How many people get sick after a tick bite? - briefly
Approximately 1 % of tick encounters result in a clinically recognized infection, with Lyme disease accounting for about 30,000 cases per year in the United States alone. Other tick‑borne illnesses—including anaplasmosis, babesiosis and Rocky Mountain spotted fever—contribute several thousand additional cases annually.
How many people get sick after a tick bite? - in detail
Tick bites are a primary route for transmission of several bacterial, viral, and protozoan pathogens. Surveillance data from the United States, Europe, and Asia provide a quantitative picture of how often exposure results in illness.
In the United States, the Centers for Disease Control and Prevention (CDC) estimate roughly 300,000 cases of Lyme disease each year. Studies of Ixodes scapularis and Ixodes pacificus show that about 1 %–2 % of attached ticks transmit the spirochete Borrelia burgdorferi to humans. Consequently, for every 100 bites by these species, approximately one to two individuals develop Lyme disease.
Rocky Mountain spotted fever, transmitted by Dermacentor species, accounts for about 5,000–6,000 reported cases annually. The infection rate per bite is lower, estimated at 0.5 %–1 % in endemic regions of the southeastern United States.
Other tick‑borne illnesses have even smaller conversion ratios:
- Anaplasmosis (caused by Anaplasma phagocytophilum): ~0.2 % of bites in the Upper Midwest and Northeast lead to disease.
- Ehrlichiosis (caused by Ehrlichia chaffeensis): ~0.1 % of bites in the southeastern and south‑central United States result in infection.
- Babesiosis (Babesia microti): ~0.2 %–0.3 % of bites in the Northeast and Upper Midwest cause clinical disease.
- Tick‑borne encephalitis (TBE) in Europe and Asia: incidence ranges from 0.1 % to 0.5 % of bites, varying by country and vaccination coverage.
Globally, the World Health Organization reports an estimated 1 million cases of tick‑borne diseases each year, with Lyme disease comprising the majority. Regional differences reflect tick species distribution, climate, and human behavior. For example, in Central and Eastern Europe, TBE incidence can reach 5 cases per 100 ,000 population, translating to roughly 2 %–3 % of bites in high‑risk zones.
Risk factors influencing the likelihood of illness include:
- Tick attachment duration: pathogens typically require ≥24 hours of feeding before transmission.
- Life stage of the tick: nymphs are responsible for >80 % of Lyme disease cases because of their small size and higher infection prevalence.
- Geographic exposure: endemic areas show higher bite‑to‑illness conversion rates.
- Host immunity: immunocompromised individuals may develop disease from lower inoculum loads.
Overall, while the absolute number of bites far exceeds the number of resulting infections, the proportion of bites that lead to clinical disease generally stays below 2 % for the most common tick‑borne pathogens, with Lyme disease representing the highest individual risk among them.