What is Toxoplasmosis?
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.
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, asmany as one half of adult cats have antibodies to the organism in theirblood 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 fedprimarily 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 arodent. The Toxoplasma organism replicates first locally in the intestinaltract of the cat, and is often contained there. The replication in theintestinal tract results in shedding of oocysts in the feces. The oocystsrepresent a hardy form of the organism that can survive in the externalenvironment for many months or even years. Other animals can becomeinfected by ingesting these oocysts, but only if large numbers areingested will disease result.In some cats, particularly if their immune defenses are compromised, theToxoplasma organisms can invade beyond the intestine and spread intovarious organs of the body. There, they may cause enough damage tocause signs of disease or may become dormant in a tissue cyst (not thesame as the oocyst form). Such tissue cysts can be infective if the infectedtissue 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.
Toxoplasmosis is difficult to diagnose in cats because the signs can be so variable. Blood testsare available that will demonstrate, by the presence of antibodies to the organism, whether acat 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 appropriateantibiotic.
How important is Toxoplasma in people?
Around 30% of the adult population has been exposed to Toxoplasma. As with infection in cats,the vast majority of people infected with this organism experience no clinical disease at all, orpossibly just mild and transient 'flu'-like signs. However, there are also some individuals wheresignificant 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 organismcarries no risk of transmission to a fetus if she subsequently becomes pregnant.
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 thereforeagain no further risk to humans.
- Although humans can be infected through exposure to, and ingestion of oocysts in the environment, a more common sourceof 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 160º -180º F (70º-82º C) throughout.
- Wash hands, utensils and surfaces carefully after handling raw meat.
- Wash all vegetables carefully.
- 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 catis excreting oocysts, they will not have become infectious (which takes more than 24 hours from when they are passed in thefeces) 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.