Zoonotic Babesia microti in the northeastern U.S.: evidence for the expansion of a specific parasite lineage

Abstract

The recent range expansion of human babesiosis in the northeastern United States, once found only in restricted coastal sites, is not well understood. This study sought to utilize a large number of samples to examine the population structure of the parasites on a fine scale to provide insights into the mode of emergence across the region. 228 B. microti samples collected in endemic northeastern U.S. sites were genotyped using published VNTR markers. The genetic diversity and population structure were analysed on a geographic scale using Phyloviz and TESS. Three distinct populations were detected in northeastern US, each dominated by a single ancestral type. In contrast to the limited range of the Nantucket and Cape Cod populations, the mainland population dominated from New Jersey eastward to Boston. Ancestral populations of B. microti were sufficiently isolated to differentiate into distinct populations. Despite this, a single population was detected across a large geographic area of the northeast that historically had at least 3 distinct foci of transmission, central New Jersey, Long Island and southeastern Connecticut. We conclude that a single B. microti genotype has expanded across the northeastern U.S. The biological attributes associated with this parasite genotype that have contributed to such a selective sweep remain to be identified.\n\nAuthor summaryBabesiosis is a disease caused by a protozoan parasite, Babesia microti, related to malaria. The disease is acquired by the bite of the deer tick, the same tick that transmits Lyme disease. Although Lyme disease rapidly emerged over a wide range within the last 40 years, babesiosis remained rare with an extremely focal distribution. Within the last decade, the number of reports of babesiosis cases has increased from an expanded area of risk, particularly across the mainland of southern New England. We determined whether the expanded risk may be due to local intensification of transmission as opposed to introduction of the parasite. Historical fragmentation of the landscape suggests that sites of B. microti transmission should have been isolated and thus evidence of multiple genetically distinct populations should be found. By a genetic fingerprinting method, we found that samples from the new mainland sites were all genetically similar. We conclude that one parasite genetic lineage has recently expanded its distribution and now dominates, suggesting that it has some phenotypic attribute that may confer a selective advantage over others.