Toxoplasmosis is a contagious disease of all species, including man. The agent, Toxoplasmosis gondii is a minute parasite, belonging to the animal kingdom of Protozoa. Most infected animals show no disease at all. The main disease observed is abortion and neonatal death in sheep. In all other species disease is rare, pneumonia, stillbirths and central nervous disease are the conditions observed. In sheep the disease is of major importance in Australia, UK, US, USSR, Turkey and Denmark.
It develops through three basic forms, a cyst, a blood born parasite, and an egg. These have special names to describe each stage:
1. Oocyst - (ooh cyst) The double layered egg found in huge numbers in the faeces of young cats. It survives for long periods in the environment. The oocyst, when mature, contains four diminutive infective parasites. These are called sporozoites. The oocyst is 10 by 12 microns in size. One micron is one thousandth of a millimetre.
2. Tachyzoite - (Tacky zoite) This rapidly multiplying tiny parasite is found in many different cells of the body. After initial rapid multiplication it bursts from the cells and is released into the bloodstream. Tachyzoites develop from the ingested oocysts. The tachyzoite is 6 by 3 microns in size.
3. Bradyzoite- (brae dee zoite) This dormant stage is found in large numbers in cysts in the muscle, nervous tissue and placenta of most animals. It is the result of a tachyzoite migrating into body tissues from the bloodstream. The cysts are not visible to the naked eye, and are less than 150 microns in diameter.
A unique stage is found in the felidae (cat) family. This includes domestic & wild cats, lions, tigers, leopards, lynx, bobcats, mountain lions and ocelots. The cat becomes infected by eating either an oocyst or an animal cyst containing bradyzoites. The parasites invade the intestine, mate and produce millions of offspring. The offspring are defecated as immature oocysts in the faeces.
The only source of important infection for sheep, cattle and horses are oocysts originating from kitten faeces.
Cats become infected after ingesting animals infected with the parasite. All animals can be a source for cats, but typically rodents and small birds are the main source. Rodents pass the organism from generation to generation through congenital infection, therefore rodents act as a long term reservoir of infection. Cats eating raw meat and the infected foetal membranes from lambing ewes can also become infected. Oocysts can be swallowed whilst grooming.
Young cats hunting for the first time are the main contributors.
After ingestion, oocysts begin to be passed after 3 to 5 days. Shedding occurs for a short period, usually 7 to 20 days, but in that time enormous numbers can be passed. Two hundred million oocysts may be excreted, with fifty grams of cat faeces containing up to 10 million oocysts.
Each new litter of kittens or previously unexposed cats introduced into an area will become infected and further contaminate the environment.
The oocyst is extremely tolerant and can survive in the environment for at least one year. They can over winter in very cold climates , but are less viable in arid environments. The tachyzoite does not survive long outside a host.
Sheep are infected by eating oocysts from pasture, hay, other feed stuffs or contaminated water. As few as 40 oocysts are required to infect a ewe. Cats nest in hay barns and grain stores, by defecated on feed it becomes infective. Feral cats have large territories and roam extensively defecating on pasture.
Direct sheep to sheep spread at lambing has not been proven. If it does occur it is of little importance. Transmission from rams at mating has been dismissed as a route of infection. Spread of oocysts by the wind has been found to be a cause of contamination.
Toxoplasmosis gondii attacks most organs of the body. Sporozoites from oocysts, or bradyzoites from tissue cysts penetrate the cells of the intestine. After invasion of a cell they multiple until the cell bursts into the blood stream. This occurs over about 5 days. The parasite (tachyzoite) then circulates in the blood for 2 to 3 weeks invading most other tissues. This includes the placenta of pregnant ewes. The development of immunity halts the circulation of the parasite. The parasite evades elimination by transforming into a tissue cyst.
The vast majority of infections occur without any signs of disease. Ewes may have a temperature during the parasitaemic (parasites in the blood) stage, and infected cats may be off colour and have mild diarrhoea.
The cells and proteins responsible for immunity do not cross into the placenta of ruminants, hence the lamb has no ability to fight infection until it is at less 60 days of age. Infection in early and mid pregnancy results in uncontrollable multiplication and foetal death. The foetus either is resorbed by the ewe or becomes mummified. Some ewes infected in mid pregnancy produce a weak or still born lamb. Most lambs infected during late pregnancy are able to mount an immune response, and are born healthy and immune. Multiplication in the placenta of the uterus and the brain of the foetus can result in the birth of weak lambs with locomotory and suckling dysfunction.
Dry dry ewes, decreased scanning to lambing percentages, abortion and the birth of mummified, stillborn, weak or uncoordinated lambs are all signs of toxoplasmosis infection. This is not the only reason for these signs however.
Occasionally only one lamb is affected from twins. Abortion usually occurs during the last 4 weeks of pregnancy.
The placental membranes of affected lambs may show characteristic lesions.
Testing the ewe is of little value as the infection may have been some time before. History and the examination of foetal fluids and histology of foetal tissues is the best diagnostic tool.
Tissue cysts are destroyed by proper cooking.
Practically little can be done to destroy oocysts, they are destroyed by exposure to boiling water and temperatures of 90oC for 30 seconds. Ammonium hydroxide (5%) is effective in ten minutes. Oocysts are resistant to sodium hypochlorite (bleach) and sodium hydroxide.
Little can be done during an outbreak. Ensure sensible precautions such as disposing of dead lambs and infected foetal membranes. Remember toxoplasma is transmissible to humans.
Move pregnant ewes away from areas were cats have reared litters of kittens and high risk areas. The paddocks of greatest risk may be speculated from grazing history.
The parasite appears to be susceptible to the antibiotic, sulphonamide. A single injection of Tribrissen ® at 1.25ml per 10 kg liveweight may be useful.
Elimination of feral cats, especially kittens may help, however the environment is already heavily contaminated.
Prevention & Control
There is no point in culling ewes that have aborted with toxoplasmosis, they should lamb normally the next season. The only rare exception are ewes that are carrying a mummified lamb. A number of other ewes will have had the infection and given birth to normal lambs but will not be identifiable.
Avoid exposure of unvaccinated pregnant ewes to oocysts by:
Feeding of monensin for the first 100 days of pregnancy at a dose of 15mg/head/day in concentrated feed reduces disease in ewes. The drug suppresses the developing oocystes in the intestinal cells. The ewe will generally develop immunity to toxoplasma however. This is generally not practical under New Zealand conditions.
Exposing non-pregnant ewes to areas were oocysts are likely to be present is possible. This method is very hit and miss, and the sheep will go onto develop tissue cysts containing bradyzoites.
The most effective means of long term control is vaccination.
Tachyzoites from an incomplete strain (S84), are used in the only currently available vaccine. The parasites in this live vaccine result in a life long immunity. They strain does not go on to develop tissue cysts, and has not been found in brain or muscle tissue 6 weeks after vaccination.
Vaccinate all ewe replacements as young as possible and do not rely on “natural” infection as these animals will develop tissue cysts.
Non-pregnant sheep can be vaccinated at any time but not closer than 6 weeks before mating. The vaccine is only required once in the ewes life.
As toxoplasma cycles from cats to birds and rodents, it is impossible to eradicate from a property. Vaccination is thus indefinite.
Operators should take special care in handling this vaccine since it contains live parasites which are transmissible to humans. Toxovax ® should not be administered by pregnant woman.
If self-injection occurs seek medical attention immediately. Instruct the doctor what has happened. Treatment is with drugs such as sulfadiazine and pyrimethamine. The product has been listed with the National Information Centre for Poisons and Hazardous Chemicals in Dunedin.
Human infection is very common, 1/2 the population of the U.S is infected, as are most New Zealanders.
Human infection is usually from the ingestion of tachyzoites or bradyzoite cysts during meat handling, or the consumption of under cooked meat. Humans can also be infected by the inadvertent ingestion of oocysts on food contaminated with cat faeces, or if poor hygiene is practiced.
Disease is very rare.
AIDS sufferers, very old, very young and patients on immunosuppressive drugs are most at risk. Pregnant woman are at risk of abortion or having a deformed foetus. People who come into contact with lambing ewes are in the highest risk of being infected.
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