Ehrlichiosis is a tick-borne disease which can be caused by either of two different organisms. Human monocytic ehrlichiosis (HME) is caused by Ehrlichia chaffeensis, which is transmitted by the lone star tick (Amblyomma americanum). Human granulocytic anaplasmosis (HGA), previously known as human granulocytic ehrlichiosis (HGE), is caused by Anaplasma phagocytophilia, also called anaplasmosis, transmitted by the deer tick (Ixodes scapularis). In New York State, most cases of ehrlichiosis have been reported on Long Island and in the Hudson Valley.
The Ehrlichiae are a group of small, gram-negative, pleiomorphic, obligate intracellular cocci that infect different blood cells in various animal species and in humans. There has recently been a reclassification of the family Anaplasmataceae to which the Ehrlichiae belong.8 According to this new classification there are two leukotrophic diseases in dogs that are caused by bacteria in the genus Ehrlichia, namely, Canine Monocytic Ehrlichiosis (caused mainly by Ehrlichia canis) and Canine Granulocytic Ehrlichiosis (caused by Ehrlichia ewingii).
In the United States, HME has so far been concentrated in the southeast and south-central regions. HGE has been found mainly in the upper midwestern and northeastern states, but also in northern California. People get ehrlichiosis from the bite of an infected tick. Evidence suggests that the lone star tick transmits HME and that the deer tick transmits HGE. The deer tick also spreads Lyme disease and babesiosis.
Ehrlichiosis is a bacterial illness transmitted by ticks. Ehrlichiosis causes flu-like symptoms. Another tick-borne infection — anaplasmosis — is closely related to ehrlichiosis. But the two have distinct differences and are caused by different microorganisms. The signs and symptoms of ehrlichiosis range from mild body aches to severe fever and usually appear within a week or two of a tick bite. If treated quickly with antibiotics, ehrlichiosis generally improves within a few days.
The bacteria are transmitted to humans by the bite of infected ticks. In the southern USA, the lone star tick (Amblyomma americanum) is thought to be responsible for transmission. In the upper Midwest, the agent of human granulocytic ehrlichiosis is transmitted by the deer tick (Ixodes scapularis), the same tick responsible for the spread of Lyme disease.
Most cases of ehrlichiosis are reported within the geographic distribution of the vector ticks (see map below). Occasionally, cases are reported from areas outside the distribution of the tick vector. In most instances, these cases have involved persons who traveled to areas where the diseases are endemic, and who had been bitten by an infected tick and developed symptoms after returning home. Therefore, if you traveled to an ehrlichiosis-endemic area 2 weeks prior to becoming ill, you should tell your doctor where you traveled.
The acute stage of the disease, occurring most often in the spring and summer, begins one to three weeks after infection and lasts for two to four weeks. Clinical signs include a fever, petechiae, bleeding disorders, vasculitis, lymphadenopathy, discharge from the nose and eyes, and edema of the legs and scrotum. There are no outward signs of the subclinical phase. Clinical signs of the chronic phase include weight loss, pale gums due to anemia, bleeding due to thrombocytopenia, vasculitis, lymphadenopathy, dyspnea, coughing, polyuria, polydipsia, lameness, ophthalmic diseases such as retinal hemorrhage and anterior uveitis, and neurological disease.
The disease responds well to treatment with doxycycline or other tetracyclines. And, fortunately, diseases due to other tickborne agents--Rocky Mountain spotted fever and Lyme disease--also respond to these drugs. Treatment with antibiotics should be considered if ehrlichiosis is suspected because delayed treatment while awaiting laboratory confirmation of the disease may increase the risk for adverse outcomes.
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