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==Introduction==

==Introduction==

Toxoplasmosis is the disease caused by ''Toxoplasma gondii'',  an intracelluler protozoan parasite. Although the definitive host is the cat, ''T. gondii'' can infect all mammals including man and is a significant cause of abortion in sheep and goats. Toxoplasmosis does not seem to cause disease in cattle.

Toxoplasmosis is the disease caused by ''[[Toxoplasma gondii]]'',  an intracelluler protozoan parasite. Although the definitive host is the cat, ''T. gondii'' can infect all mammals including man and is a significant cause of abortion in sheep and goats. Toxoplasmosis does not seem to cause disease in cattle.

[[Image:Toxoplasmosis Life Cycle.jpg|thumb|right|300px| Life cycle of ''Toxoplasma gondii''. Source: Wikimedia Commons; Author: LadyofHats (2010)]]

[[Image:Toxoplasmosis Life Cycle.jpg|thumb|right|300px| Life cycle of ''Toxoplasma gondii''. Source: Wikimedia Commons; Author: LadyofHats (2010)]]

==Transmission to Sheep==

==Transmission to Sheep==

===Oocysts in the Environment===

===Oocysts in the Environment===

As the definitive hosts of ''Toxoplasma gondii'', cats become infected when they hunt and eat infected wild rodents and birds. Rodents are a particularly important source of feline infection, as they can pass ''T. gondii'' infection to their offspring without causing clinical disease. This means that a farm may develop a reservoir of ''T. gondii'' tissue cysts with the potential to cause feline infection and massive oocyst excretion when a cat is introduced to the environment. Between days 3 and 14 post-infection, cats shed over 100 million of oocysts in their faeces. Studies have shown an association between ovine toxoplasma infection, and the contamination of feed or grazing with sporulated oocysts¹, highligting the importance of oocysts as a source of infection for sheep. It has also been demonstrated that the prevalence of ovine toxoplasmosis varies with the presence of cats on a farm².

As the definitive hosts of ''Toxoplasma gondii'', cats become infected when they hunt and eat infected wild rodents and birds. Rodents are a particularly important source of [[Toxoplasmosis - Cat and Dog|feline infection]], as they can pass ''T. gondii'' infection to their offspring without causing clinical disease. This means that a farm may develop a reservoir of ''T. gondii'' tissue cysts with the potential to cause feline infection and massive oocyst excretion when a cat is introduced to the environment. Between days 3 and 14 post-infection, cats shed over 100 million of oocysts in their faeces. Studies have shown an association between ovine toxoplasma infection, and the contamination of feed or grazing with sporulated oocysts¹, highligting the importance of oocysts as a source of infection for sheep. It has also been demonstrated that the prevalence of ovine toxoplasmosis varies with the presence of cats on a farm².

===Congenital Transmission===

===Congenital Transmission===

The signs of toxoplasmosis in sheep manifest following the exposure of a naive pregnant ewe to oocysts. The sporozoites ingested excyst in the digestive tract and penetrate the intestinal epithelium, before reaching the mesenteric lymph nodes around day 4 post-infection. Here, they cause lymphomegaly and focal necrosis before contributing to a parisitaemia from day 5. Pyrexia is associated with the development of parasitaemia.  

The signs of toxoplasmosis in sheep manifest following the exposure of a naive pregnant ewe to oocysts. The sporozoites ingested excyst in the digestive tract and penetrate the intestinal epithelium, before reaching the mesenteric lymph nodes around day 4 post-infection. Here, they cause lymphomegaly and focal necrosis before contributing to a parisitaemia from day 5. Pyrexia is associated with the development of parasitaemia.  

Following dissemination of ''T. gondii'' in the blood, many tissues become infected. Parasitaemia ends when the maternal immune response becomes effective, and protozoa start to encyst as bradizoites. In pregnant animals, the uterus is an immunoprivileged site, and the outcome of foetal infection is influenced by the stage of gestation. In early pregnancy, the foetus is unable to mount any immune response, and so cannot inhibit parasite multiplication. The foetus rapidly dies and is resorbed. In a flock, this is visible clinically as large numbers of barren ewes. In mid-gestation (70-120 days), infection can again be fatal. This causes a mummified foetus which is often twinned with a lamb that is stillborn or weak. Abortion due to infection at 70-120 days gestation tends to occur in very late pregnancy. Because the foetal immune system is well developed in late pregnancy, infection at this stage will be resisted, and the lamb will be born transiently infected but alive.

Following dissemination of ''T. gondii'' in the blood, many tissues become infected. Parasitaemia ends when the maternal immune response becomes effective, and protozoa start to encyst as bradyzoites. In pregnant animals, the uterus is an immunoprivileged site, and the outcome of foetal infection is influenced by the stage of gestation. In early pregnancy, the foetus is unable to mount any immune response, and so cannot inhibit parasite multiplication. The foetus rapidly dies and is resorbed. In a flock, this is visible clinically as large numbers of barren ewes. In mid-gestation (70-120 days), infection can again be fatal. This causes a mummified foetus which is often twinned with a lamb that is stillborn or weak. Abortion due to infection at 70-120 days gestation tends to occur in very late pregnancy. Because the foetal immune system is well developed in late pregnancy, infection at this stage will be resisted, and the lamb will be born transiently infected but alive.

===Laboratory Tests===

===Laboratory Tests===

===Pathology===

===Pathology===

Multiplication of ''Toxoplasma gondii'' in the placeneta causes multiple foci of necrosis, which limit effective function during pregnancy. After birth, these areas of necrosis are visible as white spots on the cotyledons. The intercotyledonary areas appear normal.

Multiplication of ''Toxoplasma gondii'' in the placenta causes multiple foci of necrosis, which limit effective function during pregnancy. After birth, these areas of necrosis are visible as white spots on the cotyledons. The intercotyledonary areas appear normal.

Histologically, foetal tissues may display changes. In the brain, glial foci surround a necrotic centre and represent a foeal immune response to damage initiated by parasite multiplication. An associated mild lymphoid meningitis is often seen. Focal leukomalacia is also common and is thought to be due to foetal anoxia in late gestation, caused by extensive necrosis of the placentome. Focal inflammatory lesions with diffuse lymphoid infiltrates can be found in many other tissues, including the liver, lung and heart. The kidneys and skeletal muscle are less frequently affected.

Histologically, foetal tissues may display changes. In the brain, glial foci surround a necrotic centre and represent a foetal immune response to damage initiated by parasite multiplication. An associated mild lymphoid meningitis is often seen. Focal leukomalacia is also common and is thought to be due to foetal anoxia in late gestation, caused by extensive necrosis of the placentome. Focal inflammatory lesions with diffuse lymphoid infiltrates can be found in many other tissues, including the liver, lung and heart. The kidneys and skeletal muscle are less frequently affected.

==Treatment==

==Treatment==

In the event of an outbreak, little can be done to prevent further spread since transmission is via contaminated food or water rather than sheep-to-sheep contact. Since environmental contamination is related to the behaviour of cats, numbers should be limited. Keeping an older, neutered male cat may help ward of other felines: since most cats seroconvert at a young age, adults are unlikely to shed oocysts to contribute to contamination. Rodents and other vermin transmit toxoplasmosis to cats, and so populations should also be controlled. Animals should be prevented from gaining access to feed or bedding.

In the event of an outbreak, little can be done to prevent further spread since transmission is via contaminated food or water rather than sheep-to-sheep contact. Since environmental contamination is related to the behaviour of cats, numbers should be limited. Keeping an older, neutered male cat may help ward of other felines: since most cats seroconvert at a young age, adults are unlikely to shed oocysts to contribute to contamination. Rodents and other vermin transmit toxoplasmosis to cats, and so populations should also be controlled. Animals should be prevented from gaining access to sheep feed or bedding.

Previously, monensin has been given in sheep feed in the lead up to lambing. Although this was shown to reduce perinatal lamb mortality related to ''Toxoplasma'' infection, monensin is no longer licensed for sheep and should not be used.

Previously, monensin has been given in sheep feed in the lead up to lambing. Although this was shown to reduce perinatal lamb mortality related to ''Toxoplasma'' infection, monensin is no longer licensed for sheep and should not be used.

The best method of controlling ovine toxoplasmosis is therefore by vaccination. A live vaccine containing tachyzoites of the avirulent S48 strain is available. These tachyzoites do not cause pathology or form bradyzoites or tissues cysts. A single dose of vaccine is administered intramuscularly at least 3 weeks (and up to four months) prior to mating, from an age of five months. The vaccine is known to protect agains toxoplasmosis for at least two lambing seasons.

The best method of controlling ovine toxoplasmosis is therefore by vaccination. A live vaccine containing tachyzoites of the avirulent S48 strain is available. These tachyzoites do not cause pathology or form bradyzoites or tissues cysts. A single dose of vaccine is administered intramuscularly at least 3 weeks (and up to four months) prior to mating, from an age of five months. The vaccine is known to protect against toxoplasmosis for at least two lambing seasons.

==Prognosis==

==Prognosis==

An outbreak of toxoplasmosis can cause significant lamb losses. However, ewes rarely show ill effects, and will not abort again in subsequent lambings. Vaccination gives excellent control of toxoplasmosis.

An outbreak of toxoplasmosis can cause significant lamb losses. However, ewes rarely show ill effects, and will not abort again in subsequent lambings. Vaccination gives excellent control of toxoplasmosis.

==Links==

==Links==

*[http://www.moredun.org.uk/feature-article.asp?ref=111 Toxoplasma infection: vaccination as a control strategy (Moredun Insititute)]

*[http://www.moredun.org.uk/feature-article.asp?ref=111 Toxoplasma infection: vaccination as a control strategy (Moredun Insititute)]

*[http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/52200.htm The Merck Veterinary Manual - Toxoplasmosis]

*[http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/52200.htm The Merck Veterinary Manual - Toxoplasmosis]

#Dubey, J P (2009) Toxoplasmosis in sheep — The last 20 years. ''Veterinary Parasitology'', '''163''', 1-14.

#Dubey, J P (2009) Toxoplasmosis in sheep — The last 20 years. ''Veterinary Parasitology'', '''163''', 1-14.

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