Ticks can wait for a host for years. Once they have crawled onto a person to draw blood, it can be dangerous: there are several hundred thousand cases of Lyme disease every year in Germany alone.
Malaria, dengue, West Nile fever – it is well known that mosquitoes often transmit dangerous diseases. Ticks are the second most common source of disease-causing microbes in the world – in Europe they even trump mosquitoes. In addition to tick-borne encephalitis (FSME), against which there is an effective vaccination, Lyme disease plays a particularly important role in Germany. Infections are occurring in the summer right now – and not just because a particularly large number of people are out in nature in short clothing. “This is mainly due to the tick density, which is strongly dependent on the temperature,” explains Hendrik Wilking from the Robert Koch Institute (RKI) in Berlin.
Lyme disease is the world’s most common tick-borne infection, as researchers write in the journal BMJ Global Health. In the overview of studies in recent years, they calculated that around one in seven people worldwide has antibodies against the infection in their blood, i.e. has been infected at least once. According to this, Central and Western Europe, along with East Asia, are among the most affected regions. Men over the age of 50 living in rural areas are particularly at risk.
In Germany, according to RKI data, antibodies can be found in about every sixth woman and almost a quarter of men in the 70 to 79 age group. Around 3 percent of 3- to 6-year-olds and 7 percent of 14- to 17-year-olds have been bitten at least once by a tick infected with Borrelia. Most human infections go unnoticed.
Lyme disease is caused by the bacterium Borrelia burgdorferi. It is named after the town of Lyme (Connecticut, USA), where cases of joint inflammation after tick bites were particularly common in the 1970s. The infection is distributed exclusively in the northern hemisphere – North America, Europe and Asia. In Germany, Lyme disease occurs more frequently from June to August.
In contrast to TBE, there is no vaccination against Lyme disease, but it can be treated well in the early stages with a two-week course of antibiotics. The most common symptom is the so-called wandering erythema, also called wandering redness – a ring-shaped reddening that usually occurs at the site of the tick bite. It forms days to weeks after the sting. If the infection is not treated with antibiotics at this early stage, the pathogen can also spread to other tissues and organs and cause irreparable long-term damage, for example to the nervous system, joints, heart or skin. Late forms can appear months or even years after the tick bite.
Ticks carrying Borrelia are found everywhere in Germany. The probability of catching such a specimen is not small: regionally, around a third of the animals can be infected with it. The distribution is small-scale: on one side of a forest or a fallow area, there can be a high risk of ticks for humans, while on the other side it is negligible. “It’s very, very variable,” says RKI expert Wilking. The weather also has an influence: Borrelia is more common in ticks in warm, humid months.
And there are borreliosis strongholds: in 2020, contract doctors in Saxony diagnosed Lyme disease 927 times per 100,000 insured persons. There were also many diseases in Thuringia (780), Brandenburg (707), Bavaria (637) and Saxony-Anhalt (615), according to figures from the Central Institute for Statutory Health Insurance Physician Care in Germany (Zi). A total of almost 360,000 Lyme diseases were diagnosed by the panel doctors nationwide. In addition, there is a large number of undetected infections. And: surviving Lyme disease does not protect against renewed infection.
While the TBE-causing viruses sit in the biting apparatus of the ticks, Borrelia are in the stomach – unlike the viruses, they are therefore only transmitted after a few hours of sucking time. According to the RKI, if a tick is removed within the first twelve hours after docking, the likelihood of it transmitting Borrelia to humans is significantly reduced. Borrelia is transmitted in an estimated three to six out of 100 tick bites in Germany. On average, about one in 100 stings leads to the stung person also contracting Lyme disease.
Lyme disease is primarily transmitted by the nation’s most common species of tick, the wood tick (Ixodes ricinus). It occurs practically everywhere where there are plants, including gardens and parks, according to the RKI. Contrary to what is often assumed, the eight-legged arachnids cannot fall from trees, nor can they jump. Rather, they sit on blades of grass, in bushes or on dead wood. If an animal or a person comes by, they are stripped off on contact and hold on. “Most ticks wait at a height of less than one meter, often only between 10 and 50 centimeters above the ground,” according to the RKI.
Areas such as the armpit, crook of the elbow, navel, genital area or the back of the knee are often chosen for the stitch. If the tick is not removed, it can suckle on its victim for more than a week and swell to 200 times its original weight.
It is still largely unclear how climate change will affect the tick population. In Germany, as a result of global warming, ticks are active earlier in the year and longer – but at the same time there are often long dry phases, which harm ticks. Dehydration is one of the biggest risks for ticks.
There are indications that the number of Lyme disease cases is increasing worldwide, explains the team led by Fukai Bao and Aihua Liu from Kunming Medical University in China in “BMJ Global Health”. Possible explanations included factors such as longer summers and warmer winters, fragmentation of natural areas, and more time spent outdoors with pets.
One consequence of the climate crisis can already be seen in Germany: the spread of the tick species Hyalomma rufipes, originally mainly native to the arid and semi-arid areas of Africa, Asia and southern Europe. The giant tick, which can be up to two centimeters in size, benefits from the milder winters. Unlike the common woodbuck, it hardly needs water, has eyes and actively pursues potential victims – probably up to a distance of 100 meters. Above all, however, it can transmit worse diseases than Lyme disease: Crimean-Congo fever and tick-borne typhus.
According to analyzes by the University of Hohenheim, almost every second Hyalomma tick found in Germany carries the tick spotted fever pathogen. A suspected case from 2019 has been recorded so far, in which a man from North Rhine-Westphalia probably contracted tick spotted fever after being bitten by a Hyalomma tick. A skin rash, which gave the disease its name, is typical of the infection that goes back to certain bacteria – called rickettsia. In contrast to classic typhus, tick-borne typhus is usually comparatively mild. A tick with the pathogen of Crimean-Congo fever, which can be associated with potentially fatal bleeding, has not yet been found in Germany.
The incidence of tick-borne infections in general has doubled in the past twelve years, as numerous studies have shown, the researchers write in “BMJ Global Health”. Northern Europe and Canada are particularly affected, probably due to climate change. Such diseases transmitted by living organisms, characterized by a specific geographic distribution and a high incidence and introduction of pathogens, posed a significant and growing public health problem.
Over 900 species of ticks are known worldwide, and 16 species are now native to Germany. 15 of them belong to the so-called shield ticks. The only leather tick in Germany is the pigeon tick (Argus reflexus), which has been reported to be able to survive without food for more than ten years.
