Ticks Are Spreading in the US—and Taking New Diseases With Them

“Tick-borne diseases are the most important vector-borne disease in the continental US when you tally the amount of disease transmitted to humans,” says Gonzalo Vazquez-Prokopec, a disease ecologist and associate professor at the Emory College of Arts and Sciences. “And if you go back in time, 30 to 40 years, those diseases had not even been described.”

Scientists recognize that danger even though the statistics on tick-borne diseases are widely understood to be incomplete. Of those 16 illnesses, only six are “nationally notifiable,” meaning that state health departments are required to collect data on their occurrence and funnel it to the CDC. (Those would be Lyme disease; Powassan; babesiosis, a parasitic disease that’s been called “American malaria”; anaplasmosis; spotted fever rickettsiosis, which used to be known as Rocky Mountain spotted fever; and tularemia, also known as rabbit fever.) In the agency’s last count, in 2019, before Covid disrupted the data flow to its tick programs, there were 50,865 cases of those six diseases recorded in the US, ranging from 34,945 for Lyme to 43 for Powassan. To suggest how much of an undercount that is: CDC scientists have estimated, based on insurance data, that more than 476,000 Americans are diagnosed and treated each year just for Lyme disease. That’s more than 13 times the reported number of cases.

For the unreported tick-borne diseases, “we don’t know exactly the extent of infection in humans,” says Saravanan Thangamani, a vector biologist and professor of microbiology and immunology at SUNY Upstate Medical University in Syracuse, New York. “For two reasons: The reporting structure is not there, and it’s easy to misdiagnose. When you go to a doctor with an acute febrile illness, these days they will test you for Covid and they will test you for flu. But they will not think about, ‘Should I test for the viral illnesses in ticks in the United States?’ The awareness among clinicians is not there.”

Thangamani can make that assertion because his lab runs a citizen-science program, TickMAP (for “Mapping Arthropods and Pathogens”), that accepts ticks sent in by New York residents, analyzes the species of tick and any pathogens each tick is carrying, and geolocates the results. The analysis is free. “Every year we are seeing a progressive increase in the number of ticks we’re receiving; geographic expansion—from each county, we see more ticks coming; and pathogen prevalence in the ticks is increasing,” he says.

That migration isn’t only happening within New York state. For about a decade, researchers have documented that the major disease-carrying tick species are on the move throughout the US. This is unexpected, because ticks are picky; temperature, humidity, forest cover, soil moisture, and the presence of their preferred hosts all tend to keep them confined to particular areas. The obvious answer to why they might relocate is climate change. As temperature zones move northward, it’s reasonable to think the arachnids would follow.

“Tick-borne diseases are the most important vector-borne disease in the continental US when you tally the amount of disease transmitted to humans,” says Gonzalo Vazquez-Prokopec, a disease ecologist and associate professor at the Emory College of Arts and Sciences. “And if you go back in time, 30 to 40 years, those diseases had not even been described.”

Scientists recognize that danger even though the statistics on tick-borne diseases are widely understood to be incomplete. Of those 16 illnesses, only six are “nationally notifiable,” meaning that state health departments are required to collect data on their occurrence and funnel it to the CDC. (Those would be Lyme disease; Powassan; babesiosis, a parasitic disease that’s been called “American malaria”; anaplasmosis; spotted fever rickettsiosis, which used to be known as Rocky Mountain spotted fever; and tularemia, also known as rabbit fever.) In the agency’s last count, in 2019, before Covid disrupted the data flow to its tick programs, there were 50,865 cases of those six diseases recorded in the US, ranging from 34,945 for Lyme to 43 for Powassan. To suggest how much of an undercount that is: CDC scientists have estimated, based on insurance data, that more than 476,000 Americans are diagnosed and treated each year just for Lyme disease. That’s more than 13 times the reported number of cases.

For the unreported tick-borne diseases, “we don’t know exactly the extent of infection in humans,” says Saravanan Thangamani, a vector biologist and professor of microbiology and immunology at SUNY Upstate Medical University in Syracuse, New York. “For two reasons: The reporting structure is not there, and it’s easy to misdiagnose. When you go to a doctor with an acute febrile illness, these days they will test you for Covid and they will test you for flu. But they will not think about, ‘Should I test for the viral illnesses in ticks in the United States?’ The awareness among clinicians is not there.”

Thangamani can make that assertion because his lab runs a citizen-science program, TickMAP (for “Mapping Arthropods and Pathogens”), that accepts ticks sent in by New York residents, analyzes the species of tick and any pathogens each tick is carrying, and geolocates the results. The analysis is free. “Every year we are seeing a progressive increase in the number of ticks we’re receiving; geographic expansion—from each county, we see more ticks coming; and pathogen prevalence in the ticks is increasing,” he says.

That migration isn’t only happening within New York state. For about a decade, researchers have documented that the major disease-carrying tick species are on the move throughout the US. This is unexpected, because ticks are picky; temperature, humidity, forest cover, soil moisture, and the presence of their preferred hosts all tend to keep them confined to particular areas. The obvious answer to why they might relocate is climate change. As temperature zones move northward, it’s reasonable to think the arachnids would follow.