| |
Toxoplasmosis in Cats
Toxoplasmosis, a disease of cats and other mammalian species,
is caused by a parasitic protozoan, Toxoplasma gondii.
Protozoa are single-celled organisms that are among the simplest
creatures in the animal kingdom. Although infection with Toxoplasma
is fairly common, actual disease caused by the parasite is relatively
rare.
The Life Cycle of Toxoplasma
Cats, domestic and wild, are the definitive host (host in which
the adult, or sexually mature stage, of the parasite is produced)
and are the parasite's primary reservoir of infection. Domestic
cats are important in transmission of Toxoplasma to other animals
and human beings, which become involved only as intermediate hosts
of the parasite. Consumption of raw meat tissues is another important
means of transmission.
Cats acquire Toxoplasma infection by eating any of the three
infective stages of the parasite: cyst, oocyst, or tachyzoite.
Following ingestion of cysts in infected prey (rodents or birds),
the intraintestinal infection cycle begins. This cycle occurs
only in members of the cat family. The organisms multiply in the
wall of the small intestine and produce oocysts, which are then
excreted in great numbers in the feces for two to three weeks.
Within five days the shed oocysts may sporulate, becoming infectious
to other animals and to humans. Sporulated oocysts are highly
resistant to environmental conditions and can survive in moist
shaded soil or sand for many months.
During the intraintestinal infection cycle in the cat, some
Toxoplasma organisms released from the ingested cysts penetrate
more deeply into the wall of the intestine and multiply as tachyzoite
forms. Very soon these forms spread out from the intestine to
other body sites, starting the extraintestinal infection cycle.
Eventually the cat's immune system restrains this stage of the
organism, which then enters a dormant or "resting" stage
by forming cysts in muscles and brain. Most cysts probably remain
dormant for the life of the host. The extraintestinal infection
cycle occurs not only in cats but also in the intermediate hosts
(including humans).
Most healthy exposed cats shed oocysts during acute infection
with Toxoplasma, but will not shed them after the acute infection.
Even in those few cats that do re-excrete oocysts after another
exposure to Toxoplasma, the number of oocysts shed is smaller
and may even be insufficient to transmit the parasite effectively.
Ingestion of tissue cysts in infected prey or in other raw
meat is probably the most common route by which cats are exposed
to Toxoplasma. Congenital infection (transmission from mother
to fetus) occurs in sheep, goats, and humans, but is much less
common in cats.
Return to Top
Signs
Most cats show no clinical signs of infection with Toxoplasma.
Occasionally, however, clinical disease—toxoplasmosis—occurs,
kittens and young adult cats being more often affected than older
animals. Lethargy, depression, loss of appetite, and fever are
typical early nonspecific signs. Pneumonia, manifested by respiratory
distress of gradually increasing severity, is the outstanding
sign in many cats. Hepatitis (inflammation of the liver) may cause
vomiting, diarrhea, prostration, and jaundice (yellowing of the
mucous membranes). Inflammation of the pancreas and enlargement
of lymph nodes also occur. Toxoplasmosis can also affect the eyes
and central nervous system, producing inflammation of the retina
or anterior ocular chamber, abnormal pupil size and responsiveness
to light, blindness, incoordination, heightened sensitivity to
touch, personality changes, circling, head pressing, twitching
of the ears, difficulty in chewing and swallowing food, seizures,
and loss of control over urination and defecation.
In some cases, coinfection with feline leukemia virus (FeLV)
or feline immunodeficiency virus (FIV) may predispose a cat to
develop toxoplasmosis.
Diagnosis
Toxoplasmosis may be strongly suspected by the history, signs
of illness, and the results of supportive laboratory tests. A
presumptive diagnosis may be made by demonstration of a fourfold
or greater increase in antibody titers to Toxoplasma (indicating
a recent infection) over a three- or four-week period in a cat
showing signs suggestive of toxoplasmosis. A definitive diagnosis
requires either microscopic examination of tissues or tissue impression
smears for distinctive pathologic changes and the presence of
tachyzoites or inoculation of suspect material into laboratory
mice.
The presence of significant antibody levels in a healthy cat
suggests that the cat has been previously infected and now is
most likely immune and not excreting oocysts. The absence of antibody
in a healthy cat suggests that the cat is susceptible to infection
and thus would shed oocysts for one to two weeks following exposure.
Treatment and Prevention
The two drugs that are most often used—pyrimethamine and sulfadiazine—act
together to inhibit Toxoplasma reproduction. Treatment must be
started as soon as possible after diagnosis and continued for
several days after signs have disappeared. In acute illness, treatment
is sometimes started on the basis of a high antibody titer in
the first test. If clinical improvement is not seen within two
to three days, the diagnosis of toxoplasmosis should be questioned.
Pyrimethamine may be unpalatable or toxic to some cats, even
if given in small amounts. Recently, the antibiotic clindamycin
has been reported to be effective in treating feline toxoplasmosis,
with few side-effects observed.
No vaccine is as yet available to prevent either Toxoplasma
infection or toxoplasmosis in cats, humans, or other species.
Research in this area is in progress.
Return to Top
Toxoplasma and Human Health
Although the incidence of toxoplasmosis among humans probably
has not changed significantly over the years, awareness and concern
about the disease have increased within the medical and veterinary
communities. It has been estimated that 30 to 50 percent of the
world's human population has been infected with Toxoplasma and
harbors the clinically inapparent cyst form. This encysted form
is important because, if given the opportunity, it can produce
disease in immunocompromised patients. For this reason veterinarians
are often called on to clarify the role that cats play in the
transmission of Toxoplasma to humans.
Contact with oocyst-contaminated soil is probably the major
means by which many different species—rodents, ground-feeding
birds, sheep, goats, pigs, and cattle, as well as humans living
in developing countries—are exposed to Toxoplasma. In the industrialized
nations most transmission to humans is probably due to eating
undercooked infected meat, particularly lamb and pork (in many
areas of the world, approximately 10 percent of lamb and 25 percent
of pork products contain Toxoplasma cysts). The organism may also
on occasion be present in some unpasteurized dairy products, such
as goat's milk.
Congenital infection is of greatest concern in humans. About
one-third to one-half of human infants born to mothers who have
acquired Toxoplasma during that pregnancy are infected. In general,
Toxoplasma infection of the fetus is least common (but disease
is most severe) if the maternal infection occurs during the first
trimester of pregnancy. Fetal infection is most common (but disease
is least severe, often without symptoms) if the maternal infection
occurs during the third trimester. The vast majority of women
infected during pregnancy have no symptoms of the infection themselves.
It has been estimated that Toxoplasma is responsible for over
three thousand human congenital infections in the United States
each year, most of which are symptomless. Among symptomatic individuals,
symptoms may be present at birth, or may first appear weeks, months,
or even years later (the majority of clinical cases appearing
at puberty, for example, are the result of congenital, rather
than recent, infection). Ocular and central-nervous-system disturbances,
deafness, fever, jaundice, rash, and respiratory disease, in varying
combinations, are among the more common clinical manifestations
in these patients. In immunocompromised persons—those undergoing
immunosuppressive therapy (e.g., for cancer or organ transplantation)
or those with an immunosuppressive disease such as AIDS—enlargement
of the lymph nodes, ocular and central nervous-system disturbances,
respiratory disease, and heart disease are among the more characteristic
symptoms. In these patients—especially those with AIDS—relapses
of the disease are common, and the mortality rate is high.
Return to Top
Minimizing Exposure
Tissue cysts can be destroyed by thoroughly cooking meat to
an internal temperature of 70°C (158°F) for at least 15
to 30 minutes. Freezing and thawing, salting, smoking, or pickling
will not reliably destroy cysts in meat. Restricting the access
of pet cats to rodents and birds and offering them only cooked
meat, commercially prepared cat food, and pasteurized dairy products
should prevent most transmission. (Nor should humans eat uncooked
meat or unpasteurized dairy products.) Scavenging can be discouraged
by placing secure lids on all garbage cans.
Because excreted oocysts are highly resistant to environmental
conditions and millions may be present in a single stool, contamination
of garden soil, flower beds, children's sandboxes, cats' litter
boxes, and other areas of loose, moist soil where cats defecate
may be extensive. Under such conditions transmission of oocyts
to humans can be minimized by the following measures:
Avoid contact with potentially contaminated soil, or
wear rubber gloves during contact, and follow by washing hands
vigorously and thoroughly with soap and water.
Cover children's sandboxes to prevent contamination
by cats.
Dispose of feces from litter boxes daily or every other
day to remove oocysts before they sporulate and become infective.
Disinfect potentially contaminated litter boxes with
scalding water or with dry-heat sterilization (55°C, 131°F).
Chemical disinfection does not reliably destroy oocysts.
Return to Top
Specific Recommendations for Pregnant Women
A pregnant woman (or one who contemplates pregnancy) can minimize
exposure to Toxoplasma by taking the following measures:
Exclude rare or undercooked meat and unpasteurized dairy
products from the diet.
Test household cats for antibodies to Toxoplasma. Assuming
that a cat is healthy, a positive antibody test indicates that
the animal is most probably immune and not excreting oocysts and
thus would be an unlikely source of infection. A healthy antibody-negative
cat is most probably susceptible to infection and would shed oocysts
for one to two weeks after exposure to Toxoplasma. If possible,
the cat should be tested before the woman becomes pregnant.
Have herself tested for antibodies, preferably before
becoming pregnant. A positive test would indicate past infection
that will not be transmitted to the fetus. The presence of antibodies
also lessens the likelihood that congenital transmission would
occur should she be exposed again to the parasite during pregnancy.
An antibody-negative woman would thus be at greater risk of transmitting
Toxoplasma to the fetus should she become infected during pregnancy.
Protect cats from infection (or reinfection) by preventing
access to birds, rodents, uncooked meat, and unpasteurized dairy
products.
Avoid handling litter boxes. Even if a cat is antibody-positive
and hence most likely immune, there exists a potential for reshedding
of oocysts (although in much smaller numbers than during the initial
infection). For safety, litter boxes should be changed daily or
every other day by another person to eliminate any potential for
accidental infection.
Avoid handling free-roaming cats, because the fur or
paws could be contaminated with oocysts, which might be transmitted
by hand-to-mouth contact. Any cat allowed indoors should be kept
off the bed, pillows, blankets, or other furnishings the woman
uses.
Avoid handling any cat showing signs of illness.
Wear rubber gloves if working with garden soil. Uncooked
vegetables, whether grown in a home garden or supplied commercially,
should be washed thoroughly before ingestion, in case they have
been contaminated by cat feces.
Make a habit of vigorously and thoroughly washing hands
with soap and water after contact with soil, cats, unpasteurized
dairy products, or uncooked meat or vegetables.
Return to Top
Prepared by the Cornell Feline Health Center,
Cornell University, College of Veterinary Medicine, Ithaca, New
York 14853-6401, and the American Association of Feline Practitioners.
The ultimate purpose of the Feline Health Center is to improve
the health of cats by developing methods to prevent or cure feline
diseases and by providing continuing education to veterinarians
and cat owners. Much of that work is made possible by the financial
support of friends. (c)1990 by Cornell University. Reviewed 1994.
All rights reserved. Cornell University is an equal opportunity,
affirmative aciton educator and employer.
|
