Page 197 - The Veterinary Laboratory and Field Manual 3rd Edition

166 Susan C. Cork and Mani Lejeune

cysts containing bradyzoites develops. The life contaminated with sporulated oocysts. Cats
cycle is complex. The definitive host (cats) gets may become infected by ingestion of sporulated
infected by consuming the tissue cysts in inter- oocysts from cat faeces or by ingestion of cys-
mediate hosts (prey animals). Intermediate tic stages (bradyzoites) in an intermediate host
hosts get infected through various routes of such as a rodent. The life cycle of Toxoplasma
transmission but mainly through environment gondii is illustrated in Figure 3.32. Humans may

Figure 3.32 The life cycle of Toxoplasma gondii. The cat or other wild felid species are the definitive host
for this protozoal organism. It is only in the definitive host that infective oocysts develop and are shed in
the faeces to infect other animals. (A) Humans can become infected by accidental ingestion of sporulated
(infective) Toxoplasma sp. oocysts in food or from inanimate objects contaminated with cat faeces. Healthy
adults rarely develop clinical disease but if a pregnant woman is infected the organism migrates to the pla-
centa and can cause damage to the developing foetus. (B) Healthy adult sheep and other animals may also
ingest sporulated oocysts while grazing. Pregnant sheep may abort if they become infected during gestation
unless the ewe has developed immunity, that is, she has had previous exposure to the organism (natural
infection) or has been vaccinated. (C) Cats and kittens can become infected by ingestion of sporulated
oocysts or by ingestion of paratenic hosts. The protozoal organisms undergo asexual reproduction in the
cells of the cat but only cause clinical disease in young animals or if the number of organisms is very high.

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