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.

Ixodes scapularis, the blacklegged tick, is capable of transmitting the pathogens that cause Lyme disease (Borrelia burgdorferi), babesiosis (Babesia microti), anaplasmosis (Anaplasma phagocytophilum), and to a lesser extent Powassan encephalitis (deer tick virus [DTV]). These pathogens represent significant public health problems, but little is known about the occurrence and co-infection prevalence of these pathogens in I. scapularis. Here, we used standard PCR and pathogen-specific primers to estimate the prevalence of infection of A. phagocytophilium, B. burgdorferi, B. microti, and Ehrlichia chaffeensis in questing nymph and adult I. scapularis collected from sites in Putnam and Dutchess counties in southern New York in 2011. To detect DTV infection, cell cultures were observed for the presence of cytopathic effects and positive results were confirmed via real time RT-PCR. In 466 individually sampled adult ticks, B. burgdorferi had the highest prevalence of infection (55%) followed by A. phagocytophilum (18.2%), DTV (3.4%), B. microti (3.2%), and E. chaffeensis (1.5%). Infection with two pathogens occurred in 13.3% of ticks, and 10 ticks were infected with three combinations of three pathogens. These results provide an estimate of the rate of co-infection, which then can help inform the epidemiological risk of contracting multiple zoonotic tick-borne pathogens within the Hudson Valley region of New York State.

Numerous diseases of wildlife have recently emerged due to trade and travel. However, the impact of disease on wild animal populations has been notoriously difficult to detect and demonstrate, due to problems of attribution and the rapid disappearance of bodies after death. Determining the magnitude of avian mortality from West Nile virus (WNV) is emblematic of these challenges. Although correlational analyses may show population declines coincident with the arrival of the virus, strong inference of WNV as a cause of mortality or a population decline requires additional evidence. We show how integrating field data on mosquito feeding patterns, avian abundance, and seroprevalence can be used to predict relative mortality from vector-borne pathogens. We illustrate the method with a case study on WNV in three species of small songbirds, tufted titmouse (Baeolophus bicolor), Carolina wrens (Thryothorus ludovicianus), and northern cardinals (Cardinalis cardinalis). We then determined mortality, infectiousness, and behavioral response of wrens and titmouse following infection with WNV in laboratory experiments and compared them to a previous study on WNV mortality in cardinals. In agreement with predictions, we found titmouse had the highest mortality from WNV infection, with 100% of eleven birds perishing within seven days after infection. Mortality in wrens was significantly lower at 27% (3/11), but still substantial. Viremia profiles indicated that both species were highly infectious for WNV and could play roles in WNV amplification. These findings suggest that WNV may be killing many small-bodied birds, despite the absence of large numbers of dead birds testing positive for WNV. More broadly, they illustrate a framework for predicting relative mortality in hosts from vector-borne disease.