Toxoplasmosis is a disease caused by infection with the organism called Toxoplasma gondii (T. gondii). This is a microscopic single-cell protozoal organism related to coccidia. Virtually all warm-blooded animals, including people, can be infected with this organism. It is an extremely well adapted parasite and rarely causes significant disease to the individuals it infects.
Toxoplasma occurs worldwide and infection in cats is similarly widespread. Many more cats are infected than show symptoms. 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.
Cats are usually infected by ingesting the organism present in the tissues (meat) of another infected animal known as an ‘intermediate host' which is usually a rodent. 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 faeces. 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 disease will result only if large numbers are ingested.
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. This is not the same as the oocyst form. Such tissue cysts can be infective if the infected tissue is eaten by another animal.
Although Toxoplasmosis 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 or 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 very rare cause of disease in cats.
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. 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.
As with infection in cats, the vast majority of people infected with this organism experience no clinical disease at all, or possibly just show mild and transient flu-like signs. However, there are also some individuals where significant disease does occur and one situation is 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. This is only a risk though, if the woman acquires the infection during her pregnancy. A woman who has previously been exposed to the organism caries no risk of transmission to a fetus if she subsequently becomes pregnant.
While cats are usually infected by eating infected rodents or more rarely by ingestion of oocysts from the environment, humans are most commonly infected through eating contaminated 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 if proper hygiene precautions are not followed during handling of uncooked meat, humans can become infected. Ingestion of oocysts from infected cats, for example during gardening in contaminated soil, is a much less common source of human infection.
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:
Following a few sensible environmental and meat hygiene measures can greatly reduce the risk of human infection: