It’s reminiscent of the X-Files episode ‘Ice’. A parasite (Toxoplasmosis gondii) manipulates behaviour, making rats suicidal, predisposing humans to schizophrenia or changing personalities – perhaps even producing those traits that lead you to read FT. And you probably have T. gondii lurking in your garden. Indeed, T. gondii’s been with us since our ancestors domesticated cats, its main host, 9,500 years ago. Even then, cats seemed to be more than rodent eradicators; they probably had mystical, occult significance. I would like to suggest that the parasite’s ability to change human thought and induce schizophrenia may contribute to both the cat’s iconic links with the occult and our shamanistic abilities.

T. gondii is an unpleasant, tough little parasite. It has a hardy ‘spore’ phase – called an oocyst – that can survive in soil for more than a year. When cats ingest oocysts, the parasite reproduces in the gut wall. The cat expels oocysts in the fæces in huge numbers: a 20g (0.7oz) cat stool can contain 20 million oocysts. While cats are the definitive hosts, T. gondii infects numerous other species, including humans, birds and rodents. In these intermediate hosts, the parasite reproduces outside the intestine, producing thin-walled cysts throughout the body, but especially in the brain. These can persist for life, so-called latent toxoplasmosis. The life cycle continues when a cat eats the bird or rodent.

In humans, T. gondii infestations generally trigger symptoms resembling a mild viral infection. Nevertheless, pregnant women need to be careful: T. gondii can cause numerous fœtal abnormalities including deafness, seizures and mental retardation. Furthermore, up to 60 per cent of AIDS patients who are infected with the parasite – their compromised immune systems opens them to infection – exhibit delusions, auditory hallucinations, thought disorders and other symptoms strikingly similar to those seen in serious mental illnesses and shamanistic experiences.

Indeed, in all but one of 19 studies performed since 1953 that examined the link, T. gondii infections were more common in people with schizophrenia or other serious mental illnesses compared with controls from the healthy population. A German study found that 42 per cent of people with schizophrenia had been infected, compared to 11 per cent of controls. Similarly, an Irish study found prevalences of T. gondii antibodies of 60 per cent and 45 per cent among in-patients with schizophrenia and hospital workers respectively.

As a final example of the link, adults who suffer from schizophrenia or bipolar disorder (manic depression) were more likely to have been exposed to cats in childhood than healthy controls. In one study, for instance, people who owned a cat between birth and age 13 were 53 per cent more likely to suffer from schizophrenia or bipolar disorder. Such circumstantial evidence doesn’t prove that T. gondii causes schizophrenia, but several other lines of evidence support the suggestion. For example, in mice, T. gondii infection changes levels of several chemicals in the brain linked to mood and learning as well as to schizophrenia and other mental illnesses.

Schizophrenia offers a dramatic example of how T. gondii may change mental states. However, T. gondii can change the host’s behaviour in more subtle ways that, in some species, increase the chance of transmission. For example, wild rats are very cautious, avoiding novel stimuli (one reason they’re difficult to trap) and showing a strong aversion to the smell of cat’s urine. Rats infected with T. gondii throw caution to the wind. They are more likely to approach a novel object and tend to be caught before uninfected rats are. Indeed, T. gondii-infected rats prefer areas treated with cat’s urine. In other words, T. gondii turns, in the words of a recent paper in the Proceedings of the Royal Society, rat’s “innate aversion into a ‘suicidal’ feline attraction”. Valporic acid and haloperidol (medicines used to treat bipolar disorder and schizophrenia respectively) inhibit the growth of T. gondii and were at least as effective as the standard treatments for toxoplasmosis at preventing the rats’ suicidal behaviour.

It’s also possible that T. gondii could influence human personality. For example, based on personality questionnaires, men infected with T. gondii are more likely to disregard society’s rules as well as being more suspicious, jealous and dogmatic. Infected women tend to be more warm-hearted, out- and easy-going, conscientious, persistent, moralistic and staid. Infected men and women tend to be prone to feelings of guilt.

In another study, young women with latent toxoplasmosis were more intelligent, but were also more apprehensive, self-reproaching and insecure. Two other personality traits also seemed to be more common among the women with latent toxoplasmosis: a constellation of characteristics psychologists call ‘ergic tension’ (tense, frustrated, driven and overwrought); and radicalism – they tended to be experimental, liberal and analytical. (Experimental; radical; disregarding society’s rules – sounds like a typical FT reader to me. Perhaps we should compare T. gondii infection rates among FT subscribers and those who read the Telegraph.) An easy-going, tense or liberal human host probably doesn’t increase the parasite’s chance of transmission. However, the same biochemical changes that lead rats to become suicidal could, in humans, manifest as behavioural changes.

We’ve seen that T. gondii can change behaviour in both subtle and dramatic ways. I’d now like to take a speculative leap and suggest that T. gondii’s ability to modify our mental state might contribute to the cat’s iconic position in the occult, which traces its roots to deep prehistory. For example, 4,000-year-old remains and paintings show that cats were intertwined with the cosmology and religion of ancient Egypt. Cats were, for instance, symbols of Bastet, a protective goddess with the body of a woman and a feline head.

Furthermore, ancient Egyptians produced millions of mummified mammals, birds and reptiles. Despite being done on an almost industrial scale, the mummification was often complex, including, for example, evisceration, elaborate bandaging and treatment with balms similar to that found in mummified humans. In other words, the ancient Egyptians mummified at least some cats from this time with considerable care, which further underscores their spiritual significance.

It seems, however, that the spiritual links between cats and humans might have been widespread across Middle Eastern Neolithic societies. Over the years, archæologists have found stone or clay cat figurines in Syria, Turkey and Israel. Then, in 2001, researchers excavated a 9,500-year-old burial of a 30-year-old human in Shillourokambos, Cyprus, who seemed to be from the Neolithic elite. The grave contained numerous offerings, including polished stones, axes, ochre and flint tools, while a pit nearby contained marine shells, which suggests that the person was important, a leader or shaman perhaps. Archæologists also found a complete cat skeleton lying 40cm (16in) from the man, buried at the same depth and in the same soil layer. In addition, the cat skeleton remained articulated. This suggests that the Neolithic Cypriots buried and rapidly covered the corpse around the same time as the human burial. Otherwise, scavengers would have scattered the remains. So, the researchers argue, the 8-month-old cat was probably tame.

The archæologists believe that the dual burial suggests that cats were important in the life and afterlife of people living in Shillourokambos – that Neolithic people regarded the cat as an individual and that the joint burial could imply “a strong association” between the human and the cat. Indeed, this association, especially as the human seems to be of high social status, implies a particularly special relationship, possibly involving “spiritual links”.

The long relationship between cats and humans – even their spiritual significance – makes sense. Humanity tamed cats, which were larger than the modern moggy, about 3,000 years after dogs and around the same time as our ancestors domesticated wheat and sheep. As humanity moved from a hunter-gatherer society to settled communities based around agriculture, stored grain would attract mice. Cats lived in villages and controlled vermin.

As I have argued in the pagan magazine Pentangle, Bastet might have evolved from the Middle Eastern Neolithic cat cult into a protective goddess, reflecting the cat’s critical role protecting from rodents the grain vital to the society’s survival. I also suggested that the long-standing spiritual relationship reflects differences between dog lovers and cat fanciers. Dogs hunted with the male warriors, becoming ‘man’s best friend’. Cats remained in villages, often with the wise woman and shaman. (Think about the slang use of pussy, for example.) This might have led, in turn, to their association with mysteries and magic. Of course, this would fit in well with the cat’s protective role. In time, cats became archetypes represented by Bastet and the witch’s familiar.

This close relationship between familiar and witch increases the likelihood of infection with T. gondii and, therefore, the chances of developing schizophrenia. Furthermore, people with schizophrenia endure numerous symptoms including delusions, hallucinations, unusual thought content and conceptual disorganisation. Many prophecies show conceptual disorganisation, which may help us understand our own minds. (Think of the ambiguity of the I-Ching or tarot correspondences.) Read any shamanistic account and you will encounter events and sights akin to delusions and hallucinations. Indeed, a fifth of young people with schizophrenia experience the partition delusion: the belief that people and objects can pass through a wall, ceiling and other barriers. The idea of going through barriers is, of course, a mainstay of pagan literature, fairy stories and occult lore, although obviously the concept has important symbolic meanings.

In other words, many of the symptoms of schizophrenia are similar to the experiences of shamans and witches. Women and shamans may have been more likely to contract T. gondii simply because they spent more time in the villages. The changes in mental state could result in some individuals seeing and hearing things others could not and being deemed, depending on the prevailing ideas, witches, wise women or shamans. As cats became part of the occult iconography, more wise women would have kept the animals, perpetuating the link. There’s no proof – and the sociology and anthropology is much more complex than a simple infection – but the circumstantial evidence that T. gondii contributes to our occult experiences, and the role of the cat as a familiar, seems compelling.

In ‘Ice’, Mulder and Scully discovered a parasite buried in the Arctic ice for thousands of years that infects people and leads them to kill each other. Given the occasional cases of extreme violence by people with schizophrenia, the idea doesn’t seem that fictional. Something close to the truth seems to be out there, in gardens and litter trays across the country.