Lyme disease is the most frequently reported vector-borne illness in the United States, and Pennsylvania consistently reports the highest number of annual cases nationwide. This disease is caused by the bacterium Borrelia burgdorferi, which is transmitted to humans through the bite of an infected tick. Understanding the risk begins with knowing which tick species is responsible for transmission and the likelihood that it carries the pathogen in this high-incidence state.
Identifying the Primary Vector
The primary carrier of Lyme disease in Pennsylvania is the Blacklegged Tick, also commonly referred to as the Deer Tick (Ixodes scapularis). While all active life stages—larva, nymph, and adult—will feed on humans, the nymphal stage is responsible for the majority of human infections.
The nymphal ticks are particularly hazardous because their size makes them extremely difficult to detect. They are typically no larger than a poppy seed, which allows them to feed for the necessary time to transmit the bacteria without being noticed. Unfed adult ticks are slightly larger, about the size of a sesame seed.
State-Level Prevalence Data
The percentage of Blacklegged Ticks carrying the Borrelia burgdorferi bacterium across Pennsylvania reflects a significant and uneven risk. Statewide surveillance studies consistently demonstrate a high infection rate in the tick population. The average infection rate for nymphal Blacklegged Ticks is approximately one in four, or 24.5%.
This percentage represents a state average and can fluctuate based on the specific year and region of collection. Adult ticks, which are active in the cooler months, often exhibit a higher rate of infection, with estimates reaching up to 50% in some areas. This is because adults have had more opportunities to feed on infected hosts during their two-year life cycle.
Geographic Risk Variation in Pennsylvania
While Blacklegged Ticks infected with the Lyme disease bacterium have been documented in all 67 Pennsylvania counties, the concentration of infected ticks varies significantly across the state. Rural and heavily forested areas generally report a higher incidence of human cases compared to densely populated urban centers. The northeastern counties, in particular, have frequently shown the highest overall incidence rates.
This geographic variation is strongly linked to ecological factors, specifically land use and the density of host animals. Forest fragmentation can inadvertently increase the risk of infection. Smaller, isolated forest patches tend to reduce the diversity of tick hosts, leading ticks to feed more frequently on white-footed mice. These mice are highly competent reservoirs for the Lyme bacterium, meaning they are very effective at infecting the ticks that feed on them. This concentration of feeding on infected hosts contributes to the elevated prevalence of B. burgdorferi in ticks in these specific landscapes.
Transmission Timelines and Immediate Action
The transmission of Lyme disease from an infected Blacklegged Tick to a human host is not immediate; it requires a critical period of attachment. The bacteria must migrate from the tick’s midgut to its salivary glands before transmission can occur, a process that typically takes between 36 and 48 hours of continuous feeding. Removing a tick promptly, especially within 24 hours of attachment, greatly reduces the risk of infection.
If an attached tick is discovered, it should be removed immediately using fine-tipped tweezers. The tweezers should grasp the tick as close to the skin’s surface as possible, and a steady, upward pull should be applied without twisting or jerking. After removal, the bite area should be thoroughly cleaned with soap and water or rubbing alcohol.
Individuals who have been bitten by a Blacklegged Tick should seek medical consultation to discuss the option of post-exposure prophylaxis (PEP). Pennsylvania is considered a high-incidence area, making all counties eligible for this consideration. A single dose of the antibiotic doxycycline may be offered if the tick was attached for at least 36 hours, and the removal occurred within the last 72 hours.