Acute HME syndrome

Overview

Acute HME syndrome refers to the sudden and severe manifestation of symptoms associated with human monocytic ehrlichiosis (HME), a tick-borne infectious disease caused by the bacterium Ehrlichia chaffeensis. While most cases of HME present with flu-like symptoms, acute HME syndrome is a more severe, life-threatening form that involves multi-organ dysfunction, central nervous system involvement, or even septic shock. Prompt recognition and antibiotic treatment are critical to improving outcomes.

Causes

Acute HME syndrome is caused by an infection with Ehrlichia chaffeensis, an obligate intracellular bacterium transmitted primarily by the lone star tick (Amblyomma americanum). The infection targets monocytes, a type of white blood cell, impairing immune function and triggering a systemic inflammatory response. Risk factors include:

• Tick bites, especially in southeastern and south-central United States

• Outdoor activities in wooded or grassy areas

• Immunocompromised individuals (e.g., elderly, transplant recipients)

Symptoms

The symptoms of acute HME syndrome typically appear 1–2 weeks after a tick bite and are more severe than standard ehrlichiosis. They may include:

• High fever and chills

• Severe headache

• Muscle aches (myalgia)

• Confusion, altered mental status, or seizures

• Difficulty breathing or respiratory failure

• Multisystem organ failure

• Low blood pressure or shock

• Rash: More common in children

• Hepatitis or elevated liver enzymes

Without early intervention, the infection can progress rapidly, particularly in vulnerable populations.

Diagnosis

Timely diagnosis is essential and typically based on clinical suspicion in patients with tick exposure and compatible symptoms. Diagnostic methods include:

• Polymerase chain reaction (PCR): Detects Ehrlichia DNA in blood

• Blood smear: May show morulae (bacterial clusters) in monocytes

• Serologic testing: Indirect immunofluorescence assay (IFA), though often negative early in the illness

• Complete blood count (CBC): Often shows low white cell and platelet counts

• Liver function tests: Elevated AST and ALT

Because confirmatory tests may take time, treatment should not be delayed while awaiting results if HME is strongly suspected.

Treatment

The treatment of choice for both mild and severe cases of acute HME syndrome is:

• Doxycycline: 100 mg orally or intravenously twice daily for adults (or weight-based dose in children). Treatment is typically given for 7–14 days.

Supportive care may also be needed in acute cases, including:

• Intravenous fluids and electrolytes

• Oxygen or mechanical ventilation (for respiratory failure)

• Management of shock and organ dysfunction

• Intensive care unit (ICU) monitoring for critically ill patients

Doxycycline should be started immediately upon clinical suspicion—delay in treatment increases the risk of severe complications and death.

Prognosis

With early recognition and prompt treatment, the prognosis for acute HME syndrome is favorable. Most patients respond well to doxycycline within 24–48 hours. However, delayed treatment can lead to severe complications including:

• Respiratory failure

• Neurological impairment

• Renal or hepatic failure

• Disseminated intravascular coagulation (DIC)

• Death (reported mortality rate up to 5–10% in severe cases)

Preventive measures such as tick avoidance and prompt removal of ticks remain key strategies in reducing the risk of infection.