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4. EHRLICHIOSIS

Ehrlichia body inside a stained monocyte

Two previously unknown tick-borne diseases have been discovered in the past decade: Human monocytic erhlichiosis (HME) and human granulocytic ehrlichiosis (HGE). The agents are bacteria that live inside monocytes and granulocytes, two types of circulating white blood cells. Since 1964, more than 400 confirmed cases of HME, and 170 cases of HGE have been reported, including some from northern California. Many were first thought to be Rocky Mountain Spotted Fever.

HME is caused by a bacteria named Ehrlichia chaffeensis, and HGE by another bacteria closely related to Ehrlichia equi. Similar agents cause severe disease in animal populations. The still undiscovered source of the human infections may involve domestic (HGE) and wild mammals (HME). Many cases of E. equi infection have been recorded from horses in Marin and Sonoma counties, but there is no formal reporting system so the actual numbers are not known. In one study site in the Sonoma Valley, workers found E. equi in 16 horses. A possible role of other vertebrates, like reptiles, in maintaining ehrlichia in the wild has not yet been studied.

In 1994, health workers studied a rural community in the Sonoma Valley, after several reports of locally acquired Lyme disease. Laboratory tests reported by the County and State Departments of Health Services, the Univeristy of California and the Centers for Disease Control, found that ten of the 219 residents tested had antibodies against E. chaffeensis, showing that at some time in the past they had been infected with the HME bacteria. Three of the ten reported having symptoms that could have been caused by the infection.

The ticks most often found carrying E. chaffeensis has been Amblyomma americanum and Dermacentor variabilis (see photo), while Ixodes and Dermacentor ticks have been discovered carrying the E. equi-like agent. The western black-legged tick, Ixodes pacificus is believed to transmit E. equi among horses in northern California.

Human ehrlichiosis is virtually impossible to diagnose from the symptoms alone. The first signs usually appear one to three weeks after a tick bite. Initial symptoms suggest a developing flu, with a high, non-specific fever, pains in the muscles and joints and severe headache. A skin rash may develop. These, plus a marked decrease in the numbers of thrombocytes and other white blood cells, and a history of contact with ticks, suggest a patient should be tested for ehrlichiosis. Kidney, heart or respiratory failure and nerve deterioration may occur later on, and fatalities have been reported.

Helpful reference:

Human ehrlichiosis: new emerging tick-borne diseases in California. by K. Glynn, V. Krammer and D. Vugia. in California Morbidity, January 1996.