Abstract

Ehrlichiosis is a tickborne

bacterial infection, caused by bacteria of the family Anaplasmataceae, genera "Ehrlichia" and "Anaplasma". These obligate intracellular bacteria infect and kill white blood cells.

The average reported annual incidence is on the order of 2.3 cases per million people.

Species

Five (see note below) species have been shown to cause human infection:

- "Anaplasma phagocytophilum" (which causes human granulocytic anaplasmosis,). "A. phagocytophilum" is endemic to New England and the north central and Pacific regions of the United States.

- "Ehrlichia ewingii" (which causes human ewingii ehrlichiosis). "E. ewingii" primarily infects deer and dogs (see Ehrlichiosis (canine)). "E. ewingii" is most common in the south central and southeastern states.

- "Ehrlichia chaffeensis" (which causes human monocytic ehrlichiosis). "E. chaffeensis" is most common in the south central and southeastern states.

- "Ehrlichia canis"

- "Neorickettsia sennetsu"

The latter two infections are not well studied.

In 2006, human infection by Boone County, Missouri. Symptoms: high fever,severe joint and muscle aches, vomiting. Onset of symptoms began exactly 14 days after bite occurred. Symptoms affected patient on a 12-hour cycle; beginning early evening, ending early morning. No symptoms occurred for next 12 hours. CBC presented low WBC.

Note: In 2008, human infection by Panola Mountain (Georgia, USA) "Ehrlichia" species was reported. 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.

"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.

Signs and symptoms

The most common symptoms include headache, muscle aches, and fatigue. A rash may occur, but is uncommon. Ehrlichiosis can also blunt the immune system by suppressing production of TNF-alpha, which may lead to opportunistic infections such as candidiasis.

Most of the signs and symptoms of ehrlichiosis can likely be ascribed to the immune dysregulation that it causes. A "toxic shock-like" syndrome is seen in some severe cases of ehrlichiosis. Some cases can present with purpura and in one such case the organisms were present in such overwhelming numbers that 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.

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.

3% of human monocytic ehrlichiosis cases result in death; however, these deaths occur "most commonly in immunosuppressed individuals who develop respiratory distress syndrome, hepatitis, or opportunistic nosocomial infections."

Epidemiology

Ehrlichiosis is a nationally notifiable disease in the United States. There have been cases reported in every month of the year, but most cases are reported during April–September. These months are also the peak months for tick activity in the United States.

From 2008-2012, the average yearly incidence of ehrlichiosis was 3.2 cases per million persons. This is more than twice the estimated incidence for the years 2000-2007. The incidence rate increases with age, with the ages of 60–69 years being the highest age-specific years. Children of less than 10 years and adults aged 70 years and older, have the highest case-fatality rates. There is a documented higher risk of death among persons who are immunosuppressed.

Treatment

Doxycycline and minocycline are the medications of choice. For people allergic to antibiotics of the tetracycline class, rifampin is an alternative. Early clinical experience suggested that chloramphenicol may also be effective, however, in vitro susceptibility testing revealed resistance.

Prevention

No human vaccine is available for ehrlichiosis. Tick control is the main preventive measure against the disease. However, in late 2012 a breakthrough in the prevention of CME (canine monocytic ehrlichiosis) was announced when a vaccine was accidentally discovered by Prof. Shimon Harrus, Dean of the Hebrew University of Jerusalem's Koret School of Veterinary Medicine.