Toxoplasmosis

Monograph

definition: 

Toxoplasmosis is a zoonotic contagious disease caused by a protozoan parasite and affecting warm-blooded animals.

 

Situation in America: 

Toxoplasmosis is one of the most common disease worldwide. A vast proportion of human beings is or have been infected with this parasite throughout the world.

Thus, in the United States, around 22,5% of the population 12 years old and older have already been infected. Moreover, infections are highest in warm, humid climates and at lower altitudes.

A high prevalence of infection in Central America has been related to the frequency of stray cats.

Susceptible species: 

The life cycle includes intermediate and definitive hosts. Members of the Felidae including domestic cats are the definitive hosts. Most mammals and birds can serve as intermediate hosts.

Infections are most common in cats, sheep, goats and swine. Lower infection rates are seen in dogs and horses and cattle seem to be resistant to infection. The majority of infections in humans are asymptomatic. Tne clinical cases are confined to particular population at high risk like the immuno-depressed persons and the pregnant women.

Etiological agent: 

The disease is caused by an obligate intracellular protozoan parasite Toxoplasma gondii. The parasite shows three infectious stages for both intermediate and definitive hosts :

  1. 1. Sporozoites contained in oocysts release in the feces. They are ingested by intermediate hosts and undergo divisions leading to tachyzoites
  2. 2. Tachyzoites, rapidly multiplying organisms leading to acute infection
  3. 3. Bradyzoites, slowly multiplying organisms are released and form tissue cysts leading to chronic infection

Life cycle of Toxoplasma gondii

life cycle

Source (Pearson Education, Inc.)

Methods of transmission
Source: 

Soil contaminated with oocysts from cat feces as food or contaminated water  are important sources of contamination. Contaminated pastures and unwashed fruits and vegetables are suceptible to transmit the disease too.

Tissue raw or undercooked may also be a source of contamination. Thus Pigs can become infected by ingesting  rodents or cannibalism.

Some studies have also highlighted the fact that seafood could be involved in the transmission of the parasite. 

Contaminated blood or organs during a transfusion or organ transplantation can be sources of infection.

 

Transmission: 

horizontal transmission : oral ingestion of occysts from the environment or ingestion of bradyzoites in raw or undercooked tissues

transpacental transmission : tachyzoites cross the placenta. Susceptible species to this transmission are sheep, goats, humans and small rodents. Infections are likely during the second and the third trimesters but less severe than infections that occured during the first trimester.

Flies and cockroaches can act as mechanical vectors.

Symptoms: 

Most infections are asymptomatic except for sheep and goats which are the most susceptible in livestock

Sheep, goats and pigs

Infections acquired during pregnancy can cause infertility, stillbirths, foetal abnormalities and abortion according to the stage of pregnancy.

Cats

Acute anc chronic infections have been reported. Acute disease is characterized by respiratory signs of pnuemonia, icterus and lymphadenopathy.

Chronic disease is associated with wasting and intestinal signs.

OCular toxoplasmosis is less frequent.

Cattle and horses

They are resistant to clinical toxoplasmosis.

Humans

10-20% develop lymphadenitis or a flu-like syndrom (fever, malaise, myalgia, headache, sore throat, lymphadenopathy and rash). Severe symptoms like myositis, myocarditis, pneumonitis, neurologic signs are possible but rare.

Infections during pregnancy can lead to congenital toxoplasmosis. The symptoms are usually due to infection of the developing brain and/or retina. Ocular toxoplasmosis with uveitis is often the result of an asymptomatic congenital infection.

Neurologic disease is the most common sign in immunosupressed patients.

Lesions
Macroscopic lesions: 

The most common lesions are areas of necrosis and associated inflammatory reactions in the brain, lung, liver and lymph nodes.

Foci of inflammation and necrosis in the placenta.

Microscopic lesions: 

Inflammatory and necrotic lesions in the tissues of the fetus (brain, lungs and liver) and placenta

Diagnostics
Clinical diagnosis: 

Clinical signs are generally non-specific.

Differential diagnosis: 

Abortion in sheep and goats must be differentiated from :

  • Q fever
  • Brucellosis
  • Campylobacteriosis
  • Salmonellosis
  • Bluetongue
  • Akabane disease
  • Rift Valley Fever
Laboratory diagnosis: 

- Cell culture : from muscle, brain, blood

- Histological diagnosis : direct observation in tissues (lavage or biopsies)

- PCR : helpful for detecting congenital infecitons in utero

Serological diagnosis: 

IFA (indirect fluorescent antibody) and ELISA tests are used in humans.

IgM-specific tests : helpful to know the time of infection (e. g. in a pregnant woman)

Treatment: 

Antibiotics : pregnant women, immunocompromised patients, congenitally infected infants or persons with ocular disease, animals with clinical disease

They cannot destroy tissue cysts and eradicate the parasites.

Prophylaxis: 

Cats should be kept away from sheep and goats areas.

Cats should not be fed raw or undercooked meat.

Litter boxes should be washed regurlarly to avoid sporulation of oocysts.

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

Meat should be cooked to a temperature sufficient to kill the parasite.

Fruits and vegetables should be peeled and washed thoroughly.

Hands should be washed after contact with any possible source of contamination.

 

 

Vaccines: 

A modified live vaccine has been developed for sheep in Europe and New Zeland. One vaccination is required during the lifetime of the animal. The flocks have to be vaccinated three weeks before mating.

No human or cat vaccine has been developed yet.

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