Vector-Borne and Zoonotic Diseases

In the northeastern United States, Anaplasma phagocytophilum, the agent of human granulocytic ehrlichiosis (HGE), is transmitted by the tick vector Ixodes scapularis. The white-footed mousePeromyscus leucopus is a competent reservoir for this agent, but the reservoir competence of non-Peromyscus hosts of I. scapularis has not been studied. Here, we report data confirming reservoir competence of medium-sized mammals for A. phagocytophilum. Raccoons, Virginia opossums, gray squirrels, and striped skunks were live-trapped in June-August of 1998-1999 at two locations in Connecticut. Captured animals were kept for several days at the laboratory in wire-mesh cages over water to allow naturally attached ticks to drop off. Samples of blood and serum were taken from each animal prior to its release at the site of capture. Engorged ticks collected from each animal were allowed to molt. Resulting I. scapularis nymphs and adults were tested for the presence of A. phagocytophilum DNA by polymerase chain reaction, as were the blood samples from the animals. A. phagocytophilum DNA was detected in the blood of >10% of the raccoons tested. Raccoons, opossums, squirrels, and skunks produced adult I. scapularis infected with the agent of HGE. Prevalence of infection was the highest in adult ticks fed as nymphs upon raccoons (23%) and the lowest in those fed upon skunks and opossums (5-7%). The agent was present in nymphal I. scapularis fed as larvae upon raccoons and squirrels, but not in ticks fed upon skunks or opossums. We also tested the ability of I. scapularis to transmit A. phagocytophilum to laboratory-reared white-footed mice after acquiring it from medium-sized mammals. Ticks that acquired the agent from raccoons and squirrels successfully transmitted it to mice. Thus, raccoons and gray squirrels are reservoir-competent for the agent of HGE - they become naturally infected, and are capable of transmitting the infection to feeding ticks.

To compare the incidence of Campylobacter jejuni enteritis in high- and low-poultry-density counties in Michigan between the years 1992 and 1999, an ecological study was conducted in the state of Michigan. A log-linear model was used to compare yearly, seasonal, age, and gender-specific incidence rates between county groupings. Counties with a high poultry density had a higher overall incidence of C. jejuni enteritis, particularly among children and young adults, compared with counties with low poultry density. The findings suggest that living in high-poultry-density counties is associated with higher odds for C. jejuni enteritis. This may be due to occupational exposure among poultry workers in these counties, but the findings in children suggest that indirect or environmental exposures may also play a role. Future studies should be conducted to investigate these issues.

The 1999 New York epidemic of human West Nile virus (WN) encephalitis and meningitis was preceded by a crow die-off also caused by WN infection. As one component of the subsequently developed national surveillance system, crow mortality data were collected to detect WN activity before humans might become infected. However, predicting areas at risk for human WN disease likely requires assessment of multiple factors, including the intensity and timing of crow epizootics. To identify early season measures of WN activity in crows associated with subsequent WN disease in humans, county-level crow mortality data from seven northeastern states were analyzed. A predictive model was developed based on analysis of 2000 surveillance data and then assessed for 2001. To characterize the intensity of early season WN activity in crows, 15 variables were constructed from surveillance data of 52 counties that tested at least four crows during the early season (defined as June 17-July 28, 2000). County values for each variable were dichotomized at the 75th percentile into "high" and "low" activity. Multivariate analysis indicated that "high" early season activity of two variables - density of reported dead crow sightings (reported dead crows/area) and [(WN-infected crows/tested crows) × (human population)] - were associated with report of at least one human WN disease case (for each variable: adjusted odds ratio, 6.9; 95% confidence interval, 1.2-40.6). An assessment of this model using 2001 surveillance data from 61 counties yielded similar findings. With emphasis on early season WN activity, crow surveillance may allow timely targeting of interventions to protect the public health.

The distribution of human risk for West Nile virus was determined by spatial analysis of the initial case distribution for the New York City area in 1999 using remote sensing and geographic information system technologies. Cluster analysis revealed the presence of a statistically significant grouping of cases, which also indicates the area of probable virus introduction. Within the cluster, habitat suitability for potentially infective adult mosquitoes was measured by the amount of vegetation cover using satellite imagery. Logistic regression analysis revealed satellite-derived vegetation abundance to be significantly and positively associated with the presence of human cases. The logistic model was used to estimate the spatial distribution of human risk for West Nile virus throughout New York City. Accuracy of the resulting risk map was cross-validated using virus-positive mosquito sample sites. These new epidemiological methods aid in rapid entry point identification and spatial prediction of human risk of infection for introduced vector-borne pathogens.

This work was designed to study the infection process of Rickettsia conorii in the salivary glands of experimentally infected Rhipicephalus sanguineus ticks. One hundred six uninfected engorged nymphs were intracelomically inoculated with ∼2 × 10³ plaque-forming units of a rickettsial suspension. After the molt, unfed and fed adults were dissected, and the salivary glands were extracted and processed for transmission electron microscopy observation. Three different uninfected control groups were used for (1) evaluating the impact of the inoculation procedure, (2) establishing the feeding period of infected ticks, and (3) ultrastructural characterization of the salivary glands. Overall, 75.5% (80 of 106) of the nymphs inoculated with rickettsiae died during the molt or soon after hatching into adult instars; 50% (12 of 24) of the remaining infected adults showed severe malformations compromising their viability. In apparently healthy specimens, time of engorgement was longer. The contrast with the negative control groups was statistically significant, suggesting that R. conorii exerts a strong negative effect on the vector ticks. The ultrastructural study showed that in the salivary glands of infected ticks, rickettsial growth occurs preferentially in central, peripheral, and interstitial acini cells.

Coxiella burnetii, the etiologic agent of Q fever, is a worldwide zoonotic pathogen. Although Q fever is present in the United States, little is known about its current incidence or geographic distribution in either humans or animals. Published reports of national disease surveillance, individual cases, outbreak investigations, and serologic surveys were reviewed to better characterize Q fever epidemiology in the United States. In national disease surveillance reports for 1948-1986, 1,396 human cases were reported from almost every state. Among published individual case reports and outbreak investigations, occupational exposures (research facilities, farm environments, slaughterhouses) were commonly reported, and sheep were most frequently implicated as a possible source of infection. In studies conducted on specific groups, livestock handlers had a significantly higher prevalence of antibodies to C. burnetii than did persons with no known risk. Animal studies showed wide variation in seroprevalence, with goats having a significantly higher average seroprevalence (41.6%) than sheep (16.5%) or cattle (3.4%). Evidence of antibody to C. burnetii was reported among various wild-animal species, including coyotes, foxes, rodents, skunks, raccoons, rabbits, deer, and birds. This literature review suggests that C. burnetii is enzootic among ruminants and wild animals throughout much of the United States and that there is widespread human exposure to this pathogen. Sheep and goats appear to be a more important risk for human infection in the United States than cattle or wild animals, and research studies examining the natural history and transmission risk of Q fever in sheep and goats in this country should be encouraged.

To evaluate the vector competence of Culex tarsalis Coquillett for West Nile virus (WN), females reared from larvae collected in Huntington Beach, Orange County, CA, were fed on 2-3-day-old chickens previously inoculated with a New York strain (Crow 397-99) of WN. The Cx. tarsalis mosquitoes were efficient laboratory vectors of WN, with estimated transmission rates of 81% and 91% for mosquitoes that ingested 10^6.5 or 10^7.3 plaque-forming units of WN/mL of blood, respectively. Based on efficiency of viral transmission and the role of this species in the transmission of the closely related St. Louis encephalitis virus, Cx. tarsalis should be considered a potentially important vector of WN in the western United States.

This report describes the isolation of Moellerella wisconsensis from the oral secretions of a wild raccoon in Northern California. Human enteric disease has previously been associated with this organism. This represents the first isolation of this rare enterobacterial species from a non-captive animal and only the third from a non-human source.