TICK-BORNE DISEASES IN THE UNITED STATES
Lyme disease and ehrlichiosis are not the only illnesses transmitted by ticks in the United States. Rocky Mountain spotted fever, Colorado tick fever, tick paralysis, tularemia, babesiosis, and relapsing fever are some of the other diseases of importance.
IMPORTANT TICKS AND TICK-BORNE DISEASES IN THE UNITED STATES
The deer tick (Ixodes scapularis) is found in great abundance from Virginia to Maine, as well as in Wisconsin and Minnesota, whereas its first cousin, the western deer tick (Ixodes pacificus, the black-legged tick) is active along the West Coast. The deer tick is a very small tick, much smaller than the dog tick or wood tick. Deer ticks, both adults and nymphs, are dark reddish brown and have black legs and a pear-shaped body. All stages, especially nymphs and adults, feed on people. The deer tick is the most important carrier of Lyme disease and is the only known carrier of babesiosis. It is also the primary transmitter of human granulocytic anaplasmosis.
The Lone Star tick (Amblyomma americanum) is found throughout the south and southeastern United States, with a high density in the Ozarks. Adults are about one-quarter inch long; nymphs, which are the most aggressive biters, are pinhead sized. The ticks are reddish brown, and the female has a white mark on the middle of her back. The smaller male has lacy white markings on the rear edge of his back. Lone Star ticks transmit monocytic ehrlichiosis, tularemia, and a variant form of Lyme disease, Southern tick-associated rash illness (STARI).
The American dog tick (Dermacentor variabilis) is widely distributed in the eastern half of the United States and is also found on the West Coast. It resembles the wood tick in appearance. The unfed female has silvery-gray markings on the shield on her back; the rest of the body is reddish brown. It is bigger than other ticks—approximately one-eighth inch to one-quarter inch long—and although it prefers dogs, it does bite people. The dog tick is the most important transmitter of Rocky Mountain spotted fever. It also transmits tularemia and probably transmits human granulocytic anaplasmosis. The dog tick can cause tick paralysis.
The Rocky Mountain wood tick (Dermacentor andersoni) is a hard tick that resembles the American dog tick and the Pacific coast tick. The female has silvery-gray markings on the shield on her back; the rest is reddish brown. This tick is the prime carrier of Rocky Mountain spotted fever in the West, and it also transmits tularemia and Colorado tick fever (mountain fever). It is the most important cause of tick paralysis in the United States.
Relapsing fever ticks (Ornithodoros hermsi, O. parkeri, O. talaje, O. turicata) are soft ticks that transmit relapsing fever, a spirochetal disease. Their bites can be painful. Relapsing fever ticks are widely scattered west of the Mississippi River. Adults are oval-shaped and colored gray to pale blue. Larvae and nymphs are gray.
The brown dog tick (Rhipicephalus sanguineus) is found throughout the United States wherever you find dogs. Although suspected of carrying ehrlichiosis, these ticks are probably not disease transmitters. The male is uniformly dark brown. The female is brown but the shield on her back is darker than the rest of her body.
EHRLICHIOSIS & ANAPLASMOSIS
These diseases are caused by bacteria transmitted by ticks. Two types of human infections are recognized in the United States. The infections have the same symptoms and the same treatment but differ in their geographic distribution because of the fact they are transmitted by different species of ticks.
- Human monocytic ehrlichiosis (HME), caused by Ehrlichia chaffeensis, is transmitted by Lone Star ticks (Amblyomma americanum). These ticks are found predominantly in the Southeast and Mid-Atlantic states.
- Human granulocytic anaplasmosis (HGA), previously known as human granulocytic ehrlichiosis (HGE), is a tick-associated disease caused by a species of bacteria called Anaplasma phagocytophilum. HGA is transmistted to humans by the bite of the deer tick and western black-legged tick. Deer ticks infected with A. phagocytophilum are found in the New England and North Central United States while black-legged ticks infected with A. phagocytophilum are found in northern California. States reporting the highest incidence of HGA in 2001-2002 were Rhode Island, Minnesota, Connecticut, New York, and Maryland. Overseas, cases of HGA have been documented in many countries in Europe, as well as Argentina, Japan, Malaysia, and Venezuela.
Symptoms of the two infections are practically identical. Most patients have a nonspecific flulike illness with chills, fever, headache, muscle aches, and malaise. In 15% of the cases, however, the disease is more severe and patients can develop kidney failure, pneumonia, and neurologic changes such as seizures and coma. The fatality rate is 2% to 5%.
Suspect the diagnosis in anybody with a febrile illness (with the symptoms above) who has a low white blood cell count (leukopenia), low platelet count (throbocytopenia) and abnormal liver function tests, and who has been in an endemic area, especially between April and September. The white blood cells of some patients will have characteristic inclusion bodies in their white blood cells. Antibody tests may be used to establish a diagnosis.
When a case of ehrlichiosis or anaplasmosis is suspected, treatment should be started immediately. The drug of choice is doxycycline, 100 mg twice daily, for 10 to 14 days.
PREVENTIONThis is a tick-borne illness that can be prevented by insect bite prevention measures, utilizing permethrin on clothing and DEET applied to exposed skin.
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