Cats, toxoplasmosis and (human) pregnancy

'So now that you are pregnant, you'll be getting rid of your cat.' Many a mother-to-be has been startled to be told that a new addition to the family is supposed to come with the subtraction of a well-loved pet and friend. The reason? Well, the old wives' tale is that cats shed germs. The immune systems of males and non-pregnant females can cope with these germs. But because a pregnant woman's immune system is compromised anyway by coping with the fetus inside her, she and the baby are vulnerable.

Like many examples of 'folk wisdom', the above example is both correct and inaccurate. There is a risk that a cat might pass an infection called Toxoplasmosis to an unborn child, but the risk is very small, and can be brought down much further with the sort of basic hygiene that an expectant mother should practice in any case. So anyone who wants to save a cat from being shipped off to relatives or the nearest animal shelter should arm themselves with all the facts. (Though remember that ultimately the decision rests with the expectant mother and cat's owner.)

First the bad news Ö the risk may be small but it is serious and very real.

What is Toxoplasmosis?

Toxoplasmosis is an illness which can be spread to both pets and humans, and in fact is the most common parasitic infection in mammals. It is caused by a single-celled parasite called Toxoplasma gondii (T. gondii).

A mammal with a healthy and functional immune system generally copes with toxoplasmosis with little difficulty. The only symptoms might be a feeling of lethargy and something resembling mild flu. However, an individual with a severely weakened or immature immune system - such as a prenatal child - runs much higher risks. How Toxoplasmosis affects a body depends on where the T.gondii parasite spreads. It can produce pneumonia in the lungs, and inflammation or even blindness in the eyes. In the brain it might produce encephalitis, mood swings, or even mental health issues; and all these symptoms can be spread from mother to fetus. Furthermore, if the parasite becomes embedded in a developing child it can be very hard to root out.

So if all mammals get Toxoplasmosis, why pick on cats?

Like humans with fully-functional immune systems, most cats can deal with the T.gondii and remain healthy without showing any signs of illness. Cats which are immuno-compromised, for example infected with FIV or feline leukemia virus (FELV), are more at risk of developing a full clinical case of toxoplasmosis.

So one problem with a cat and a pregnant woman sharing the same household is that it is hard to tell if an otherwise healthy cat has an active T.gondii infection, because it will show few signs of it. The second problem is that T.gondii has a special affinity with cats. Cats are what biologists call the 'definite host' for the parasite. Humans and other mammals are 'intermediate hosts' - the parasite can live in them, but but can't produce viable eggs. T.gondii needs to be in a cat intestine before producing the eggs (technically the 'oocysts') by which the infection is spread.

How do cats get infected?

If a cat catches and eats a small mammal infected with T.gondii, that cat will be infected and become a host for the parasite. T.gondii passes from the cat's stomach and makes itself at home in the small intestine. There the cells multiply, and three to ten days after infection, the cat itself becomes infectious, and begins shedding eggs. This infectious stage will last for about two weeks.

During this period some T.gondii breach the intestinal wall and get into the rest of the catís body. This is the trigger that alerts the feline immune system to the intruder, and the catís body begins to mount an effective defence. Active T.gondi cells are destroyed, and others go dormant, forming cysts in muscle fibre and other organs, but generally becoming harmless both to the cat and other species. (Except in the case of an extreme immune breakdown, such as HIV in humans or FIV in cats.)

While infectious, the cat sheds the parasitic eggs in its feces. This is the only way that it passes on infection - it remains safe to pet or otherwise handle a cat with even a current T.gondii infection. Once out of the cat's body the oocytes are not immediately dangerous. On exposure to oxygen they undergo a process called sporulation. (The production of spores.) This takes 1-5 days, and these spores are not only what spreads the infection, but they are also very tough and resistant to damage. They can persist in a friendly environment - such as garden soil - for up to two years.

When a human or other mammal ingests T.gondii, the parasite skips the intestinal infection and goes directly to the rest of the body. Here again, a fully-functional immune system catches and deals with the infection, and the surviving parasites are walled off in cysts. The T.gondii in cysts are dormant but not dead. Eating - or even handling - meat containing the cysts is one way that both humans and cats get toxoplasmosis. Unlike cats, humans and other mammals cannot spread toxoplasmosis in feces, because the parasite in their body doesn't produce active spores.

So what are the risks?

First of all remember that the infection works two ways - you can infect your cat with T.gondii if you feed your pet infected meat. So if you are worried about T.gondii a good first step is to ensure your cat doesn't get it. Don't feed your cat raw meat, and ensure there are no rodents around the house which might carry the cysts.

The next thing to remember is that a cat is infectious for (at most) two to three weeks in its life. A cat which has already had T.gondii in the past has the necessary antibodies to prevent an active infection ever happening again. So if a cat has already got over a T.gondii infection the only way it can spread Toxoplasmosis to you is if you eat it - raw.

Your cat is not the only source of infection you have to worry about. It's unlikely that a cat will get infected for a fortnight in just the wrong nine months of its life, and even if it is, this is a known risk easily guarded against. The problem is other people's cats - the ones that left spores in the soil for an unwary gardener to encounter a year later. Also, spores in soil can get into animal feed. So also beware of undercooked meat from the animals that the spores ended up in, unpasteurized milk, or a healthy-looking bowl of poorly-washed organic salad. And always wash your hands after handling raw meat.

To get toxoplasmosis, you have to eat spores which have directly or indirectly come out of a cat's bottom, which is where basic hygiene comes in. An obvious red zone is the cat's litter tray. Cleaning this should be done with gloves in any case, and if the cat's human is a pregnant woman, this is one of the first jobs that should be delegated. However, if this is not possible the risk can still be kept low with proper use of gloves, a surgical mask and a good wash afterwards. Also remember that oocytes are not dangerous for the first 24 hours, so a promptly-emptied litter tray is a very low-risk tray.

How do I know if I or my cat is infected?

You probably won't. There's a chance that either you and your cat have met and disposed of T.gondii already, and were not even aware of it. If that's the case, the risk of your cat giving you second dose of the parasite is remote.

Most cats which get infected with T.gondii will show no or mild symptoms and will recover by themselves. However if for some reason T.gondii infection becomes full-blown toxoplasmosis the vet will perform a blood test and examine the feces for eggs. After diagnosis, he will prescribe a course of antibiotics. Clindamycin is the treatment of choice but the vet might use other drugs such as pyrimethamine and sulfadiazine which inhibit T.gondii reproduction. Most cats will recover within few days. If you know that your cat has been infected in the past this is not altogether bad news. It is highly unlikely ever to be infectious again.

Eggs in feces are a sign of infection, but not necessarily by T.gondii. However a simple blood test will reveal if there are any antibodies to that particular parasite in the cat's blood. If there is a good antibody response to the parasites and the cat is otherwise healthy, it shows that the cat has previously had T.gondii and is now immune. This animal will not spread toxoplasmosis.

The cat may respond to the test with a certain type of antibody reaction which tells the vet that infection is ongoing, and the cat is likely to shed oocysts and be infectious. If no antibodies are present, it means that the cat has no previous infection with T.gondii. This is good news, but it also means that it can still catch the parasite and become infectious. There is no vaccine.


Note: This information is for guidance only. It is not intended to replace consultation with a licensed veterinary practitioner or a doctor.

 
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