What is anaplasmosis?

Anaplasmosis is a disease caused by the bacteria Anaplasma phagocytophilum, transmitted to humans by tick bites.  It is less common than Lyme disease in California but is transmitted by the same tick, the western black-legged tick (Ixodes pacificus).  On the east coast of the United States, the tick vector is the deer tick (Ixodes scapularis).  It is treatable with antibiotics and rarely fatal.   A person who has been bitten by a tick and develops symptoms of anaplasmosis should see their physician.

How is anaplasmosis transmitted?

Anaplasmosis is transmitted through the bite of an infected tick.

If I am bitten by a tick, will I get anaplasmosis?

Anaplasmosis is uncommon in San Mateo County.  Years of testing ticks in San Mateo County have shown less than 1% of adult western black-legged ticks tested positive for bacteria that cause anaplasmosis.   Checking for ticks after hiking or camping and removing ticks promptly will reduce the risk of acquiring the disease.

What are the signs and symptoms of anaplasmosis?

• Fever
• Headache
• Muscle pain
• Malaise
• Chills
• Nausea / Abdominal pain
• Cough
• Confusion

People will rarely develop all symptoms, and symptoms vary from person to person.  Symptoms usually develop 1-2 weeks following a tick bite. 

For more information, visit the CDPH website.

For more information, visit the CDC website.

Page last reviewed: February 20, 2025