Aetiology

Carl Zimmer over at The Loom has post discussing Toxoplasma, and how it can affect behavior--and may play a role in the development of schizophrenia. (Check out his post for the details).

As Zimmer mentions, this is a controversial hypothesis. Though support is growing for acceptance of the idea that infectious agents can cause all kinds of diseases that have traditionally been deemed "genetic" or "lifestyle" diseases, it is a slow and uphill battle. It also happens to be an interest of mine. I discussed another putative cause of schizophrenia, Chlamydia species, here. These type of links are very difficult to confirm using traditional Koch's postulates, however. For example, Zimmer notes that estimates suggest approximately 50% of the world's population is infected with Toxoplasma--yet a very small number of us develop schizophrenia. Therefore, it's likely that development of schizophrenia (and many other diseases) result from a combination of human genetics and other factors, such as infection. For example, multiple sclerosis is an autoimmune disease, thought to result from a combination of host genetics and an infectious agent.

Really, it's even messier than that, because the genetics of the pathogen--what proteins it expresses, for example--can also play a role in the development of specific diseases. Add in a possible time lag of up to 50 years or so between initial exposure to the microbe and development of the disease, and you can see why this field is such a mess--and why there's so much skepticism in the first place regarding the role of infectious agents when they're so far removed from the final outcome.

However, this has been shown fairly conclusively in a number of cases. For example, one of the earliest viruses investigated was the Rous sarcoma virus, a virus capable of causing tumors in chickens. Today, we know that the human papilloma virus (HPV) is the major cause of cervical cancer, while Hepatitis B can cause liver cancer, and Epstein-barr virus has been implicated in Burkitt's lymphoma.

Microbes can cause other types of disease as well. Lyme disease, caused by the spirochete Borrelia burgdorferi, was first identified due to a cluster of juvenile arthritis cases, and can also cause neurologic symptoms. Another spirochete, Treponema pallidum, has long been known to cause neurologic symptoms: Treponema is the causative agent of syphillis. If left untreated, can result in permanent damage to almost any system in the body--up to 20-50 years after initial infection. And of course, infection with the rabies virus can lead to strange behaviors, including hydrophobia, hallucinations, and dementia.

I could go on and on...Helicobacter and gastric ulcers and cancer; HHV-8 and Kaposi's sarcoma; Streptococcus pyogenes and rheumatic fever/rheumatic heart disease...point being, to badly paraphrase, there are more things that contribute to disease development, Horatio, than are dreamed of in your philosophy. Zimmer highlights one fascinating link--it may hold up upon further scrutiny, and it may not. Time will tell. But for me, ti's just incredible to think that despite all our work in the past 150-odd years on infectious agents that cause acute, generally self-resolving disease (such as colds, or flu, or food poisoning), we may only be scratching the surface when it comes to the roles these agents play in human disease.

Parasites, Cancer, Epidemiology