Description

Two clinically described, tick-borne infections are human monocytic ehrlichiosis (HME), most commonly caused by Ehrlichia chaffeensis, and human granulocytic anaplasmosis (HGA), caused by Anaplasma phagocytophilum. HME can also be caused by two other Ehrlichia species in the United States: ewingii and emuris-like agent.

Epidemiology

• HME typically occurs in the midwest, south central, and southeastern United States, mirroring the pattern of Rocky Mountain spotted fever (RMSF). In addition, it has been found in Europe, South America, Asia, and Africa.
• HGA typically occurs in the north central, northeastern United States, and northern California, similar to Lyme disease. Most patients are infected during April through September, the months of greatest tick and human outdoor activity.
• A second peak of HGA occurs from late October to December.

General Prevention

• Avoid tick-infested areas.
• Clothes should cover arms and legs.
• Use tick repellents, but with caution in young children.
• A thorough body search should always be done after returning from a tick-infested area:
  • If a tick is found, the area should be cleaned with a disinfectant, and the tick should be removed immediately.
  • To remove the tick, grasp the tick at the point of origin with forceps, staying as close to the skin as possible.
  • Applying steady, even pressure, slowly pull the tick off the skin. After the tick has been removed, clean the skin with a disinfectant.
• Instruct parents to seek medical attention only if symptoms develop.
• No vaccine is available.

Pathophysiology

• Obligate intracellular, pleomorphic, gram-negative bacteria.
• Transmission to humans by a tick vector
• Incubation period from 5 to 14 days for HME and 5 to 21 days for HGA
• HME infects monocytes and macrophages, whereas HGA infects neutrophils.
• The bacteria reside and divide within cytoplasmic vacuoles of circulating leukocytes, called morulae.
• There is overinduction of the inflammatory and immune response, resulting in clinical manifestations of disease, including multiorgan system involvement.

Etiology

• HME is transmitted by Amblyomma americanum, the lone star tick. The white-tailed deer is the major reservoir.
• HGA is transmitted by Ixodes scapularis, the black-legged or deer tick, or the Western black-legged tick (Ixodes pacificus). Small mammals such as the white-footed mouse are the major reservoirs.
• Congenital infection is very rare but has been described in case reports.