February 01, 2010
5 min read

Toxoplasmosis in animal and man

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Several abortifacient disease agents of sheep and goats are known to be pathogenic for humans. This includes bacterial diseases – such as brucellosis, Q fever and Mammalian Chlamydiosis – as well as protozoan (toxoplasmosis) and viral (Rift valley fever). Other zoonotic pathogens, such as Listeria and Salmonella spp, may cause abortions in sheep and goats occasionally. Toxoplasma gondii, found worldwide, causes infections in carnivore and omnivore animal species and in humans. It is one of the main three causes of abortion in sheep; in some countries, it is the main cause.

Arnon Shimshony, DVM
Arnon Shimshony

From 1909, when Nicolle and Manceaux described a micro-organism in a rodent, the gondi (Ctenodactylus gondi), in Tunisia, more than 50 years were necessary to fully elucidate the complex life cycle of T. gondii, highlighting the major role of cats in its dissemination. It took almost as long to identify the first effective treatments and set up the first serodiagnostic methods.

Infections with T. gondii are an important public health concern, causing eye disease both congenital and acquired. The definitive hosts (animals in which the organism reproduces sexually) for T. gondii are members of the Felidae family (mainly domestic cats); they excrete oocysts in their feces. Livestock, and all warm-blood animals, are intermediate hosts, infected by the ingestion of the oocysts. When infected, they will harbor infective tissue cysts in their muscles and brains for the rest of their lives; carnivorous animals and humans may get infected from eating infected meat or organs. Toxoplasmosis is included in the World Organization for Animal Health’s list of notifiable animal diseases.

T. gondii is an obligate intracellular protozoan parasite in the phylum Apicomplexa.

The major forms of the parasite are:

  • Oocysts (containing sporozoites), which are shed in the feces.
  • Tachyzoites, rapidly multiplying organisms found in the tissues.
  • Bradyzoites, slowly multiplying organisms found in the tissues.
  • Tissue cysts: walled structures, often found in the muscles and central nervous system (CNS), containing dormant T. gondii bradyzoites.

Carnivores and omnivores, including humans, can become infected when they eat raw or undercooked tissues containing tissue cysts or, occasionally, tachyzoites. Both herbivores and carnivores may ingest infective oocysts in food or water, inhale them in aerosols, or come into contact with contaminated soil. T. gondii can cross the placenta in some species, particularly sheep, goats and humans.

T. gondii undergoes an asexual reproductive cycle in all species. The tissue cyst or oocyst wall is dissolved during digestion, releasing bradyzoites or sporozoites, which enter the lamina propria of the small intestine and begin to multiply as tachyzoites. The tachyzoites can disseminate to extraintestinal tissues within a few hours of infection, via the lymph and blood. They can enter nearly any cell and multiply; the host cell eventually ruptures and the released tachyzoites enter new cells. As host resistance develops, the tachyzoites begin to disappear from the visceral tissues and form “resting” bradyzoites within tissue cysts. These cysts are found most often in the skeletal muscles, brain and myocardium. They generally do not cause a host reaction and can persist for life.

In Felidae, the parasites simultaneously undergo a sexual cycle of replication. After ingestion, some of the bradyzoites multiply within the epithelial cells of the small intestine. After numerous cycles of asexual reproduction, these bradyzoites initiate the sexual cycle, which results in the formation of an unsporulated oocyst. The oocyst is excreted in the feces and sporulates in the environment. After sporulation, the oocyst contains two sporocysts, Cats usually shed oocyts for one to two weeks.

Infections in animals

Members of the Felidae, including domestic cats, are the definitive host. Most mammals and birds can serve as intermediate hosts. Among domestic animals, infections are most common in cats, sheep, goats and swine. Lower infection rates are seen in dogs and horses. Cattle seem to be relatively resistant to infection.

Most infected cats are asymptomatic, but generalized acute, subacute and chronic (months to years) infections have been reported, particularly in young or immunocompromised animals. Respiratory, hepatic and pancreatic involvements occur. CNS signs, particularly common in older cats, vary with the site of the lesion. Ocular signs are common.

Most infected dogs are asymptomatic; clinical toxoplasmosis is most common in puppies or in immunosuppressed older dogs. Toxoplasma encephalitis are most severe in dogs aged younger than 6 months. Acute hepatitis is often fatal. Ocular signs may include retinitis, uveitis and iridocyclitis.

In other animal species, infections are generally asymptomatic, although outbreaks with generalized infections, abortions, stillbirths and neonatal mortality are occasionally reported in swine. Fever, encephalitis, ataxia and retinal degeneration have been reported in horses.

Clinical toxoplasmosis is seen most often in sheep and goats when infected during pregnancy and are manifested by abortions, stillbirths, or mummification or resorption of the fetus. Congenitally infected lambs may be uncoordinated, weak and unable to nurse. Abortions and diseased lambs or kids do not recur during subsequent pregnancies. Preventive measures may help to reduce congenital disease in sheep; in Europe and New Zealand, a modified live vaccine is available for sheep.

Toxoplasmosis is communicable only in Felidae. Antibodies have been found in 15% to 58% of the cats in the United States and 25% to 100% worldwide.

Infections are particularly common in strays and less prevalent in pets. No vaccine is available for cats; if possible, they should be kept away from pastures and barns where pregnant sheep and goats are kept.

After an abortion, the placenta and abortion products should be removed, and the area cleaned and disinfected.

To prevent cats from becoming infected and serving as sources of infection for humans, they should not be fed raw or undercooked meat. Indoor cats are less likely to become infected, due to reduced exposure to rodents.

Infection in humans

This review is not meant to address the clinical disease in humans in detail. In immunocompetent non-pregnant individuals, infection with T. gondii is usually asymptomatic. Approximately 10% to 20% of patients develop lymphadenitis or a mild, flulike syndrome. Severe symptoms, including myositis, myocarditis, pneumonitis and neurologic signs, are possible but rare. Ocular toxoplasmosis with uveitis, often unilateral, can be seen in adolescents and young adults, as the result of an asymptomatic congenital infection or the delayed result of a postnatal infection.

Infections acquired during pregnancy can lead to congenital toxoplasmosis of the infant, affecting the developing brain and/or retina. The incidence and severity of the disease vary with the stage of pregnancy.

Toxoplasmosis is often severe in immunosuppressed patients. Neurologic disease is the most common sign, particularly in reactivated infections and may lead to coma and death. In patients with HIV/AIDS, toxoplasmosis is often a reactivated rather than a new infection.

Humans become infected with T.gondii mainly by ingesting uncooked meat containing viable tissue cysts or by ingesting food or water contaminated with oocysts from the feces of infected cats.

Children might be exposed to oocysts in cat excrements deposited in sandpits. Circumstantial evidence suggests that oocyst-induced infections in humans are clinically more severe than tissue cyst-acquired infections.

T. gondii infection is one of the most common infections in humans; in worldwide serological surveys, 3% to 80% of healthy adults have been exposed to this parasite. Infection is asymptomatic in 80% to 90% of non-pregnant, immunocompetent individuals and usually causes mild disease in the remainder.

Toxoplasmosis is not directly communicable from person to person, except in utero. Infection of the fetus is particularly likely during the second and third trimesters.

Most cases are sporadic, but small epidemics may occur, usually associated with contaminated food or water. Immunity appears to be lifelong unless the individual becomes immunosuppressed.

Prevention of human infection

The risk of infection can be reduced by proper food preparation and good hygiene. Meats should be cooked to a temperature sufficient to kill T. gondii; the internal temperature of beef, lamb and veal steaks or roasts should reach at least 145°F (63°C), and pork, ground meat and wild game should be cooked to 160°F (71°C). Pregnant women and others at risk should wear gloves when gardening and during other soil or sand contact.

To help prevent transmission of T. gondii to humans, cats should be fed only commercial pet food or well-cooked meats. Indoor cats are less likely to transmit toxoplasmosis than outdoor cats. Litter boxes should be cleaned daily to reduce the risk of oocyst sporulation and rinsed with boiling water. Pregnant women should avoid cleaning the litter box; if this is unavoidable, they should use gloves, then wash their hands.

Arnon Shimshony, DVM, is Associate Professor at the Koret School of Veterinary Medicine Hebrew University of Jerusalem, Rehovot, and is the ProMED-mail Animal Diseases Zoonoses Moderator. Dr. Shimshony was Chief Veterinary Officer, State of Israel, from 1974 to 1999.