In 2022, doctors recorded Britain’s first confirmed case of tick-borne encephalitis virus.
A 50-year-old man was cycling in the North Yorkshire Moors National Park in England, famous for its vast expanses of woodland and purple heather. At one point during his run, at least one blacklegged tick attached itself to his skin. Five days later, the cyclist presented the symptoms commonly associated with a viral infection: fatigue, muscle pain and fever.
At first he seemed to recover, but a week or so later he began to lose motor coordination. An MRI revealed that she had developed encephalitis, or inflammation of the brain. It was tick-borne encephalitis (TDE), a life-threatening disease that experts say is spreading to other regions largely due to global warming.
Over the past 30 years, the average temperature in Britain has risen by around 1 degree Celsius (1.8 °Fahrenheit) compared to historical normal. Several studies have shown that various tick-borne diseases have become more common due to climate change. Public health authorities are particularly concerned about GTD because of its rapid spread from country to country and because its mortality is higher than that of other tick-borne diseases, such as Lyme disease.
Gábor Földvári, an expert at the Center for Ecological Research in Hungary, said the impact of climate change on TSG is undeniable.
“This is a very common problem that was non-existent 20 or 30 years ago,” he added.
Ticks can’t survive more than a couple of days in freezing temperatures, but they can thrive in fairly warm conditions if there is enough moisture in the environment. As the planet’s average temperatures rise and winters become less cold, ticks become active months earlier than usual. Climate change affects each stage of the tick life cycle: egg, six-legged larva, eight-legged nymph and adult, by lengthening the period in which they can feed on people and animals. Even a fraction of a degree global warming creates more opportunities for ticks to breed and carry disease.
“The number of ticks that survive the winter is increasing, and in the spring there is a lot of tick activity,” said Gerhard Dobler, a doctor working at the German Research Center for Infectious Diseases. “This could increase contact between infected ticks and humans, and cause more illness.”
Since its discovery in the 1930s, the virus has been detected mainly in Europe and regions of Asia, such as Siberia and northern China. The same type of tick carries the disease in those areas, but the subtype of the virus varies by region. In places where the virus is endemic, tick bites are the main cause of encephalitis cases, although it is also possible to become infected by consuming unpasteurized milk from tick-infected cattle. TSG has not been found in the United States, although some Americans have been infected on visits to Europe.
According to the World Health Organization, between 10,000 and 12,000 cases of the disease occur each year in Europe and North Asia. The total number of cases worldwide may be lower than the true number because counts are unreliable in countries where the population has little knowledge of the disease and health departments are not required to report cases to the government. But experts say there has been a clear rise since the 1990s, especially in countries where the disease was rare.
“We see an upward trend in human cases,” said Dobler, noting increases in Austria, Germany, Estonia, Latvia and other European countries.
TSG is not always life-threatening. On average, about 10% of infections lead to a severe form of the disease that often requires hospitalization. But once severe symptoms occur, the disease can no longer be cured. The mortality rate among those who develop severe symptoms ranges from 1 to 35%, depending on the virus subtype, the most lethal being that of the Far East. In Europe, for example, 16 deaths were recorded in 2020 out of around 3,700 confirmed cases.
Up to half of severe TGD survivors have persistent neurological problems, such as insomnia and aggression. Many infected are asymptomatic or have mild symptoms, Dobler said, so the actual number of cases in some regions could be 10 times higher than the projections included in the reports.
Although there are two vaccines against TSG, their application is low in regions where the virus is new. None of the vaccines cover the three most frequent subtypes, and a 2020 study proposed the development of a new vaccine that would provide greater protection against the virus. In Austria, for example, the vaccination rate against TGD is almost 85%, according to Dobler, and even so the number of infected people maintains an upward trend, which in his opinion is an indication of the influence of climate change on the disease.
In central and northern Europe, where average annual temperatures have risen by about 2°C (3.6°F) in the past decade relative to pre-industrial times, documented cases of the virus have been on the rise in recent decades, which according to some experts is evidence that the increase in temperatures in the world has favored a greater activity of ticks. It has also been noted that these parasitic arachnids have been moving further north and to higher altitudes as temperatures rise to favorable levels in previously inhospitable territories. The northern regions of Russia are an excellent example of how far encephalopathy-transmitting ticks have spread. In mountainous areas of Germany, such as Bavaria, and Austria, where there were no ticks before, the number of reported cases has multiplied by 20 in the last 10 years.
The widening specter that the virus looms over Europe, Asia, and now parts of Britain, highlights the dangers of tick-borne disease. The British cyclist, who was the first case of infection within its borders, survived encephalitis, but the episode serves as a warning to the region: while the virus remains rare, it may not be for much longer.