Ticks require an animal host and blood meals to survive and reproduce. This lifestyle leads to the transmission of disease from one host to another. The American dog tick can transmit several pathogens, including Rocky Mountain spotted fever and tularemia. This tick can also cause tick paralysis. Three diseases are spread by the bite of the blacklegged tick: human babesiosis, human granulocytic ehrlichiosis and Lyme disease. Lyme disease is caused by the spirochete Borrelia burgdorferi, a corkscrew-shaped bacterium. It is associated with the bite of the blacklegged tick. The disease was first recognized in a group of arthritis patients from the area of Lyme, Connecticut in 1975. The emergence of Lyme disease is associated with reforestation, increased white-tailed deer populations and populations of small animal hosts. Deer are the principal hosts for adult blacklegged ticks. White-footed mice, chipmunks, and birds are important hosts for the larval and nymphal stages. Most cases of Lyme disease occur in the spring and summer months when blacklegged ticks in the nymph stage. Nymphs are about the size of a pinhead, difficult to spot, and are active during the late spring and summer months when most people are outdoors. Adult blacklegged ticks are more heavily infected with B. burgdorferi than the nymphs. In Maine, the adult tick population in certain sites may be more than 50% infected.
Signs and symptoms of Lyme disease
Localized infection - within 2 to 32 days of a tick bite (typically 8 or 9 days), 70-80% of patients develop a red rash known as erythema migrans (EM). The rash gradually expands over a period of a week or more. Rashes vary in size and shape, and may occur anywhere on the body, common sites are the thigh, groin, trunk and armpits. Many rashes reach 6 inches in diameter but some can be 8-16 inches or more in size. The rash often remains red; swelling, blistering, scabbing or central clearing may occur, resulting in a “bulls-eye” appearance. The rash may be hot to the touch but it is usually not painful. Mild nonspecific, flu-like symptoms may be experienced along with the rash. In most cases, symptoms occur during summer months when the nymphal stage of Ixodes is active. In some cases there are no early symptoms.
Disseminated infection - The course and severity of Lyme disease is variable. Days or weeks after the bite of an infected tick, multiple rashes, migratory joint and muscle pain, intermittent arthritic attacks, debilitating malaise and fatigue, neurologic or cardiac problems may occur. Early neurologic symptoms develop in about 15% of untreated patients, including paralysis of facial muscles (Bell’s palsy), meningitis (fever, stiff neck, and severe headache), and radiculoneuropathy (pain in affected nerves and nerve roots). A year or more after the tick bite in untreated or inadequately treated individuals, symptoms of persistent infection may include numbness or tingling of the extremities, sensory loss, weakness, diminished reflexes, disturbances in memory, mood or sleep, and an intermittent chronic arthritis (typically swelling and pain of the large joints, especially the knee). Approximately 50-60% of untreated patients develop arthritis and about 10% of these will progress to chronic arthritis.
Consult a physician if you suspect Lyme disease. In the absence of a rash, Lyme disease may be difficult to diagnose because its symptoms and signs vary among individuals and mimic those of many other diseases. A diagnosis of Lyme disease is made on signs, symptoms, history, and supporting laboratory data. A blood test to detect antibodies to Lyme disease pathogens can aid in the diagnosis of the disease. Some newer tests for the serodiagnosis of Lyme disease appear to be more sensitive and specific for antibodies to B. burgdorferi, but tests should not be used as the sole criterion for a diagnosis, especially during the early stages of the disease. Antibodies can usually be detected 3-4 weeks after infection. Negative results do not necessarily exclude Lyme disease, but reliability of the test does improve in later stages of the disease. Patients are known to have detectable antibodies for years following successful treatment but can still be re-infected and develop Lyme disease with subsequent tick bites, requiring another course of treatment.
Lyme disease is treated with several antibiotics, including tetracycline, doxycycline, amoxicillin, penicillin, cefuroxime, or ceftriaxone. Early treatment can prevent the later symptoms of Lyme disease from developing. Oral antibiotics are effective in treating most cases of Lyme disease. In some cases, antibiotics may be given intravenously. Patients treated in the early stages of the disease usually completely recover. Full recovery is possible for patients treated in the later stages of the disease but resolution of symptoms may take weeks or months. Persistent signs and symptoms after treatment have been reported in some patients, though the cause is unclear and controversial. Possible reasons for treatment failure include co-infection with another tickborne pathogen or unrecognized involvement of the central nervous system at the time of initial treatment. A study published in July 2001 in the New England Journal of Medicine showed that a single 200mg dose of the antibiotic doxycycline, largely prevented early Lyme disease. The antibiotic had to be given within 72 hours after a deer tick bite. This study was conducted in Westchester County, New York, where there is a very high incidence of Lyme disease. Of those who received doxycycline, 30% developed minor side effects, primarily nausea and vomiting. They also found that the only people likely to develop Lyme disease after tick bites were those who had been bitten by deer tick nymphs that had been attached for 72 hours or more and were thus engorged with or full of blood. Of those who had an engorged nymphal deer tick attached for at least 72 hours, 25% of those who did not receive antibiotics developed erythema migrans, the bull’s eye rash typical of Lyme disease.
Human Granulocytic Ehrlichiosis (HGE)- Ehrlichiosis, which is due to a rickettsia-like organism that infects white blood cells, may cause a flu-like illness with high fever, particularly in elderly individuals. This disease is transmitted by the blacklegged tick. Several locally acquired cases have been confirmed in Maine. HGE may also affect dogs and horses.
Babesiosis - Babesia microtii, a red-cell parasite, usually causes only mild or asymptomatic infection, but in the elderly or individuals with no spleen, it may result in a severe, malaria-like illness. This disease is transmitted by the blacklegged tick. Several locally acquired cases have been confirmed in Maine.
Powassan Virus- Four human cases of Powassan virus have been found in Maine. The disease, takes the form of infection and inflammation of the brain (encephalitis and meningitis). Most infections do not result in disease. Powassan virus has been isolated from Ixodes cookei, “the woodchuck tick,” which is thought to be the primary vector.
Tick Paralysis- The feeding by certain Dermacentor ticks can cause a progressive paralysis, which is reversed upon removal of the tick. Recovery is usually complete. The paralysis is not caused by a disease pathogen, but by a toxin produced by the tick. Paralysis begins in the extremities with a loss of coordination. It progresses to the face with corresponding slurred speech, and finally shallow, irregular breathing. Failure to remove the tick can result in death by respiratory failure. The American dog tick has been known to cause tick paralysis.