Cats, toxoplasma and shizophrenia

On biological complexity

A recent article in the February publication of DVM news magazine (a veterinary multimedia news source) discusses a study that linked a cat parasite; toxoplasma gondii, to the risk of schizophrenia in humans (affects about two million individuals a year or 1% of the US population).

This story is interesting because it illustrates just how interrelated and messy the world of life can be. It also exemplifies just how deeply a parasitic organism can penetrate into a larger ecosystem. By utilizing differing mechanisms to weave a tangled web toxoplasma gondii has become so enmeshed with warm-blooded animals and their environment that it would be extremely difficult or next to impossible to eradicate- and you thought politicians were bad!

The study the article refers to is discussed in the American Journal of Psychiatry and is titled “Selected infectious agents and risk of schizophrenia among US military personnel.” It forms part of an increasing number of studies that seem to be finding a possible association between people exposed to toxoplasma gondii (based on collected serum samples) and an increased risk for developing schizophrenia.

This particular study stands out in that it is larger than most, serum samples were collected in many cases before a diagnosis of schizophrenia was given and the subjects (military personnel) could be clearly compared with non-schizophrenic subjects from the same population. Though the link was small and there are some important confounding factors, combined with other studies these results shed some light on one possible infectious trigger to schizophrenia in predisposed (genetically?) individuals.

Notes on T gondii

Toxoplasma gondii is a well known ubiquitous intracellular parasite that has a rather complex and fascinating life cycle involving a variety of intermediary hosts (i.e.; pigs, goats, insects, humans, and chickens) as well as the environment (i.e.; water and soil). Felines are definitive hosts, which mean this is where toxoplasma will eventually sexually reproduce and form the oocysts that can infect other organisms on a journey that eventually takes some back to infect other cats.

In humans the CDC notes that it is known as one of the most common human infections in the world affecting over 500 million people. In the US it is the third leading cause of death related to food borne illness. Though about 60 million people in the US carry the parasite, most healthy individuals have no symptoms although pregnant women and imuno-compromised people are at higher risk for problems.

Infected cats usually shed oocysts soon after weaning and only for a short time. Though oocysts shed from infected cats spread the illness, the most common form of exposure in humans is through the ingestion of raw or undercooked meat (usually pork, goat, and lamb). Interestingly, there is no correlation between increased toxoplasmosis and cat ownership (there are a couple of studies that seem to relate adult schizophrenia and childhood exposure to cats in the household- not enough epidemiological power to make anything out though).

Because this organism is so common and the infection to humans is usually between intermediaries and not cats, it turns out that simple hygienic measures do the most to minimize problems. Feeding cats cooked food, cleaning there litters every day, reducing stray populations are some effective measures to reduce infection and environmental contamination. In humans eating cooked meats (and pasteurizing goats milk), cleaning ones hands after outdoor activities such as gardening, covering children’s sandboxes and controlling insect mechanical vectors (i.e.; houseflies, cockroaches). It should be noted that the biggest risk of oocysts exposure is in warm, moist, or tropical climates and lower in arid and frigid areas of the world.

The bigger picture

The January 2008 American Journal of Psychiatry editorial Alan Brown MD, MPH suggests that the idea parasitic infections can play some role in psychological diseases is not as strange as it sounds and does have parallels with other infectious disease processes.

Dr Brown notes that “Authorities from several diverse disciplines, including infectious disease, neonatlogy, pediatrics, neurology, and obstetrics and gynecology have long known that infections during prenatal and postnatal life have many neuropsychiatric sequelae, including behavioral problems, mental retardation, learning disabilities, and mood alterations.”

Improved data bases are allowing for more refined statistical analysis and seem to be finding associations between “prenatal exposure to influenza, toxoplasma gondii, genital/reproductive pathogens, and immunologic disturbances with an increased risk of schizophrenia.” Though still in its infancy, there are animal model studies that also support a link between behavioral anomalies consistent with what is seen in schizophrenia and “anti-viral like” inflammatory responses. So, the association with infectious disease and schizophrenia, if it exists, is not due just to a feline linked parasite.

If the body of evidence eventually indicates that infections do in fact impact schizophrenia in some cases then preventative measures as well as different treatment approaches could be used to reduce this mental illness in some people. Dr. Brown notes that “several infections that have been associated with this disorder can be effectively treated with antibiotics and prevented by vaccination and by minimizing the occurrence of risk factors for these infections.” That is the good news.

This is only a small part of the picture as further epidemiological, molecular genetics, genomics, brain imaging and human developmental (looking for precursors) studies expand researchers’ ability to discover new factors involved with schizophrenia risk as well as enhance their search for new treatments.

Though informative, these findings could easily be misconstrued by many who don’t have a biological background as an indictment against our beloved pet cats. It doesn’t emphasize that though toxoplasma has “chosen” felines as its definitive host where it reproduces sexually and sheds its oocysts, it is in fact an organism that takes advantage of many other animals and environmental factors –as intermediary hosts- in an elaborate “macro- bio-ecological” parasitic relationship.

These overlapping characteristics are the M.O. of innumerable organisms from viruses to larger organisms and illustrate how some of them don’t work under our taxonomic classifications- species are more blended from their viewpoint. Therefore it behooves people to take the bigger picture into account, use common sense –to have a “situational” awareness of the world around us- and most important remember that cats still rule!

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Ref:

Greene E Craig. Infectious disease of the dog and cat (3^rd edition). WB Saunders. Philadelphia, Pennsylvania. 2007

Niebuhr, D.W., Millikan, A.M., Cowan, D.N., Yolken, R., Li, Y., Weber, N.S. (2007). Selected Infectious Agents and Risk of Schizophrenia Among U.S. Military Personnel. American Journal of Psychiatry, 165(1), 99-106. DOI: 10.1176/appi.ajp.2007.06081254