NEW YORK (AP) – In 2022, doctors registered the first confirmed case of tick-borne encephalitis virus in the United Kingdom.
It started with a bike ride.
A 50-year-old man was mountain biking in the North Yorkshire Moors, a national park in England known for its vast woodland and purple heather. At some point during his ride, at least one black-legged tick burrowed into his skin.
Five days later the mountain biker have developed symptoms often associated with a viral infection – fatigue, muscle pain, fever.
At first he appeared to be on the mend, but about a week later he began to lose coordination. An MRI scan revealed that he had developed encephalitis, or swelling of the brain. He had contracted tick-borne encephalitis (TBE), a potentially deadly disease that experts say is spreading to new regions primarily because of global warming.
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EDITOR’S NOTE: This story is part of a collaboration between The Associated Press and Grist exploring the intersection of climate change and infectious diseases.
In the last 30 years Britain has grown roughly 1 degree Celsius warmer (1.8 degrees Fahrenheit) on average compared to the historical norm. Studies have shown that several tick-borne diseases are becoming more common due to climate change. Public health officials are particularly concerned about TBE, which is more deadly than more well-known tick-borne diseases like Lyme disease because it has spread rapidly from country to country.
Gábor Földvári, an expert at the Center for Ecological Research in Hungary, said the impact of climate change on TBE is unmistakable.
“It’s a really common problem that wasn’t there 20 or 30 years ago,” he added.
Ticks cannot survive in sub-zero temperatures for more than a few days, but they can survive in very warm conditions as long as there is enough humidity in the area. As the earth warms on average and winters become milder, ticks become active earlier in the year. climate change affects ticks at every stage of their life cycle – Egg, six-legged larva, eight-legged nymph, and adult – by increasing the length of time ticks actively feed on humans and animals. Even a fraction of a degree of global warming gives ticks more opportunities to breed and spread disease.
“The number of hibernating ticks is increasing and tick activity is high in spring,” says Gerhard Dobler, doctor at the German Center for Infection Research. “This could increase contact between infected ticks and humans, causing more disease.”
Since the virus was first discovered in the 1930s, it has been found primarily in Europe and parts of Asia, including Siberia and the northern regions of China. The same tick species 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. However, the virus can also be transmitted by eating raw milk from tick-infected cattle. TBE was not found in the United States, although some Americans have contracted the virus while traveling in Europe.
According to the World Health Organization, there are in between 10,000 and 12,000 cases of the disease in Europe and North Asia every year. The total number of cases worldwide is likely to be underestimated as case numbers in countries where the population is poorly informed about the disease are unreliable and local health authorities are not required to report cases to the government. But experts say there has been clear upward trend since the 1990sespecially in countries where the disease used to be rare.
“We are seeing an increasing trend in 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 progress to a severe form of the disease, often requiring hospitalization. However, once severe symptoms appear, there is no cure for the disease. The mortality rate for people who develop severe symptoms ranges from 1 to 35 percent, depending on the virus subtype, with the Far Eastern subtype being the deadliest. In Europe for example 16 deaths were registered by around 3,700 confirmed cases in 2020.
Up to half of survivors of severe TBE suffer from persistent neurological problems such as insomnia and aggressiveness. Many infected people are asymptomatic or develop only mild symptoms, Dobler said, so the actual number of cases in some regions could be up to 10 times higher than reports estimate.
Although two TBE vaccines are in circulation, coverage in regions where the virus is new is low. None of the vaccines cover all three most common subtypes, and a Study 2020 called for the development of a new vaccine that offers greater protection against the virus. In Austria, for example, the TBE vaccination rate is almost 85 percent, said Dobler, and yet the number of human cases continues to rise – in his opinion a sign of the influence of climate change on the disease.
In central and northern Europe, where the average annual temperatures have been for the last decade about 2 degrees Celsius above pre-industrial times (3.6 degrees Fahrenheit), documented cases of the virus have been increasing over the past few decades – evidence, some experts say, that rising global temperatures are encouraging the emergence of more active ticks. The parasitic arachnids have also been found to migrate further north and to higher altitudes as formerly inhospitable terrain warms to their preferred temperature range. The northern parts of Russia are a prime example of where TBE-infected ticks have migrated north. Some hitherto tick-free mountains in Germany, Bavaria, and Austria report a 20-fold increase in cases over the past 10 years.
The growing shadow of the virus across Europe, Asia and now parts of the UK makes the dangers of a tick-borne disease all the clearer. The British cyclist, who was the first locally acquired case, survived his TBE but the episode is a warning to the region: while the virus is still rare, it may not remain so for long.
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