You would think that you’re in total control of your thoughts and behaviours, but for some, this is not always the case..
These pretty pathetic looking purple things to the left are parasitic individuals known as Toxoplasma Gondii. The parasite causes Toxoplasmosis and can infect many hosts, but I will mainly be concentrating on the cat/rat life cycle and the potential to impact humans.
Basic Lifecycle of T. gondii:
Let’s start with the feline host. If an infected cat (pet or wild) defecates it releases oocysts into the environment. Oocysts are spores which contain the parasites; they can be very long lasting in the environment and become infective within a few days. If a rat comes along and ingests food contaminated with these oocysts, the parasite has entered its next host. Within the rat the oocysts quickly develop into tachyzoites which are the mobile and asexual form of the parasite shown here in the picture. These move through the rat until they locate neural or muscle tissue where they develop into bradyzoites (tissue cysts).
If this rat, containing potentially huge numbers of tissue cysts is ingested by the cat, the parasite has returned to another feline host. In the cat, the bradyzoites eventually develop into the oocysts and are released into the environment and so completing the life cycle.
For an illustrated version of the Toxoplasma Gondii life cycle or any other parasitic life cycle DPDx do great, visual diagrams.
It is common to underestimate the effects single celled organisms can have on our health and wellbeing, I mean, what can one cell do that is so bad? Well the answer to that is A LOT. To help ensure that their life cycle is completed the parasites change the behaviour of the rat host. Rats have innate instincts and characteristics which cause them to avoid potential predators like cats. For example, if a rat can smell the odours given off by cats they will actively attempt to get as far away from this smell as possible or take shelter in a safe place. However, when infected with the Toxoplasmosis parasite this instinct changes, and rats instead are attracted to this smell and will persue it to get closer to the feline in question. So the parasite is actually changing the brain of the rat host so that its normal instincts are not only forgotten but also replaced by completely different and life threatening ones.
M. Berdoy, J. P. Webster and D. W. Macdonald conducted a study investigating these behavioural changes brought about by the T. gondii parasite. The rats tended to show much more risky behaviour when infected with the parasite than without and in doing so increased their chances of being caught and eaten by a cat. These changes to the brain are considered to be the work of the bradyzoites in the neural tissue in the brain. By altering the behaviour of the rat host so that it is no longer as acutely aware of risky behaviour, places and stimuli means that the parasite is increasing its likelyhood of getting into its next host and surviving. The parasite has taken control of its host and is driving them to danger and potential death to ensure its long term success.
Changing Human Behaviour
The prevalence of Toxoplasmosis in the human population is between 20-80% depending on the region, which is potentially a very large proportion of some populations. It is predicted that roughly 80% of French people are infected, which is a huge amount of people. Cats are a hugely popular pet across the globe and have become one of the most widely invasive species in existence. With our relationship with cats becoming increasingly close the T. gondii parasite has found itself inside humans instead of its normal host range. Humans can become infected in a number of ways for example, by ingesting contaminated water or food (spores or cysts in undercooked meat), through organ transplants or from their mothers via the placenta.
So it comes to question whether these odd behavioural changes brought about by the parasite could potentially occur in us humans when infected. This concept has been studied quite a few times, but one good study was done by J. Flegr et al. in which they performed a personality test (specifically Cattell’s questionnaire) on 224 men and 170 women. They found that the men and women who did have Toxoplasmosis did have altered behaviour. The characteristics that were seen to change as a result of the infecti0n were “Superego strength” (conscientious, moralistic), “protension” (suspicious/ jealous), “affectothymia” (outgoing/ warm), “shrewdness” and a “high strength of self sentiment” (controlled/ over powering). These behaviours are being brought upon by the parasitic cysts found in the brain tissue and like in rats are leading to more risky behaviours. Another study by J. Flegr et al. looked further into this concept at more specific situations and found that people infected with T. gondii are significantly more likely to get involved in car crashes than uninfected people.
Toxoplasmosis in Foetuses:
Besides these personality changes, an infection with Toxoplasma has no serious effects on those infected unless you are immunosuppressed. The big issues come in pregnancy. A normal infection is controlled by cells of the adaptive immune response, including crucially, cytotoxic T cells. These act by destroying cells that are infected with the parasite and so regulate the infection. The problem is that foetuses do not have these cells as they cannot be passed from mother to foetus, but the parasites can. Therefore, the infection cannot be controlled in the unborn child and this can have severe effects. If the mother has a T.gondii infection before or soon after conception the foetus is in nearly all cases miscarried. If an infection occurs later in pregnancy the chance of still birth is hugely increased. If the child does survive birth it is likely that the child will soon die and if not they generally will have impaired vision and severe learning difficulties.
As these effects are so severe there are measures in place to detect T. gondii infections so don’t worry too much! Women are routinely checked for the infection when pregnant. If a woman does have an infection it is generally caught before the parasite could have spread to the foetus. These women are given drugs like spiramycin to control the infection and decrease the chance of spread to the foetus.
So next time you’re feeling particularly aggressive or risky maybe take some time to consider whether it is in fact you driving these emotions or if your inner parasite is taking control..