Human monocytotropic ehrlichiosis
Tick-borne disease caused by Ehrlichia chaffeensis infecting monocytes.
Human monocytotropic ehrlichiosis is a tick-borne illness caused by the bacterium *Ehrlichia chaffeensis*, which lives only inside host cells and targets monocytes and macrophages. People typically catch it from the bite of an infected tick, most often the lone star tick (*Amblyomma americanum*) in the United States.
Common symptoms include fever, headache, general discomfort, and muscle aches. A rash is more frequent here than in human granulocytic anaplasmosis. Lab results often show low platelet counts, low white blood cell counts, and elevated liver enzymes. The illness can range from mild or symptom-free to life-threatening. It may affect the central nervous system, and in people with weakened immune systems, it can lead to a severe septic or toxic shock-like condition.
Diagnosis can be tricky because tick exposure is often missed. For patients in high-risk areas who spend time outdoors, doctors should keep a high suspicion. Antibody tests may come back negative early on, so polymerase chain reaction (PCR) testing is the preferred lab method. If ehrlichiosis is suspected, treatment should start right away without waiting for lab confirmation, since prompt doxycycline improves outcomes. Doxycycline is the main treatment. Early pregnancy complicates therapy; rifampin has been used in pregnant patients or those allergic to doxycycline.
In the United States, the disease occurs across the south-central, southeastern, and mid-Atlantic states, where white-tailed deer and lone star ticks are common. It also appears in California, carried by *Ixodes pacificus* and *Dermacentor variabilis* ticks. Nearly 600 cases were reported to the CDC in 2006. From 2001 to 2002, the highest incidence was in Missouri, Tennessee, and Oklahoma, and among people over 60.
- Caused by
- Ehrlichia chaffeensis
- Primary cells affected
- Monocytes and macrophages
- Common symptoms
- Fever, headache, malaise, myalgia
- Laboratory abnormalities
- Thrombocytopenia, leukopenia, elevated liver tests
- Treatment of choice
- Doxycycline
- Primary vector
- Lone star tick (Amblyomma americanum)
- Geographic range in us
- South-central, southeastern, mid-Atlantic states; also California
Lore & Background
Human monocytotropic ehrlichiosis is caused by Ehrlichia chaffeensis, an obligate intracellular bacterium that primarily infects monocytes and macrophages. Humans usually acquire the infection through the bite of infected ticks, particularly the lone star tick (Amblyomma americanum) in the United States. The severity of the illness can range from minor or asymptomatic to life-threatening, with central nervous system involvement possible and a serious septic or toxic shock-like picture developing especially in patients with impaired immunity.
Reader's Guide
Human monocytotropic ehrlichiosis is significant as a tick-borne illness that can cause severe disease, particularly in immunocompromised individuals. Its diagnosis is challenging because tick exposure is often overlooked and serologies can be negative in the acute period; polymerase chain reaction is the laboratory diagnostic tool of choice. Prompt treatment with doxycycline is critical, as delayed therapy is associated with worse outcomes. The disease occurs across the south-central, southeastern, and mid-Atlantic United States, with nearly 600 cases reported to the CDC in 2006. Incidence was highest in Missouri, Tennessee, and Oklahoma, and in people older than 60. The bacterium also occurs in California in Ixodes pacificus and Dermacentor variabilis ticks.
Did You Know?
- The most common symptoms are fever, headache, malaise, and muscle aches.
- Rash is more common in human monocytotropic ehrlichiosis than in human granulocytic anaplasmosis.
- Doxycycline is the treatment of choice, and treatment should not be delayed while waiting for laboratory confirmation.
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