Tick-Borne Diseases In North America

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Special thanks to Tom Bush of GoodGear

Lyme Disease

Lyme disease was first identified in 1982 after an epidemic of arthritis near Old Lyme Connecticut in the mid 1970's. Borrelia burgdorfi (a bacteria) was isolated from the midgut of the deer tick and patients were found to have antibodies to the same bacteria.

Lyme disease is the leading vector-borne disease in the United States with more than 50,000 cases reported between 1982 and 1992. Forty-eight states have reported the disease with most cases from northeastern coastal states, the upper midwest and northern California. This distribution coincides with the known range of several closely related species of Ixodes ticks. Cases of Lyme disease outside the range of these tick species suggest that other vectors are also responsible for disease transmission.

Most human infections occur between May and August with an increase in July when peak tick activity and human outdoor activity coincide. All three stages of tick development (larval, nymph and adult) bite humans and the nymph is primarily responsible for transmission of Lyme disease to humans because of its small size. Most people don't know they have been bitten and twenty-five percent of known cases are in children under 11 years old.

There are several stages of Lyme disease and it affects many body systems. Clinically it can be divided into the early stage (onset days to weeks after exposure), the secondary stage (days to months), and late stages (months to years).

Symptoms usually begin with bullseye rash at the site of the bite an average of 7 days after exposure. The lesion may be warm to touch and described as burning or itching. Other symptoms include enlarged lymph nodes, fever and malaise. Most people do not develop all symptoms and asymptomatic infection can occur. Some people will not develop a rash. Lesions fade after about a month without treatment. With antibiotic treatment lesions resolve after several days. Round, red lesions occurring hours after tick bite are secondary to hypersensitivity reaction and should not be confused with the rash of Lyme disease.

Later stages of Lyme disease involve a secondary rash and nearly constant fatigue. Fever is also common. Muscle and joint aches may mimic a viral illness. These later symptoms are intermittent and rapidly change. Neurologic and cardiac symptoms can also occur in later disease.

There aren't any reliable tests that definitively diagnose early Lyme disease. Your health care provider will rely on your symptoms and history and maybe some lab tests to make the diagnosis.

Treatment is straightforward. Many antibiotics are effective against the organism that causes Lyme disease and should be taken for several weeks. Late disease will require longer treatment.

The risk of Lyme disease is low after tick bite because most people remove the tick before it has a chance to transmit disease. If transmission of disease does occur, the majority of people will develop the characteristic bullseye rash and seek treatment.

Relapsing Fever

Tick-borne relapsing fever is caused by an acute bacterial infection characterized by recurrent episodes of fever separated by afebrile periods. It occurs worldwide and is transmitted by soft ticks (Argaside) belonging to the genus Ornithodoros. These ticks feed on wild rodents and can remain alive and infectious for many years without feeding. They generally feed at night and usually attach for less than one hour. The bite is seldom painful and often goes unnoticed. In Mexico and the southwestern United States, these ticks live in the guano of cave floors and human infection is often associated with spelunking or camping in caves.

The majority of cases of relapsing fever in the United States have been reported in remote undisturbed areas from Colorado to California. In general only sporadic cases are diagnosed and several common-source epidemics have been described involving exposure in mountain cabins or in caves. The disease is more common in men and occurs mostly in the summer, but cases can occur year-round.

After an incubation period of about 7 days the illness characteristically begins with and abrupt onset of fever lasting about 3 days followed by an afebrile period of variable duration with a relapse of fever and other clinical symptoms. Fever is often accompanied by shaking chills, severe headache, muscle and joint pain, abdominal pain, nausea and vomiting. An itchy bump may develop at the bite but may be absent by the onset of symptoms. A rash may develop in some cases but is not a reliable sign. The temperature is usually more than 103.5 F and patients may develop an enlarged liver or spleen.. Neurologic involvement occurs less often and includes altered sensorium and numbness of extremities. Left untreated, the primary fever breaks in 3 to 6 days with drenching sweats followed by an afebrile period of about 8 days. Three to five releapses typically occur without treatment. Each succeeding relapse is less severe. Death is rare and limited mainly to infants and the elderly.

The recurrent fever is caused by the ability of the organism to undergo antigenic variation (it can change the way it looks to your immune system and evade detection). Just as your body fights off one invader it pops up as another closely related threat. Here we have a tick that can go years between blood meals, feeds for a short time at night with a painless bite and can transmit an organism that plays a game of hide a seek with your immune system. Who could imagine a better science fiction villain?

Diagnosis can be made by observing the organism in a drop of blood under a microscope. This does not always show the organism. Other blood tests may be helpful but lack laboratory standardization. Your health care provider will have to rely on the history you give to suspect this one. Tell them if you think you have been exposed to ticks. Treatment with specific antibiotics is effective against relapsing fever.

Rocky Mountain Spotted Fever

Rocky Mountain Spotted Fever (RMSF), an acute febrile illness caused by infection with Rickettsia rickettsii, is the most common fatal tick-borne disease in the United States. Many species of ixodid ticks have been implicated as vectors of RMSF.

The American dog tick (Dermacentor variabilis), the Wood tick (Dermacentor andersonii) and the Lone Star tick (Amblyomma americanum) harbor the organism in their gut and transmit it to humans during a blood meal.

The illness was first described in 1873 in Montana and Idaho and was soon recognized throughout the Rocky Mountains. For unknown reasons, the incidence of RMSF decreased dramatically in the Rocky Mountain states and is now most prevalent in the south Atlantic coastal and western south central states. North Carolina, South Carolina, Oklahoma and Tennessee had the highest incidence in 1990.

RMSF is not easy to diagnosis early in its course. The classic presentation of rash, fever and history of tick exposure is present in only 3% of patients seeking treatment during the first 3 days of illness . A week after being bitten by an infected tick (range 3 to 12 days) victims develops fever (almost 100%), headache, fatigue, muscle aches and often nausea, vomiting and anorexia. This can easily be mistaken for common viral illness.

The rash in RMSF results from dissemination of the organism through the skin. It typically develops first on the wrists, hands, ankles and feet, spreading rapidly centrally to cover most of the body, including palms, soles and face. As the organism spreads through the circulation it causes bleeding of microscopic vessels leading to collapse of the vascular system and death.

Neurologic involvement in RMSF ranges from mild headache to serious focal or generalized disorders of cerebral function. Headache is common and complaints of severe headache in adults should raise your suspicion of RMSF. Many patients report the worst headache of their lives. Seizures may develop during the acute phase of the illness but rarely persist. Lethargy and confusion are common and may progress to stupor or coma. Death usually occurs between days 8 and 15 of illness.

Diagnosis of RMSF must be made on the basis of clinical presentation and history in order to ensure timely therapy. The course of RMSF is one of advancing severity. Most viral illness do not get worse on days 3 through 6. The most significant factor in death from RMSF is delayed diagnosis and initiation of appropriate antibiotic therapy.

Laboratory confirmation of RMSF has generally relied on blood tests detecting an antibody increase to the offending organism. These tests do not detect disease until 7 to 10 days after symptoms begin. This may be to late. Work is being done to find tests that can help diagnose early disease.

Only a few antibiotics are effective against the organism responsible for RMSF. Treatment should continue for 7 to 10 days. Talk with your primary care provider to choose the best medication for you or your child.

Ehrlichiosis

Two tick-borne infectious diseases with clinical features similar to RMSF have recently been recognized in the United States. The initial recognition of human Ehrlichiosis was based on the finding of organisms in white blood cells in a man bitten by ticks in Arkansas in 1986. The organisms resembled those found in canine Ehrlichiosis, a tick-borne infection that wiped out 2/3 of military dogs during the Vietnam War and it was thought that humans could acquire the dog disease. It turned out that human Ehrlichiosis in the United States is caused by Ehrlichia chaffeensis, a closely related species.

Ehrlichia chaffeensis, the causative agent of human Ehrlichiosis, is found mostly in the southern and south-central United States. Both Dog ticks (Dermacentor variabilis) and Lone star ticks (Amblyomma americanum) have been implicated as vectors of the disease. Another closely related species, Ehrlichia equi, infects different types of white blood cells and is reported mostly from the upper midwestern and northeastern states where Lyme disease is common. Transmission by deer ticks and dog ticks is suspected. Further work is needed to fully characterize the host-vector relationship of Ehrlichia species.

Both forms of Ehrlichiosis have symptoms ranging from subclinical infection to life-threatening disease. They are more difficult to diagnose clinically than RMSF because of infrequent rash. For this reason Ehrlichiosis may be considered "Rocky Mountain Spotless Fever". The most common symptoms are fever, headache and muscle aches.

As part of an investigation of patients who had Ehrlichia infections in a golf oriented retirement community in Tennessee, researchers found evidence of past infection in people who had noticed no symptoms. Community members who frequently saw deer in their yards had and increased incidence of antibodies to Ehrlichia (they had evidence of previous infection). Among men who golfed, the risk of infection was significantly greater for players who reported higher scores (better golfers had less incidence of exposure). Golfers who retrieved balls hit off the course were more likely to show evidence of past infection. There was a significantly lower risk of infection among those who always used insect repellent. The community had been recently built in a dense forest directly adjacent to a wildlife-management area.

Diagnosis of Ehrlichiosis should be made on a clinical basis and confirmed retrospectively by blood tests. There are no widely available blood test to positively identify the organisms responsible for human Ehrlichiosis. Both types of Ehrlichiosis usually respond to antibiotics quickly enough for the response to be helpful in supporting the diagnosis.

Colorado Tick Fever

Ticks transmit a wide variety of viruses to humans. Only Colorado Tick Fever (CTF) occurs very often in the United States. Ground squirrels are the main vertebrate reservoirs and Dermacentor andersonii is the primary vector that transmits the disease to humans. The disease occurs in the mountains of 11 western states primarily between late march and early October.

CTF is usually a self-limited febrile illness that is probably more prevalent than the 200 to 300 cases reported annually. Many cases are not reported or are simply diagnosed as a viral illness. Three to six days after exposure, the illness usually begins with abrupt onset of fever, chills severe headache, sensitivity to light and muscle aches. Anorexia, nausea, vomiting, and abdominal pain are common. Patients may experience an episode of fever followed by brief afebrile period of 2 to 3 days and then another febrile period. A rash may develop but is not a common feature. Rare complications may occur in children under the age of 10. Nervous system infection, bleeding and involvement of abdominal organs has been reported.

Lab tests are not generally helpful or widely available during the clinical course of illness. No specific treatment exists for CTF and therapy is primarily supportive. Infection generally confers lifelong immunity.

Babesiosis

Babesiosis is a malaria-like illness caused by a parasite (Babesia microti) that invades Red blood cells. Most Babesia infections have occurred in the summer months on islands offshore the northeastern United States. It has also been reported in the southeast, northern midwest and west coast states. The ecology of Babesiosis parallels that of Lyme Disease with deer ticks (Ixodes scapularis) being the primary vector in both diseases. Babesiosis occurs primarily in the elderly or in those with underlying medical problems.

Symptoms of Babesiosis begin gradually with anorexia and fatigue followed by fever, sweats and muscle aches. Most patients do not recall tick bite and symptoms begin about one week after exposure. Severity ranges from a mild self-limited illness to serious disease with anemia, kidney failure and decreased blood pressure. Rash is not a feature of the illness. A large percentage of patients with Babesiosis have evidence of concurrent Lyme disease. Theoretically, coinfection with Lyme disease Babesiosis and Ehrlichiosis can occur.

Identification of parasites in red blood cells confirms the diagnosis of Babesiosis. Differentiating Babesia from the organism that causes malaria may be difficult.. Treatment with a 7 day course of antibiotics has been effective and is usually reserved for seriously ill patients. Blood transfusion may produce rapid improvement in severe cases.

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