NEW YORK (AP) — In 2022, doctors recorded the first confirmed case of the UK-acquired tick-borne encephalitis virus.
It started with a bike ride.
A 50-year-old man was riding a mountain bike in the North Yorkshire Moors, a national park in England known for its vast expanses of woodland and purple heather. At some point during his journey, at least one blacklegged tick burrowed into his skin. Five days later, the mountain biker developed symptoms commonly associated with a viral infection: fatigue, muscle pain, fever.
At first, he seemed to be getting better, but about a week later, he began to lose his coordination. An MRI revealed that she had developed encephalitis, or inflammation of the brain. He had been infected with tick-borne encephalitis, or TBE, a life-threatening disease that experts say is spreading to new regions due in large part to global warming.
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EDITOR’S NOTE: This story is part of a collaboration between The Associated Press and Grist that explores the intersection of climate change and infectious disease.
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Over the past 30 years, the UK has gotten about 1 degree Celsius (1.8 degrees Fahrenheit) warmer on average compared to the historical norm. Studies have shown that various tick-borne diseases are becoming more prevalent due to climate change. Public health officials are particularly concerned about TBE, which is deadlier than better-known tick-borne diseases like Lyme, because of how quickly it has jumped from country to country.
Gábor Földvári, an expert at the Hungarian Ecological Research Center, said the effects of climate change on TBE are unmistakable.
“It is a very common problem that was absent 20 or 30 years ago,” he added.
Ticks cannot survive for more than a couple of days in freezing temperatures, but they can persevere in very hot conditions as long as there is sufficient moisture in the environment. As the Earth warms on average and winters get milder, ticks become active early in the year. Climate change affects ticks at every stage of their life cycle (egg, six-legged larva, eight-legged nymph, and adult) by extending the time that ticks actively feed on humans and animals. Even a fraction of a degree of global warming creates more opportunities for ticks to breed and spread disease.
“The number of overwintering ticks is increasing and in the spring there is a lot of tick activity,” said Gerhard Dobler, a doctor working at the German Center for Infection Research. “This can increase contact between infected ticks and humans and cause more illness.”
Since the virus was first discovered in the 1930s, it has been found mainly in Europe and parts of Asia, including Siberia and the northern regions of China. The same type of tick transmits the disease in these areas, but the virus subtype, of which there are several, varies by region. In places where the virus is endemic, tick bites are the leading cause of encephalitis, although the virus can also be contracted by consuming raw milk from tick-infected cattle. TBE has not been found in the United States, although some Americans have contracted the virus while traveling in Europe.
According to the World Health Organization, there are between 10,000 and 12,000 cases of the disease in Europe and North Asia each year. The total number of cases worldwide is likely to be an undercount, as case counts are unreliable in countries where the population has little awareness of the disease and local health departments are not required to report cases to the government. But experts say there has been a clear uptick since the 1990s, especially in countries where the disease used to be rare.
“We see a growing trend of human cases,” Dobler said, citing rising cases in Austria, Germany, Estonia, Latvia and other European countries.
TBE is not always life-threatening. On average, about 10 percent of infections develop into the severe form of the disease, which often requires hospitalization. However, once severe symptoms develop, there is no cure for the disease. The mortality rate among those who develop severe symptoms ranges from 1 to 35 percent, depending on the subtype of the virus, with the Far Eastern subtype being the deadliest. In Europe, for example, 16 deaths were recorded in 2020 out of approximately 3,700 confirmed cases.
Up to half of severe TBE survivors have persistent neurological problems, such as insomnia and aggression. Many infected people are asymptomatic or develop only mild symptoms, Dobler said, so the true number of cases could be up to 10 times higher in some regions than reports estimate.
Although there are two TBE vaccines in circulation, uptake of the vaccine is low in regions where the virus is new. No vaccine covers the three most prevalent subtypes, and a 2020 study called for the development of a new vaccine that offers greater protection against the virus. In Austria, for example, the TBE vaccination rate is close to 85 percent, Dobler said, and yet the number of human cases continues to rise, a sign, he said, of the influence of climate change on the disease.
In central and northern Europe, where average annual temperatures for the past decade have been about 2 degrees Celsius above pre-industrial times (3.6 degrees Fahrenheit), documented cases of the virus have increased in recent decades — evidence, some experts say, that rising global temperatures lead to more active ticks. Parasitic arachnids are also observed to move further north and to higher altitudes as the previously inhospitable terrain warms to their preferred temperature range. The northern parts of Russia are an excellent example of where TBE-infected ticks have moved north. Some previously tick-free mountains in Germany, Bavaria and Austria are reporting a 20-fold increase in cases in the last 10 years.
The growing shadow of the virus across Europe, Asia and now parts of the UK highlights the dangers of tick-borne diseases. The UK cyclist who was the first case of the country-acquired disease survived his bout with TBE, but the episode serves as a warning to the region: while the virus is still rare, it may not stay that way for long.
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