Several abortifacient disease agents of sheep and goats are known to be
pathogenic for humans. This includes bacterial diseases such as
brucellosis, Q fever and Mammalian Chlamydiosis as well as protozoan
(toxoplasmosis) and viral (Rift valley fever). Other zoonotic pathogens, such
as Listeria and Salmonella spp, may cause abortions in sheep and
goats occasionally. Toxoplasma gondii, found worldwide, causes
infections in carnivore and omnivore animal species and in humans. It is one of
the main three causes of abortion in sheep; in some countries, it is the main
From 1909, when Nicolle and Manceaux described a micro-organism in a
rodent, the gondi (Ctenodactylus gondi), in Tunisia, more than 50 years
were necessary to fully elucidate the complex life cycle of T. gondii,
highlighting the major role of cats in its dissemination. It took almost as
long to identify the first effective treatments and set up the first
Infections with T. gondii are an important public health concern,
causing eye disease both congenital and acquired. The definitive hosts (animals
in which the organism reproduces sexually) for T. gondii are members of
the Felidae family (mainly domestic cats); they excrete oocysts in their feces.
Livestock, and all warm-blood animals, are intermediate hosts, infected by the
ingestion of the oocysts. When infected, they will harbor infective tissue
cysts in their muscles and brains for the rest of their lives; carnivorous
animals and humans may get infected from eating infected meat or organs.
Toxoplasmosis is included in the World Organization for Animal Healths
list of notifiable animal diseases.
T. gondii is an obligate intracellular protozoan parasite in the
The major forms of the parasite are:
- Oocysts (containing sporozoites), which are shed in the feces.
- Tachyzoites, rapidly multiplying organisms found in the tissues.
- Bradyzoites, slowly multiplying organisms found in the tissues.
- Tissue cysts: walled structures, often found in the muscles and
central nervous system (CNS), containing dormant T. gondii bradyzoites.
Carnivores and omnivores, including humans, can become infected when
they eat raw or undercooked tissues containing tissue cysts or, occasionally,
tachyzoites. Both herbivores and carnivores may ingest infective oocysts in
food or water, inhale them in aerosols, or come into contact with contaminated
soil. T. gondii can cross the placenta in some species, particularly
sheep, goats and humans.
T. gondii undergoes an asexual reproductive cycle in all species.
The tissue cyst or oocyst wall is dissolved during digestion, releasing
bradyzoites or sporozoites, which enter the lamina propria of the small
intestine and begin to multiply as tachyzoites. The tachyzoites can disseminate
to extraintestinal tissues within a few hours of infection, via the lymph and
blood. They can enter nearly any cell and multiply; the host cell eventually
ruptures and the released tachyzoites enter new cells. As host resistance
develops, the tachyzoites begin to disappear from the visceral tissues and form
resting bradyzoites within tissue cysts. These cysts are found most
often in the skeletal muscles, brain and myocardium. They generally do not
cause a host reaction and can persist for life.
In Felidae, the parasites simultaneously undergo a sexual cycle
of replication. After ingestion, some of the bradyzoites multiply within the
epithelial cells of the small intestine. After numerous cycles of asexual
reproduction, these bradyzoites initiate the sexual cycle, which results in the
formation of an unsporulated oocyst. The oocyst is excreted in the feces and
sporulates in the environment. After sporulation, the oocyst contains two
sporocysts, Cats usually shed oocyts for one to two weeks.
Infections in animals
Members of the Felidae, including domestic cats, are the
definitive host. Most mammals and birds can serve as intermediate hosts. Among
domestic animals, infections are most common in cats, sheep, goats and swine.
Lower infection rates are seen in dogs and horses. Cattle seem to be relatively
resistant to infection.
Most infected cats are asymptomatic, but generalized acute, subacute and
chronic (months to years) infections have been reported, particularly in young
or immunocompromised animals. Respiratory, hepatic and pancreatic involvements
occur. CNS signs, particularly common in older cats, vary with the site of the
lesion. Ocular signs are common.
Most infected dogs are asymptomatic; clinical toxoplasmosis is most
common in puppies or in immunosuppressed older dogs. Toxoplasma encephalitis
are most severe in dogs aged younger than 6 months. Acute hepatitis is often
fatal. Ocular signs may include retinitis, uveitis and iridocyclitis.
In other animal species, infections are generally asymptomatic, although
outbreaks with generalized infections, abortions, stillbirths and neonatal
mortality are occasionally reported in swine. Fever, encephalitis, ataxia and
retinal degeneration have been reported in horses.
Clinical toxoplasmosis is seen most often in sheep and goats when
infected during pregnancy and are manifested by abortions, stillbirths, or
mummification or resorption of the fetus. Congenitally infected lambs may be
uncoordinated, weak and unable to nurse. Abortions and diseased lambs or kids
do not recur during subsequent pregnancies. Preventive measures may help to
reduce congenital disease in sheep; in Europe and New Zealand, a modified live
vaccine is available for sheep.
Toxoplasmosis is communicable only in Felidae. Antibodies have
been found in 15% to 58% of the cats in the United States and 25% to 100%
Infections are particularly common in strays and less prevalent in pets.
No vaccine is available for cats; if possible, they should be kept away from
pastures and barns where pregnant sheep and goats are kept.
After an abortion, the placenta and abortion products should be removed,
and the area cleaned and disinfected.
To prevent cats from becoming infected and serving as sources of
infection for humans, they should not be fed raw or undercooked meat. Indoor
cats are less likely to become infected, due to reduced exposure to rodents.
Infection in humans
This review is not meant to address the clinical disease in humans in
detail. In immunocompetent non-pregnant individuals, infection with T.
gondii is usually asymptomatic. Approximately 10% to 20% of patients
develop lymphadenitis or a mild, flulike syndrome. Severe symptoms, including
myositis, myocarditis, pneumonitis and neurologic signs, are possible but rare.
Ocular toxoplasmosis with uveitis, often unilateral, can be seen in adolescents
and young adults, as the result of an asymptomatic congenital infection or the
delayed result of a postnatal infection.
Infections acquired during pregnancy can lead to congenital
toxoplasmosis of the infant, affecting the developing brain and/or retina. The
incidence and severity of the disease vary with the stage of pregnancy.
Toxoplasmosis is often severe in immunosuppressed patients. Neurologic
disease is the most common sign, particularly in reactivated infections and may
lead to coma and death. In patients with HIV/AIDS, toxoplasmosis is often a
reactivated rather than a new infection.
Humans become infected with T.gondii mainly by ingesting uncooked
meat containing viable tissue cysts or by ingesting food or water contaminated
with oocysts from the feces of infected cats.
Children might be exposed to oocysts in cat excrements deposited in
sandpits. Circumstantial evidence suggests that oocyst-induced infections in
humans are clinically more severe than tissue cyst-acquired infections.
T. gondii infection is one of the most common infections in
humans; in worldwide serological surveys, 3% to 80% of healthy adults have been
exposed to this parasite. Infection is asymptomatic in 80% to 90% of
non-pregnant, immunocompetent individuals and usually causes mild disease in
Toxoplasmosis is not directly communicable from person to person, except
in utero. Infection of the fetus is particularly likely during the second and
Most cases are sporadic, but small epidemics may occur, usually
associated with contaminated food or water. Immunity appears to be lifelong
unless the individual becomes immunosuppressed.
Prevention of human infection
The risk of infection can be reduced by proper food preparation and good
hygiene. Meats should be cooked to a temperature sufficient to kill T.
gondii; the internal temperature of beef, lamb and veal steaks or roasts
should reach at least 145°F (63°C), and pork, ground meat and wild game
should be cooked to 160°F (71°C). Pregnant women and others at risk
should wear gloves when gardening and during other soil or sand contact.
To help prevent transmission of T. gondii to humans, cats should
be fed only commercial pet food or well-cooked meats. Indoor cats are less
likely to transmit toxoplasmosis than outdoor cats. Litter boxes should be
cleaned daily to reduce the risk of oocyst sporulation and rinsed with boiling
water. Pregnant women should avoid cleaning the litter box; if this is
unavoidable, they should use gloves, then wash their hands.
Arnon Shimshony, DVM, is Associate Professor at the Koret School of
Veterinary Medicine Hebrew University of Jerusalem, Rehovot, and is the
ProMED-mail Animal Diseases Zoonoses Moderator. Dr. Shimshony was Chief
Veterinary Officer, State of Israel, from 1974 to 1999.