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Tick-borne diseases in Minnesota

Larissa Minicucci and Jeffrey Hahn


Minnesota provides suitable habitat for several tick species, but of particular interest is the blacklegged tick (also known as the deer tick), Ixodes scapularis, which can be involved in the transmission of several diseases to humans. Lyme disease is the most recognized of these diseases. The disease was first described in Connecticut in 1977 and is now the most commonly reported vector-borne disease in the United States.

Three other diseases that can result from the bite of a blacklegged tick have also been reported in Minnesota. Human anaplasmosis and babesiosis are being reported with increasing frequency, sometimes occurring as co-infections in individuals with Lyme disease. Additionally, the first case of Powassan virus infection was reported in Cass County in June 2008.

One additional disease, Rocky Mountain spotted fever, is very rare in Minnesota, but more commonly encountered in the South Atlantic states. It is transmitted by the American dog tick (wood tick), Dermacentor variabilis. Given its isolated nature in Minnesota, only diseases transmitted by the blacklegged tick will be discussed. For more information on Rocky Mountain spotted fever, see the Center for Disease Control and Prevention (CDC).

The organism

Lyme disease

The organism that causes Lyme disease is a spirochete bacterium known as Borrelia burgdorferi. This bacterium has been identified in both the vector tick and in the blood, skin, joint fluid, and cerebral spinal fluid (CSF) of patients with Lyme disease.


Anaplasmosis is a rickettsial disease caused by Anaplasma phagocytophilum, a type of bacteria that infect white blood cells. The disease was formerly known as human granulocytic ehrlichiosis or HGE.


The organism that causes babesiosis in humans invades the red blood cell to cause infection. While many protozoan Babesia species have been linked to human infection, Babesia microti is the most commonly identified cause of babesiosis in humans in the United States.

Powassan Encephalitis

The virus is a flavivirus related to tick-borne encephalitis viruses found in Northern Europe and Asia. It was initially described in a patient in Powassan, Ontario fifty years ago.

The vector


Black legged tick female (left) and male (right)
Jeff Hahn, Univ. of Minnesota

The blacklegged tick, I. scapularis, is the vector of Lyme disease, human anaplasmosis, babesiosis, and Powassan encephalitis.

You can generally recognize an unengorged adult female blacklegged tick from its black legs and scutum (the area behind the head) and its orange to reddish body. It is about 1/10 inch long or generally smaller than the American dog tick (commonly referred to as wood tick), and it has long mouthparts. An adult male is about 1/16 inch long and dark brown. Adult males attach but do not feed. Nymphal deer ticks are a bit larger than a poppy seed and are dark and teardrop-shaped.

Identifying ticks based solely on size and color is often challenging, especially when you encounter immature and/or ticks filled with blood. It is important to know what species of tick has bitten you as the American dog tick and other ticks are not known to transmit any diseases in Minnesota. (In rare instances, the American dog tick may transmit Rocky Mountain spotted fever in southern Minnesota.) If you ever have any doubt as to which tick you have encountered, submit a sample to an expert for identification.

The life cycle of a blacklegged tick generally takes two to three years in Minnesota. Eggs are deposited by adult females during the spring, which hatch a month later into six-legged, pinhead-sized larvae. The larvae feed once on the blood of host animals, such as a white-footed mouse, during summer, taking about three to five days to complete their blood meal. Lyme disease, human anaplasmosis, and babesiosis are not passed on to the larvae by infected adult females so they can only acquire any of these diseases by feeding on infected hosts. Powassan virus, however, can be passed from the female tick to her offspring, and are therefore a potential source of infection. After feeding, larvae molt to eight-legged nymphs, the second immature stage, which overwinter until the following spring.

During the late spring and early summer, these nymphs feed on host animals, staying attached for about three to five days. They also prefer white-footed mice, the primary source of disease infection, but will also feed on a variety of animals, including humans and dogs. Nymphs are very small, about the size of a poppy seed. The nymphs then molt into adults. Adult females feed either during the fall or the following spring, staying attached for about five to seven days when taking a blood meal. Adults are active outdoors even at temperatures as low as the mid to upper 30'so F but tend to be most active in warmer temperatures.

A blacklegged tick can only transmit disease to humans through a bite. They can not do so by just crawling on a person. Even when biting, a blacklegged tick must stay attached for at least 12-24 hours (in the case of human anaplasmosis) or at least 24-48 hours (in the case of Lyme disease) before it is able to transmit disease. Only nymphal and adult female blacklegged ticks transmit these diseases. The exception to this rule is Powassan virus which can be transmitted all tick stages (larvae, nymphs and adults) within minutes following tick attachment.

More information on Minnesota Ticks

diagram of the life cycle of a blacklegged tick

Blacklegged tick life cycle Centers for Disease Control and Prevention

Illness in Minnesota

Minnesota map with orange areas

Areas of highest risk for Lyme disease in Minnesota
Minnesota Dept. of Health

High risk areas for tick exposure in Minnesota include the north central, east-central and southeastern regions of the state, also extending into some northwestern counties. Greatest risk is found within hardwood or mixed hardwood forests, which provide suitable habitat for blacklegged ticks. Risk of bites from these ticks in Minnesota is highest during the spring, early summer, and fall months.

From 1986 - 2007, nearly 9,000 cases of Lyme disease have been reported in Minnesota with over 1,200 cases reported in 2007 alone. As a general trend, tick-borne diseases have been increasing in frequency each year in the state. In 2007, over 300 cases of human anaplasmosis and just over 20 cases of babesiosis were reported in Minnesota. Powassan encephalitis is very rare in Minnesota, with only two cases reported as of 2009. Tick-borne diseases peak during the months of June through August, coincident with the tick life cycle and increased human exposure due to outdoor activity.

For current information on tick-transmitted disease statistics, see the Minnesota Department of Health.

Clinical disease in humans

Lyme disease

Some people infected with B. burgdorferi may not develop any clinical signs of illness. However, 70-80% of people with Lyme disease will develop an erythema migrans (EM) rash, an expanding, red circular rash, often with an area of central clearing. This rash usually occurs 3-30 days after a tick bite and may be accompanied by fever, headache, and muscle aches.

After the early stage of disease, individuals may progress to an early disseminated infection if treatment is not administered. More body systems can become affected resulting in possible secondary EM lesions, arthritis, meningitis or neuropathy, migratory musculoskeletal pains, and cardiac arrhythmias.

The late disseminated form of Lyme disease occurs weeks to months after infection in some untreated patients. Typical manifestations at this point may include chronic fatigue, joint swelling in one or more large joints, or neurologic signs.


Clinical signs of human anaplasmosis are often non-specific. Fever, chills, headache and muscle ache may develop 5-21 days after a bite from an infected tick. While most cases are self-limiting, severe complications can occur in individuals who are elderly or immunocompromised, and fatalities from human anaplasmosis have been documented in Minnesota. Chronic infection has not been described.


In humans, babesiosis is often asymptomatic. However, it can cause fever, muscle aches, headaches, nausea, and anemia, especially if an individual is co-infected with Lyme disease or human anaplasmosis or in those with an immunocompromised status. Like human anaplasmosis, babesiosis can be severe and lead to death. Occasionally, the organism may persist and cause subsequent symptoms weeks to months later, if untreated.

Powassan Encephalitis

Its symptoms in humans can be confused with mosquito-borne encephalitis, particularly West Nile and LaCrosse encephalitis. Patients may experience fever, headache, nausea, loss of memory and difficulty speaking. Long-term effects have been reported to be common and are potentially severe.

Clinical disease in animals

Of all the domestic animals, dogs are most commonly identified with these tick-borne diseases. In dogs exposed to Lyme disease, many will not show any clinical signs of illness. However, some may develop fever and lameness. In complicated cases, dogs may develop acute kidney failure or show neurologic manifestations. Dogs do not develop the EM rash seen in people. Similarly, dogs exposed to anaplasmosis may not demonstrate any clinical signs, but may develop fever, lethargy, and bleeding disorders. One of the most consistent findings in canine anaplasmosis is low platelet counts (thrombocytopenia). Babesiosis in dogs is rare and is caused by a different organism than that causing human disease. If dogs do demonstrate clinical signs consistent with babesiosis, anemia is often the primary sign of illness. Powassan encephalitis has not been reported in pets.

Horse owners should likewise be alert to tick-borne diseases, especially anaplasmosis, which can be severe in these animals. Signs and symptoms include fever, inappetence, listlessness, and limb edema. As in dogs, thrombocytopenia as well as anemia and low white blood cell counts are common, but infection of white blood cells (granulocytes) is often more easily detected in horses than other animals and humans.


In endemic areas such as Minnesota, Lyme disease can be diagnosed in people based solely on the presence of the EM rash and a history of a blacklegged tick bite or exposure to blacklegged tick habitat. In individuals with later manifestations of disease, those in which the EM is absent, or in people without a history of a tick bite, laboratory testing is recommended. Specifically, a two-tier testing protocol should be used. The first step involves serologic (blood) testing to evaluate exposure to the bacteria that causes Lyme disease. Positive or equivocal results on this first test should be supported with a Western Blot test.

Anaplasmosis can be detected through PCR testing or visualized in white blood cells on a blood smear. In addition, serology is used to assess whether a patient has developed specific antibodies, the body's response to the infection.

Babesiosis can be diagnosed by visualization of the organism on a blood smear or through serologic testing or PCR.

For Powassan encephalitis diagnosis is based on detection of the virus in cerebrospinal fluid, and demonstration of serum antibody.

Diagnosis for all three diseases should also be based on potential exposure to blacklegged ticks in an endemic area.

In animals, similar diagnostic testing is utilized, but more often relies on serologic testing. A rapid in-clinic test exists to assess exposure to Lyme disease and anaplasmosis in dogs.


Lyme disease, anaplasmosis, and babesiosis are all treatable with antibiotic or antimicrobial therapy. Prognosis is best if individuals are treated early in their illness. The type of medication, however, will vary based on the illness, the presenting clinical signs, and concurrent illness. It is strongly encouraged that individuals with a suspicion of a tick-borne illness consult their physician for appropriate diagnosis and treatment. Likewise, if your pet is demonstrating signs of a tick-borne illness, it is recommended that you consult your veterinarian. There is, however, no specific treatment for Powassan encephalitis, and there is no vaccine to prevent it. Patients are hospitalized and given supportive care to guard against further complications.


Protect yourself when you are outside in areas known to be infested with blacklegged ticks:

  1. When in the woods, walk on trails and avoid moving through grassy areas. Ticks are commonly found in brush and low-lying vegetation near the forest floor, and you increase your risk of ticks finding you by moving through such areas.
  2. Wear protective clothing, such as long-sleeved shirts and pants that are light-colored. To maximize your protection, tuck your pants into your socks to prevent ticks from moving underneath them. The ticks will crawl up the outside of your pant legs, giving you more time to see and remove them before they bite.
  3. Use repellents to protect yourself. DEET (N,N-diethyl-meta-toluamide) is a very effective repellent that you can apply to your clothes as well as exposed skin. It is not necessary to use concentrations higher than 20 - 30 % - there is no evidence that increasing percentages are more effective. Another effective repellent is permethrin. Unlike DEET, apply permethrin only to clothing. An advantage of permethrin is that it can last through several washings, so it does not need to be reapplied for every outing. Consult your veterinarian to determine which tick preventive products are best for your dog. Ideally, dog owners in the Upper Midwest should use topical tick preventatives from March through November. Repellents approved for humans should not be used for pets.
  4. It is very important to check yourself for ticks after you have been in tick-infested areas, especially during summer when immature blacklegged ticks are active. Because of their very small size, nymphs can be easily overlooked.
  5. Several Lyme disease vaccines are available for dogs. Consult your veterinarian to determine if your dog should be vaccinated and if so, which vaccine should be used. There are currently no Lyme disease vaccines for humans.
  6. If you do find a tick attached to your skin, carefully remove it with tweezers. Grasp it around the head as close to the skin as possible and gently yet firmly pull it out. Save any ticks that are found biting so an expert can identify them later. Only blacklegged ticks are known vectors of disease in Minnesota.

Selected References:
Steere AC, Lyme Disease. New England Journal of Medicine, 2001. 345: p. 115-125.
Steere AC, M.S., Hardin JA, Ruddy S, Askenase PW, Andiman WA, Erythema Chronicum Migrans and Lyme Arthritis The Enlarging Clinical Spectrum. Annals of Internal Medicine, 1977. 86: p. 685-698.
Minicucci LA. Lyme Disease, in Maxcy Rosenau Last Public Health and Preventive Medicine. Wallace RB, ed. McGraw Hill, 15th ed., New York. 2008;386-392.
Merck Veterinary Manual,
Wormser GP, Dattwyler RJ, Shapiro ED, et al. The Clinical Assessment, Treatment, and Prevention of Lyme Disease, Human Granulocytic Anaplasmosis, and Babesiosis: Clinical Practice Guidelines by the Infectious Diseases Society of America. Clinical Infectious Diseases, 2006. 43: 1089-1134.

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