Emerging tick-borne infections: rediscovered and better characterized, or truly 'new' ?

The emergence of Lyme borreliosis as a public health burden within the last two decades has stimulated renewed interest in tick-borne infections. This attention towards ticks, coupled with advances in detection technologies, has promoted the recognition of diverse emergent or potentially emerging infections, such as monocytic and granulocytic ehrlichiosis, local variants of spotted fever group rickettsioses, WA-1 babesiosis, or a Lyme disease mimic (Masters' Disease). The distribution of pathogens associated with well-described tick-borne zoonoses such as human babesiosis due to Babesia microti or B. divergens seems wider than previously thought. Bartonellae, previously known to be maintained by fleas, lice or sandflies, have been detected within ticks. Purported 'new' agents, mainly identified by sequencing of PCR products and comparison with those sequences present in GenBank, are being increasingly reported from ticks. We briefly review the diversity of these infectious agents, identify aetiological enigmas that remain to be solved, and provide a reminder about 'old friends' that should not be forgotten in our pursuit of novelty. We suggest that newly recognised agents or tick/pathogen associations receive careful scrutiny before being declared as potential public health burdens.

What is Babesia microti?

Babesia microti (Apicomplexa: Piroplasmida) has historically been considered a common parasite of Holarctic rodents. However, human babesiosis due to this species has generally been limited to the northeastern seaboard of the United States and Minnesota and Wisconsin. The absence of reports of B. microti babesiosis from sites where the agent is enzootic, such as in western Europe, remains unexplained. Previous work focusing on the 18S rDNA demonstrates little sequence diversity among samples from allopatric host populations across a wide geographical area. It may be that genetic diversity is underestimated due to sample size or the gene analysed. Accordingly, we collected blood or spleen samples from American or Eurasian animals with parasites that were morphologically consistent with B. microti, amplified the 18S rDNA and beta-tubulin gene, and conducted phylogenetic analysis. Surprisingly, what was considered to be 'B. microti' by microscopy appears to be a diverse species complex. We identify 3 distinct clades within this complex, including parasites from non-rodent hosts. Rodent parasites comprise 2 clades, one representing zoonotic isolates, and the other apparently maintained in microtine rodents, and therefore their morphological detection within animals from a site does not necessarily imply a risk to public health.

Enzootic Babesia microti in Maine.

Human babesiosis in the northeastern United States caused by Babesia microti (Apicomplexa: Piroplasmida) is mainly reported from coastal New England sites, where deer ticks (Ixodes dammini) are common. However, the piroplasm has been detected in microtine rodents elsewhere in association with I. angustus or other nidicolous ticks, suggesting that the agent is widely distributed but zoonotically significant only where a human-biting "bridge" vector is present. To determine whether this piroplasm may be enzootic in areas where I. dammini is absent, we surveyed small mammals collected from 2 sites in Maine, where I. angustus or I. muris is common but I. dammini is not. Of 43 chipmunks, voles, deer mice, and shrews examined, 3 (6.9, 95% confidence interval 0 to 14.5) were parasitemic, as determined by blood smear or polymerase chain reaction targeting a piroplasm-specific portion of the 18S ribosomal DNA gene. Phylogenetic analysis of the sequenced amplification products demonstrates the presence of 2 forms of B. microti. We conclude that B. microti may be enzootic in the absence of I. dammini but that human risk relates to dense infestations of this human-biting tick.

[Detection of natural foci of babesiosis and granulocytic ehrlichiosis in Russia]. / Vyiavlenie v Rossii prirodnykh ochagov babezioza i granulotsitarnogo érlikhioza.

The use of microscopy, infection of golden hamsters and the polymerase chain reaction made it possible to find out that about 30% of common red-backed voles (Clethrionomys glareolus), inhabiting the taiga forests of the southern part of the Western Urals (the Chusovskoi district of the Perm region), were infected with Babesia microti and simultaneously (a third of them) with Ehrlichia (Cytoecetes) phagocytophila, the causative agent of granulocytic ehrlichiosis. The sequencing of 18S rDNA of strain "Mys", isolated in Russia, revealed its identity to American B. microti strain GI, pathogenic for humans. The main vector supporting the circulation of B. microti in the natural foci in the region where these investigations were conducted was, seemingly, the tick Ixodes trianguliceps, Thus, for the first time the data proving the presence of reservoir hosts infected with B. microti and granulocytic E. phagocytophila, pathogenic for humans, in Russia were presented.

Infection of dogs in north-west Spain with a Babesia microti-like agent.

During 1996 a small, ring-shaped, piroplasm was observed in blood smears from 157 dogs in north-west Spain. None of them had previously been in areas endemic for Babesia gibsoni, which was until recently the only small piroplasm known to parasitise dogs. Haematological and serum biochemistry analyses showed that almost all the dogs had an intense regenerative haemolytic anaemia and that in some cases there was evidence of renal failure. A molecular study was made of a sample of the parasite obtained in June 2000. The phylogenetic analysis showed an identity of 100 per cent with the new piroplasm, provisionally denominated as Theileria annae, and 99 per cent with Babesia microti and B. microti-Japan. The results confirm the previous observation of a new form of piroplasm (Theileria annae) which causes disease in dogs in Europe and suggest that it is endemic among the canine population in north-west Spain.

Enzootic transmission of deer tick virus in New England and Wisconsin sites.

To determine whether rodents that are intensely exposed to the deer tick-transmitted agents of Lyme disease, human granulocytic ehrlichiosis, and human babesiosis are also exposed to deer tick virus (DTV), we assayed serum samples from white-footed mice (Peromyscus leucopus) and meadow voles (Microtus pennsylvanicus) in sites densely infested by deer ticks. To conduct serosurveys, we developed an enzyme-linked immunosorbent assay (ELISA) and Western blot assay by cloning, expressing, and purifying a portion of the DTV envelope glycoprotein (DTV rE) for use as test antigen. Sera from mice and voles trapped in Massachusetts, Rhode Island, and Wisconsin were screened by ELISA for IgG reactive to DTV rE. Samples that were positive or borderline by ELISA were subsequently analyzed by immunoblotting. Samples reactive in both assays were considered to be positive. Three percent of 264 mouse samples collected from sites in Rhode Island, 3.8% of 52 samples from mice trapped in Wisconsin, and 3.9% of 282 samples collected from mice trapped on Nantucket Island, MA were positive. No samples from either Great Island, MA, or voles from any study site were reactive. A reverse transcriptase-polymerase chain reaction yielded molecular evidence of DTV infecting questing adult deer ticks in sites where seroreactive mice were trapped, but not from ticks collected where serologic evidence of virus perpetuation was absent. White-footed mice appear to be exposed to DTV in certain sites where other deer tick-borne agents perpetuate. This virus may be maintained in the same enzootic cycle.