Climate connections is a collaboration between Grist and The Associated Press which explores how climate change is accelerating the spread of infectious diseases around the world and how mitigation efforts require a collective, global response. Learn more here.
In 2022, doctors recorded the first confirmed case of tick-borne encephalitis virus acquired in the UK.
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 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 often associated with a viral infection: fatigue, muscle pain, fever.
At first he appeared to be on the mend, but about a week later the man began to lose his coordination. An MRI revealed that he had developed encephalitis, or brain swelling. He had been infected with tick-borne encephalitis, or TBE, a life-threatening disease that experts say is spreading to new areas largely due to global warming.
Over the past 30 years, the UK has become roughly 1 degree Celsius warmer on average relative to the historical norm. Studies have shown that several 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 such as Lyme due to how quickly it jumped from country to country.
Gábor Földvári, an expert at the Center for Ecological Research in Hungary, said the effects of climate change on SBW are unmistakable.
“It’s a very common problem that was absent 20 or 30 years ago.”
How climate change is making us sick
Ticks cannot survive more than a few days in sub-zero temperatures, but they are able to persevere in very hot conditions as long as there is enough moisture in the environment. 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 extending the time 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 tick activity is high,” said Gerhard Dobler, a doctor who works at the German Center for Infection Research. “This can increase contact between infected ticks and humans and cause more disease.”
Since the virus was first discovered in the 1930s, it has mainly been found in Europe and parts of Asia, including Siberia and parts of northern China. The same type of tick carries the disease in these regions, but the virus subtype – of which there are several – varies by region. In places where the virus is endemic, tick bites are the main cause of encephalitis, although the virus can also be contracted by consuming raw milk from tick-infected cattle. TBE was not found in the United States, although a few Americans contracted the virus while traveling in Europe.
According to the World Health Organization, there are 10,000 and 12,000 cases of the disease in Europe and North Asia every year. The total number of cases worldwide is likely an underestimate, as case counts are unreliable in countries where people have little awareness of the disease and where local health departments are not required to report cases to the government. But experts say there has been a sharp increase since the 1990sespecially in countries where the disease was once rare.
“We are seeing an upward trend in human cases,” Dobler said, citing rising cases in Austria, Germany, Estonia, Latvia and other European countries.
TBE is not always fatal. On average, about 10% of infections progress to the severe form of the disease, which often requires hospitalization. Once severe symptoms develop, however, there is no cure for the disease. The death rate among those who develop severe symptoms ranges from 1% to 35%, depending on the virus subtype, 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 survivors of severe TBE have persistent neurological problems, such as insomnia and aggression. Many infected people are asymptomatic or only develop mild symptoms, Dobler said, so the true number of cases could be up to 10 times higher in some areas than reports estimate.
Although there are two TBE vaccines in circulation, vaccination coverage is low in regions where the virus is new. Neither vaccine covers the three most common subtypes, and one study 2020 called for the development of a new vaccine that offers better protection against the virus. In Austria, for example, the TBE vaccination rate is close to 85%, Dobler said, and yet the number of human cases continues to rise – a sign, he believes, of the influence of climate change on the disease.
(Next read: Mosquitoes move to higher altitudes, so does malaria)
In central and northern Europe, where over the past decade mean annual temperatures have been about 2 degrees Celsius above pre-industrial times, documented cases of the virus have increased in recent decades – evidence, some experts say, that rising global temperatures are conducive to more active ticks. The parasitic arachnids are also noted to move further north and higher in elevation as the once inhospitable terrain warms to their preferred temperature range. The northern regions of Russia are a prime example of where TBE-infected ticks have settled. And some previously tick-free mountains in Germany and Austria are reporting 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 highlights the dangers of tick-borne diseases. The British cyclist who was the country’s first case of the disease survived his battle with TBE, but the episode serves as a warning to the region that although the virus is still rare, it may not remain so for long.
