C612 - TOXOPLASMOSIS
C612 - TOXOPLASMOSIS
| Nature of the disease |
| Toxoplasmosis is an infectious disease caused by the
coccidia Toxoplasma gondii. The definitive host is a Felid but it can
affect a wide range of species as
intermediate hosts including humans. It frequently manifests as abortion
and stillbirths in sheep and by encephalitis, pneumonia and neonatal mortality
in all species.
Apart from the losses in sheep the impact is relatively small and the importance of the disease lies in its potential threat to humans. |
| Classification |
| FAO List C disease |
| Susceptible species |
| Toxoplasmosis naturally occurs in a wide range of
domestic and wild animals and birds. Domestic cats predominate as reservoir for
zoonotic transmission as they are the
definitive hosts (were the parasite reproduce).
Pigs, sheep, goats and humans are the more susceptible to the disease, cattle and horses being less susceptible. Within each species, young animals and those with a weaker immune system (e.g. sick, pregnant, old) are more susceptible to the disease. Clinically, sheep flocks are commonly affected with abortions and neonatal deaths being seen. Within the Pacific region, serological surveys indicate that goats are the species most frequently exposed to infection. |
| Distribution |
| Toxoplasmosis occurs in most parts of the world. However, it seems to be absent from, at least, French Polynesia, Kiribati and Vanuatu. |
| Clinical signs |
| Cattle Recent observations showed that toxoplasmosis is uncommon in cattle. Calves are more susceptible than adults and the clinical signs of orally affected calves include diarrhea, anorexia, poor weight gain, depression, weakness, dyspnea and fever. In some casejust a lymphadenopathy. Congenitally affected calves show fever, dyspnoea, coughing, sneezing and neurological signs. If the disease occurs in adults, symptoms include fever, dyspnoea, and nervous signs, followed by lethargy. Stillborn calves and neonatal deaths may be seen.
Pigs
Sheep
Horses
Cats
In young cats dyspnoea associated with bronchopneumonia may be seen. Infection may be fatal in young kittens.
Birds
Humans
|
| Post-mortem findings |
| In cattle, multiple, proliferative necrotic granulomas
in the lungs and nervous system are characteristic. If there is visceral
involvement pneumonitis, hydrothorax, ascites, lymphadenitis, intestinal
ulceration and necrotic foci in internal organs may be seen.
In sheep there may be involvement of the uterine wall, placenta and foetus. In the foetus focal necrotic lesions may be seen in the brain, liver, lungs and in the foetal membranes. In horses haemorrhagic lesions may be seen in the brain and spinal cord. These may also be seen in sheep. |
| Differential diagnosis |
| There is a wide range of symptoms and lesions that
may be produced with toxoplasmosis in domestic animals and the disease may be
difficult to recognise.
Clinical syndromes of encephalopathy in new-born animals occur with vitamin A
deficiency and as congenital defects after vaccination of pregnant dams with
attenuated bluetongue and hog cholera vaccines.
Neurological signs can also be produced by:
There are also a wide range of cause of respiratory distress that may need to be considered. |
| Specimens required for diagnosis |
| Blood
samples and faeces should be collected to establish the diagnosis in
cats.
Tissue samples collected from animals and aborted foetuses should include brain and lungs. In humans, the disease is diagnosed by serology. Laboratory tests available include serology ( ELISA, Fulton test, Indirect Fluorescent Antibody test (Ig G), isolation, DNA-probe with PCR and microscopic demonstration of organisms in smear or sections. The demonstration of cysts does not establish a causal relationship to clinical illness, since cysts may be found in both acute and chronic infections. However, finding tachyzoites in blood or body fluids confirms active infection. |
| Transmission |
 Cats are responsible for maintaining and spreading
infection. The sexual reproduction of the parasite only occurs in this specie.
Cats faces contain occysts that can infect all species through food
contamination. Cats can become infected by eating chronically infected birds,
rodents, raw meat, etc.There are three modes of spread by which animals can become infected:
Transmission to humans occurs via eating raw or undercooked meat containing cysts; by ingesting sporulated oocysts from cat faeces; or transplacentally. Freezing of meat to -20o C for 2 days or heating to 60o C kills cysts. Under appropriate environmental conditions, oocysts passed in cat faeces can remain infective for a year or more.
|
| Risk of introduction |
| Toxoplasmosis is most likely to be introduced to new
areas with deliberate or accidental importation of cats. Unapparent infection is
common in cats.
The risk of introduction by contaminated meat exists but the probability of transmission of the disease is low if the meat is properly cooked. |
| Control / vaccines |
| Cats can be treated with a 5-7 day course of
sulphamezathine. Antimicrobials such as spiramycin, clindamycin sulphonamides,
diphenylsulphones and pyrimenthamine have been shown to be effective against
trophozoites and can be used against acute and chronic toxoplasmosis.
In humans, the treatment of choice is pyrimethamine plus either trisulphapyrimidines or sulfadiazine. Folic acid is given to avoid the haematological effects of pyrimethamine-induced folate deficiency. Children's play areas should be protected from cat and dog faeces. Pregnant women should take measures to prevent infection by avoiding contact with cat faeces and avoid working in soil or gardens that could be contaminated by cats, etc. and by thoroughly cooking meat. Hands should be washed after handling raw meat and before eating or touching the face. Vaccines are under development for both humans and animals. |
| References |
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