BackgroundDeer tick virus, DTV, is a genetically and ecologically distinct lineage of Powassan virus (POWV) also known as lineage II POWV. Human incidence of POW encephalitis has increased in the last 15 years potentially due to the emergence of DTV, particularly in the Hudson Valley of New York State. We initiated an extensive sampling campaign to determine whether POWV was extant throughout the Hudson Valley in tick vectors and/or vertebrate hosts.MethodsMore than 13,000 ticks were collected from hosts or vegetation and tested for the presence of DTV using molecular and virus isolation techniques. Vertebrate hosts of Ixodes scapularis (black-legged tick) were trapped (mammals) or netted (birds) and blood samples analyzed for the presence of neutralizing antibodies to POWV. Maximum likelihood estimates (MLE) were calculated to determine infection rates in ticks at each study site.ResultsEvidence of DTV was identified each year from 2007 to 2012, in nymphal and adult I. scapularis collected from the Hudson Valley. 58 tick pools were positive for virus and/or RNA. Infection rates were higher in adult ticks collected from areas east of the Hudson River. MLE limits ranged from 0.2-6.0 infected adults per 100 at sites where DTV was detected. Virginia opossums, striped skunks and raccoons were the source of infected nymphal ticks collected as replete larvae. Serologic evidence of POWV infection was detected in woodchucks (4/6), an opossum (1/6), and birds (4/727). Lineage I, prototype POWV, was not detected.ConclusionsThese data demonstrate widespread enzootic transmission of DTV throughout the Hudson Valley, in particular areas east of the river. High infection rates were detected in counties where recent POW encephalitis cases have been identified, supporting the hypothesis that lineage II POWV, DTV, is responsible for these human infections.

TOC Summary: The epidemiologic pattern and limited laboratory testing indicate that this virus lineage might account for most of these illnesses.

Blacklegged ticks, Ixodes scapularis Say, were collected from 27 sites in eight New York State counties from 2003 to 2006 to determine the prevalence and distribution of tick-borne pathogens in public-use areas over a 4-yr period. In total, 11,204 I. scapularis (3,300 nymphs and 7,904 adults) were individually analyzed using polymerase chain reaction to detect the presence of Borrelia burgdorferi (causative agent of Lyme disease), Anaplasma phagocytophilum (formerly Ehrlichia phagocytophila, causative agent of human granulocytic anaplasmosis), and Babesia microti (causative agent of human babesiosis). Overall prevalence of B. burgdorferi, A. phagocytophilum, and B. microti was 14.4, 6.5, and 2.7% in nymphs and 45.7, 12.3, and 2.5% in adult ticks, respectively. Rates varied geographically and temporally during the time period examined, and were related to measurements of tick density. Average rate ofpolymicrobial infection for nymphs and adults, respectively, was 1.5 and 8.5% overall, with 0.5 and 6.3% coinfection of B. burgdorferi and A. phagocytophilum, 1.0 and 1.5% B. burgdorferi and B. microti, and 0.05 and 0.6% A. phagocytophilum and B. microti. Thirty-three individual adult ticks from seven study sites in Westchester, Putnam, Dutchess, and Rockland counties tested positive for simultaneous infection with all three agents by multiplex polymerase chain reaction assay.

The population densities of many organisms have changed dramatically in recent history. Increases in the population density of medically relevant organisms are of particular importance to public health as they are often correlated with the emergence of infectious diseases in human populations. Our aim is to delineate increases in density of a common disease vector in North America, the blacklegged tick, and to identify the environmental factors correlated with these population dynamics. Empirical data that capture the growth of a population are often necessary to identify environmental factors associated with these dynamics. We analyzed temporally- and spatially-structured field collected data in a geographical information systems framework to describe the population growth of blacklegged ticks (Ixodes scapularis) and to identify environmental and climatic factors correlated with these dynamics. The density of the ticks increased throughout the study’s temporal and spatial ranges. Tick density increases were positively correlated with mild temperatures, low precipitation, low forest cover, and high urbanization. Importantly, models that accounted for these environmental factors accurately forecast future tick densities across the region. Tick density increased annually along the south-to-north gradient. These trends parallel the increases in human incidences of diseases commonly vectored by I. scapularis. For example, I. scapularis densities are correlated with human Lyme disease incidence, albeit in a non-linear manner that disappears at low tick densities, potentially indicating that a threshold tick density is needed to support epidemiologically-relevant levels of the Lyme disease bacterium. Our results demonstrate a connection between the biogeography of this species and public health.