Overview
Feral cats are a continuing problem in Tasmania. They present
three specific problems the effects of which have been difficult
to get information about and to date we have unable to be quantify.
However, the problems caused are well documented and figures
from non-Tasmanian sources can be applied to Tasmania.
The three specific problems presented by feral cats are:
1. Animal Health - spread of disease to sheep;
2. Human Health - spread of disease to humans; and
3. Predation of native vertebrates.
1. Animal Health - Spread of Disease to Sheep
Cats carry two diseases, Toxoplasmosis and Cycospiridosis,
that can have a major impact on farming.
Toxoplasmosis is a disease caused by a protozoan parasite
of cats. The organism, Toxoplasma gondii reproduces in the
cat's gut, and the infective stage of the parasite, the oocyst,
is passed in the cat's faeces. Cats are therefore, the definitive
host of the disease.
The disease can be spread from the ingestion of the Toxoplasma
gondii organism in its infective form (oocyst). Oocysts are
very small and environmentally resistant and can remain infective
in the environment for up to one year. Once the oocyst is
inside the human or animal it can cause sudden disease or
become quiescent (tissue cyst) for months to years.
The major vectors in the spread of Toxoplasmosis are cats
as they are the only animals that can shed the oocysts in
their faecal material. Oocysts require an incubation period
of one to five days after excretion before they become infective.
Sheep become infected by ingesting the oocyst when they graze
infected where cats have defecated or from hay or grain similarly
contaminated. Non-pregnant sheep that have been infected with
Toxoplasmosis develop a lifelong immunity and are no longer
at risk of Toxoplasmosis induced abortion. However the tissue
cysts remain in the muscles of the sheep.
In sheep, infection in ewes in early pregnancy causes inflammation
of the placenta resulting in foetal death and often abortion.
Infection in later pregnancy causes a live infected lamb to
be born which is often weak.
Toxoplasmosis is well documented as causing great financial
loss in the agricultural industry, being a major cause of
abortion and stillbirth in sheep flocks in Britain, Australia
and New Zealand. Toxoplasmosis is the most frequently diagnosed
cause of abortion in sheep and accounts for approximately
80% of sheep abortions in the UK. Abortions are higher in
colder areas, and while 30% of ewes in a flock aborting would
be considered at the higher end of the scale it is not unknown.
Toxoplasmosis occurs in isolated occurrences in Tasmania
throughout the lambing season. While the economic impact to
the industry is small. High lambing loss percentages can have
a devastating financial impact on individual farm businesses
in any one season. Anecdotal evidence suggests that some farm
businesses no longer run ewes and lambs because of toxoplasmosis
but instead run wethers for wool production only.
The disease is known to affect other animal species however
the impact is relatively small. Cats reinfect themselves by
consuming an animal that contains the tissue cysts, typically
some type of rodents and birds. Once the tissue cyst is back
inside the cat it goes through a cycle that ultimately results
in the release of the oocyst back into the environment.
The other disease Cyclosporidiosis is similar in many ways
to toxoplasmosis, it is a reportable disease in some countries
and can be transferred to sheep (and humans) by cats in a
similar way to Toxoplasmosis. Sheep found with Cyclosporidiosis
cannot be exported. It is not known whether Cyclosporidiosis
is present in Tasmania or present in the feral cat population
here.
2. Human Health - Spread of Disease to Humans
Humans and other animals may act as intermediate hosts for
the disease. Toxoplasmosis causes a variety of disease syndromes
in humans, ranging from flu-like symptoms to miscarriage or
birth defects in infants when women are exposed during pregnancy.
Humans can catch toxoplasmosis in a number of ways:
• By inadvertently swallowing the organism Toxoplasma
gondii. This contact might be initiated by cleaning cat litter
boxes, gardening where cats defecate, handling hay and straw
which has been defecated on by cats or playing in a sandbox
in which a cat has defecated.
• By consuming raw or undercooked meat of an infected
animal.
• Humans can also pick up the Toxoplasma organism when
handling a lambing ewe, newborn lamb or the afterbirth and
then ingesting the organism or through open wounds.
In Australia about 0.2-0.5% of women of child bearing age
show evidence of acute toxoplasmosis.
Both major hospitals the Launceston General Hospital and
Royal Hobart (emergency and infectious diseases departments)
as well as the Public Health Department were unable to provide
figures for incidences of Toxoplasmosis. As it is not one
of the 64 diseases on the Public Health Register, as it does
not cause epidemic problems, it is not reportable. Consequently
none of the institutions keep numbers/statistics on this disease.
3. Predation of Native Vertebrates
Research on feral cats on small islands shows that the impact
on native vertebrates is high as the cats will eat lizards/birds
etc that are specific to that area whereas in the mainland
areas, while feral cats continue to eat birds and lizards,
when these numbers decrease they are adaptable enough to eat
other vertebrae, therefore competing with and replacing other
predators in the food chain. This is of concern to primary
producers and it is recognised as a very real threat given
the health problems in the Tasmanian devil population.
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