Causes

Ehrlichiosis and anaplasmosis are caused by different bacteria.

Ehrlichiosis is caused by different species of ehrlichia bacteria. The Lone Star tick — found in south-central, southeastern and eastern coastal states — is the primary carrier of bacteria causing ehrlichiosis. Black-legged ticks, commonly called deer ticks, in the Upper Midwest are less common carriers.

Anaplasmosis is caused by the bacterium Anaplasma phagocytophilum. It's primarily carried by deer ticks in the Upper Midwest, northeastern states and central Canadian provinces. It's also carried by the Western black-legged tick in Western coastal states and other tick species in Europe and Asia.

The ehrlichia and anaplasma species belong to the same family of bacteria. Although each bacterium appears to have a specific target among immune system cells in the host, all of these infectious agents generally cause the same symptoms.

Tick bites and infection

Ticks feed on blood by latching onto a host and feeding until they're swollen to many times their normal size. Ticks can pick up bacteria from a host, such as a deer, and then spread the bacteria to another host, such as a human. The spread of the bacteria from the tick to the host probably occurs about 24 hours after the tick has begun feeding.

Other ways bacteria spreads

Spread of the bacteria causing ehrlichiosis or anaplasmosis is possible through blood transfusions, from mother to fetus, or through direct contact with an infected, slaughtered animal.