The 6 worst tick-borne diseases in North America

When ticks bite humans and other animals, they are out for blood. It’s the only thing parasitic arachnids need to meet their nutritional needs, and they can be ruthless in their pursuit of a meal. A tick that remains attached to its host until fully submerged can stay there for a week, increasing its weight by 200 to 600 percent.

This long feeding time makes ticks ideal transmitters of diseases, which are passed on to hosts via the parasites, viruses or bacteria they carry. According to the Centers for Disease Control (CDC), ticks spread more than a dozen diseases in the United States, some of which can have serious, even fatal, consequences for their human victims. Here are some of the worst.

the black-legged tick on a blade of grass.
Blacklegs are known to cause Lyme disease. CDC

1) Lyme disease

Lyme disease is not only the most common tick-borne disease in the United States, but it is also the disease most commonly spread by any “vector” species, which includes mosquitoes, sandflies, blackflies, and triatomines (lip of kissing) except ticks. Lyme is caused by Borrelia burgdorferi AND Borrelia mayonii bacteria, and about 30,000 to 40,000 cases in the US are reported each year.

Vector: black-legged ticks (Ixodes scapularis) in the northeast and upper midwest; western black-legged tick (Ixodes the peacemaker) along the Pacific coast.

Geographical range: Lyme is named for the Connecticut town where an outbreak of unexplained illness in 1975 led to the study and documentation of the disease, but it is often found throughout the northeastern states, the upper Midwest, and the Mid-Atlantic states. Cases have also been reported in California, Oregon and Washington. Genetic sequencing has led researchers to conclude that Lyme disease has likely been circulating in North America for 60,000 years, long before the first humans arrived on the continent.

Symptoms: Fever, chills, headache, fatigue, swollen lymph nodes and muscle and joint pain. A characteristic bullseye rash appears in about 70 to 80 percent of infected people, appearing anywhere from three to 30 days after a bite.

Prognosis: If left untreated, Lyme can spread to the joints, heart and nervous system, but early detection and treatment can stop its progression. Most cases of Lyme disease can be cured with a two- to four-week course of oral antibiotics, but patients sometimes have symptoms of pain, fatigue, or difficulty thinking that last more than six months after they finish treatment.

2) Rocky Mountain spotted fever

The CDC has labeled Rocky Mountain Spotted Fever (RMSF) as the deadliest tick-borne disease in the world, and 4,000 to 6,000 cases are reported in the US each year. Although the incidence of the disease has increased since it was discovered in the 1920s, the death rate has decreased. Currently, about 5 to 10 percent of cases are fatal.

Vector: American dog ticks (Dermacentor variable) in the eastern, central, and western US, Rocky Mountain wood ticks (Dermacentor andersoni) in the Rocky Mountain States and brown dog ticks (Rhipicephalus sangunineus) in the southwest.

Geographical range: Reported nationwide, but more than 60 percent of cases occur in five states: Arkansas, Missouri, North Carolina, Oklahoma and Tennessee.

Symptoms: Fever, headache, rash (usually occurs two to five days after a bite), nausea, vomiting, stomach pain, muscle aches and lack of appetite.

Prognosis: Treatment with doxycycline antibiotics within five days usually results in complete recovery. Delayed treatment can lead to more serious illness requiring hospitalization, and those who recover from advanced infections can suffer life-changing damage, including amputations, hearing loss, paralysis or mental disability. If left untreated, RMSF can cause death within eight days of the onset of symptoms.

3) Powassan virus

Named after the town in Ontario, Canada, where a young man died of the disease in 1958, Powassan is rare but deadly. Infections have increased in the US in recent years. From 2011 to 2015, an average of 8.8 cases per year were reported; from 2016 to 2020 the average increased to 26.8 per year.

Vector: black-legged ticks (Ixodes scapularis) and the earth tick (Ixodes cookei).

Geographical range: Cases have been reported mostly in the Northeast and Great Lakes regions.

Symptoms: Many people infected with the virus have no symptoms, but those who do usually experience fever, headache, vomiting and weakness. In its severe stages, Powassan can cause infection of the brain (encephalitis) or the membranes surrounding the brain and spinal cord (meningitis), which can lead to confusion, loss of coordination, difficulty speaking, seizures and death.

Prognosis: There is still no effective drug treatment for Powassan virus.

Rocky Mountain Wood tick on a blade of grass.
Rocky Mountain wood ticks can carry Rocky Mountain Spotted Fever, which can be fatal if left untreated. CDC

4) Relapsing tick fever

Transmitted to humans through bacteria carried by infected soft ticks, TRBF occurs primarily in the western US and is associated with sleeping in rustic cabins and vacation homes, where rodent infestations attract ticks. Unlike hard ticks, soft ticks only bite for a short time, often emerging at night to feed on hosts while they are sleeping – making their bite much more difficult to prevent or discovered.

Vector: Several ticks in the genus Ornithodoros, incl Ornithodoros hermsi.

Geographical range: Reported in 15 states: Arizona, California, Colorado, Idaho, Kansas, Montana, Nevada, New Mexico, Ohio, Oklahoma, Oregon, Texas, Utah, Washington, and Wyoming.

Symptoms: Headaches, muscle and joint pain, combined with a dangerously high fever that usually lasts three days, subsides for a week, and then returns for three days. As the name of the disease suggests, without treatment, this pattern can repeat (or restart) many times.

Prognosis: With antibiotic treatment, patients usually recover within a few days, although in some rare cases neurological problems and respiratory distress requiring intubation have been reported. TRBF can also cause more serious symptoms in pregnant women.

5) Anaplasmosis

Second only to Lyme disease in frequency, with nearly 8,000 cases reported each year in the U.S., anaplasmosis is spread by ticks infected with the bacteria Anaplasma phagocytophilum.

Vector: black-legged ticks (Ixodes scapularis) in the northeast and upper midwest; western black-legged tick (Ixodes the peacemaker) along the Pacific coast

Geographical range: It is most commonly reported in the northeastern and midwestern states

Symptoms: Onset may occur only one to two weeks after a bite. Early symptoms include fever, chills, severe headache, muscle aches, nausea, vomiting, diarrhea and loss of appetite.

Prognosis: Prompt treatment with doxycycline antibiotics can prevent serious illness. Untreated, anaplasmosis can lead to respiratory failure, bleeding problems, organ failure, and death. Delayed treatment, advanced age and a weakened immune system increase the risk of serious illness.

6) Heartland Virus

A tick-borne virus discovered in 2009, Heartland is relatively rare, with only 50 reported cases. But a recent study in Georgia found that one in every 2,000 tick specimens collected carried the virus. Antibodies to the disease have been detected in deer, raccoons, coyotes and moose in 13 different states.

Vector: Lone Star Tick (Amblyomma americanum)

Geographical range: Most commonly reported in the Midwest and South, Heartland Virus has been identified in Arkansas, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Missouri, North Carolina, Oklahoma, and Tennessee.

Symptoms: Fever, fatigue, loss of appetite, headache, nausea, diarrhea and muscle or joint pain. Symptoms can last up to two weeks after a bite appears and often require hospitalization. Some people experience low white blood cell counts, low platelet counts, and elevated liver enzyme levels. The virus has been fatal for some elderly patients with underlying medical conditions.

Prognosis: There is still no effective drug treatment for Heartland virus.

Read more: The Hunter’s Guide to Ticks—The Woodland’s Most Disgusting and Disgusting Bloodsuckers

lone star tick on a blade of grass
The lone star tick can cause meat allergies in humans. CDC

7) Alpha-Gal syndrome

Also known as red meat allergy, Alpha-Gal syndrome (AGS) is a serious, potentially life-threatening allergic reaction to alpha-gal, a sugar molecule found in red meat and dairy products. Growing evidence suggests that the recently discovered condition may be caused by the bite of the lone star tick, but other tick species have been linked to AGS outside the US, and more research is needed to determine the role ticks play in its spread.

Vector: Lone Star Tick (Amblyomma americanum)

Geographical range: AGS is most commonly reported in adults living in the southern, eastern, and central US

Symptoms: Hives or itchy rash; nausea or vomiting; heartburn; diarrhea; cough, shortness of breath, or difficulty breathing; a drop in blood pressure; swelling of the lips, throat, tongue or eyelids; dizziness or fainting; and severe stomach pain. Symptoms appear 2 to 6 hours after eating meat or dairy.

Prognosis: Reactions range from mild to severe to life-threatening in cases where anaphylactic shock requires immediate emergency care. Laboratory tests that identify AGS antibodies are useful in diagnosing the condition, but there is currently no effective treatment beyond eliminating meat from the diet.

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