Infection prevalence of two vector-borne zoonotic pathogens in the tick vectors across a gradient of reported human incidence rates

Background/Question/Methods

Lyme disease, caused by Borrelia burgdorferi, has been established in the northeastern US for the past 30 years while Anaplasmosis, caused by Anaplasma phagocytophilum, is an emerging disease in the northeastern US. Both bacteria circulate among wildlife host species and transmission among animal host species and humans occur through infected tick bites. In New York State (NYS), reported cases of Lyme disease and Anaplasmosis are highest in counties in the southeastern part of the state. In the Capital region of NYS, Lyme disease cases have increased rapidly over the past decade while areas north and west of the Capital (Outskirts region) appear to have much lower case reporting. A similar pattern is seen with Anaplasmosis. This suggests a radiating pattern for both diseases from Southeastern NY, north to the Capital and west to the Outskirts. We hypothesized that infection prevalence for both pathogens would track the pattern of disease spread, with highest infection prevalence in Southeastern NY, followed by the Capital region, and lowest in the Outskirts region. We tested questing nymphal ticks for the presence of B. burgdorferi and A. phagocytophilum using nested polymerase chain reaction.

Results/Conclusions

Of 657 ticks tested across 18 sites within these three regions, we found 117 ticks infected with B. burgdorferi (average of 17.8%). The Capital region had the highest infection prevalence (26.4%, 46/174), followed by Southeastern NY (19.3%, 44/228), and Outskirts (10.5%, 27/255) (χ² = 18.3, p = 0.0001). On average, A. phagocytophilum was present in 7.6% of ticks tested (29/381). A. phagocytophilum infection rates were 14.9% (17/114), 4.7% (6/128), and 4.3% (6/139) for Capital, Outskirts, and Southeastern NY respectively (χ² = 12.3, p = 0.002). Highest co-infection occurred in the Capital region (12.3%, 95% CL: 6.9% - 19.8%), followed by Southeastern NY (0.72%, 95% CL: 0.02% - 3.9%) with the Outskirts having no co-infection. High co-infection prevalence in the Capital region is primarily due to a single site having >80% infection for A. phagocytophilum. Low infection prevalence of both pathogens in the Outskirts supports our hypothesis. However, higher infection prevalence for B. burgdorferi and A. phagocytophilum in the Capital region compared to Southeastern NY did not support our hypothesis. These results suggest that tick infection prevalence may not always be a good indicator of human infection rates.