Ehrlichiosis

Etiology

The genus Ehrlichia includes Ehrlichia chaffeesis, which causes human monocytic ehrlichiosis (HME), and Ehrlichia ewingii, which causes Ehrlichia ewingii ehrlichiosis. Other species within this genus include Ehrlichia canis and Ehrlichia ruminantium, which are predominantly veterinary pathogens but may also occasionally infect humans, and the recently described EMLA (Ehrlichia muris-like agent), which also has associations with human disease. Ehrlichia are intracellular, gram-negative organisms transmitted via tick bite and can replicate in both the tick and the infected host. In addition to humans, other hosts include dogs, cattle, sheep, goats, rodents, and deer.

Epidemiology

The frequency of Ehrlichia diagnoses is increasing, possibly due to increased recognition and diagnostic availability and the expansion of the regions in which the most common tick vector, the Lone Star tick (Amblyomma americanum), resides. Most Ehrlichia infections in the United States are due to Ehrlichia chaffeesis, with Ehrlichia ewingii reported less frequently. Ehrlichia infections occur predominantly in areas where the Lone Star tick (Amblyomma americanum) is prevalent, such as the South Central United States. According to the Centers for Disease Control and Prevention data, over 30% of reported Ehrlichia infections in the United States were from Oklahoma, Missouri, and Arkansas. Infections are reported most commonly during the summer months, coincident with the adult and nymphal stages of the tick life cycle in which ticks are most likely to bite humans. Infections are also reported most commonly in the elderly. However, children are known to experience milder or subclinical infections, and thus, reports may underrepresent this population.[4][5]

Pathophysiology

Infection with Ehrlichia species begins with the intracellular uptake of the infectious extracellular form of the organism, the elementary body (EB) or dense core (DC). The elementary body/dense core is then taken up by endocytosis, where the organism replicates and matures to form a reticulate body or reticulate core (RB/RC) and then morula before redifferentiating into an elementary body/dense core that leaves the infected host cell to spread infection. During this process, Ehrlichia utilizes many immune evasion mechanisms, including suppressing apoptosis of host cells, modulation of chemokine and cytokine responses, and down-regulation of host pattern recognition receptors that might enable clearance of the infection. Ehrlichia preferentially infects peripheral blood leukocytes, with E chaffeensis associated with human monocytic cells, including monocytes and macrophages, and neutrophil infections reported in E ewingii. Multiorgan involvement can occur, with organisms detected in the spleen, lymph nodes, bone marrow, and peripheral blood. The clinical manifestations of Ehrlichia infections appear to be due to the host's inflammatory response to the infection rather than direct damage from the bacteria.

History and Physical

Patients with Ehrlichia infections, regardless of organism, typically present with fever, malaise, headache, body aches, and chills. Symptoms typically begin 1 to 2 weeks after a tick bite, at a median of 9 days. It is important to collect history regarding travel to endemic areas and outdoor activities of patients with these non-specific flu-like symptoms during the summer months. Prompt recognition and treatment of ehrlichiosis and other tick-borne illnesses may improve outcomes. Other findings may include gastrointestinal symptoms, such as nausea, vomiting, diarrhea, and rash. The rash is more common in children and typically presents 5 days into illness as a maculopapular rash, petechiae, or diffuse erythema. Cough and respiratory symptoms are more common in adults than in children. Like rickettsial infections, central nervous system involvement can also occur in up to 20% of patients, including meningitis and meningoencephalitis.

In some patients, symptoms may progress to acute respiratory distress syndrome (ARDS) or a sepsis-like or shock-like presentation with cardiovascular instability and coagulopathy. However, overall mortality is much lower in ehrlichiosis than in rickettsial disease. The Centers for Disease Control and Prevention (CDC) reports 1% to 3% mortality rates since 2000 in patients who present for medical care of ehrlichiosis. Importantly, many patients may be infected with ehrlichiosis but not present for medical evaluation, and thus, this may be an overestimate of mortality. Patients who are immunocompromised, elderly, or have received pretreatment with sulfonamide antibiotics are predisposed to more severe illness.[6][7][8][9]

Evaluation

Laboratory findings in ehrlichiosis may include leukopenia, thrombocytopenia, and mild to moderately elevated transaminases. Hyponatremia may also be a finding. In some cases, morulae may be present in the peripheral blood or other body fluids of a patient with ehrlichiosis. However, this is not considered a diagnostic of ehrlichiosis. Cerebrospinal leukocytosis and mildly elevated protein levels may also occur. In most centers, diagnosis of ehrlichiosis occurs via polymerase chain reaction (PCR), which is particularly useful in these infections given their predilection for infecting circulating leukocytes. Treatment with doxycycline can decrease the sensitivity of PCR, so samples should be collected before initiation of treatment if possible without significantly delaying therapy. Serology may also be an option; however, culture is not a routine test to diagnose ehrlichiosis.

Treatment / Management

Doxycycline is the preferred drug for the treatment of ehrlichiosis, as well as for rickettsial infection. There is no clinical data to support alternative agents for ehrlichiosis, although in vitro studies suggest that rifampin may be an option. However, as there is no demonstrated clinical efficacy of rifampin in treating ehrlichiosis and as rifampin has no efficacy against rickettsial infections, which can be indistinguishable from ehrlichiosis in the early stages of illness, doxycycline remains the drug of choice. Patients with ehrlichiosis typically defervesce in 48 to 72 hours after initiation of doxycycline. Treatment courses are typically 5 to 7 days, based on illness severity. Clinical studies have demonstrated that tooth staining is a rare cosmetic complication of doxycycline at the doses and duration used to treat ehrlichiosis and rickettsial infections. Concerns regarding age and potential tooth staining should not delay treatment with doxycycline in children with suspected ehrlichiosis. There is no recommendation for post-tick bite prophylaxis against ehrlichiosis in endemic areas.[10][11]

Differential Diagnosis

The differential diagnoses for ehrlichiosis include the following:

• Bacterial sepsis  

• Endocarditis

• Hepatitis

• Influenza

• Kawasaki disease

• Leptospirosis

• Murine typhus

• Q fever

• Toxic shock syndrome

• Typhoid fever

Prognosis

Ehrlichiosis generally responds well when diagnosed and treated promptly, with symptomatic improvement within 24 to 48 hours. However, in those with additional health conditions, severe infection, or who experience a delay in treatment, the disease can be serious or even fatal; this is why empiric therapy of suspected ehrlichiosis should commence immediately, even before the finalizing of laboratory results.[12]

Complications

In rare instances, eg, in patients with a weak immune system (as can be the case with HIV or cancer), ehrlichiosis can lead to serious complications. These include:

• Brain conditions, including confusion, seizures, or coma
• Hemorrhage
• Heart failure
• Respiratory failure
• Kidney failure
• Septic shock

These complications may require additional treatment, including respiratory support, IV fluids, or kidney dialysis. Complications are more prevalent in the elderly or people with pre-existing health conditions.[13]