New bacterium-cancer link
Chlamydia psittaci is a bacterium which causes the zoonotic disease psittacosis in humans. This is commonly acquired from birds, and generally causes symptoms similar to influenza: fever, chills, cough, muscle aches, and possibly pneumonia. It's diagnosed rarely in the U.S., averaging about 50 cases/year (though quite likely, it is underdiagnosed). It's in the same genus as Chlamydia pneumoniae, which has been linked to the development of heart disease and Alzheimer's, and Chlamydia trachomatis, which, in conjunction with human papilloma virus, has been linked to the development of cervical cancer. As mentioned above, new evidence appears to implicate their psittacian cousin in the development of ocular lymphoma:
In the study, Dr. Changhoon You from the Asan Medical Center in Seoul, South Korea, compared chlamydia infection in 33 people with ocular adnexal lymphoma, or OAL, and 21 people with a comparable but non-cancerous condition called non-neoplastic ocular adnexal disease.
He found the Chlamydia psittaci strain was present in 78 percent of the cancer patients, but only in 23 percent of those in the comparison group.
In a previous study conducted in Italy, the bacteria were found in 80 percent of people with the lymphoma and in none of those in a comparison group of healthy people.
Note that these findings have, as yet, only been published in conference proceedings, and not in a medical journal. As such, I don't have the answers to the obvious question: what is the temporality here? Are C. psittaci truly a cause, meaning they'd necessarily have to precede the development of the cancer, or do they take advantage and infect the damaged tissue?
A few other publications have already addressed this purported link. This paper from Italy found a correlation between cancer and the presence of C. psittaci DNA. Additionally, antibiotics to treat this infection appeared to improve their clinical conditions. However, other studies have since found conflicting results. This publication found no evidence of C. psittaci DNA in any of their cases; they suggest possible geographic differences as a reason for the discrepancy in results. Similar results were found in this study from Florida. But, again, these researchers treated C. psittaci-positive ocular adnexal lymphoma patients with antibiotics, and, similar to results in the Italian study, saw clinical improvement. The article doesn't say whether the newest research tested the effectiveness of antibiotics in their C. psittaci-positive patient group. (I assume not, or they would have reported that bit as well).
So, what does it all mean? The intersection between infectious and chronic disease epidemiology is a tricky one. Even when we know an infection can cause a particular disease (such as Helicobacter pylori and gastric cancer), there's never a 100% correlation between the presence of the infectious agent and development of the disease. H. pylori infects people every day without causing cancer, and people who are Helicobacter-free go on to develop gastric carcinoma. It gets even tougher to determine a cause-effect relationship for pathogens that may rarely cause a particular chronic disease, especially if they're pathogens that are routinely under-diagnosed. The chronic disease literature is awash with potential infectious etiologies: multiple sclerosis, for instance, has been associated with measles, Epstein-barr virus, human herpesvirus 6, HTLV-1, generic "coronaviruses," hepatitis B, canine distemper virus, varicella-zoster, JC virus, Chlamydia pneumoniae, Candida albicans, and probably others I'm forgetting. Nothing convincing has been shown yet--but what if several of those agents could cause MS? Are we missing the forest for the trees here? Imagine if we dismissed Streptococcus pneumoniae and Staphylococcus aureus as causes of pneumonia, because neither was present in all (or even most) cases of pneumonia. Bring in genetic risk factors and strain-to-strain variation in pathogens, and you can see why this intersection is desperately in need of some traffic signals. I don't know whether the current link between C. psittaci and ocular lymphoma will be repeated in larger studies or not, but I hope those who are so quick to dismiss infectious etiologies of their favorite diseases will at least think twice before doing so.