- Lyme disease is the most common tick-borne disease in the northern US.
- The tick that's linked to Lyme disease is local to the northeast, while other regions see different illnesses.
- In the South and central US, spotted fevers spread by various dog ticks are a greater risk than Lyme.
Tick season typically starts in early spring and lasts through fall in most regions of the US, causing flu-like illnesses that may have long-term health consequences.
Depending on where you live, the first signs of tick-borne illness can vary. At least nine different tick species in the US are capable of spreading disease, according to surveillance by the Centers for Disease Control and Prevention.
Overall, around 50,000 cases of various sicknesses linked to tick bites are reported to the CDC each year. Some tick-borne diseases are so rare that there are only a handful of reported cases nationwide each year, so Insider made a map featuring the most common tick-borne diseases.
Lyme disease is the most common sickness spread by ticks in the US, but the total number of cases is concentrated in regions where the blacklegged tick thrives.
For the purpose of the map, Insider used different scales to describe the incidence of Lyme disease and Rocky Mountain spotted fever, another tick-borne disease. States were considered to have a high incidence of Lyme if they had 100 or more cases per million people in 2019, but the high-incidence baseline for spotted fevers was 50 cases per million people. States with fewer than 10 cases per million people for each disease are denoted in grey.
Lyme disease is most prevalent in the Northeast and Midwest
Cases of Lyme disease are typically reported in states where the tick species Ixodes scapularis is looking for a host. Also known as the deer tick, this black-legged tick originated in the northeast and has spread down the coast and to the Midwest as the climate has warmed.
Maine, Vermont, and New Hampshire reported between 800 and 1200 cases per million people in 2019, which was the most recent year with complete surveillance data compiled by the CDC.
High-incidence case counts — at least 100 cases per million people — have been reported as far west as Minnesota and down the eastern seaboard. Cases drop off once you go south of Virginia, where a different species of tick thrives.
Rocky Mountain spotted fever sweeps the south, but at lower rates than Lyme
Although it's less common than Lyme, a tick-borne illness called Rocky Mountain spotted fever sickens thousands of people in the southern US every year.
Five states — Arkansas, Missouri, North Carolina, Tennessee, and Virginia account for more than 50% of spotted fever cases in the US, according to the CDC. The illness is most common in Arkansas, where more than 350 cases per million people were recorded in 2019.
A few different ticks can spread Rickettsia bacteria, a group that causes spotted fevers. Rocky Mountain spotted fever is the most common such illness in the US, according to the CDC, but most diagnostic tests can't distinguish similar bacterial species.
The bites of the American dog tick, brown dog tick, and the geographically-named Rocky Mountain wood tick can all cause spotted fevers. The American dog tick is widely distributed east of the Rockies, while the brown dog tick has driven up RMSF transmission rates in Arizona in recent years.
Some tick species can cause multiple illnesses
The blacklegged tick may be known for spreading Lyme disease, but it can also transmit bacteria and viruses that cause other illnesses.
The most common of these secondary infections is anaplasmosis, a flu-like illness that is reported at rates as high as 500 cases per million people in Maine — under half of the state's incidence of Lyme disease, but greater than most states' rates of RMSF.
Massachusetts used to be a high-incidence Lyme state like its neighbors, but the number of reported cases of Lyme disease in the state dropped drastically in 2016. In the meantime, the rate of anaplasmosis in the state has surpassed that of Lyme: same tick, different bacteria.
Ehrlichiosis, another lesser-known tick-borne illness, is spread by the lone star tick in the southern and central US. In Missouri, the annual rate of ehrlichiosis is nearly as high as the rate of RMSF. The disease has been reported as far west as Nebraska, but most cases occur in the eastern half of the country.