Climate Communications It is a collaboration between Grist and Associated Press It explores how climate change is accelerating the spread of infectious diseases around the world, and how mitigation efforts require a collective global response. Read more here.
In 2022, doctors recorded the first confirmed case of tick-borne encephalitis virus acquired in the UK.
It started with a bike trip.
A 50-year-old man was cycling in the mountains in the North Yorkshire Moors, a national park in England known for its vast expanses of woodland and purple heather. At some point in his journey, at least one tick pierced his skin. Five days later, the mountain biker developed symptoms commonly associated with a viral infection: fatigue, muscle aches, and fever.
At first, he seemed to be on the mend, but after about a week, the guy started to lose coordination. An MRI scan revealed that he had encephalitis, or swelling of the brain. He contracted tick-borne encephalitis, or TBE, a deadly disease that experts say is spreading to new areas due in large part to global warming.
Over the past 30 years, the UK has become about 1°C warmer on average than it has been by historical standards. Studies have shown that many tick-borne diseases are becoming more prevalent due to climate change. Public health officials are particularly concerned about TBE, which is more deadly than known tick diseases like Lyme because of the way it has jumped quickly from country to country.
Gabor Foldvari, an expert at the Center for Environmental Research in Hungary, said the effects of climate change on TBE are unequivocal.
“It’s a really common problem that has been absent for 20 or 30 years.”
How does climate change make us sick?
Ticks cannot survive more than two days in sub-zero temperatures, but they are able to persevere in very warm conditions as long as there is sufficient moisture in the environment. As the Earth’s average temperature warms and winters become milder, ticks are becoming active earlier in the year. Climate change affects ticks at every stage of their life cycle—eggs, 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 winter ticks is increasing, and in the spring there is a lot of tick activity,” said Gerhard Doppler, a doctor who works at the German Center for Infection Research. “This may increase contact between infected ticks and humans and cause more diseases.”
Since the virus was first detected in the 1930s, it has been found primarily in Europe and parts of Asia, including Siberia and the northern regions of China. The same type of tick carries the disease in these areas, but the subtype of the virus — 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 transmitted by ingesting raw milk from tick-infected cattle. TBE has not been found in the United States, although a few 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 low 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 increase since the 1990s, especially in countries where the disease was once uncommon.
“We’re seeing an increasing trend of human cases,” Doppler 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 the acute form of the disease, which often requires hospitalization. However, once severe symptoms appear, there is no cure for the disease. The fatality rate for those with 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 recorded in 2020 out of about 3,700 confirmed cases.
Up to half of survivors of severe tuberculosis develop chronic neurological problems, such as insomnia and aggressiveness. Many infected people are asymptomatic or only have mild symptoms, Doppler said, so the true number of cases could be up to 10 times higher in some areas than reported estimates.
While there are two TB vaccines in circulation, vaccine uptake is low in areas where the virus is new. Neither vaccine covers all three of the most common subtypes, and a 2020 study called for the development of a new vaccine that offers higher protection against the virus. In Austria, for example, the TBE vaccine rate is close to 85 percent, Doppler said, yet the number of human cases continues to trend upward — a sign, in his view, of the impact of climate change on the disease.
(Read next: Mosquitoes move to higher altitudes — and so does malaria.))
In central and northern Europe, where average annual temperatures in the past decade have been nearly 2 degrees Celsius higher than in pre-industrial times, documented cases of the virus have spiked in recent decades — some experts say rising global temperatures are driving increased tick activity. Parasitic spiders are also observed moving north and higher in elevation as the previously unfavorable terrain warms to their preferred temperature range. Northern parts of Russia are a prime example of where ticks infected with TBE have moved to. Some previously tick-free mountains in Germany and Austria have reported 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 casts the dangers of tick-borne disease into stark relief. A cyclist in the UK who was the first locally acquired case of the disease survived a bout of TBE, but the episode serves as a warning to the region: although the virus is still rare, it may not last that way for long.