Los Angeles County West Vector & Vector-Borne Disease Control District
What is ehrlichiosis?
Ehrlichiosis is the general name used to describe several bacterial
diseases that affect animals and humans. These diseases are caused by the
organisms in the genus Ehrlichia. Worldwide, there are currently four
ehrlichial species that are known to cause disease in humans.
Human ehrlichiosis due to Ehrlichia
chaffeensis was first described in 1987. The disease occurs primarily in
the southeastern and south central regions of the country and is primarily
transmitted by the lone star tick, Amblyomma americanum (see photo to
Human granulocytic ehrlichiosis (HGE)
represents the second recognized ehrlichial infection of humans in the United
States, and was first described in 1994. The name for the species that causes
HGE has not been formally proposed, but this species is closely related or
identical to the veterinary pathogens Ehrlichia equi and Ehrlichia
phagocytophila. HGE is transmitted by the blacklegged tick (Ixodes
scapularis) and the western blacklegged tick (Ixodes pacificus) in
the United States.
Ehrlichia ewingii is
the most recently recognized human pathogen. Disease caused by E. ewingii
has been limited to a few patients in Missouri, Oklahoma, and Tennessee, most of
whom have had underlying immunosuppression. The full extent of the geographic
range of this species, its vectors, and its role in human disease is currently
How do people get ehrlichiosis?
In the United States, ehrlichiae are transmitted by the bite of an
infected tick. The lone star tick (Amblyomma americanum), the
blacklegged tick (Ixodes scapularis), and the western blacklegged tick
(Ixodes pacificus) are known vectors of ehrlichiosis in the United
States. Ixodes ricinus is the primary vector in Europe.
What are the symptoms
The symptoms of ehrlichiosis may resemble
symptoms of various other infectious and non-infectious diseases. These
clinical features generally include fever, headache, fatigue, and muscle aches.
Other signs and symptoms may include nausea, vomiting, diarrhea, cough, joint
pains, confusion, and occasionally rash. Symptoms typically appear after an
incubation period of 5-10 days following the tick bite.
It is possible that many individuals who become infected with ehrlichiae do not
become ill or they develop only very mild symptoms.
In the United States,
where do most cases of ehrlichiosis occur?
Most cases of ehrlichiosis are reported within the geographic
distribution of the vector ticks (click on map below). Occasionally, cases are
reported from areas outside the distribution of the tick vector. In most
instances, these cases have involved persons who traveled to areas where the
diseases are endemic, and who had been bitten by an infected tick and developed
symptoms after returning home. Therefore, if you traveled to an ehrlichiosis-endemic
area 2 weeks prior to becoming ill, you should tell your doctor where you
Areas where human ehrlichiosis may occur based on approximate distribution of
vector tick species
(Click on Map)
How is ehrlichiosis diagnosed?
A diagnosis of ehrlichiosis is based on a combination of clinical signs
and symptoms and confirmatory laboratory tests. Your doctor can send your blood
sample to a reference laboratory for testing. However, the availability of the
different types of laboratory tests varies considerably. Other laboratory
findings indicative of ehrlichiosis include low white blood cell count, low
platelet count, and elevated liver enzymes.
How is ehrlichiosis
Ehrlichiosis is treated with a tetracycline antibiotic, usually
Can a person get
ehrlichiosis more than once?
Very little is known about immunity to ehrlichial infections.
Although it has been proposed that infection with ehrlichiae confers long-term
protection against reinfection, there have been occassional reports of
laboratory-confirmed reinfection. Short-term protection has been described in
animals infected with some Ehrlichia species and this protection wanes
after about 1 year. Clearly, more studies are needed to determine the extent and
duration of protection against reinfection in humans.
(Information and images on this page from the (Centers
for Disease Control and Prevention)