Ehrlichiosis

Ehrlichiosis is a tick-borne infection of mononuclear cells and granulocytes that affects various mammals, including mice, cattle, dogs, deer, horses, sheep, goats, and humans. Ehrlichia are obligate, intracytoplasmic bacteria that resemble Rickettsia and are tiny gram-negative organisms.^[1]

• One of the most common Ehrlichia species infecting humans is Ehrlichia chaffeensis, which mainly infects human monocytes and macrophages (human monocytic ehrlichiosis - HME).
• Anaplasma phagocytophilum causes human granulocytic anaplasmosis (HGA).
• Ehrlichia species that infect granulocytes are called human granulocytic ehrlichiosis (HGE).

The clinical manifestations of HME, HGA and HGE are the same.

Epidemiology

• Ehrlichial pathogens are distributed globally, mainly in temperate regions, and have been reported throughout Europe.^[2]
• More common in males than females.
• Most often occurs in young adults.
• Elderly patients are more likely to develop severe infections.

Presentation

• More than 90% of patients give a history of having been bitten by several ticks.
• Incubation period is 1-4 weeks and infection presents as a febrile illness with headache, malaise, myalgia, arthralgia and rigors.^[3]
• May also have nausea, vomiting and anorexia.
• Infection may also lead to confusion or a non-specific rash, which can occur anywhere on the body and not necessarily at the site of the tick bite.
• There are no specific findings on clinical examination but mild hepatomegaly, lymphadenopathy and splenomegaly may occur.

Differential diagnosis

• Any cause of an influenza-like illness.
• Other possible diagnoses to consider are babesiosis, malaria, meningitis, rocky mountain spotted fever and typhoid fever.^[1]

Investigations

• Blood count and film: leucopenia, thrombocytopenia.^[4] Characteristic morulae (cytoplasmic vacuoles in which the Ehrlichia species grow in peripheral leucocytes) are diagnostic.
• Mild-to-moderate elevations of aminotransferase levels may be present.^[5]
• An indirect fluorescent antibody test is used for diagnosis. The diagnosis of HME or HGE is made by a single elevated IgG immunofluorescent antibody (IFA) Ehrlichia titre or by demonstrating an increase between acute and convalescent IFA Ehrlichia titres.^[1]
• Polymerase chain reaction technology provides the most sensitive and specific serology testing but this is a research tool and only available in selected hospitals.

Management^[1]

• Doxycycline is very effective in arresting progression of either form of human ehrlichiosis and is continued for 14 days.
• Rifampin may be useful in patients unable to take doxycycline, eg for children and in pregnancy.

Complications

• Systemic complications may involve the respiratory tract (sore throat, cough, pulmonary infiltrates, acute respiratory distress syndrome), the digestive system (nausea, vomiting, abdominal pain, gastrointestinal bleeding), or hepatitis.
• Other possible complications with severe infection include meningitis, pericarditis, renal failure, and disseminated intravascular coagulation.

Prognosis

• The prognosis is excellent in otherwise healthy people with normal immunity.
• Many cases may be sub-clinical and self-limiting.

Prevention

• Wear light-coloured clothes, tuck trousers into socks, use insect repellent.
• Regularly examine the body for ticks and remove ticks promptly (a feeding period of 3-48 hours is required for the disease to be transmitted).