When it comes to problems caused by ticks, Lyme disease hogs a lot of the limelight. But various tick species carry and transmit a collection of other pathogens, some of which cause serious, even fatal, conditions.
In fact, the number of tick-borne disease cases is on the rise in the United States. The range where various species of ticks live in North America may be expanding due to climate change. Researchers continue to discover new pathogens that live in ticks. And new, invasive tick species keep turning up.
In my career as a public health entomologist, I’ve been amazed at the ability of ticks to bounce back from all the ways people try to control them, including with pesticides. Ticks excel at finding new ecological niches for survival. So people and ticks frequently cross paths, exposing us to their bites and the diseases they carry.
Here are some of the lesser-known, but growing, threats from ticks.
Ticks can spread bacterial diseases
Certain very small species of bacteria that can cause human diseases, such as rickettsia, ehrlichia and anaplasma, live in ticks. Ticks ingest these bacteria when they drink animals’ blood. Then when the ticks take a subsequent blood meal, they pass the bacteria along to the next animal or person they feed on.
Probably the most well known of these bacterial diseases is Rocky Mountain spotted fever, the most frequently reported rickettsial disease in the U.S., with about 6,000 cases each year. The number of diagnoses seems to be increasing nationwide, especially among Native Americans, probably due to exposure on reservations to free-roaming dogs that can carry ticks.
When people get sick with Rocky Mountain spotted fever, they usually come to a clinic with three things: fever, rash and history of tick bite. They may also report severe headache, chills and muscle pains, and gastrointestinal symptoms such as abdominal pain and diarrhea. A skin rash is usually present after a few days, but not always. Mental confusion, coma and death can occur in severe cases. Untreated, the mortality rate is about 20%; and even with treatment, 4% of those infected die.
Not all tick species are effective transmitters of the rickettsia bacteria. Even within the vector species, often only 1% to 5% of ticks in an area are infected. So getting bitten by a tick that passes rickettsia bacteria on to you is like getting stuck with a needle in a haystack. The primary carriers are the American dog tick in the eastern U.S. and Rocky Mountain wood tick in the West. The brown dog tick has also recently been shown to be a vector.
In most tick-borne diseases, the tick needs to feed for some amount of time before any pathogens it’s carrying are transmitted to the animal whose blood it’s eating. Rocky Mountain spotted fever organisms generally take between one and three hours for transmission to occur, so attached ticks need to be removed quickly. Doctors usually prescribe the antibiotic doxycycline to treat Rocky Mountain spotted fever, which works quite well if the disease is recognized early.
Ehrlichiosis is another bacterial disease transmitted from ticks to people. In the U.S. it’s most commonly caused by Ehrlichia chaffeensis bacteria, carried by lone star ticks which are common in the eastern U.S. Ehrlichia bacteria infect a type of blood cell called leukocytes. Human monocytic ehrlichiosis occurs mostly in the southern and south-central U.S.; 1,642 cases were reported to the CDC in 2017.
Ehrlichiosis patients usually have fever, headache, muscle aches and a progressive low white blood cell count. As opposed to Rocky Mountain spotted fever, people get a rash only about 20% to 40% of the time. Doctors usually treat ehrlichiosis with doxycycline.
Another tick-borne bacterial disease to worry about is human granulocytic anaplasmosis. In human granulocytic anaplasmosis, Anaplasma phagocytophilum bacteria infects a type of white blood cell called granulocytes. It mostly occurs in the upper midwestern and northeastern U.S., and the incidence is increasing, with 5,762 cases of human granulocytic anaplasmosis reported to the CDC in 2017.
Symptoms include fever, headache, muscle aches and progressive low white blood cell count. It’s the deer tick Ixodes scapularis – famously also responsible for Lyme disease – that transmits the Anaplasma bacteria to humans. There’s the unlucky chance that a bite from a deer tick could infect you with both diseases. Again, recommended therapy is doxycycline.