Study: Number of ticks, tick-borne pathogens on rise in Estonia

Tick (Ixodes ricinus).
Tick (Ixodes ricinus). Source: NOVA/ERR

The number of ticks as well as the diversity and spread of ticke-borne pathogens in Estonia have all increased significantly over the past ten years, it appears from the results of a recent study conducted by the National Institute of Health Development (TAI).

The number of ticks as well as the diversity and spread of ticke-borne pathogens in Estonia have all increased significantly over the past ten years, it appears from the results of a recent study completed by the National Institute of Health Development (TAI).

With help from the public, TAI in summer 2020 mapped out the most tick-prone parts of the country. During a campaign lasting from May 20 through August 31 that year, TAI received more than 14,500 reports regarding a total of 23,064 tick finds across the country.

A surprising amount — 35 percent — of all mapped out tick finds were in people's backyards, nearly the same amount as in forests, brush and unmaintained roadsides combined.

According to Julia Geller, director of TAI's Virology and Immunology Department, this challenges the previous widespread belief that ticks are only a concern in wooded areas, brush or other unmaintained areas.

Of 6,437 physical ticks to be sent in to TAI, 3,559 were included in the institute's pathogen study sample and tested for the presence a wider array of tick-borne pathogens, including those that cause neoehrlichiosis, human granulocytic anaplasmosis (HGA), tick-borne relapsing fever (TBRF) and tick-borne rickettsioses in addition to the more well-known tick-borne encephalitis (TBE) and Lyme disease, or Lyme borreliosis.

"In order to ensure comparable study results, we compiled random samples so that more or less equal numbers of ticks would be included from each county," Geller explained. "We took samples based on four criteria: location, condition, developmental stage and sex of the tick."

The presence of at least one pathogen was detected in 62 percent of all tested ticks, with the highest percentage of infected ticks found in Tartu County (77 percent) and the lowest in Ida-Viru County (39 percent).

One fifth of tested ticks were carrying several pathogens. The highest number of ticks carrying several pathogens were found in Tartu, Harju, Viljandi and Lääne-Viru counties.

Found in 35 percent of all tested ticks, bacteria from the Rickettsia genus proved the most common tick-borne pathogen in Estonia. Rickettsia bacteria were detected in more than half of ticks sent in from Valga and Tartu counties.

Tick-borne rickettsioses are caused by bacteria belonging to the spotted fever group (SFG) of the genus Rickettsia, which also includes Rocky Mountain spotted fever (RMSF), a disease most widespread in the contiguous U.S.

While many people infected with tick-borne rickettsiae don't experience symptoms or fall ill, others may experience flu-like symptoms including fever, muscle aches, headaches and respiratory issues; in more serious cases, meningitis can develop. Tick-borne rickettsioses are typically treated with antibiotics.

Lyme disease-causing bacteria increasingly prevalent

A total of 28 percent of ticks were found to carry the Borrelia bacterium, which causes Lyme disease. Borrelia was most widespread in ticks found in Lääne County, detected in 41 percent of those tested.

Compared with previous study results, the presence of Borrelia bacteria in ticks has increased throughout Estonia over the past 15 years, in some places by as much as two- to threefold.

Following infection, it can take up to two to three weeks for initial symptoms of Lyme disease to appear. The signature symptom is a red spot or bullseye that forms around the site of an infected bite, but may not always be present. Other symptoms may include fever, headache, general weakness, joint and muscle aches and weight loss. Lyme disease can typically be treated with a course of antibiotics, but in more complicated cases may require hospitalization.

Compared with previous study results, the prevalence of the pathogen behind human neoehrlichiosis is also on the rise, and its distribution has spread as well; it can be found in all 15 Estonian counties, and the neoehrlichiosis pathogen was identified in 19 percent of all analyzed ticks. It was most prevalent in Viljandi County.

Most cases of infection with neoehrlichiosis pathogens are asymptomatic, however symptoms can include headache, nausea and vomiting, muscle aches and stiffness in the neck, chills, night sweats and repeated bouts of high fever, as well as localized or migrating joint or muscle pain. A skin rash resembling erythema nodosum or rosacea may also occur. Neoehrlichiosis is treated with antibiotics.

Vaccine exists for tick-borne encephalitis

Of all of the pathogens tested for, the tick-borne encephalitis virus (TBEV) was the least prevalent, detected in just two ticks tested as part of the study. One of the two TBEV-infected ticks was discovered in a wooded area in Võru County; the other in a backyard in Hiiumaa.

The low prevalence in tested specimens does not indicate a decrease in prevalence of the virus in nature, but rather its disproportionate and scattered distribution pattern, which in turn depends on the existence and prevalence of any outbreaks.

"Once more cases of infection with [the TBEV] have started popping up in a given area, others living in the same area must be vigilant as well," Geller warned.

Tick-borne encephalitis (TBE) can severely impact the central nervous system and causes paresis, paralysis as well as memory impairment and cognitive dysfunction. More than 70 percent of patients with TBE require hospitalization, and the disease often involves long-term neurological complications. As there is no specific cure, treatment for TBE is aimed at alleviating symptoms.

TBE is the only ticke-borne disease that can be prevented via vaccination. In Estonia, children as young as one year of age can be vaccinated against TBE.


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Editor: Aili Vahtla

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