XXI. Getting Toxo, quick and easy – risk factors for toxoplasmosis
Another topic we studied long-term in our laboratory, but none too systematically, regards the effect of various risk factors on the likelihood of catching Toxoplasma. When we had our test subjects complete questionnaires, we almost always included questions related to possible risk factors that the person might have come into contact with Toxoplasma. Among other things, we discovered that in Czech population, the main risk of infection comes not from having a cat, but a rabbit. And we were surprised at how many Toxo positives kept rabbits themselves, or at least their family did. For example, in our group of 3255 soldiers of the mandatory military service, 33% of the respondents said they raised rabbits for meat (76). There are two reasons why raising rabbits is a significant risk factor for Toxoplasma infection. First off, it’s possible that the rabbit is entirely blameless, and that it merely indicates a country rather than city lifestyle – and the country lifestyle is associated with characteristics like closer contact with soil, which increase the risk of infection. Of course we tried to verify this possibility, and so, in our analysis, we included information about whether the subject lives in a village. It turned out the effect of the rabbit was apparent even after we filtered out the size of the settlement, in which that person spent their childhood or was currently living in. Still, this isn’t definite proof that the rabbit is a direct source of infection. Even when a subject answers that he grew up in a small village of less than two thousand people, it’s possible that his family lived a typical city lifestyle on the third floor of an apartment complex. On the other hand, a person who wrote that he lives in Prague, the capital, may actually live in a small village which has been administratively joined to Prague, but actually reflects a country lifestyle. So it’s possible that we can determine a country lifestyle more reliably based on the raising of rabbits, than according to the size of a person’s settlement.
Of course, raising rabbits could be a direct risk factor, rather than merely an indication of certain lifestyle. Raising rabbits is associated with killing, skinning and cooking the rabbit, and during this process, infection can easily occur. Blood from the infected animal can get into scratches or abrasions on the skin, and this is an easy route of infection. A large percentage of rabbits in the country are infected by Toxoplasma. It’s undoubtedly because they’re fed with hay that was generally stored in silos or on dirt. Cats often loiter in these places, and probably leave behind droppings with Toxoplasma cysts. Contaminated hay can not only infect the rabbits, but also their owners.
Just having a cat isn’t a very significant risk factor, though it is a source of infection to a certain extent. If the cat is kept indoors and given cooked food, it’s hard to see how it could get infected. Even a cat free to go outside the house, that occasionally catches a mouse in the garden or neighboring field for a midday snack, actually isn’t a likely source of infection for its human. An infected cat releases Toxoplasma cysts only during a brief period of days to weeks after the initial infection, before developing resistance to subsequent infections. Furthermore, when one follows basic hygiene and regularly cleans the litter box, it’s hard to catch Toxoplasma from an infected cat. The cysts which the cat releases are not yet infectious; it takes several days for them to mature. Perhaps this is a safety mechanism for Toxoplasma, so that the parasite does not infect the cat which just released the cysts (Box 88 Why parasites fear their own children, and how they defend against them). If a person keeps the litter box clean, the risk of infection should be low. Of course, pregnant women should certainly leave cleaning the litter box to someone else; though they’re unlikely to be infected, the consequences of toxoplasmosis reaching the fetus can be tragic (see Box 57 How dangerous is Toxoplasma in pregnant women?).
Our studies show that aside from rabbits and cats, there exist other sources of infection, some of which are quite important. Gardening is a very strong risk factor. If a person
Box 88 Why parasites fear their own children, and how they defend against themParasites are in danger from their own children. A parasite that reaches a host has found an island of plenty, surrounded by the wasteland that is the rest of the world. Many parasites produce an immense number of eggs in their life time, but most of the offspring never find a host, and die. If a parasite reaches the correct host, then it’s like winning the lottery – he’s often the only winner out of a million losers that has the chance to pass down his genes to the next generation. Evolution programmed the parasite to take full advantage of his opportunity. For example, the parasite must reproduce at the right rate, so that it can produce the maximum number of offspring within the host’s lifespan. But that means that it cannot reproduce too slowly (which would produce too few offspring), nor too quickly (this would soon kill the host and also stop the parasite from reproducing). However, problems can occur in three cases. Firstly, when a host already infected by one parasite is infected by another strain, leading to superinfection; secondly if mutants appear among its offspring; and lastly, if the parasite in question reproduces sexually. In all three scenarios population residing in the host would become genetically variable. This creates a situation in which these different parasites compete to reproduce. But this race is destructive: the fastest reproducing, i.e. the most virulent, strain wins in that it creates the most offspring per day, but it also ends up prematurely quickly killing the host. This brings an end to the reproduction of both this strain, and of all the others, so that the parasites actually churn out fewer offspring overall. In parts of the host population where a parasite is highly prevalent, superinfection and the resulting destructive race (for the highest virulence) is more likely to occur. This explains why viruses whose genetic information is encoded in RNA, are often nastier to their hosts than are DNA viruses. RNA replication involves incomparably more mutations than DNA replication, so the many mutated strains of RNA viruses have a much more intensive battle for speedy reproduction. The fact that most parasites reproduce asexually, or even not all, inside their host may be a preventative measure against this destructive race. They reproduce sexually only to create offspring which are to leave the body to seek another host. Asexual reproduction is a safer method to employ inside the host, because it produces genetically identical individuals which cannot compete amongst each other and kill off their single host. Offspring from sexual reproduction swiftly leave the host – natural selection has proven that sexual reproduction resulting in variable offspring is necessary for success (or rather a draw) in the coevolutionary battle with a host – and therefore cannot initiate this destructive competition. This may also explain why Toxoplasma cysts must mature for several days before becoming infectious. An infected cat which releases Toxoplasma cysts in its feces, can wash its rear with no danger of being re-infected. And the Toxoplasma found inside the cat needn’t fear competition with its genetically distinct offspring that would lead to premature destruction of the host |
tends his own garden, then it’s very likely that his or the neighbors’ cats will dig a latrine in the garden bed, or rather several small latrines. The cysts in the dirt can remain infectious for several years, so when our gardener eventually munches on a carelessly washed carrot, he may easily be infected.
Another significant source of infection is the consumption of raw or undercooked meat. Based on our results, it seems that this infection source is quite prevalent in the Czech Republic. It’s not that raw meat is a common dish, but rather that it is often sampled during preparation. For example, regularly tasting the dough for liver dumplings is a surefire way of getting infected. It’s also possible for a cook who has abrasions on her skin to get infected by merely handling the raw meat. It’s remarkable that men are usually infected by Toxoplasma in their childhood (when they’re playing with dirt or in the sandbox), whereas women have an increased risk of infection when they’re starting a family and begin cooking very often. The consumption of raw meat as part of a dish isn’t a serious source of infection in the Czech Republic, because the most common raw meat dish is steak tartar, made from minced beef. Toxoplasmosis isn’t prevalent in factory farmed cattle, and furthermore, cysts found in beef are usually small and probably not very infectious. The situation is probably different in countries that eat undercooked or even raw mutton or pork. In Czech Republic, of course, the greatest source of infection could be raw rabbit meat. And then custom of giving children raw liver as a source of iron is therefore a reliable way of infecting them with Toxo, particularly when it’s raw pig liver and the animal comes from a homestead, or a modern bio-farm.
Aside from these known risk factors, we discovered two more. One of them is the already mentioned blood group. People with AB blood are significantly more likely to be infected than people with type A, B, or even O blood, which carries the lowest risk of infection (76). Soviet researchers already noticed this, but their article was published only in Russian, so this risk factor is virtually unknown.
The other risk factor, which we found quite shocking, is the number of blood donations. In several groups of blood donors, we tested possible risk factors and repeatedly found that toxoplasmosis is much more prevalent among blood donors – a sector of the population that should be very healthy – than among control groups obtained from state serological surveys. A more detailed analysis revealed that the probability of infection increases with the number of blood donations, not with how long an individual has been donating blood. Our hypothesis is that frequent blood donations somehow weaken the immune system, and that at least the blood donors who don’t maintain the advised intervals between donations suffer from some degree of immunosuppression, and therefore are more susceptible to Toxoplasma infection. This probably extends to infection by pathogens in general. This means that blood donors – and particularly paid blood donors – should be more closely monitored, to ensure that people don’t donate too frequently. It might even be worthwhile limit the absolute number of blood donations (Box 89 Asking too many (bad) questions, will bring you lots of (nonsense) answers).
Box 89 Asking too many (bad) questions, will bring you a lot of (nonsense) answersWhen studying risk factors, we discovered yet another interesting phenomenon (95). It turned out that the usual epidemiological practice of verbally questioning the subject regarding his contact with each risk factor is very unreliable. When we conducted the same epidemiological study on the same population using both verbal questioning and a written form, we got significantly different results. We employed verbal questioning when screening our students for toxoplasmosis. The person conducting the intradermal test asked each student whether they often come into contact with cats and whether they eat or sample raw meat. The results showed that among people who didn’t have toxoplasmosis, there existed a strong correlation between replying “yes” to both questions. Those who said that they eat or taste raw meat almost always replied that they often come into contact with cats. But when we mailed the same people a questionnaire which included these two questions, the correlation between cats and raw meat completely disappeared. The simplest explanation is that people are very suggestible, and when a doctor or researcher asks them about these two questions, they reply in the affirmative. Maybe some hypochondriac subjects already believe that they’re Toxo positive, or perhaps our students think that their affirmative reply will please the questioner. In contrast, when we ask about the same two risk factors in an impersonal questionnaire, it rules out much of this psychological effect, and there’s no correlation between the answers. Therefore, our results indicate that one should be cautious when considering verbally acquired answers, and whenever possible, rather employ a written questionnaire.
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Another interesting finding from studying Toxoplasma risk factors regarded the psychological traits of cat owners and raw meat consumers (95). Since we discovered in the mid-90s that Toxo positive and negative people differ in a number of psychological factors, we decidedly needed to ascertain whether the same factors correlated with keeping cats or eating raw meat. That would mean that the association between Toxo and particular psychological traits is because people with a certain personality profile are more likely to be cat and meat lovers, which increases their chances of being infected. So we calculated the statistical relationship between Cattell’s psychological factors, and cats and raw meat. We found that keeping cats, as well as eating raw meat, does correlate with certain psychological traits – but different traits than those which change in humans with latent toxoplasmosis. Specifically, we noticed that people who kept cats had lower emotional maturity (factor C, Emotional stability) than did humans who didn’t, and people who answered that they ate raw meat expressed a more phlegmatic personality (factor Q4, Tension). Therefore, our fear that the relationship between Toxo and the psyche could be due to a psychological effect on the risk of infection – as opposed to the possibility that infection alters the psyche – wasn’t validated. At least our current results don’t support it. But I must admit that we’ve conducted this study on only one, fairly small subject group, so we need to verify the results on others. We have the data for it, but time seems a bit harder to get hold of.
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