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→Transmission to Sheep
===Transmission to Sheep===
===Transmission to Sheep===
The ingestion of tissue cysts by cats is of great
significance, in terms of spread of infection to other
animals (Figure 2). Oocysts may be shed continuously
in the faeces from 4 until 14 days after infection,
with an expected peak output of tens of millions at
6-8 days9. Recrudescence of infection may occur if
the cat is experimentally stressed'0 and perhaps also
through unrelated illness. This can result in the
re-excretion of oocysts in smaller numbers for a
shorter time than in a primary infection.
Cats acquire infection as a result of hunting so that
many will have seroconverted by adulthood. Although
less than 1% may be shedding oocysts at any one
time5, infection may be more prevalent in young cats
taking up hunting for the first time.
Female feral cats can produce two to three litters
a year, each ofup to eight kittens, and may rear their
young communally". Numbers of young cats are
also dependent upon the density of breeding adults.
In rural areas male cats may have territories of 60-80
hectares (250-200 acres) while females usually only
occupy a 10th of this area"1. In an urban environment
these territories are considerably smaller'2. The area
occupied by feral cats is influenced by the supply
of food, which includes mice, voles, shrews, rats,
rabbits and small birds".
Ingestion
small animals I CATS
INFECTED
WARM-BLOODED
HOSTS
pregnant sheep
ingestion
OOCYSTS contaminate
environment
(e.g. livestock feed and
pasture)
abortion
Figure 2. The spread ofToxoplasma infection to susceptible
pregnant sheep from infected cat faeces deposited in the
environment I
0141-0768/90/
080509-03402.00/0
0 1990
The Royal
Society of
Medicine
510 Journal of the Royal Society of Medicine Volume 83 August 1990
The most important sources of feline infection are
chronically-infected birds and rodents6"13, particularly
the latter because they can pass T. gondii infection
from generation to generation without causing
overt clinical disease14'16. In this way a reservoir of
T. gondii tissue cyst infection can exist in a particular
location for a long time, with the potential for
infecting cats and triggering massive oocyst excretion.
The available epidemiological and experimental
evidence suggests that, in the UK, sheep are
frequently maintained in an environment significantly
contaminated with oocysts and that infection follows
ingestion of infected food17"8. Perhaps the most
common source of infection is contaminated pasture.
Certainly, fields treated with manure and bedding
from farm buildings where cats live can cause
infection'9. Careless storage of farm feeds may also
pose a risk20. Fifty grams of infected cat faeces may
contain as many as 10 million oocysts9. If in a
hypothetical situation this was evenly dispersed
throughout 10 tonnes of concentrated animal feed
then each kilogram could contain between five and
25 sheep-infective doses21. The extent of environmental
contamination with T. gondii oocysts is thus
related to the distribution and behaviour of cats.
Measures to reduce environmental contamination
by oocysts should be aimed at reducing the number
of cats capable of shedding oocysts. This would include
attempts to limit their breeding. If male cats are
caught, neutered and returned to their colonies the
stability ofthe colony is maintained; fertile male cats
do not challenge the neutered males12 and breeding
is controlled. Thus the maintenance ofa small healthy
population of mature cats will reduce oocyst excretion
as well as help to control rodents. Sheep feed should be
kept covered at all times to prevent its contamination
by cat faeces.
==Signalment==
==Signalment==