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| | ===Transmission to Sheep=== | | ===Transmission to Sheep=== |
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| − | The ingestion of tissue cysts by cats is of great
| + | Infected cats shed oocysts continuously between days 3 and 14 post-infection. During this time, tens of millions of oocysts may be shed. The main sources of feline toxoplasma infection are chronically infected birds and rodents. Rodents are particularly important since they can pass ''T. gondii'' infection to their offspring without causing clinical disease. This means that a location may develop a reservoir of ''T. gondii'' tissue cysts with the potential to cause feline infection and massive oocyst excretion. In turn, environments may easily become contaminated with a high oocyst burdedn/ |
| − | significance, in terms of spread of infection to other
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| − | animals (Figure 2). Oocysts may be shed continuously
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| − | in the faeces from 4 until 14 days after infection,
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| − | with an expected peak output of tens of millions at
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| − | 6-8 days9. Recrudescence of infection may occur if
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| − | the cat is experimentally stressed'0 and perhaps also
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| − | through unrelated illness. This can result in the
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| − | re-excretion of oocysts in smaller numbers for a
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| − | shorter time than in a primary infection.
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| − | Cats acquire infection as a result of hunting so that
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| − | many will have seroconverted by adulthood. Although
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| − | less than 1% may be shedding oocysts at any one
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| − | time5, infection may be more prevalent in young cats
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| − | taking up hunting for the first time.
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| − | Female feral cats can produce two to three litters
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| − | a year, each ofup to eight kittens, and may rear their
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| − | young communally". Numbers of young cats are
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| − | also dependent upon the density of breeding adults.
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| − | In rural areas male cats may have territories of 60-80
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| − | hectares (250-200 acres) while females usually only
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| − | occupy a 10th of this area"1. In an urban environment
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| − | these territories are considerably smaller'2. The area
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| − | occupied by feral cats is influenced by the supply
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| − | of food, which includes mice, voles, shrews, rats,
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| − | rabbits and small birds".
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| − | Ingestion
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| − | small animals I CATS
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| − | INFECTED
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| − | WARM-BLOODED
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| − | HOSTS
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| − | pregnant sheep
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| − | ingestion
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| − | OOCYSTS contaminate
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| − | environment
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| − | (e.g. livestock feed and
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| − | pasture)
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| − | abortion
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| − | Figure 2. The spread ofToxoplasma infection to susceptible
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| − | pregnant sheep from infected cat faeces deposited in the
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| − | environment I
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| − | 0141-0768/90/
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| − | 080509-03402.00/0
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| − | 0 1990
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| − | The Royal
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| − | Society of
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| − | Medicine
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| − | 510 Journal of the Royal Society of Medicine Volume 83 August 1990
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| − | The most important sources of feline infection are
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| − | chronically-infected birds and rodents6"13, particularly | |
| − | the latter because they can pass T. gondii infection
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| − | from generation to generation without causing
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| − | overt clinical disease14'16. In this way a reservoir of
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| − | T. gondii tissue cyst infection can exist in a particular | |
| − | location for a long time, with the potential for
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| − | infecting cats and triggering massive oocyst excretion.
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| | The available epidemiological and experimental | | The available epidemiological and experimental |
| | evidence suggests that, in the UK, sheep are | | evidence suggests that, in the UK, sheep are |