- Climate change and population growth have driven humans and ticks closer together in recent decades.
- As technology advances, scientists can identify new viruses and bacteria transmitted by ticks.
- Invasive species can also bring new diseases or affect the spread of local viruses.
There's always been a multitude of viruses and bacteria that can infect humans via tick bites, but until recently, scientists haven't had the resources to detect and identify every new tick-borne pathogen.
Improvements in technology and renewed scientific curiosity have led researchers like David Wang, a molecular microbiology professor at Washington University in St. Louis, to study new disease-causing agents.
Emerging diseases are not necessarily new to nature, Wang told Insider, but genome sequencing technology has enabled researchers to distinguish known viruses and bacteria from those not yet documented in scientific literature.
In the US, at least two new tick-borne viruses have been identified since 2009, after samples from infected people were sent to the Centers for Disease Control and Prevention for testing.
"There are probably many more viruses and other pathogens that are transmitted by ticks and other insect vectors," Wang told Insider. "There's lots of cases of people who have unexplained fevers, and probably many of these are caused by some kind of virus."
Two new tick-borne viruses discovered in the Midwest
Most tick-borne diseases present with similar, flu-like symptoms in humans: fevers, fatigue, and body aches.
Because the symptoms are so nonspecific, it's not obvious when an undocumented virus or bacterium is causing the infection, Wang said.
That was the case for Heartland virus, which was first identified in western Missouri in 2009. Two patients were treated for known tick-borne infections, but when they didn't get better, a doctor sent blood samples to a lab that classified the new virus.
Although Heartland virus disease is uncommon compared to other infections, scientists in the Midwest have been on the lookout for cases since it was discovered. There have been more than 50 cases of severe Heartland virus disease reported to the CDC — and testing for it revealed another virus circulating in the region.
Bourbon virus was first documented in 2014 in Bourbon County, Kansas, with similar symptoms to Heartland. Some people who have been infected later died, according to the CDC. Both viruses are transmitted by the lone star tick, which has expanded its reach from the southeast to the entire eastern half of the US in recent years.
Gonzalo Vazquez-Prokopec, an associate professor who studies vector-borne diseases at Emory University, told Insider that the tick has been spotted as far north as Canada as temperatures have warmed.
Climate change not only influences the geographic and seasonal distribution of ticks, but it also has driven humans deeper into tick territory. A 2016 report by the CDC found that as winters get milder and the human population grows, there's a greater likelihood of human-tick interactions in general, which could also contribute to reports of new diseases.
The Asian longhorned tick may carry a new virus that causes deadly fevers
A few new tick-borne viruses have been identified in Asia in recent years: the Songling virus in China, Alongsham virus in Malaysia, and the Yezo virus in Japan were all classified between 2019 and 2021.
Most of the insects that are known to spread these newfound viruses are not found in the US. However, one invasive species called the Asian longhorned tick has been spreading through North America since at least 2010.
The Asian longhorned tick was first documented in the US in 2017 on a farm in New Jersey, and has since been found in at least 16 more states, according to the CDC.
A study of Asian longhorned ticks collected in Virginia in 2021 found that the invasive pest can be infected with the Bourbon virus. It's possible that the tick co-fed on a host (such as a deer) that had been previously bitten by a local tick, according to findings published in the journal Pathogens.
"That's potentially concerning, because if that [Asian longhorned] tick spreads in the US, we don't know what the geographic limits are going to be," Wang said.
The species is also thought to be a host for the virus that causes severe fever with thrombocytopenia syndrome, a hemorrhagic fever that can be fatal. Hundreds of cases have been identified in China each year since the virus was described in 2011, but it hasn't been documented in the US to date.