Out of all the zoonotic diseases which humans can get from cats, Toxoplasmosis is the most well known and publicised one. However, there is a huge amount of misunderstanding about this disease and about the role in which the cat plays in causing human disease.
Toxoplasmosis refers to disease caused by infection with a single-celled microscopic parasitic organism called Toxoplasma gondii. Infection is widespread throughout the world but T gondii is a very well adapted parasite, meaning it rarely causes significant disease in an infected host.
T gondii is unusual in that virtually any warm-blooded animal can be infected with the parasite, but it is cats that are required to complete the life-cycle of T gondii. Although cats are important in the life cycle and epidemiology of T gondii infections, human infections are often nothing to do with the cat itself and often result from ingestion of undercooked meat containing the parasite.
Occasionally, if an immune response does not prevent widespread replication of the organism, T gondii may cause clinical disease in humans, cats, or other animals.
What is toxoplasmosis?
Toxoplasmosis is a disease caused by infection with Toxoplasma gondii (T gondii), a coccidian parasite.
Toxoplasmosis in humans
In humans with a normally functioning immune system, clinical signs of toxoplasmosis are usually mild and may pass either undetected or just cause mild ‘flu’-like symptoms. Toxoplasmosis is most severe in certain ‘high risk’ groups of individuals whose immunity is impaired. This group includes:
- Babies (foetuses) developing in the womb prior to birth
- Babies and young children
- Very elderly people
- Pregnant women (because of the risk to their baby)
- Immunosuppressed people – for example, those receiving anticancer therapy, those with acquired immunodeficiency syndrome (AIDS) and organ transplant recipients on immunosuppressive treatment
In this group of ‘high risk’ people, infection can be associated with severe illness including encephalitis (inflammation of the brain), abortion, stillbirth, birth defects (including mental retardation) and other problems affecting the nervous system and eyes.
Toxoplasmosis in cats
Although infection with T gondii rarely causes disease in cats, signs of illness can sometimes be seen and these include:
- Fever, loss of appetite, weight loss, lethargy
- Pneumonia causing breathing difficulties
- Inflammatory eye problems (uveitis and retinitis)
- Liver disease (hepatitis) causing jaundice
- Neurological (nervous) signs (eg, tremors or seizures)
- Less common signs of illness reported include: lymph node enlargement; vomiting and diarrhoea; muscle pain
Infection in a pregnant cat can cause severe signs of illness in the offspring such as foetal death, abortion, stillbirths and death of young kittens, but this is unusual.
How common is toxoplasmosis?
In cats, infection with T gondii is much more common in outdoor cats that are active hunters, and in cats that are fed undercooked or raw meat. In general, depending on their lifestyle, between 20-60% of cats will be infected with T gondii, but very few of these will ever show clinical signs.
Similarly, in humans, it is estimated that more than 500 million people are infected worldwide, but again the vast majority of these result in no significant clinical signs. Human infection is more common in some countries than others. For example, in the USA and UK around 20-30% of people are infected at some point in their lives, whereas approximately 80% of people in France and Germany are infected. People who have been infected with T gondii develop antibodies against the organism and these can be detected in a blood test.
Once an animal has been infected with T gondii, it will probably remain infected for life, as the parasite can be present in microscopic cysts present in tissues (typically in the muscle) where it will remain dormant, but ‘hidden’ from the immune response.
Life cycle and infection with T gondii
The life cycle of T gondii is quite complex (see figure 1-3) and involves two types of host: definitive host (the cat, including wild cat species and domestic cats), and intermediate hosts (other animals).
Definitive host for T gondii
Cats (wild cat species and domestic cats) are the definitive hosts, meaning they are the only animals in which replication of T gondii can result in the production of oocysts (eggs), which are then shed in the faeces.
Most cats are infected by eating meat containing T gondii cysts – this can include raw or inadequately cooked meat (eg, beef, lamb, pork) or, more commonly, prey species (eg, rodents such as mice or voles). A few days after a cat has been infected for the first time it will start to shed millions of oocysts (eggs) in its faeces. The oocysts are only shed for a short period of time, typically less than 14 days, before the body’s immune response stops oocyst production altogether. It is rare for a cat to re-shed oocysts in faeces after their first infection, and when this does occur it usually results in a much smaller number of oocysts being shed. Even drug therapy that profoundly suppresses the cat’s immune system is rarely effective in triggering re-shedding of oocysts.
At the same time that T gondii establishes infection in the intestine of a cat, it will also disseminate throughout the body with a rapidly dividing form of the organism called tachyzoites. An effective immune response halts this replication and the organism then transforms into bradyzoites which remain ‘hidden’ from the immune system in tissue cysts (most commonly in muscles). If a cat fails to develop an effective immune response, continued replication of tachyzoites may lead to severe inflammation and signs of disease.
Intermediate hosts for T gondii
Other warm blooded mammals, including humans, can act as intermediate hosts for T gondii. Intermediate hosts can become infected, but never produce oocysts. Infection can be acquired either through:
- Ingestion of an oocyst passed in a cat’s faeces – these are not immediately infectious to other animals, and must first undergo a process called sporulation, which takes between one and five days. Oocysts are very resistant though and can survive in soil or water for up to 18 months or more.
- Ingestion of tissue cysts from another animal – humans, for example, can also be infected if undercooked or raw meat is eaten that contains T gondii tissue cysts (for example beef, lamb or pork meat from an animal infected by eating oocysts from the environment).
When intermediate hosts are infected, the organism initially spreads through the body as tachyzoites (as during infection in cats), and then an immune response will result in the formation of tissue cysts (containing bradyzoites). Again, in rare cases where an ineffective immune response allows continued replication of tachyzoites, severe inflammation and clinical disease may develop.
T gondii and human pregnancy
If a pregnant woman is infected with T gondii during her pregnancy, there is a risk that the infection may spread to the unborn foetus. This will only happen if the woman is infected during pregnancy, and prior infections carry no such risk.
In around 20-50% of women infected during pregnancy, infection will be passed on to the foetus. In many cases, even if the foetus is infected, it will remain asymptomatic, but in a minority of cases the infection can result in abortion, birth defects, neurological problems or ocular problems. However, these problems can only occur if a woman contracts toxoplasmosis for the first time during her pregnancy.
What are the sources of infection for people?
In most cases, people become infected via one of two routes:
- Ingestion of oocysts from the environment (eg, through contact with soil containing sporulated oocysts, or through eating contaminated fruit or vegetables)
- Ingestion of meat containing tissue cysts. Fresh meat is most risky since freezing meat for several days will kill most tissue cysts.
Other routes of infection are less commonly implicated but include:
- Ingestion of sporulated oocysts through contact with contaminated water
- Ingestion of raw (unpasteurised) goats milk can transmit the organism
- Inhalation of sporulated oocysts on dust particles (extremely rare)
- It is generally believed that infection cannot be transmitted by a bite or scratch from an infected cat.
Does contact with a cat increase the risk of infection?
Research indicates that contact with cats (or owning a cat) does not increase the risk of T gondii infection in humans. Studies have shown that:
- It is rare to find cats shedding oocysts in their faeces – for example one study of more than 206 cats showed nearly 25% had been infected with T gondii, but none of them were shedding oocysts in their faeces
- Vets working with cats are no more likely to be infected with T gondii than the general population including people not in contact with cats
- Contact with cats generally has little or no influence on the probability of people being infected with T gondii whereas consuming raw meat significantly increases the risk of acquiring infection
- Stroking a cat will not spread infection from cats to people (even when shedding oocysts in their faeces, oocysts cannot be found in the cat’s coat)
- The risk of infection from cats is very low, except in young children playing in soil contaminated with sporulated oocysts
- Most people are probably infected through ingestion of undercooked meat – especially goat, mutton and pork
Newer strains of T gondii have been identified that are highly contagious, with infection being efficiently passed between intermediate hosts (species other than cats). Consequently, some scientists think that cats are becoming less important in the spread of this infection.
Reducing the risk of transmission of T gondii from a cat to its owner
Although the risk of transmission of T gondii from a cat to its owner is very low, this can be reduced further and/or its consequences minimised by adopting the following recommendations:
- People in ‘high risk’ groups (eg, pregnant women, immunosuppressed individuals, young children) should not have contact with or handle the cat’s litter tray
- Empty litter trays daily, so that oocysts do not have sufficient time to sporulate (become infective) while in the litter tray
- Wear gloves when handling cat litter and wash hands thoroughly after cleaning the litter tray
- Use litter tray liners if possible and periodically clean the litter tray with detergent and scalding water (which kills oocysts)
- Dispose of cat litter safely. For example, seal it in a plastic bag before putting it with other household waste
- Cover children’s sandpits when not in use to prevent cats using them as litter trays
- Feed only properly cooked food or commercial cat food to your cat to avoid infection
- Gloves should be worn when gardening and hands thoroughly washed after contact with soil which may contain sporulated (infectious) oocysts.
- When handling food, to prevent exposure to oocysts and tissue cysts, hands should always be washed thoroughly afterwards.
- Fruit and vegetables should be thoroughly washed before eating to remove any oocysts present on their surface.
- All food preparation surfaces and utensils should be cleaned with detergent in hot water before and after use to inactivate any tissue cysts.
- Meat should be cooked to a minimum of 58°C for 10 minutes or 61°C for four minutes to kill the tissue cysts. Microwaving is not a safe way to kill tissue cysts as the heating is uneven
- Freezing meat at –12°C to -20°C for three days kills tissue cysts as does gamma irradiated food
- If drinking a non-potable water, boil or filter before drinking to remove oocysts
Toxoplasmosis and altered human behaviour
There is current interest in the possibility that infection with T gondii can be a potential cause of altered human behaviour, including schizophrenia and suicidal tendencies. While this is an important line of research, current information should be interpreted with caution because of the often small numbers of people studied and their different backgrounds. A cause and effect is far from being established with T gondii infection and these disorders and other factors may well have been responsible for some of the changes observed.
The risks of acquiring toxoplasmosis from a cat are extremely small and most people are infected through other routes (such as eating undercooked meat). Simple everyday hygiene measures can be taken to reduce the risks of infection (from cats and other sources) making it safe to own and enjoy owning a cat.