Thimerosal: It doesn't cause autism, okay guyz?

Happy X-mas! I got you all a blog post. It's sort of late and last minute, but I hope you still like it. I am both tired and deadly afraid of screwing it up. The methanol/menthol kerfuffle of '09 (see previous post) is haunting me something awful. Baby steps, folks. Baby steps.

Thimerosal is a mercury-containing organic compound that was developed and patented in 1928 by a chemist named Morris Kharasch. Somewhere along the way it started to be called thiomersal as well, and it's commercial name is Merthiolate. The 'thio' in the name refers to the presence of a sulfur atom linking ethylmercury (the 'mer' part) to the rest of the molecule. Presumably someone got pissy about the lack of an 'o' in the 'thi' part of thimerosal.

In addition to its use as a topical antiseptic in ointments and the like, thiomersal is added to some multi-use vaccines so that nasty bugs don't grow in them and subsequently grow in us. Multi-use vaccines might sound like a real bad idea but I'm guessing that they are much less expensive, last longer, and generally don't cause infections because they contain preservatives like thimerosal.

There are a bunch of people out there who believe that thiomersal is responsible for autism. Unfortunately, this is not the case. There is this neat thing called herd immunity, whereby if a certain proportion of people get vaccinated against a thing then the few who don't get vaccinated against the thing don't tend to catch the thing because everyone else keeps the thing from being able to survive and get from person to person. When people fail to vaccinate their children out of misplaced fears that they will develop autism or be more likely to die of SIDS, then herd immunity can be compromised, and we start to see things like measels and rubella showing up in places where they previously had not been seen for a long while. This is not a good thing. Check ya later, getting-slightly-off-topic soapbox.

Although thiomersal does not cause autism, it does represent a source of environmental exposure to mercury, which is probably not a good thing. Since we tend to get vaccinated when we are relatively small and still developing, a bit of mercury can have a much greater impact than it would on a fully grown adult. Once inside us, thiomersal is attacked by enzymes and ethylmercury is released.

Ethylmercury is an organic mercury compound, meaning that atoms of mercury are covalently linked to hydrocarbons instead of just hanging out on their own. Organic mercury compounds are better able to move about inside of us and tend to hang out for longer periods of time, resulting in greater exposure. Ethylmercury is not thought to be as toxic as methylmercury (the form responsible for Minamata disease and most cases of environmental mercury poisoning), in part because it doesn't stick around for as long, but it still may be bad news.

Thiomersal has been phased out of many antiseptic products, and is currently being removed from vaccines in many countries. It's probably a good thing, so long as the replacements for vaccine preservation do their job and people don't interpret it as an autism-related move.

- Madsen KM, Lauritsen MB, Pedersen CB, et al. Thimerosal and the occurrence of autism: negative ecological evidence from Danish population-based data. Pediatrics. 2003 Sep;112(3 Pt 1):604-606. [pdf]
- DeStefano F. Vaccines and autism: evidence does not support a causal association. Clin Pharmacol Ther. 2007 Dec;82(6):756-759. [link]
- Vennemann MM, Höffgen M, Bajanowski T, Hense HW, Mitchell EA. Do immunisations reduce the risk for SIDS? A meta-analysis. Vaccine. 2007 Jun 21;25(26):4875-4879. [link]
- Geier DA, Sykes LK, Geier MR. A review of Thimerosal (Merthiolate) and its ethylmercury breakdown product: specific historical considerations regarding safety and effectiveness. J Toxicol Environ Health B Crit Rev. 2007 Dec;10(8):575-596. [pdf]
- http://en.wikipedia.org/wiki/Thiomersal