STD parasites infected with viruses
Last month’s case study about virophages, where a species of amoeba was found to be infected with a virus, which in turn was infected with a virus, was interesting but really pretty much an exercise in “Wow! That’s interesting” as opposed to something that might feel clinically important. Not so today! Via Boing Boing, from an article on Live Science, a report about viruses that infect Trichomonas, the causative agent of a very common venereal disease.
Trichomonads are interesting in their own right; they are eukaryotes like us, classified in the broad group of single-celled flagellated protozoans, however they are anaerobes. A little cursory research indicates that this class of protozoans lack fully functioning mitochondria. This suits the lifestyle of Trichomonas vaginalis, which colonizes the surfaces of the vaginal tract in humans. It is then transmitted between sexual partners and can spread through the population via non-monogamous means. On its own, Trichomonas infection is not particularly severe, however women infected with this organism are more susceptible to other STD infections, and pregnancies are significantly more risky with a high frequency of preterm complications and miscarriages.
Researchers at Harvard Medical School recently published their findings about Trichomonas in the medical journal PLOSOne, which have been summarized in the Live Science article above. They found that about 80% of clinical Trichomonas isolates were themselves infected with an endosymbiotic virus now called Trichomonasvirus. Comparison of disease between individuals infected with the virus-free Trichomonas tended to have less significant disease than those infected with virus-containing Trichomonas. This offered preliminary results to suggest that the virus conferred virulence traits to Trichomonas in the process of disease. This is not a new idea (virus-infected isolates of Vibrio cholerae are able to cause cholera, whereas virus-lacking isolates are avirulent), however this is one of the first demonstrations of this relationship with a eukaryotic pathogen and the development of disease. With this critical observation in mind, the researchers began to examine the mechanism.
They found that virus-infected Trichomonas was detected by Toll-like receptor 3 on the epithelial cells of the vaginal tract. According to lecture, TLR-3 is a pattern recognizing protein that specifically recognizes double-stranded RNA molecules, which is what the genome of Trichomonasvirus is composed of. Binding of TLR-3 triggers a cascade of soluble messengers including type I Interferon, resulting in an inflammatory cascade, which have previously been found to be involved in preterm labor and HIV susceptibility. Alarmingly, the standard of care for Trichomonas infection in women, the anti-protozoan antibiotic metridnazole, exacerbated these effects. The authors recommend that a new approach in the treatment of these infections might be necessary. Specifically targeting the virus infection of Trichomonas might help to avoid some of the complications of these STDs.
Bonus time, as I am becoming depressed over the lack of commenting here on the blog. Note that this will be a limited time offer, only through the end of this week (noon on Friday November 16.) For bonus credit on the clicker quiz–same deal as before–comment below by naming a protozoan disease of humans, and the etiologic agent of that disease. We will see some of these in lab this week, so this is timely. Duplicate entries will be disallowed!