A highly invasive tick species is spreading across the United States, carrying with it a previously unknown but potentially crippling infection that is causing scientists concern. First identified in New Jersey in 2017, the Asian longhorned tick has been detected in at least 21 states. With rising temperatures extending tick season and broadening their habitat, scientists fear that these invasive parasites may catalyze the spread of ehrlichiosis, a bacterial infection that is already increasing in the U.S.Americans have linked ticks for decades with Lyme disease. But as climate change continues to reshape ecosystems and new invasive populations find a toehold, the risk landscape is evolving. Ehrlichiosis, previously rare, is now emerging as a major public health problem—and the experts say the risks are likely to increase.Indigenous to East Asia, the Asian longhorned tick is an opportunistic species. It came to other nations such as Australia, New Zealand, and various Pacific islands before it arrived unobtrusively in the United States. Scientists speculate that it most likely rode in on imported animals or livestock, but their point of entry is not known.Since its New Jersey discovery, the tick has consistently gained a foothold throughout the Northeast, Midwest, and South. Michigan had its first sighting during this summer, indicating how rapidly its range is spreading. Modeling research indicates that the majority of North America—southern Canada to almost every nook and cranny of the continental U.S.—poses acceptable habitat for this resilient species.What's more frightening about the longhorned tick is that it can reproduce without mating. One female will produce thousands of offspring, which enables populations to increase quickly. Unlike most indigenous animals, this tick is also able to co-feed on the same host with other ticks and retrieve and pass along pathogens very efficiently.What is Ehrlichiosis?The tick-transmitted disease ehrlichiosis, caused by the bacterium Ehrlichia chaffeensis, has risen in the shadows of recent years. A bite produces its effects usually one to two weeks later, in the form of fever, chills, muscle pain, headaches, and crushing fatigue.Though most of them recover with early treatment, the disease becomes life-threatening if neglected. In serious situations, it develops into respiratory failure, brain and nervous system injury, uncontrollable bleeding, or organ failure. Approximately 60% of patients need to be hospitalized, and about 1% die from the disease, as per the CDC.Confirmed cases went from only 200 in 2000 to over 2,000 in 2019. Experts, however, believe these figures are merely the tip of the iceberg. Rutgers University studies indicate that as much as 99% of infection may go undetected or unreported, masked by nonspecific flu-like symptoms doctors may attribute to other explanations.Is Climate Change Contributing To Tick-Borne Diseases?The biology of ticks is closely coupled with the weather. In the past, long, frigid winters kept tick populations low, both in terms of their numbers and in terms of the diseases they were spreading. Climate change is rewriting that season script, though."We are losing winter," says Connecticut's tick-testing program director, Goudarz Molaei. "And this tick, like others, will be active year-round."Warmer, shorter winters mean ticks come out of hibernation earlier and remain active for longer periods, raising the chances of human contact. The CDC just released word of an all-time high number of emergency room visits in July for tick bites—illustrating how rapidly the risks are increasing.The longhorned tick is not coming alone. A study in iScience reported seven non-native tick species arriving in Connecticut during 2019-2023. They rode in on livestock, pets, or travelers and are potential carriers of domestic and exotic diseases.Several of these ticks carried bacteria similar to those causing Rocky Mountain spotted fever, a lethal disease recently starting to appear in Connecticut even though it has been nonexistent there historically. Other invasive ticks are recognized carriers of Theileria orientalis, a cattle-decimating parasite not yet shown to pose danger to humans.Worldwide, ticks spread over 17% of infectious diseases in humans, the World Health Organization finds. In the United States, they are responsible for 77% of all vector-borne illnesses, case loads more than doubling over the past 15 years. The introduction of invasive ticks merely increases the stakes.Why Invasive Ticks Are Different?In contrast to indigenous species, invasive ticks present two such significant threats: they increase the range of current diseases and bring with them the potential to introduce entirely new ones. With their explosive ability to breed and thrive in a variety of climates, longhorned ticks are particularly well-suited to redefine the U.S. disease map.Their diet also distinguishes them. Though they usually prefer livestock, they will consume humans if given the chance. And when there is more than one tick species that feeds on a single animal, they can share pathogens with each other in what is called "co-feeding transmission," multiplying the chances of new disease dynamics.How Ticks Spread Diseases?Ticks are not born with pathogens. They pick them up after they feed on an infected animal like deer, rodents, or livestock. Ticks being infected have the ability to transmit the pathogen to the next host—occasionally human.America has about 50 native tick species that will bite humans and transmit diseases such as Lyme disease, babesiosis, anaplasmosis, and Rocky Mountain spotted fever. Invasive ticks, however, expand that menu of potential harm by introducing pathogens from elsewhere in the world into American habitats.Is There Any Way to Avoid Tick-Borne Disease?As scientists track the spread of invasive ticks, members of the public can act to minimize individual risk. Long sleeves and long pants in tick-infested areas, repellents that have DEET or permethrin, and careful tick checks after going outside are the most effective measures.Medical professionals also advise travelers to report recent travel when they go for medical attention for tick bites, as invasive ticks from other countries are often hard to distinguish from local ones. Prompt diagnosis and treatment of ehrlichiosis and other diseases can be the difference between life and death.The outbreak of invasive ticks in the United States illustrates how global change, fueled by climate change, trade, and travel, reconfigures local health threats. Within a decade, a species foreign to most Americans is now a possible cause of a major public health epidemic. Experts predict it's only a matter of time before other invasive ticks catch local or native pathogens, or even create new ones not previously observed in the US.