Ehrlichiosis is a disease caused by several tick-borne bacterial species in the genus Ehrlichia (pronounced err-lick-ee-uh) which were first recognized in 1935. Over the next several decades, since these veterinary pathogens that caused disease in dogs, cattle, sheep, goats, and horses were identified. Currently, three species of Ehrlichia in the United States and one in Japan are known to cause disease in humans; others will likely be recognized in the future as methods of detection improve.
There are two
major types of human ehrlichiosis in the U.S.; human monocytic ehrlichiosis
(HME), caused by E. chaffeensis; and human granulocytic ehrlichiosis (HGE),
caused by E. phagocytophila. Clinically, they are difficult to differentiate.
Symptoms for both include fever, headache, malaise, and muscle aches. Rashes
occur more frequently with HME than HGE. Treatment for both are with antibiotics
in the tetracycline family, most commonly doxycycline. Fatalities are rare (2-3%
of diagnosed cases), but can occur as the result of complications from
infection. Complete diagnosis requires serological or molecular tests to
differentiate Ehrlichia species, but treatment should begin after clinical
diagnosis. Asymptomatic infections probably occur with all Ehrlichia. Improved
diagnostic tools and an increased awareness of ehrlichiosis are revealing that
these infections are more common than previously suspected.
Both horses and dogs are susceptible to Ehrlichia,
although the species involved vary (see below for details). In dogs, the
clinical signs for different types of ehrlichiosis are similar and difficult to
separate clinically. These include fever, epilepsy, incoordination, lethargy,
anemia, and bleeding episodes. Asymptomatic infections are probably common. The
clinical signs in horses vary more between the two types of ehrlichiosis. Signs
of equine granulocytic ehrlichiosis (EGE, E. phagocytophila) include fever,
lethargy, anorexia, ataxia and limb edema. Equine monocytic ehrlichiosis
(Potomac horse fever, E. risticii) most often manifests as colitis (inflamation
of the colon), resulting in diarrhea, colic, loss of appetite, depression, and
Biology of Ehrlichia
Ehrlichia are bacteria,
related to Rickettsia, and are obligate intracellular parasites, meaning they
can not survive outside of a cell. These bacteria have only recently begun to
receive much research attention, and there are still many questions about their
transmission cycles and reservoir hosts. There are likely to be taxonomic
revisions of Ehrlichia and Anaplasma as further research occurs. Many Ehrlichia
are tick-borne, although there are some species which use other invertebrates as
intermediate hosts, such as snails and helminths. The transmission cycles for
some Ehrlichia species have not yet been determined.
Ehrlichia are often
differentiated based on the mammalian cell type they infect. Monocytes,
granulocytes, and neutrophils are most frequently involved, and the common name
of the resulting disease reflects the cell type (e.g. monocytic or granulocytic
ehrlichiosis). More than one species of Ehrlichia can cause disease in most
Species of Ehrlichia in the U.S.
Disease: humans (HME), rarely monocytic erhlichiosis in dogs.
Vectors: Amblyomma americanum (lone star tick); possibly Dermacentor
variabilis (American dog tick)
Distribution: HME has been diagnosed from
every state in the U.S. except the Dakotas. It is more common in the
southeastern U.S., largely congruent with the distribution of A.
Reservoir hosts: probably white-tailed deer, rodents and/or
Disease: granulocytic ehrlichiosis
in humans (HGE), horses (EGE), dogs, cattle.
Vector: In the eastern U.S.,
Ixodes scapularis (black-legged tick or deer tick). Elsewhere, other members of
the I. ricinus group (I. ricinus, I. pacificus, I. persulcatus).
Distribution: U.S., Europe. In the U.S., it has been reported from areas
where I. scapularis and I. pacificus are present, predominately in the
northeast, midwest and California. Cases have been identified in Florida, but
the level of transmission is unclear.
Reservoir hosts: rodents, possibly
A note on species nomenclature: Initially, the agent of HGE was
identified as an Ehrlichia but not named. Later, it was determined that the
agent of HGE, E. equii (agent of equine granulocytic ehrlichiosis), and E.
phagocytophila (agent of ehrlichiosis in cattle and deer in Europe), were
genetically almost identical. The name E. phagocytophila has priority, and all
three are now generally considered as E. phagocytophila. Some literature may
differentiate between the three, and revision of the group is likely to change
some generic and specific names.
primarily dogs (canine monocytic ehrlichiosis).
sanguineus (brown dog tick), possibly A. americanum.
Disease: primarily dogs (canine
granulocytic ehrlichiosis). Human infection is rare.
but likely to be R. sanguineus or A. americanum.
the southcentral states in the United States.
Horses (Potomac horse fever or equine monocytic ehrlichiosis); has also been
isolated from dogs. An equine vaccine is available, but protection is of short
duration and booster inoculations are required.
Vector: unknown but not
tick-borne. Snails and helminths may be involved as intermediate
Distribution: much of North America, particular the east coast;
Europe. More common along major waterways and in summer.
Other Ehrlichia have been described and may be agents of
disease for livestock and wild animals. The taxonomic status of some of these
are unclear and the transmission cycles are largely unknown.
Typically, there are 1-5 reported cases of human ehrlichiosis
in Florida each year. More cases probably occur, but are not severe enough to
seek medical attention or are not confirmed by laboratory tests. Veterinary
cases are not always reported, but both canine and equine ehrlichiosis also
occur in Florida.
In August 2001 an unusual cluster of 4 ehrlichiosis
cases in Jefferson county occurred, prompting the county health office to
declare a medical alert. The conditions that led to this cluster are unknown,
but it is likely to be related to tick populations and tick-human contact. The
species of Ehrlichia involved was confirmed in one case, and is probable in the
others, as E. chaffeensis.
Several species of ticks which transmit
Ehrlichia spp. are present in Florida. These include I. scapularis, A.
americanum, and D. variablis.
Prevention and Management
with any vector-borne pathogen, the primary disease preventative measure is to
minimize contact with the vectors. For humans, protective clothing, such as long
pants and socks tucked into pants will reduce tick contact; repellents
containing DEET are effective against most ticks. Permethrin-based repellents
can be sprayed on boots and clothing. For dogs, there are various treatments in
sprays, spot-ons and collars (active ingredients include permethrin, fipronil,
amitraz). Permethrin and pyrethroid based sprays and spot-ons for horses will
reduce tick bites.
For all host species, thorough tick checks and
grooming to remove attached ticks will reduce transmission of tick-borne
pathogens. Use fine-tipped forceps to remove ticks; grasp the tick near the skin
and pull straight back. Do not squeeze the abdomen or apply heat or petroleum
products; this may cause the tick to regurgitate into the host!
population reduction is difficult and it is unclear how effective it will be in
reducing infection rates. Various methods have been tested, including vegetation
management, acaracide treatment, host exclusion, and host treatment. Treatment
of hosts, via treated feed or feeding stations that apply acaricide to hosts,
are promising methods for population reduction of ticks which feed on
Bakken, J. S. and J. S. Dumler. 2001.
Proper nomenclature for the HGE agent. Emerging Infectious Diseases 7: 486. http://www.cdc.gov/ncidod/eid/vol7no3/bakken.htm
Health Topics: http://www.cdc.gov/ncidod/dvrd/ehrlichia/Index.htm
J. H. et al. 1999. The Human ehrlichioses in the United States. Emerging
Infectious Diseases 5: 635-642. http://www.cdc.gov/ncidod/eid/ vol5no5/mcquiston.htm
This document is Fact Sheet ENY-662 (IN191), one of a series of the
Entomology and Nematology Department, Florida Cooperative Extension Serivce,
Institute of Food and Agricultural Sciences, University of Florida. Date first
published: February 2001. Reviewed: August 2006. Please visit the EDIS Web site
Lord and C. Roxanne Rutledge Connelly, Assistant Professors, Florida Medical
Entomology Laboratory, Vero Beach, FL; Entomology and Nematology Department,
Cooperative Extension Service, Institute of Food and Agricultural Sciences,
University of Florida, Gainesville,
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