Animals that play roles in human parasitic infections


Trichinella spiralis is one of the more clinically important roundworm parasites. In addition to humans, other natural vertebrate hosts include swine, dogs, cats, mice, raccoons and foxes.

Trichinellosis is most commonly acquired by human through consumption of raw or undercooked pork products infected with Trichinella spiralis.

Humans can acquire an infection with larvae of various Trichinella species by ingesting raw meat containing encysted Trichinella larvae (from pigs, wild boars, horses, and other species).

Clinical manifestations of trichinellosis show intestinal and muscular symptoms and are in two phases: the intestinal phase whose incubation period is one to seven days and the extra intestinal phase whose incubation period seven or more days.

Effective prevention of trichinellosis can be through proper animal husbandry practices as well as adequate cooking of meat. Freezing pork usually kills T.spiralis effectively. Heat exceeding 80ËšC kills trichinellae in meat. The safest methods are to boil or fry the meat sufficiently.

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Important disease control measures include prophylactic inspection of domestic and wild animal meat for Trichinella infection. Feeding swine and other susceptible domestic animals, only cooked scraps and controlling the population of infected rodents.

But if disease already exists, treatment of trichinellosis is with Benzimidazoles, mebendazole and albendazole, which are active only against the adult worm.


Baylisascariasis is caused by the nematode parasite Baylisascaris procyonis. The North American raccoon is the definitive host for B. procyonis. Humans are infected as accidental intermediate hosts.

Raccoons are infected by B. procyonis in one of two ways, depending on the age of the raccoon. Juvenile raccoons are typically infected by the ingestion of eggs in the environment. In contrast, adult raccoons are usually infected through ingesting larvae in the flesh of paratenic hosts, such as squirrels and rodents.

Humans are infected by ingesting B. procyonis eggs, typically at the sites of raccoon defecation. After ingestion, larvae penetrate the intestinal mucosa and migrate rapidly to the liver and then to the lungs where they gain access to the pulmonary veins and are then distributed through the systemic circulation to the tissues. Larvae do not mature into adult worms in human hosts.

In humans, Baylisascariasis results in death or severe neurologic sequelae.

Due to the relatively poor efficacy of treatment, preventative strategies are increasingly important. When possible, raccoons should be prevented from frequenting areas of human habitation. If an area is known to be contaminated by raccoon feces, the preferred method of disinfection is with direct flames. Education of the public regarding the dangers of raccoon contact is important. Children should be closely monitored for ingestion of soil while playing outside. If ingestion of raccoon feces is suspected in a child, prophylactic treatment should consist of immediate administration of albendazole


In the human host, especially children, the tissue migrating larvae of roundworms, Toxocara canis and T. cati, can cause serious complications including visceral larva migrans and ocular larva migrans.

In the both animals and the aberrant host, ingestion of embryonated eggs initiates infection. Dogs and cats can also acquire the infection by eating earthworms.

In humans, ingested eggs hatch in the small intestine and penetrate the intestinal wall in the same way as in the definitive host, but the larvae, unable to mature, migrate through the body for a prolonged period. Infective larvae, which do not mature into adult worms in humans, can persist for years after becoming encapsulated within granulomas.

Besides ingestion of soil contaminated with embryonated eggs, infection can also occur after consumption of raw or undercooked meat infected with Toxocara larvae, as well as by ingestion of vegetables and salads which have been grown in contaminated soil.

Exposure to Toxocariasis occurs primarily in overcrowded urban areas where children are in close contact with dogs and cats. Public education and control efforts should be directed at limiting exposure of children to soil contaminated with Toxocara eggs in play parks, school playgrounds, home gardens and other areas where children assemble.

Dogs and cats should be restricted from entering public areas where children play. Dog owners should clean up after their pets have defecated and have their pets dewormed regularly. Children should wash their hands after playing in a park or coming in close contact with dogs, especially puppies and cats.


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The typical life cycle of a digenetic trematode comprises of an asexual phase in mollusks, usually gastropods, and a sexual phase in vertebrates, the definitive hosts. Many species have a second intermediate host in which the sexual phase begins.

Humans are among the mammals that are the definitive hosts in which the Clonorchis sinensis and Opisthorchis species undergo sexual reproduction.

The flukes reproduce asexually in several species of snails, which are the first intermediate hosts. Various freshwater fish and crustaceans serve as the second intermediate hosts. In addition to humans, naturally occurring definitive hosts include dogs, pigs, cats (both domestic and wild), martens, badgers, minks, weasels and rats.

Clinical signs are usually limited to tender hepatomegaly and lymphadenopathy.

Weight loss and epigastric pain are the primary complaints, with ascities, jaundice and a palpable abdominal mass notable on exam.

Proper preparation of freshwater fish, such as cooking or freezing, destroys the metacercariae and prevents infection. As fecal-oral inoculation can occur, transmission can also be reduced by strict hygiene measures. There is no role for chemoprophylaxis.



Taeniasis and cysticercosis are diseases resulting from infection with parasitic tapeworms belonging to Taenia species. Two Taenia spp are most commonly responsible for human infection. These are the pork tapeworm Taenia solium and the beef tapeworm Taenia saginata.

Infection with adult tapeworms of either T. solium or T. saginata causes Taeniasis in humans. The metacestode or larval stage, of Taenia solium causes the tissue infection, cysticercosis.

The ingestion of cysticerci from raw or undercooked meat facilitates the transmission of T. solium from pigs to humans

The complete life-cycle of Taenia solium involves two hosts: the pig and the human, whereas that of Taenia saginata involves the cow and the human. Humans are the definitive hosts and the small intestines harbor the adult tapeworm. Infection can either be through the accidental ingestion of embryonated eggs passed in the feces of an infected person with the adult tapeworm, or through the eating of raw or undercooked meat containing cysticerci. Both pigs and cattle act as intermediate hosts for these parasites and are infected through eggs realeased by an infected person.

In humans, cysticerci turns into adult tapeworms which live in the small intestines for years causing Taeniasis, which is an infection with the adult tapeworm which usually remains confined to the small intestine.

The most effective means of preventing infection is to ensure that meats are cooked well before consumption. Good hygiene and sanitation are highly effective in decreasing the risk of infection associated with fecal-oral transmission. Improvements in sanitation and public health care are essential for preventing the further spread of disease. Pigs should be stopped from roaming freely and contacting human feces will help reduce human-to-pig transmission.

Vaccines aimed at preventing infection in pigs may play a role in efforts to control the spread of disease

Hydatid disease

Hydatid disease, also called hydatidosis or echinococcosis, is a cyst-forming disease resulting from an infection with the metacestode or larval form, of parasitic dog tapeworms from the genus Echinococcus. The vast majority of human diseases are from Echinococcus granulosus and Echinococcus multioccularis which cause cystic echinococcosis and alveolar echinococcosis, respectively.

Hydatid disease is caused by infection with the larval form of E. granulosus (and/or E. multiocularis) and results in the formation of cysts within various host tissues. The complete life cycle of Echinococcus granulosus requires two hosts. Domestic dogs act as the primary definitive host of the mature adult worms and a single infected dog may harbor millions of adult worms within its intestines. Other canines such as wild dogs, wolves, foxes and jackals may also act as a definitive host harboring the adult tapeworms. Intermediate hosts become infected with the larval form of the parasite and include a wide range of herbivorous animals, primarily sheep, cattle, pigs, goats and horses. The life cycle is completed by the ingestion of one or more cysts and its contents by the canine host through the consumption of infected viscera of sheep and and/or other livestock.

Humans may become infected though the ingestion of food and/or water contaminated with infective eggs released in the feces of dogs harboring the adult tapeworm(s). Once ingested, the eggs release oncospheres capable of actively penetrating the intestinal mucosa. These oncospheres gain access to the blood stream via the hepatic portal vein and migrate to various internal organs where they develop into cysts. Hydatid cysts most often localize within the liver and the lungs; however, cysts may also form in the bones, brain, skeletal muscles, kidney and spleen.

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Similar to E. granulosus, the complete life cycle of E. multiocularis also requires two hosts. The primary definitive host for E. multiocularis is the fox, although the parasite may also infect wild and domesticated dogs and occasionally cats. Rodents such as field mice, voles and ground squirrels act as natural intermediate hosts and acquire infection by ingesting infective eggs released into the environment.

Echinococcus granulosus and Echinococcus multiocularis are the two species most often identified in human hydatid disease.

The most effective means to control hydatid disease in humans and eliminate the consequences of Echinococcus infections in livestock is through the broad- range education of people living in endemic regions. Education to prevent the feeding of infected viscera to dogs is essential for controlling the spread of infection from livestock to dogs. Most human infections are due to close contact with infected dogs. Deliberate actions aimed at reducing the rate of dog infection in endemic regions will undoubtedly reduce the number of human infections. In addition, the reduction and removal of stray and unwanted dogs, as well as the regular treatment of dogs with antihelminthic drugs, will facilitate the widespread efforts geared towards controlling disease transmission.

The development of vaccines designed to prevent infection of either or both the definitive and intermediate host(s) offers the greatest possibility of success in the control and eradication of hydatid disease in both the livestock and human populations.

D. latum

Diphyllobothrium latum is the broad or fish tapeworm of humans. Definitive hosts include humans, dogs, cats, pigs, and other fish-eating mammals. The adult tapeworm lives primarily in the ileum, attached to the intestinal mucosa.

Diphyllobothrium latum Life cycle requires two intermediate hosts. Unembryonated eggs, are continuously discharged from the gravid proglottids through the uterine pores and pass in the feces.

Embryonation occurs in fresh-water and, in one to two weeks, the ciliated coracidium hatches through the operculum and swims about. In order for development to proceed, the coracidium must be ingested (within 12 to 24 hours) by the first intermediate host, copepods of the genera Diaptomus or Cyclops, in which development of the procercoid is completed. The procercoid averages about 500 μm in length. When an infected copepod is ingested by freshwater fish, including pike, perch, trout, and salmon, the liberated procercoid burrows through the intestinal wall and into the flesh of the fish, where it develops into a plerocercoid.

Infection of humans occurs when fish harboring plerocercoids are consumed. Upon their release in the intestine of the definitive host, the parasites attach to the intestinal wall and develop to sexual maturity in 5-6 weeks.

Prevention of humans from acquiring Diphyllobothrium latum is through proper cooking of fish before consuming it.


Toxoplasmosis is an infection caused by the protozoan obligate intracellular parasite Toxoplasma gondii.The majority of immunocompetent adults, pregnant women and children infected with Toxoplasma gondii experience no or mild symptoms during acute infection. Infants of women who seroconvert during pregnancy may develop congenital toxoplasmosis. Immunocompromised individuals are at risk for reactivation of latent infection, including potentially fatal encephalitis.

The house cat and other members of the family Felidae serve as definitive hosts in which the sexual stages of the parasite develop.

The life cycle of Toxoplasma gondii begins when a cat ingests Toxoplasma-infected tissue from an intermediate host, usually a rodent. Tissue cysts within the muscle fibers or brain are digested in the cat's digestive tract. The parasite then undergoes sexual development, multiplies in the intestine of the cat and is eventually shed in cat feces, mainly into litter boxes and garden soil.

A human may become infected in one of the following ways:

1. Accidentally ingesting oocysts in cat litter.

2. Ingesting tissue cysts within raw or undercooked meat (lamb, pork and beef), drinking unpasteurized milk, contaminated water, or unwashed fruits or vegetables.

3. Direct transmission of tachyzoites from mother to fetus through the placenta (congenital infection) or, rarely, by blood transfusion or solid organ transplantation from a positive donor to a previously uninfected host.

Prevention of infection with T.gondii can be done as follows:

Protect children's play areas from cat and dog feces. Cover sandboxes when not in use to avoid cat defecation.

Wash hands thoroughly after contact with soil that may be contaminated with animal feces.

Control flies and cockroaches as much as possible. They can spread contaminated soil or cat feces onto food.

Avoid rubbing eyes or face when preparing food, especially raw meat or poultry.

After food preparation, wash hands thoroughly with soap and water and clean the work tops.

Avoid ingesting raw or undercooked meat or poultry, raw eggs and unpasteurized milk. Fruits and vegetables should be peeled or thoroughly washed.

And during pregnancy,

Pregnant women should have their serologic testing for Toxoplasma antibodies that may be repeated several times during the pregnancy depending upon initial results.

Avoid exposure to cat feces by not handling the cat litter box. If the litter box must be changed, rubber gloves should be worn to avoid contact with the litter. Hands should be washed thoroughly with soap and water afterwards.

Use work gloves when gardening and wash hands afterwards.

And for HIV-Infected Individuals

Patients with HIV disease should have Toxoplasma antibody titers checked. If the results are positive and if the CD4+ T-cell count is less than 200 cells/μL, patients should be given prophylactic antibiotics.

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