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What is Toxoplasmosis?
Toxoplasmosis is a disease
caused by infection with the microscopic organism called Toxoplasma
gondii (T. gondii). This is not a new disease; it was first described
in 1908. Virtually all warm-blooded animals, including most pets, livestock,
and people, can be infected with this organism. It is an extremely well
adapted parasite and rarely causes
significant disease to the individuals it infects.
How common is Toxoplasma in cats?
Toxoplasma occurs worldwide
and infection in cats is similarly widespread. Many more cats are infected
than show symptoms. In research studies, as many as one half of adult cats
have antibodies to the organism in their blood indicating that they have
been exposed to the infection at some time. Infection rates are higher in
free-roaming and stray cats. In contrast, infection is uncommon in pet cats
that do little or no hunting, and are fed primarily or exclusively
commercial cat foods.
How is Toxoplasma
transmitted?
Cats are usually infected
by ingesting the organism present in the tissues (meat) of another infected
animal (an ‘intermediate host’), usually a mouse, bird, or other small
animal For indoor cats, the
most likely source is uncooked meat scraps. The Toxoplasma organism
replicates first locally in the intestinal tract of the cat, and is often
contained there. The replication in the intestinal tract results in shedding
of oocysts in the feces. The oocysts represent a hardy form of the organism
that can survive in the external environment for many months or even years.
Other animals can become infected by ingesting these oocysts, but only if
large numbers are ingested will disease result.
In some cats, particularly
if their immune defenses are compromised, the Toxoplasma organisms can
invade beyond the intestine and spread into various organs of the body.
There, they may cause enough damage to cause signs of disease or may become
dormant in a tissue cyst (not the same as the oocyst form). Such tissue
cysts can be infective if the infected tissue is eaten by another animal.
How
do people get Toxoplasmosis?
While cats are usually
infected by preying on infected rodents (or more rarely by ingestion of
oocysts from the environment), humans are most commonly infected through
food. Sheep, cattle and pigs grazing on contaminated pastures, or fed oocyst-contaminated
food, can also develop the encysted form of the organism in body tissues. If
infected meat is not adequately cooked, or poor hygiene precautions are
adopted during handling of uncooked meat, humans can become infected.
Ingestion of oocysts from infected cats, for example during gardening in
contaminated soil, is a less common source of human infection.
What disease does
Toxoplasma cause in cats?
Although Toxoplasma
is a relatively common infection, it usually causes no disease in infected
cats. However, if the cat’s immune system is not working properly,
Toxoplasma may continue to replicate, spread and cause damage to tissues.
When this happens a variety of different clinical signs can develop
including ocular (eye) disease, respiratory disease, diarrhea, liver disease
and neurological signs. Such disease may be acute (rapid in onset) or more
chronic with periods of illness interspersed with periods of some recovery.
It is important to remember that Toxoplasma is a rare cause of disease in cats.
 How
can you diagnose and treat Toxoplasmosis?
Toxoplasmosis is difficult
to diagnose in cats because the signs can be so variable. Blood tests are
available that will demonstrate, by the presence of antibodies to the
organism, whether a cat has been exposed to the organism. But these tests do
not necessarily mean that Toxoplasma
is the cause of any disease since most exposed cats do not develop disease.
When Toxoplasmosis is suspected in a cat, it is usually treated with a
course of an appropriate antibiotic.
How important is Toxoplasma
in people?
Around 30% of adults in the
U.S. and Europe have antibodies to Toxoplasma, which means they have
been exposed to the parasite. As with infection in cats, the vast majority
of people infected with this organism experience no clinical disease at all,
or possibly just mild and transient ‘flu’-like signs. However, there are
also some individuals where significant disease does occur and two
situations are particularly important. If a pregnant woman acquires Toxoplasma
infection during her pregnancy, the infection may be transmitted to the
fetus, and sometimes causes severe damage. Again, this is only a risk if the
woman acquires the infection during her pregnancy. A woman who has
previously been exposed to the organism carries no risk of transmission to a
fetus if she subsequently becomes pregnant.
Ideally, women who have frequent contact with cats should be
serologically tested for prior exposure to Toxoplasma gondii before
becoming pregnant, because if they are already seropositive, they are not at
risk for acquiring a primary infection during pregnancy.
Statistics show that of the 4 million babies born in the United
States each year, only 1200 suffer any effects from toxoplasmosis.
Most of these babies only have mild problems, such as a rash or an
eye infection. Nonetheless, the
parasite can cause grave damage to a growing fetus.
In rare instances, babies have developed mental retardation and
anemia.
The other segment of the
population at high risk for the disease is persons with compromised immune
system. People with AIDS,
people receiving chemotherapy, diabetics, alcoholics, and people receiving
drugs to prevent rejection of a transplanted organ are at higher risk for
contracting toxoplasmosis. Usually,
these people have been exposed to the Toxoplasma parasite earlier in
life, and their weakened immune system is allowing the parasite to multiply
unchecked.
How can human infection be
avoided?
Although cats are
essential to complete the life-cycle of T. gondii, numerous surveys
have shown that people who own cats are not themselves at a higher risk of
acquiring infection. There are several reasons for this:
·
Many pet
cats will never be exposed to Toxoplasma and therefore cannot pass
infection on to humans.
·
Even if a
cat does become infected with Toxoplasma, it will only shed the
oocysts (eggs) in its feces for a short period (approximately 10 days) after
initial exposure. Following this there is no further significant oocyst
shedding and therefore again no further risk to humans. Oocysts become
infectious after 1 or 2 days. Since cats are very fastidious and are
unlikely to leave feces on their fur for two days, it is unlikely that
humans become infected from direct contact with cats themselves.
·
Although
humans can be infected through exposure to, and ingestion of oocysts in the
environment, a more common source of infection appears to be infected meat.
Following a few
sensible environmental and meat hygiene measures can greatly reduce the risk
of human infection:
·
Cook all
meat thoroughly - at least 160 -180 F (70 –82 C) throughout. When cooking,
avoid tasting meat before it is fully cooked. Be aware that microwaving
meat is not guaranteed to kill Toxoplasma.
·
Avoid
eating unpasteurized dairy products
·
Wash hands,
utensils (cutting boards, knives) and surfaces (sinks, counters) carefully
after handling raw meat.
·
Wash all
vegetables carefully, especially those grown in backyard gardens.
·
Wear gloves
when gardening in soil potentially contaminated by cat feces.
·
Empty cat
litter trays daily, dispose of litter carefully, and disinfect with boiling
water. If this is done every day, even if a cat is excreting oocysts, they
will not have become infectious (because it takes at least 24 hours for
oocysts that are passed in the feces to “ripen” and become infectious) by
the time the litter is changed.
·
Discourage
pet cats from hunting, and avoid feeding them raw or undercooked meat.
·
Cover any
children's sand boxes to prevent cats using them as a litter tray.
·
Boil water
from ponds and streams when camping/hiking.
Updated 2/9/06 |
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