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→Clinical Signs
===Clinical Signs===
===Clinical Signs===
The disease toxoplasmosis in sheep usually manifests
following a primary infection of a pregnant ewe,
resulting in placental invasion and transplacental
infection of the foetus. Typical clinical signs are
abortion and production of stillborn or weak lambs
often along with a small,mummified foetus. Placental
tissue from infected ewes may also show characteristic
gross white spot lesions which are visible to
the naked eye and are areas of necrosis in the tissue
which will limit its effective function in supporting
the pregnancy (Buxton, 1990). Sheep may become
infected from the consumption of sporulated oocysts.
Once ingested, the oocysts excyst in the small intestine,
releasing sporozoites which quickly invade and
multiply within the cells of the gut differentiating
into tachyzoites. The tachyzoites can be found
multiplying within mesenteric lymph node cells by
day 4 following infection (Dubey, 1984). Toxoplasma
is an obligate intracellular parasite and uses a process
of endodyogeny to multiply within host cells
(Ferguson, 2009). The parasitized cells then rupture
releasing tachyzoites that invade other host cells. A
common clinical sign is an elevated temperature in
the animal which is observed co-incident with the
appearance of tachyzoites in the mesenteric lymph
nodes and the fever may last for a further week,
during which time tachyzoites may be detected in
the circulation (Dubey and Sharma, 1980; Wastling,
Nicoll and Buxton, 1993). In the pregnant ewe, the
tachyzoites find their way to the placenta where they
invade and multiply within the maternal caruncular
septa in the placentome and from there they invade
the adjoining foetal trophoblast cells (Buxton and
Finlayson, 1986). The immune system of the sheep
is modulated during pregnancy in order to prevent
rejection of the semi-allogeneic foetus. This manifests
as a damping down of the pro-inflammatory
immune responses such as interleukin 2 (IL-2) and
interferon gamma (IFNc) at the maternal-foetal
interface (Innes and Vermeulen, 2006; Entrican and
Wheelhouse, 2006). This change in the immune
environment of the placenta provides a favourable
location for the parasite to establish and multiply.
===Laboratory Tests===
===Laboratory Tests===