Babesia microti was first recognized as a potential cause of human infection in New England in 1969. Before this, human cases of babesiosis involving other Babesia species were recognized in other places, chiefly Europe. The appearance and persistence of Babesia microti have been associated with high deer populations, increased human contact with the deer tick, Ixodes scapularis, and the presence of Lyme disease.
[caption id="attachment_316" align="alignleft" width="300" caption="Ixodes scapularis, image by Stuart Meek"][/caption]
In 2003, a paper published in the American Journal of Tropical Medicine and Hygiene noted “Human babesiosis generally is detected in sites where Ixodes ticks are endemic only after Lyme disease has become well established” (Krause et al, “Increasing Health Burden of Human Babesiosis in Endemic Sites," 68(4)). Why?
It’s tempting to explain away the late appearance of babesiosis by saying, well, physicians aren’t familiar with it, no one is looking for it, and most cases are mild anyway, so it’s simply being missed. As well, no one’s collecting data on diagnosed cases, so the incidence is unknown.
A current article published on SouthCoastTODAY.com discusses the work of Stephen Rich at the University of Massachusetts Amherst's Laboratory of Medical Zoology (Clark, “Much to Learn About Babesia’s Spread”). Rich’s research on Ixodes scapularis suggests that Babesia microti is spreading inland at a much slower rate than Borrelia burgdorferi, the organism that causes Lyme disease.
Rich’s work tells us that the organism truly is not well established in new locales – there’s not as much of it there. This likely explains why cases of babesiosis lag behind cases of Lyme disease, but it still raises a big question mark. If ticks and Lyme are spreading, why is Babesia not spreading just as quickly? The answer isn’t clear, but it appears that something is slowing down transmission of Babesia from host to host.
Rich doesn’t say that Babesia is absent in places where ticks and Lyme have appeared, he says there is a “much lower incidence” (qtd. in Clark). I suspect that incidence will rise over time – it may take years – and eventually make babesiosis a significant health concern over a much larger geographical area. I hope surveillance and medical knowledge stay ahead of it. If nothing else, the risk of contamination of the blood supply should fuel increased interest in this organism.