Society, the Individual and Medicine

Parasitic Diseases   To Infectious Disease theme page

1. Core Knowledge:

‘Parasite’ derives from the Greek para sitos meaning 'around food', which referred sarcastically to the type of person who dines at the expense of another.

A parasite lives on or in another species, getting everything it needs from the host. Human parasites especially thrived once people ceased being nomadic and stayed in one place, kept livestock close by and discarded their waste (especially sewage) near to where they lived.

“All too often, parasites are able to dine on us because we have dined on them. Arriving at the table unannounced and unseen, they are not just at dinner, they are dinner.” (Rosemary Drisdelle. Parasites. Berkeley, U California Press, 2010, p. 36). 

2. Nice to Know:

Here is an overview of the major classes of human parasite and the conditions they cause

Class of organism

Intestinal conditions

Systemic conditions

Subkingdom: Protozoa
(single cellular eukaryotes)

   Phylum: Sarcomastigophora

      Subphylum Mastigophora

Giardia lamblia


Trypanosoma spp.


Entamoeba histolytica

      Subphylum Apicomplexa






Acamthamoeba keratitis

Subkingdom: Metazoa (multicellular organisms)

   Phylum: Nematoda: roundworms

      Subphylum: Filaria




Onchocerciasis: river blindness

Guinea worm


Pinworm: enterobiasis


   Phylum: Platyhelminthes (flatworms)

      Subphylum: Cestodes (tapeworms)

Intestinal tapeworms

Hydatid disease


      Subphylum: Trematodes (flukes)

Intestinal flukes


Liver flukes

3. Additional Information:

The following summary table is drawn from a variety of sources, including Dr. Drisdelle’s book and the definitive web source, the CDC parasites web pages.  




Subkingdom: Protozoa

   Phylum: Sarcomastigophora

      Subphylum: Mastigophora

Giardia lamblia

Carried by various animals including dogs, cats, sheep, cows and (notoriously) beavers, so never drink downstream of a beaver dam! In 1990, the town of Creston, BC was drawing its water from the pristine mountain stream above the town. But a beaver moved upstream of that and caused an outbreak.

Giardiasis, or “beaver fever”. Parasites coat the inside wall of the intestine of an infected human, blocking the absorption of nutrients and causing diarrhoea, flatulence, cramps. The attack lasts a few days, but can then recur intermittently.
An infected patient passes millions of cysts, and as few as 10 can infect a person, so transmission occurs rapidly if the water supply filtration system is not working properly.

Trypanosoma brucei;
T. brucei gambiense (in West & Central Africa);
T. brucei  rhodesiense
(in East Africa)

Infected Tsetse fly (Glossina) deposits eggs in saliva when it bites a victim. Bushbuck deer are the reservoir.
Luckily there are no tsetse flies in the New World, so T. gambiense stayed in Africa.

Sleeping sickness. Symptoms: edema, tenderness, fever, headache, joint & muscle pain; lymphadenopathy. 
Trypomastigotes invade lymph nodes; feet & hands swell, intermittent rash & fever. Invades the brain and leads to fatigue & apathy. Sleep leads to coma and then death.


Trypanosoma cruzi

The vector is the ‘kissing bug’ (Triatoma infestans). This is native to Central and Latin America and gets its name because it typically bites the thin skin at the corner of a victim’s mouth to get a meal of human blood. It defecates as it feeds, and the T. cruzi trypomastigotes it carries are deposited on the victim’s skin. They enter the body and spread to lymph nodes and disperse through the blood, invading and destroying nerve and muscle cells.
The kissing bug typically lives in crevices in homes, coming out at night to feed. The substandard or dilapidated houses of poor people are often infested. It is hence considered a neglected disease of poverty. Perhaps 8 to 11 million people are infected, most without knowing it.

American trypanosomiasis, or Chagas disease (named after Carlos Chagas, who discovered the parasite in 1909 in Rio de Janeiro). The disease can kill within weeks; the heart muscle cells become so damaged that victims die of heart failure, or else nerve function is damaged and internal organs fail.
The disease can be spread via blood donation or organ transplantation – this was a common mode of transmission in Latin America until routine blood screening was established in the 1990s.

   Phylum: Apicomplexa

Cryptosporidium parvum

Water-borne parasite that originates from cattle and other animal feces, including Canada geese. Spring run-off brings debris from fields including animal wastes.
Chemical water purification kills bacteria and viruses but not necessarily parasites, which have to be filtered out. The C. parvum oocytes with their sporozoites inside are tiny (1-2 μ), light and buoyant so can be carried for long distances in the spring run-off. This led to an outbreak of around 7,000 cases in North Battleford, Sask, in 2001

Cryptosporidiosis. The merozoites build niches in intestinal cell walls and destroy them. Pain and fever, and diffuse watery diarrhoea continue for days. 

Cyclospora cayetanensis

Newly discovered parasite that lives on fruit: raspberries or blackberries, especially imported from tropical countries. Full life cycle not known. Related to Toxoplasma gondii and to Cryptosporidium.  

Abdominal cramps, watery diarrhoea, fever.

Toxoplasma gondii

Cats are the natural host; vectors for the cysts include rodents or birds caught by cats, shed in their feces. Picked up by farm animals. Transmitted to humans via undercooked meat or unpasteurized milk. Can also spread from fecally contaminated water system when filtration is inadequate: a 1995 outbreak in Victoria, BC was traced to the water supply.

Transmission is intriguing: the cat is the only host in which T. gondii can reproduce sexually. Healthy rodents naturally avoid cats, but rodents infected with T gondii lose this fear, so get caught by a cat, infecting it. It turns out that the cysts of T gondii invade the rodent’s brain which seems to lead to the change in behaviour. This increases the chance a cat will catch the rat, allowing the parasite to complete its sexual reproduction cycle.

Toxoplasmosis. In some tropical countries, 50% of the population carry antibodies, but CDC estimates that 60 million Americans are carriers. Mostly poorer people, so this is a neglected infection of poverty.
Soporozoites invade white blood cells & macrophages. A healthy immune system can contain them for years, so many people do not know they have the parasite. Common symptoms include fever, swollen lymph nodes, fatigue & muscle aches. Ocular toxoplasmosis causes retinal lesions & impaired vision. Neonatal toxoplasmosis passed from pregnant mother to fetus.
There is debate over possible effects on human behaviour and psychology. One summary held that infected people became more apprehensive, and that men became suspicious, jealous, and dogmatic. Curiously, women tended to become more warm hearted and outgoing (Flegr J. Schizophrenia Bull 2007; 33(3): 757-60)

Plasmodium spp.:
P. fulciparum; P. vivax;
P. malariae; P. ovale

In 1898 Ronald Ross, a British physician in the Indian Medical Service, proved that the Plasmodium was transmitted by female Anopheles mosquitoes. (At about the same time, Walter Reed, a US army surgeon, showed that Aedes mosquitoes transmitted yellow fever).

Malaria. Around one million people die from malaria each year, most of them young children in sub-Saharan Africa.
Sporozoites multiply in the liver and enter blood cells, consuming them from the inside. Symptoms include flu, fatigue, aching joints, and recurring fever (coinciding with successive generations of sporozoites).

Acanthamoeba spp.

A microorganism living in the biofilm typically adhering to water pipes. From there, they can get into contact lens cases.

Acamthamoeba keratitis, and eye infection; Granulomatous amebic encephalitis (GAE), an infection of the brain and spinal cord; disseminated infections of skin or lungs.

Subkingdom Metazoa

   Phylum:  Nematoda

      Subphylum: Filaria (nematode roundworms)

The Guinea worm: Dracunculus mediensis

Guinea worm extraction in African village

A water-borne parasite reported in Biblical times (see Numbers 21: 6-8): “And the Lord sent fiery serpents among the people; and much of the people of Israel died. Therefore the people came to Moses and said, We have sinned, for we have spoken against the Lord, and against thee; pray unto the Lord that he take away the serpents from us. … And the Lord said unto Moses, make thee a fiery serpent and set it upon a pole: and it shall come to pass, that everyone that is bitten, when he looketh upon it, shall live…” (Here is one possible origin of the Aesculapian symbol).

There has been an extended effort to eradicate D. mediensis, beginning in India in 1980. Since 1984, the CDC has led the worldwide program. The Gates Foundation has provided financial support and former president Jimmy Carter has been a spokesperson. India achieved eradication in 1996 and the Guinea worm has now been eradicated in all but four countries.

Dracunculiasis or Guinea worm disease. The patient feels no symptoms for a year, then develops an itchy rash, vomiting and diarrhoea. Secondary infections can arise from attempts to remove the worm (see below).
The vector is a water cyclops (water flea) that eats the larvae of D. mediensis and is accidentally swallowed by humans when drinking from a water hole, notably in an arid area. The larva penetrates the intestinal wall and the worm grows to a length of about one metre over the period of a year. Mature females create blisters under the skin that eventually bursts. Intense itching and burning stimulates the victim to find water to soothe the sensation, and the worm (the end of which now protrudes from the lesion) releases her eggs into the water, to be consumed by a cyclops and recommence the cycle.
A traditional remedy is to grab the end of the worm as she releases her eggs, and wind it around a twig. It is then necessary to wait a few days until she has more eggs to release and some more of the worm can be dragged out.

Filarial nematode worms Wuchereria bancrofti, Brugia malayi or B. timori

Filariasis in African child

Filaria require an insect or crustacean vector. Parasites are transmitted to humans through the bite of an infected mosquito. They develop into adult worms in the lymphatic vessels

Lymphatic filariasis: an infection of filarial worms in the lymphatic system causing lymphedema.  Late stage disease presents as elephantiasis, a painful, disfiguring swelling of the legs and genital organs.
The infection can be treated with drugs, but surgery for hydrocele may be required. 

Onchocerca volvulus

A roundworm transmitted by black flies (Simulium damnosum), which thrive in fast-flowing water. The flies deposit their eggs on water plants in the river; larvae hatch and attach to rocks under the water. After maturing, they pupate and become adult flies.

Onchocerciasis, or river blindness. Humans pick up larvae in the river; the larvae burrow under the skin and mature into adult worms, mate and produce young (microfilariae) which die, causing itchy blisters on the skin. If they are in the eye they cause the characteristic blindness. Meanwhile the worm continues to produce microfilariae for up to 15 years.

Strongyloides stercoralis

Feces contaminated soil.
Can survive 70 years in human host.

Disseminated strongyloidiasis. Dyspepsia; cutaneous larva currens; eosinophilia; chronic diarrhoea & weight loss.

Ascaris lumbricoides (large intestinal nematode or roundworm)

Ascaris being excreted PR.

Very common: may infect 25% of world population. 12-40 cm long. Transmitted via stools of infected people, via dirty hands, unwashed fruit or vegetables. Eggs can lie in soil for years

Lie in the small intestine but can crawl around (e.g. coming out of the nose or climbing up a tear duct). Large numbers of worms can tie in knots and block the intestine. They absorb nutrients, leading to vitamin deficiency.

Baylisascaris procyonis

Caused by a roundworm found in raccoons. Raccoons acquire B. procyonis from infected intermediate hosts; they are not affected and pass numerous eggs in their feces. The eggs mature in moist soil. Other foraging animals get infected and the larvae migrate through the bloodstream to the brain where they mature into worms, causing bizarre behaviour before killing the animal. While infected they are more likely to fall prey to another raccoon, perpetuating the cycle.

Children are most at risk, playing in an area frequented by raccoons, including urban back yards. Eggs can be transmitted from hand to mouth. Symptoms of Baylisascaris include nausea, tiredness, liver enlargement and loss of coordination.  

Trichinella spiralis

Trichinosis typically comes from raw or under-cooked pork.
Pigs often acquire the parasite from eating raw garbage or rats which are also carriers (apparently pigs enjoy a plump rat for dinner).
Very difficult to test meat because it implies destroying the meat.

Tiny parasite whose larvae live inside the muscle cells of the host (any carnivore). So small that one gram of meat can contain a thousand larvae. Muscle pain & weakness; fever. Larvae can survive dormant inside a cell for 30 years. Larvae invade intestines then migrate via the bloodstream to muscle cells: in the heart, in the face, etc.

The common pinworm (or threadworm) Enterobius vermicularis

The most common nematode infection. The worm’s complete life cycle occurs in the human host. Adult worms live in the colon; the female moves to a child’s anal area and deposits eggs. These itch; the child scratches and gets eggs under the fingernails. The eggs are transferred to a fomite or directly to another person, and some may be conveyed to the mouth. When swallowed they mature in the intestines and the cycle begins in a new host.

Enterobiasis: anal itching, especially at night. Can lead to pelvic inflammatory disease or vaginitis.

Necator americanus
(a helminth)

Hookworm eggs are passed in the feces from an infected host. Falling on damp and fertile soil, they hatch in a few days and release larvae which feed on bacteria and grow to about 0.5 mm in length. These can penetrate the skin (e.g. bare foot) of a passing human.
This is why they were common among plantation slaves (and also poor whites) in the southern US in the nineteenth century.

The worm rides the bloodstream to the lungs, then crawls up the airway to the throat and is swallowed. They latch on to the intestine and grow to 2 or 3 cm long. Often thousands of worms live in the intestine. They feed on blood and absorb nutrients passing through the gut. This causes anemia and protein deficiency.
This was debilitating to the health of millions of poor southern people; among other economic effects, this meant that many civil war confederate soldiers were sick, listless and useless in battle.

New World screw worm fly:  Cochliomyia hominivorax

A large fly that lays eggs in a wound on an animal or person. The eggs hatch and maggots invade the host to feed on the tissue, creating a suppurating wound. They mature, drop to the ground and burrow down to pupate. In a week the adult flies emerge.
The fly has been eradicated from the southern US and Mexico by releasing millions of sterile (irradiated) males; as the female mates only once the population crashes.

Myiasis – maggot infestation.
Maggots invade the host – commonly wild animals, such as the umbilical cord of newborns, but can also invade the nasal sinus of humans. Infection of the resulting wounds can kill the animal.


   Phylum: Platyhelminthes (flatworms)

      Subphylum: Cestodes (tapeworms)

The intestinal tape worm Echinococcus granulosus, and E. multilocularis (a parasite of foxes).

A parasite of dogs or farm animals, transmitted fecal-orally.  Transmission to crops eaten by humans either via rain run-off or by insect vectors. Other cases reported following improper disposal of animal carcasses infested with the parasite (e.g. if buried on a farm where they died). Transmission can occur if a domesticated farm dog eats infected carcass and then transmits it to food crops or directly to humans. 

Hydatid disease. As with other tapeworms, hatches in the intestine, and larva migrates to lung, liver, bone or brain. Builds into a cyst that grows over a period of 10 or so years and can press on other organs or, if it breaks, cause an allergic reaction.  Can be mistaken for a cancer.
With E. multilocularis, buds break off and spread to other parts of the body including the brain, often with fatal results.

Taenia saginata, The beef tapeworm.
T. solium, the pork tapeworm

Fecal-oral transmission among cattle. Larvae infest beef muscle cells. Humans can ingest eggs or larvae from rare or under-cooked beef.
T. solium has a similar existence in pigs: it burrows through the intestine, floats through the blood stream and ends up in muscle cells in which it builds a small cyst in which the larva grows.
An infected person passes eggs which, if hand hygiene is not good, can be passed onto other people via direct contact.

The pork tapeworm causes cysticercosis, in which the larval cysts infect brain, muscle, or other tissue, and are a major cause of adult onset seizures. Adult worm attaches in the small intestine and can grow up to 3 metres in length. The infected person passes eggs for years and the worm may cause few problems. However, trouble begins if larvae migrate to muscle cells or to the brain, where they form into spherical cysticerci enclosures that cause headaches, cognitive decline, depression, and at times hallucinations and bizarre behavioural symptoms.


      Subphylum: Trematodes (flukes)

Fasciola hepatica

Worm that infests snails living in fresh water, in which we grow products like watercress (which the snails also enjoy). Often picked up by grazing sheep, which get liver fluke. Fully grown the worm is 3 cm long and lives in bile ducts.

Disease called “sheep liver rot” and has similar effects on human liver: inflammation, swelling, obstruction. Fever, anemia, abdominal swelling.

Schistosoma hematobium;
S. mansoni;
S. japonicum

Water-borne; snails living in ponds. These occur naturally, but accelerated via construction of dams (e.g. Akosombo dam in Ghana, Aswan High dam in Egypt). These offer perfect breeding waters, creating epidemics of schistosomiasis.  Snails in water are a reservoir; cercaria penetrate human skin and enter circulation. 

Schistosomiasis (formerly called Bilharzia, after Theodor Bilhars who first observed the eggs). Intestinal:
Portal hypertension; splenomegaly; CNS lesions. Fever, headache, muscle weakness, listlessness.
bladder infections; hematuria

Anisakis simplex

Pickled herring (Solomon Gundy; rollmops).  Parasitic worms that burrow into the stomach wall. Man is an accidental host, for these worms are naturally in the sea, passing between fish as a larger eats a smaller and then defecates, passing parasite eggs that are eaten by krill for the cycle to begin again. A similar parasitic worm, Pseudoterranova decipiens, passes between cod and seals and is also sometimes picked up by humans.
Freezing the fish can kill the worm, but beware that sushi which is traditionally not supposed to be frozen!

Diarrhea, abdominal cramps, chest pain.



Extra stuff…

Sarcoptes scabiei

The female scabies mite burrows into human skin, building a long winding tunnel in which she lays eggs. These hatch, mites mate and the colony grows, causing “the seven-year itch”.
The mites crawling on the skin surface are easily transferred to another person. This often occurs in sexual contact, so scabies can be viewed as an STI. .

Scabies: skin rash with red blisters; causes unrelenting skin itching.


Updated February 10, 2015