Powassan virus lineage 2 (deer tick virus) is an emergent threat to American public health, causing severe neurologic disease. Its life cycle in nature remains poorly understood. We use a host-specific retrotransposon-targeted real time PCR assay to test the hypothesis that white-footed mice, considered the main eastern U.S. reservoir of the coinfecting agent of Lyme disease, is the reservoir for deer tick virus. Of 20 virus-infected host-seeking nymphal black-legged ticks 65% fed on shrews and none on mice. The proportion of ticks feeding on shrews at a site is positively associated with prevalence of viral infection, but not the Lyme disease agent. Viral RNA is detected in the brain of one shrew. We conclude that shrews are a likely reservoir host for deer tick virus and that host bloodmeal analysis can provide direct evidence to incriminate reservoir hosts, thereby promoting our understanding of the ecology of tick-borne infections.

Specific ticks seem to locally serve as vector for characteristic microbial assemblages (guilds) comprising spirochetes, piroplasms, ehrlichiae, and arboviruses. Borrelia andersoni and Anaplasma phagocytophilum are intensely transmitted between cottontail rabbits. To test the hypothesis that a piroplasm may also be maintained in rabbits, we sampled these hosts from Nantucket Island, Massachusetts and tested their blood and tissues by a polymerase chain reaction for evidence of infection. Surprisingly, the agent of bovine redwater and of European human babesiosis, Babesia divergens, was detected in 16% of the rabbits sampled during 1998–2002 (>99% sequence similarity in the 18S ribosomal DNA). The vector of B. divergens on Nantucket appears to be Ixodes dentatus, a rabbit- and bird-feeding tick that may feed on humans. Although the risk of human infection appears to be minimal, an autochthonously acquired Kentucky case due to this rabbit agent was recently reported. Physicians should entertain the diagnosis of babesiosis due to B. divergens for severe hemolytic febrile syndromes in American patients exposed to sites where rabbits are common.

Parasites from diverse hosts morphologically identified as Babesia microti have previously been shown to belong to a paraphyletic species complex. With a growing number of reports of B. microti-like parasites from across the world, this paper seeks to report on the current knowledge of the diversity of this species complex. Phylogenetic analysis of 18S rDNA sequences obtained from GenBank shows that the diversity of the B. microti species complex has markedly increased and now encompasses at least five distinct clades. This cryptic diversity calls into question much of our current knowledge of the life cycle of these parasites, as many biological studies were conducted before DNA sequencing technology was available. In many cases, it is uncertain which B. microti-like parasite was studied because parasites from different clades may occur sympatrically and even share the same host. Progress can only be made if future studies are conducted with careful attention to parasite identification and PCR primer specificity.

Deer tick transmitted Borrelia burgdorferi sensu stricto (Lyme disease) and Babesia microti (babesiosis) increasingly burden public health across eastern North America. The white-footed mouse is considered to be the primary host for subadult deer ticks and the most important reservoir host for these and other disease agents. Local transmission is thought to be modulated by less reservoir competent hosts such as deer diverting ticks from feeding on mice. We measured the proportion of mouse fed or deer fed host-seeking nymphs from 4 sites during 2 transmission seasons by bloodmeal remnant analysis using a new retrotransposon-based qPCR assay. We then determined the host that was associated with the infection status of the tick. During the first year, the proportion of mouse-fed ticks ranged from 17% on mainland sites to 100% on an island, while deer-fed ticks ranged from 4% to 24%. The proportion of ticks feeding on mice and deer was greater from island sites than mainland sites (on average 92% vs 43%). Mouse-fed ticks decreased significantly during year two in 3 of 4 sites (most <20%), while deer-fed ticks increased for all sites (75% at one site). Overall, ticks were more likely to be infected when they had fed on mice (OR=2.4 and 1.6 for Borrelia and Babesia respectively), and less likely if they had fed on deer (OR=0.8 and 0.4). We conclude that host utilization by deer ticks is characterized by significant spatiotemporal diversity, which may confound efficacy tests of interventions targeting reservoir hosts. Importance White-footed mice are thought to be the most important reservoir host for the deer tick-transmitted pathogens that cause Lyme disease and human babesiosis because they are the primary host for immature ticks. Transmission would be reduced, however, if ticks feed on deer, which are not capable infecting ticks with either pathogen. By directly measuring whether ticks had fed on either mice or deer using an new qPCR assay to detect remnants of host DNA leftover from the larval bloodmeal, we demonstrate that host utilization by ticks varies significantly over time and space and that mice often feed fewer ticks than expected. This has implications for our understanding of the ecology of these diseases and for the efficacy of control measures.

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 Variable number tandem repeat (VNTR) markers. The genetic diversity and population structure were analysed on a geographic scale using Phyloviz and TESS, programs that utilize two different methods to identify population membership without predefined population data. 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.