The study, part of the international OH4Surveillance project, collected ticks between April and October 2025 across five counties: Harju, Tartu, Lääne-Viru, Pärnu and Saare. Researchers aimed to gather at least 1,000 ticks per region and analyze which pathogens they carried.

Most ticks were collected in Rakvere, Kuressaare and Tabasalu. Nearly all were identified as sheep ticks (Ixodes ricinus), with only one taiga tick (Ixodes persulcatus) found. Both species can transmit the bacteria that causes Lyme disease as well as the tick-borne encephalitis (TBE) virus.

Lyme disease-causing bacteria were most widespread in Lääne-Viru County, where nearly half of analyzed ticks tested positive. The lowest rate was recorded in Saare County, where about one in five ticks carried the bacteria.

Maria Vikentjeva, chief specialist at the Health Board's Department of Communicable Diseases, said the high prevalence of Lyme disease in Lääne-Viru County has been consistent for several years, suggesting a high proportion of infected ticks in the area.

"It may be linked to local environmental conditions," she explained. "For example, the Rakvere area is very suitable for ticks and has a lot of host animals that can carry pathogens."

Not just in the wilderness

Vikentjeva added that suburban and more enclosed green areas may further support transmission.

"A similar pattern has been seen in earlier studies: ticks in urban environments can carry more pathogens than in the forest because they repeatedly feed on the same animals," she noted.

The findings also reinforce that ticks are no longer just a rural or wilderness risk. They are increasingly common in city parks and on recreational trails.

"Ticks have become urbanized," Vikentjeva warned. "People need to be alert anywhere they spend time in nature, both in cities and outside them."

A similar trend was observed in a 2023 study by the National Institute for Health Development (TAI).

In Saaremaa, Lyme disease was less prevalent, but the TBE virus was detected more frequently than in other regions. Vikentjeva said the reason why isn't fully clear, and Saaremaa data covers only one year, compared with multiple years' worth from Lääne-Viru County.

The TBE virus occurs in localized clusters, she explained, noting that "if an infected animal enters an area, a new viral outbreak can occur."

Overall, the virus was found in 0.4 percent of analyzed ticks. Saaremaa County recorded the highest regional rate last year, while none was detected in Lääne-Viru County.

Vaccination and prevention both key

Occasionally, a single tick can carry multiple pathogens.

"In our data, this was under 1 percent of cases," Vikentjeva said. "We found only three ticks carrying both Lyme disease and TBE pathogens."

She added that warm, early spring weather has already activated ticks this year, warning that recent conditions have been very favorable for ticks.

While people can be vaccinated against TBE, no vaccine exists for Lyme disease, making prevention essential.

Always check for ticks after spending time outdoors, and remove any ticks promptly. The less time a tick remains attached, the lower the risk of infection.

Last year, Estonia recorded 3,276 cases of Lyme disease and 143 cases of TBE.

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