What is toxoplasmosis?
Toxoplasmosis is a disease caused by infection with the organism called Toxoplasma gondii (T. gondii), a microscopic single-cell organism which is one of the most common parasites of animals. Although virtually all warm-blooded animals, including man, can be infected with this organism, it is an extremely well adapted parasite and rarely causes significant disease to the individuals which it infects.
How is Toxoplasma transmitted?
Cats are usually infected by eating the organism present in the tissues (meat) of another infected animal (an ‘intermediate host’), which is usually a rodent. The organism replicates locally in the intestinal tract of the cat, and also replicates within the body. The replication in the intestinal tract results in shedding of oocysts (eggs) in the faeces, but an immune response rapidly develops which halts both shedding of eggs and replication of the organism in the body. Despite the immune response, infection still persists in the form of microscopic cysts present in some tissues of the body, although this does not usually result in any disease.
The oocysts (eggs) shed in the faeces are very resistant, and can contaminate the environment for several years. Other animals become infected by eating these eggs and then, as with cats, the organism replicates in the body and cysts develop within certain tissues. Cats are particularly important, as they are the only animal in which T. gondii replicates in the intestinal tract resulting in shedding of eggs in the faeces, and therefore cats are essential to the life-cycle of the organism.
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 the food chain. 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 and if infected meat is not cooked adequately enough, or poor hygiene precautions are adopted during handling of uncooked meat, humans can become infected. Ingestion of oocysts, for example during gardening in contaminated soil, is a less common source of human infection.
How common is Toxoplasma in cats?
The proportion of cats infected with Toxoplasma varies according to their life-style. Because of the way in which the organism is transmitted, infection is much more common in stray, feral, farm cats, and others which engage in a lot of hunting or are fed a lot of raw meat. As many as 60% of these cats may be infected with Toxoplasma. In contrast, infection is uncommon in pet cats which do little or no hunting, and which are fed primarily or exclusively commercial cat foods.
What disease does Toxoplasma cause in cats?
Although Toxoplasma is a relatively common infection, it usually causes no disease in infected cats. Rarely, cats fail to develop an adequate immune response to the organism which may allow it to continue to replicate and cause damage to tissues. When this happens a variety of different clinical signs can develop including ocular (eye) disease, respiratory disease, diarrhoea, hepatitis and nervous signs. It is important to remember that Toxoplasma is a rare cause of disease though.
How can you diagnose and treat toxoplasmosis?
Toxoplasmosis is difficult to diagnose in cats. Blood tests are available which will show whether a cat has been exposed to the organism, but these tests do not necessarily mean that Toxoplasma is the cause of any disease (as most exposed cats do not develop disease). When toxoplasmosis is suspected in a cat, this can be treated with a course of an appropriate antibiotic.
How important is Toxoplasma in man?
Around 30% of the adult population in the UK have been exposed to Toxoplasma. As with infection in cats, the vast majority of people infected with this organism experience no clinical disease at all, or possibly just mild and transient ‘flu’-like signs. However, as with cats 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 foetus, 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 foetus 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 faeces for a short period (approximately 10 days) after initial exposure. Following this there is no further significant oocyst shedding and therefore again no further risk to humans.
- Although humans can be infected through exposure to, and ingestion of oocysts in the environment, a more common source of 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 to at least 70°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 faeces.
- Empty cat litter trays daily, dispose of litter carefully, and disinfect with boiling water. If this is done every day, even if a cat is excreting oocysts, they will not have become infectious (which takes more than 24 hours from when they are passed in the faeces) by the time the litter is changed.
- Discourage pet cats from hunting, and avoid feeding them raw/undercooked meat.
- Cover any children's sand pits/boxes to prevent cats using them as a litter tray.