ehrlichiosis

ehrlichiosis, any of a group of illnesses stemming from a tick-borne bacterial infection that affects humans and other animals, especially dogs. Ehrlichiosis is caused by bacteria in the genus Ehrlichia, mainly E. canis, E. chaffeensis, E. ewingii, and E. muris eauclairensis. It primarily infects white blood cells and can range from mild to life-threatening if not treated promptly.

Ehrlichiosis occurs in places inhabited by both the bacteria that cause the disease and their vectors, namely the lone star tick (Amblyomma americanum) and the blacklegged tick (Ixodes scapularis; also known as the deer tick). Such places include Africa, Eurasia, and North and South America. Infections in humans most commonly occur in the southeastern and south-central United States. Since 2000 the incidence of the disease in these regions has increased dramatically, from about 200 cases reported annually to nearly 2,100 in 2019 alone. This increase is likely associated with the growing population of the lone star tick, the primary vector for E. chaffeensis and E. ewingii.

Symptoms

Ehrlichiosis in humans typically begins one to two weeks after a tick bite and often presents with flu-like symptoms, such as fever, chills, headache, muscle aches, fatigue, and sometimes nausea or vomiting. In more severe cases, patients may experience confusion, respiratory issues, bleeding problems, or a rash—especially in children. If left untreated, the illness can progress to life-threatening complications involving the heart, lungs, kidneys, or brain.

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In animals, particularly dogs, ehrlichiosis follows a similar pattern. During the acute phase, dogs may exhibit fever, lethargy, loss of appetite, swollen lymph nodes, or bleeding disorders (e.g., nosebleeds). Some dogs enter a subclinical phase with no visible signs, while chronic ehrlichiosis may lead to severe weight loss, anemia, joint pain, neurological symptoms, and even bone marrow failure. The severity of symptoms in both humans and animals can vary widely depending on the individual’s immune response, age, and how quickly treatment is started.

Diagnosis and treatment

Diagnosis of ehrlichiosis is based on a combination of clinical symptoms, recent tick exposure, and laboratory tests. A complete blood count (CBC) often reveals low white blood cell and platelet counts; other tests may reveal elevated or abnormal liver enzymes, which can raise suspicion for the disease. More specific tests, such as polymerase chain reaction (PCR) assays or indirect immunofluorescence antibody tests, can confirm the presence of Ehrlichia bacteria.

Early treatment is crucial, and therefore if ehrlichiosis is suspected doctors often begin therapy before test results return. The primary treatment is the antibiotic doxycycline, which is typically administered in human patients for at least five days and for an additional three days after fever has subsided. Prompt treatment usually leads to rapid recovery, but delays can result in serious complications, especially in older individuals, immunocompromised patients, or animals in the chronic phase of infection. Supportive care, including fluids and pain management, may be necessary in more severe cases.