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Sheep are often kept in an environment that is significantly contaminated with oocysts, and infection follows ingestion of infected food, primarily contaminated pasture. Fields treated with manure or bedding from buildings to which cats have access result in high levels of ovine toxoplasmosis, and insecure storage of supplementary feeds also poses a risk.
 
Sheep are often kept in an environment that is significantly contaminated with oocysts, and infection follows ingestion of infected food, primarily contaminated pasture. Fields treated with manure or bedding from buildings to which cats have access result in high levels of ovine toxoplasmosis, and insecure storage of supplementary feeds also poses a risk.
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As the definitive hosts of ''Toxoplasma gondii'', cats become infected when they hunt and eat infected wild rodents and birds. Between days 3 and 14 post-infection, cats shed over 100 million of oocysts in their faeces. Only 200 oocysts are required to cause abortion in naive pregnant sheep
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As the definitive hosts of ''Toxoplasma gondii'', cats become infected when they hunt and eat infected wild rodents and birds. 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 oocysts<sup>1</sup>, 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 farm<sup>2</sup>.
During this period a cat may shed over 100 million
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oocysts and experimental studies in sheep have shown
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that a dose of only 200 oocysts may cause abortion in
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previously naı¨ve pregnant sheep (McColgan, Buxton
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and Blewett, 1988). The importance of oocysts as a
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source of infection for sheep, has been supported by
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studies showing an association with infection and
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contamination of feed or grazing land with sporulated
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oocysts (Plant, Richardson and Moyle, 1974;
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Faull, Clarkson and Winter, 1986) and also work
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showing an association with cats on farms and
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prevalence of T. gondii in sheep (Skjerve et al. 1998).
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Further studies looking at development of specific
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antibodies in sheep, as an indicator of exposure to
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T. gondii, have shown that there is an increase in seroprevalence
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associated with age. This indicates that
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there is extensive environmental contamination with
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T. gondii oocysts and that most infections in sheep
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occur following exposure to the parasite after birth
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(Waldeland, 1977; Blewett, 1983; Lunden,Nasholm
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and Uggla, 1994). Recent studies have indicated that
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there is widespread environmental contamination
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with T. gondii oocysts (Dabritz et al. 2007).
      
===Congenital Transmission===
 
===Congenital Transmission===
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