Primary Tickborne Disease Concerns in Pennsylvania
The primary tickborne diseases of concern in Pennsylvania are Lyme disease (caused by Borrelia burgdorferi), followed by anaplasmosis (caused by Anaplasma phagocytophilum), babesiosis (caused by Babesia microti), and emerging concerns for Powassan/deer tick virus. All are transmitted by the blacklegged tick (Ixodes scapularis), which is endemic throughout all 67 Pennsylvania counties 1, 2.
Disease-Specific Priorities
Lyme Disease - The Dominant Threat
- Highest prevalence: Pennsylvania consistently reports the highest number of Lyme disease cases in the United States 3
- Infection rates in ticks: 45-47% of adult blacklegged ticks carry B. burgdorferi statewide 4, 3
- Geographic distribution: Confirmed in all 67 Pennsylvania counties, though infection rates are lower in western regions 3
- Clinical impact: Erythema migrans is the most common manifestation, with extracutaneous disease (joints, nervous system, heart) occurring less frequently than in earlier decades 1
Anaplasmosis - Rapidly Emerging
- Geographic expansion: Pennsylvania is among the highest-incidence states in the northeastern U.S., with cases expanding from eastern into central and western Pennsylvania over the past decade 2, 5
- Infection rates in ticks: 1.7-12% of blacklegged ticks carry A. phagocytophilum, with variant-ha being more prevalent than variant-v1 4, 6
- Mortality risk: Case-fatality rate <1% overall, but 7% of hospitalized patients require ICU admission 2
- Clinical presentation: Fever, headache, myalgia with characteristic leukopenia and thrombocytopenia; rash is rare, distinguishing it from other tickborne diseases 2
Babesiosis - Moderate Concern
- Infection rates: 3-16% of blacklegged ticks carry B. microti depending on geographic location within Pennsylvania 4, 3
- Coinfection significance: Frequently occurs with Lyme disease (2-5% coinfection rate), which can complicate treatment since beta-lactam antibiotics used for Lyme disease won't treat babesiosis 1, 2
Powassan/Deer Tick Virus - Emerging Threat
- Low but present: 0.05-2% infection rate in adult blacklegged ticks 4, 7
- Clinical significance: Though rare, this virus causes severe neurological disease with high morbidity and mortality when infection occurs
- No specific treatment: Unlike bacterial tickborne diseases, no effective antimicrobial therapy exists
Critical Coinfection Patterns
Coinfections occur at rates significantly higher than expected by chance alone 4, 6:
- B. burgdorferi + B. microti: 2-5% of ticks
- B. burgdorferi + A. phagocytophilum: 1.5-3% of ticks
Clinical Implications of Coinfection
- If Lyme disease treated with beta-lactam antibiotics (e.g., amoxicillin, ceftriaxone) in a patient with unrecognized anaplasmosis coinfection, anaplasmosis symptoms will persist 2
- Leukopenia or thrombocytopenia in a patient with Lyme disease should raise immediate suspicion for A. phagocytophilum coinfection 2
- Delayed response to doxycycline in suspected anaplasmosis should prompt evaluation for babesiosis coinfection 2
Seasonal and Geographic Risk Patterns
Peak transmission occurs June-November, with bimodal peaks in June-July (nymphal stage) and October (adult stage emergence) 2. The blacklegged tick is the sole vector for all four major pathogens in Pennsylvania, making tick avoidance the cornerstone of prevention 1, 2.
Emerging Considerations
The invasive Asian longhorned tick (Haemaphysalis longicornis) has been detected in Pennsylvania with evidence of B. burgdorferi and B. microti infection, though its role in human disease transmission remains under investigation 8. This represents a potential future concern but is not currently a primary threat compared to I. scapularis-transmitted diseases.