Some diseases still seem mysterious, with misunderstandings and unresolved questions. One of these contentious conditions is Toxoplasmosis, the best-known disease that humans can pick up from cats.
The cause of Toxoplasmosis is a tiny one-celled organism called Toxoplasma Gondii.
The parasite forms cysts in the muscles of warm blooded creatures (nearly all species, from mice to humans and many in between). Then if cats eat flesh that contains these cysts (e.g if they catch mice), the tiny parasites develop inside the cat, producing eggs ("oocysts") that are passed in the cats' faeces.
These invisible eggs become infectious if eaten by mammals (including humans) after 1 - 5 days, remaining infectious in the environment for 18 months or more, long after all traces of cat faeces have disappeared.
Humans pick up infection by ingesting these eggs (from cat faeces or contaminated soil) or more commonly by ingesting uncooked meat (e.g. pork) that contains the cyst-form of the parasite. It is this latter aspect that is often forgotten. The risk of humans picking up this disease from undercooked meat is far higher than the risk of picking it up directly from cats.
People who have been infected with T gondii are often unaware of the infection (there are rarely signs of illness), but they develop antibodies against the organism that can be detected. Blood tests show that in Ireland, USA and UK, around 20-30% of people have been infected with Toxoplasmosis at some point in their lives, whereas approximately 80% of people in France and Germany are positive.
The fact that so many more people are infected in continental European countries is because they are far more likely to eat raw, undercooked or cured meat (if you have ever ordered a burger or steak in France or eaten breakfast in Germany you will know what I mean).
The truth is that it is difficult to directly pick up this disease from cats. While 20-60% of cats are positive on blood tests (meaning that they have antibodies, showing that they have been infected at some stage), almost none will be shedding the parasite. The eggs are only shed in their faeces when they are first infected (and even then, only for up to two weeks). After this, they stop shedding eggs and are no longer infectious to humans. By the time they test positive, most cats have stopped shedding the organism.
The temptation can be to get rid of "positive cats" when there is absolutely no reason to do so. In fact, because cats are territorial, it's arguably better to keep positive cats, because they will keep away roaming cats who could be newly infected and still shedding eggs in their faeces.
To emphasize the point that cats are usually innocent, there is no correlation between cat ownership and human disease caused by this parasite. Vets working with cats are no more likely to have a positive blood test than people who are not in contact with cats.
So if most people show no signs of illness when infected with Toxoplasma, what is the problem in any case? In most healthy adults, the immune system deals promptly with the parasite, but in any situation where the human immune system is not as effective as usual, the parasite can indeed cause serious health issues. Examples of such situations include babies and young children, very elderly people, pregnant women (because of the risk to their unborn baby), and immunosuppressed people (such as people with HIV, those on anticancer therapy, or organ transplant recipients on immunosuppressive treatment).
In these individuals, infection can cause severe illness including problems affecting the nervous system and eyes. There has also been talk (but no proof) of a correlation between Toxoplasma infection and complex mental conditions like schizophrenia. It's because of these potential rare but very serious complications that patients and doctors are often very vocal about the disease.
The risk to unborn babies is the issue that worries most people. If a woman has been infected with Toxoplasmosis prior to pregnancy (i.e. one of the 20-30% in the population) then the risk is minimal, as her immune system will be primed to eliminate the organism, However if she encounters her first infection during pregnancy, in 20-50% of cases, infection will be passed on to the foetus. Even if this happens, in most cases, there are no adverse effects, but in a minority of cases, there may be abortion, birth defects, and neurological or eye problems.
So what should be done to minimize this tiny risk?
First, prevent ingestion of eggs from the environment (eg, through contact with soil containing Toxoplasma eggs, or through eating contaminated fruit or vegetables). Vulnerable people should avoid cleaning out cat litter trays, and wash their hands after gardening.
Second, prevent ingestion of meat containing tissue cysts. Fresh meat is most risky since freezing meat for several days will kill most tissue cysts. So vulnerable people should cook meat thoroughly, and be careful to wash hands if undercooked meat is ever handled.
There is no need to get rid of cats. As in many other situations, the knee jerk response to this strange tiny parasite is the wrong way to try to prevent problems.