Toxoplasmosis
is a disease caused by a one-celled parasite called Toxoplasma gondii.
It is a prevalent parasite of cats throughout the world, although other
species can become infected. The
cat is unique in that it is the definitive host of the parasite, meaning that
the organism must pass through the cat to complete all stages of its life
cycle.In humans, it may affect many different organs of the body, causing many
different types of clinical signs. The
respiratory system is commonly involved and pneumonia may result.
The most common finding is a mild, flu-like illness that lasts a few
days. Most people recover
uneventfully. Even if the patient
sees a physician, the illness is often attributed to the flu unless special
blood tests are run.
Contributing
Factors
Cats
usually become infected by feeding on rodents, some types of insects, and by
ingestion of undercooked meat. Although
cats who go outdoors are at somewhat increased risk of becoming infected, indoor
cats may also contract the disease if they are allowed to eat raw meat.
Prevalence
Once
the immune system has been exposed to the toxo organism, antibodies will be
produced. Antibodies (also called
globulins) play a protective role in the immune system.
Exposure
to the toxoplasma organism will result in the production of antibodies.
Antibodies are the defense agents of the immune system and are produced
in response to immune system stimulation. The
presence of antibodies means that the person or cat has been exposed; it does
not necessarily mean that any disease occurred.
It is estimated that about 50% of domestic cats in the United States have
been exposed to toxoplasmosis. It
is also estimated that about 33% of the U.S. population has been exposed.
However, detection of antibodies against toxo only means that the person
or cat has been exposed at some point in its life; it does not mean that active
toxoplasmosis is present.
The
disease toxoplasmosis occurs in about 140 babies per million births in the
United States, or about 14 per 100,000. A
like number of children will be infected with the toxoplasmosis organism at the
time of birth and develop disease later in life.
Therefore, the combined incidence of congenital and acquired
toxoplasmosis in the United States is 28 per 100,000 or 0.028%.
Although this is indeed a real disease with dire consequences, it should
be noted that its incidence is very small, especially in light of how many
people have evidence of exposure (presence of toxoplasma antibodies).
Clinical
Signs
In
humans, clinical signs are rarely apparent when the immune system is normal. If signs of illness are present, they are usually
self-limiting and nonspecific. The majority of clinical signs result from
replication of the organism within tissues.
The
respiratory system may be involved, giving the impression of a flu-like illness.
In other cases, it may appear similar to mononucleosis, with fever and
swollen lymph nodes. In humans with weakened immune systems (such as with AIDS or
patients receiving chemotherapy), the consequences of toxoplasmosis can be
devastating, eventually leading to death of the individual.
In
cats, the most common signs are related to inflammatory changes in the eyes.
The respiratory system, liver and central nervous system may also be
involved, with signs of pneumonia, jaundice, or seizures.
Diagnosis
Owners
worried that they have been infected with toxoplasmosis should seek consultation
with their physician about testing.
Veterinarians
are frequently asked to test a cat that belongs to a pregnant woman for
toxoplasmosis. Pregnant women
should know the following concerning toxoplasmosis testing:
1.
A screening test for toxoplasma antibodies can be performed on both the
pregnant woman and the cat. A
negative result means that the woman (and/or the cat) has not been exposed to
the toxoplasma organism. However,
it does not infer that either the woman or the cat has any immunity to
toxoplasmosis in the event of a future exposure.
In fact, it means just the opposite.
Both are susceptible to infection.
2.
A single antibody titer that is positive, performed on the woman and/or
the pet cat, means that there has been exposure to the toxo organism at some
time. In order to determine whether
there is active infection, a second test must be run 2-4 weeks later.
Be aware that no test is available to determine that infection has been
acquired by contact with cats.
a.
If the two tests give similar results, there has been an infection in the past
and a certain degree of immunity exists.
b.
If the second test is significantly higher that the first, there is a strong
possibility that an active case of toxoplasmosis is in progress.
c.
It is very important that both tests be performed by the same testing laboratory
in order to properly compare results.
3.
The direct means of documenting the possibility of transmission of
toxoplasmosis requires that we microscopically examine a fecal sample from the
cat looking for the eggs of the toxoplasma organism.
Because these eggs are very tiny (even under a microscope) and because
the cat may not be shedding eggs today but may do so in the future, multiple
examinations must be done during the course of pregnancy, preferably once
weekly. This is not a very high yield procedure, meaning that it can
be difficult to detect the parasites, and they can be missed by this diagnostic
technique.
Treatment
Clindamycin
hydrochloride is the drug of choice for treating cats with toxoplasmosis.
There is no evidence to indicate that the drug is able to completely
remove the organism from the cat’s body, although most cats are improving
within 2-3 days of starting the drug. Infections
that involve the eyes or central nervous system are more difficult to treat.
In general, better results are achieved if the cat is treated for four
weeks or longer.
Prognosis
Cats
who respond to clindamycin therapy and are negative for feline leukemia virus
and feline immunodeficiency virus have a good prognosis.
Those with weakened immune systems, advanced stages of the disease, or
infections of the nervous system have a guarded prognosis for full recovery.
Transmission
to Humans
If
a pregnant woman contracts toxoplasmosis, it may affect the unborn baby.
It is this form of the disease that has the most dire consequences
because the baby may be affected for life.
As
stated above, several species may develop the disease toxoplasmosis, including
humans and dogs, but the organism can only complete its life cycle in the
domestic cat. This means that the
cat may be infected with the toxo organism and transmit it to other cats or to
other species, including humans. Owners
should be aware that human infection resulting from direct contact with infected
cats is extremely unlikely. In
order for this to occur, the following must happen:
1.
The cat must be infected with the toxo organism, and most cats are not.
In order for this to occur, the cat must eat something infected with it.
This usually occurs as a consequence of carnivorous feeding on mice or
ingestion of undercooked meats, such as pork or mutton.
Other sources of infection for cats include cockroaches and earthworms.
2.
The cat must be shedding the organism in its feces.
This occurs for only about a 10-day period. It usually only occurs once in the cat's lifetime.
3.
The organism must "incubate" in the cat's feces for 1-5 days before it
is infective to humans. This
"incubation" must occur after the feces leaves the cat's body and have
access to oxygen (i.e. in the litter box or in soil).
Therefore, fresh cat feces cannot cause transmission of the infection to
humans.
4.
The toxo organism must be swallowed by the person being infected.
It is not spread to humans through the air.
The
toxo organism may also be transmitted to humans by eating raw or undercooked
meats, especially pork or mutton. Since
many hamburgers from fast-food restaurants are made of beef diluted with pork,
most authorities feel that human infection occurs much more frequently by this
method than by association with cats.
Although
this is indeed a real disease with dire consequences, it should be noted that
toxoplasmosis occurs infrequently, especially in consideration of the numbers of
people with Toxoplasma antibodies. In
AIDS patients with toxoplasmosis, the disease is generally considered to arise
from reactivation of a prior infection rather than establishment of a new
infection. For this reason, it is
often not necessary to remove cats from the household of HIV-infected people.
The
incidence of toxo antibodies in U.S. veterinarians is not different than that of
the rest of the population.
Prevention
At
this time, researchers are investigating the utility of a mutant strain of toxo
as a potential vaccine source. Until
such time as a vaccine would be widely used, prevention of infection is
indicated. Practical means of
preventing infection include:
1.
Do not allow your cat to eat mice or poorly cooked meat.
Feeding a commercial cat food and not allowing your cat outdoors
virtually eliminates any possibility of the cat becoming infected.
2.
Clean all feces from your cat's litter box daily.
Even if the cat's feces are infected with toxo oocysts, they must
incubate for 1-5 days before becoming infectious.
To be extra safe, pregnant women should not clean the litter box.
3.
When working in soil (flowerbeds) where cats might defecate, wear gloves to keep
from getting oocysts on your hands.
4.
Avoid eating raw or poorly cooked meats. Be
especially careful of fast-food hamburgers.
Wash hands thoroughly after handling raw meat while cooking. Clean cutting boards, knives and all utensils involved in
preparation of raw meat. Since this
is probably more of a threat to your baby than your cat, special attention
should be paid here.
5.
Wash all fruits and vegetables prior to eating.
6.
Keep children's sandboxes covered. Outdoor
cats will frequently use the sandbox for defecation.
Even if the feces are scooped out, the sandbox may remain contaminated
with parasites.
Summary
1.
Toxoplasmosis that affects babies is quite rare.
The incidence in the United States is 0.028% of all births.
2.
Feed commercial cat food and keep your cat indoors.
3.
Clean the litter box of all feces daily.
Pregnant women should not clean the box.
4.
Wash your hands following cleaning of the litter box and handling of soil
after gardening.
5.
Do not eat undercooked meats.