cycle to continue.

Hookworms. The most common hookworms are Ancylostoma duodenale and Necator americanus. Adults attach to the walls of the jejunum and females lay large numbers of eggs that are passed out with the faeces. The eggs hatch in the soil and infect man by usually burrowing through the soles of the feet. The larvae then migrate to infect the heart and lungs before passing into the tracheae, pharynx and then the small intestine.

Strongyloides stercoralis. Females live in the mucosal glands of the small intestine. Eggs hatch in these glands and the larvae are passed with the faeces into the soil. As with other hookworms, infection results from the larvae burrowing into the skin. The rest of the life cycle is as for A. duodenale and N. americanus.

Ascariasis. Adult worms of Ascaris lumbricoides live in the small intestine where they lay large numbers of eggs that are passed out with the faeces. Unlike the hookworms, the eggs are the infectious form in which the larvae develop. When ingested, the eggs hatch in the jejunum, penetrate the mucosa and are carried through the hepatic circulation to the heart and lungs. They again enter the stomach via the tracheae and oesophagus before growing to adulthood in the small intestine. Pneumonitis and intestinal obstruction may accompany heavy infestations.

Toxocariasis. The disease results from the accidental infection of man with eggs of the ascarid roundworm of the dog, Toxocara canis, and cat, T. cati. The life cycle is the same as that of Ascaris but the invasive larvae become arrested in various tissues where they are phagocytosed. In the process they induce marked eosinophilia and local tissue reaction commonly involving the liver and eye.

Trichurias. Trichuris trichiura ("whipworm") inhabits the caecum where they attach to the mucosa. Eggs from the mature worms are passed with the faeces and develop in the soil. When swallowed, the eggs hatch in the small intestine and the developing larvae pass directly to their attachment sites in the large intestine. Heavy infections can cause abdominal pain and chronic bloody diarrhoea that may result in rectal prolapse.

Snail-Mediated Helminthiases

This important group of snail-transmitted helminthiases is all caused by trematodes (flukes) that undergo a complicated cycle involving various species of land or aquatic snails. The most significant of these fluke infections is schistosomiasis and over 200 million people are estimated to be infected world-wide. The three common species infecting man, Schistosoma mansoni, S. japonicum and S. haematobium have similar life cycles. Eggs are passed in the urine (S. haematobium) or faeces (Schistosoma mansoni and S. japonicum) and hatch in natural waters. Miracidia hatch from the eggs, penetrate suitable snails and develop two generations of sporocysts. The last of these then produces fork-tailed cercariae. These cercariae penetrate the skin when a new host comes into contact with the contaminated water. Once through the skin the cercariae shed their tails and become schistosomulae that then migrate through the tissues to the liver. Here male and female flukes copulate and migrate to either the bladder or rectum where eggs are laid. Schistosomiasis can result in chronic liver, spleen and bladder damage.

Fascioliasis. Fasciola hepatica is found in most herbivores (but primarily sheep) that graze in wet pasturage where the intermediate host, snails of the genus Lymnaea, are found. F. hepatica eggs, shed from the infected primary host, mature into the embryonated form in the environment. These then hatch and release a motile miracidia that seeks out and penetrates the tissue of the intermediate snail host. Cercaria are produced in the snail that when released into the environment can encyst to produce metacercariae. In temperate climates man is often infected by eating wild watercress on which metacercariae have collected. After being ingested, the metacercariae pass through the duodenal wall and penetrate the liver capsule. Following maturation of the young flukes, the adults finally come to lie in the bile ducts or adjacent liver tissue. Here they cause severe damage to the biliary tract and eggs are passed with the bile into the faeces to continue the cycle.

Helminth infections acquired through the gastrointestinal tract

Trichinosis. Trichinella spiralis is the cause of trichinosis in man. The nematode circulates between rats and pigs with man becoming infected from eating raw or inadequately cooked pork products. Encysted larvae in the meat excyst (hatch) in the intestine and develop into minute adults in the mucosa. These mature and the females deposit larvae that then migrate through the tissues to reach skeletal muscles in which they encyst. Human infections may be asymptomatic but can include fever, orbital oedema, myalgia and eosinophilia. In the extreme, infection can be fatal through myocarditis or encephalitis.

Enterobiasis. Enterobias vermicularis is a small thread-like "pinworm" mainly infecting young children. The female emerges to the perianal region usually at night and lays some 10,000-15,000 eggs and then dies. In the process they cause severe pruritis (itching). The embryonated eggs are infectious on ingestion and hatch in the duodenum. The larvae pass to the caecum where they mature into adults. Because of the pruritis, children often re-infect themselves from eggs under their fingernails. Bedding is also a source of infection and can be a means of spreading the organism in families and institutions such as orphanages and boarding schools.

Taenia solium (pork tapeworm). The adult lives in the small intestine of man that is the definitive host. Segments of the worm pass through the anus and release large numbers of eggs that can survive for long periods outside of the body. When ingested by pigs, the eggs hatch and each releases an onchosphere that migrates through the intestinal wall and blood vessels to reach striated muscle where encystment occurs. When inadequately cooked pig meat is eaten by man, excystment occurs in the small intestine and an adult cestode (worm) develops. If the eggs are released into the upper intestine of man (e.g. through regurgitation) they can invade the host setting up a potentially dangerous larval infection known as cysticercosis in muscle and other sites.

T. saginata (beef tapeworm). This also infects man through cattle. The life cycle is similar to T. solium and in both species the adult tapeworm can grow up to 10 meters in length.

Hydatidosis. This is caused by the tapeworm Echinococcus granulosus. The adult worm inhabits the small intestine of dogs from which the eggs of the species are passed. These eggs can be ingested by herbivorous animals and hatch in the duodenum. The embryos enter the circulation where they are carried to various sites to develop into cysts. Dogs become infected when they eat contaminated offal. Humans are infected if they accidentally ingest eggs from infected dogs and the liver is the most common site of infection in which hydatid cysts form.


Baker, J.R. and Muller, R (Eds). Advances in Parasitology. Academic Press, London.
Knight, R. (1982). Parasites Diseases in Man. Churchill Livingstone, London.
Peters, W. and Gilles, H.M. (1995) Colour Atlas of Tropical Medicine and Parasitology. Mosby-Wolfe.

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