Ehrlichiosis

Ehrlichiosis
ICD-10	A79.8
ICD-9	082.4
DiseasesDB	31663
MedlinePlus	001381
MeSH	D016873

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Raviteja Guddeti, M.B.B.S. [2]

Synonyms and keywords: Sennetsu fever; human granulocytic ehrlichiosis; Anaplasma phagocytophilum; Ehrlichia phagocytophila; human monocytic ehrlichiosis

Overview

Ehrlichiosis is a tickborne bacterial infection, caused by bacteria of the family Anaplasmataceae, genera Ehrlichia and Anaplasma. These obligate intracellular bacteria infect and kill the white blood cells. Ehrlichiae are transmitted to humans by the bite of an infected tick

• On August 3, 2011, infection by a yet-unnamed bacterium in the genus Ehrlichia carried by deer ticks that has caused flu-like symptoms in at least 25 people in Minnesota and Wisconsin was reported; human ehrlichiosis was thought to be very rare or absent in Minnesota and Wisconsin. The new species, which is very similar genetically to an Ehrlichia species found in Eastern Europe and Japan called E. muris, was identified at Mayo Clinic Health System's Eau Claire hospital.
• In 1991 Dr. Aileen Marty of the AFIP was able to demonstrate the bacteria in human tissues using standard stains, and later proved that the organisms were indeed Ehrlichia using immunoperoxidase stains.

Pathophysiology

• Ehrlichia are transported between cells through the host cell filopodia during initial stages of infection, whereas, in the final stages of infection the pathogen ruptures the host cell membrane.
• Most of the symptoms of Ehrlichiosis can likely be ascribed to the immune dysregulation that it causes.
• Early in infection, production of TNF-alpha, a cellular product that promotes inflammation and immune response, is suppressed. Experiments in mouse models further supports this hypothesis, as mice lacking TNF-alpha I/II receptors are resistant to liver injury caused by ehrlichia infection.
• Late in infection, however, production of this substance can be upregulated by 30 fold, which is likely responsible for the "toxic shock-like" syndrome seen in some severe cases of ehrlichiosis.

Causes

Five species of the genus Ehrlichia have been shown to cause human infection:

• Anaplasma phagocytophilum (which causes human granulocytic anaplasmosis, formerly known as human granulocytic ehrlichiosis).
• Ehrlichia ewingii (which causes human ewingii ehrlichiosis). E. ewingii primarily infects deer and dogs (see Ehrlichiosis (canine)).
• Ehrlichia chaffeensis (which causes human monocytic ehrlichiosis).
• Ehrlichia canis
• Neorickettsia sennetsu

The latter two infections are not well studied.

Epidemiology and Demographics

• The average reported annual incidence is 0.7 cases per million population.
• A. phagocytophilium is endemic to New England and the north central and Pacific regions of the United States.
• E. chaffeensis is most common in the south central and southeastern states.
• E. ewingii is most common in the south central and southeastern states.

Risk Factors

Risk factors for ehrlichiosis include:

• Living near an area with a lot of ticks
• Owning a pet that may bring a tick home
• Walking or playing in high grasses

Natural History, Complications and Prognosis

Complications

• Respiratory distress syndrome
• Hepatitis
• Kidney damage
• Lung damage
• Other organ damage
• Seizure
• Candidiasis
• Opportunistic nosocomial infections
• Death

Diagnosis

Symptoms

• Chills
• Fever
• Headache
• Nausea
• Muscle aches / myalgias
• Fatigue

Other possible symptoms include:

• Diarrhea
• Fine pinhead-sized areas of bleeding in the skin (petechial rash)
• Flat red rash (maculopapular rash)
• General ill feeling (malaise)

A rash appears in fewer than half of cases.

Physical Examination

Vitals

• Elevated body temperature
• Increased heart rate
• Blood pressure

Skin

• Rash
• Purpura - can be a presenting feature.

Head

• Swollen lymph nodes

Laboratory Findings

• Complete blood count (CBC)
• Granulocyte stain
• Fluorescent antibody test

Treatment

• Doxycycline is the drug of choice.
• For people allergic to drugs of the tetracycline class, rifampicin is an alternative.

• Early clinical experience suggested that chloramphenicol may also be effective, however in vitro susceptibility testing revealed resistance.

References

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