Tick-Borne Diseases
Ehrlichiosis
During the past 10 years, two tick-borne diseases caused by Ehrlichia
spp. have been recognized in the United States. Human Monocytic
Ehrlichiosis (HME) was first described in 1986. It is caused
by E. chaffeensis, which was only discovered in 1991. Human
Granulocytic Ehrlichiosis (HGE), an alternative form of HME, was
recognized as a new disease in 1993. Its causative agent is still uncertain;
however, it is similar to another Ehrlichia equi described from
horses.
Both types of ehrlichiosis have similar clinical symptoms, ranging from a
mild illness to a severe, life-threatening disease. Typically, infected
individuals have fever, headache, malaise, chills, sweating, muscle aches,
nausea, and vomiting. Also, either disease may cause leukopenia,
thrombocytopenia, anemia, and liver function abnormalities. Treatment with
anitbiotics is effective when initiated early in the course of infection.
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Figure 1. Reported cases of Human Monocytic Ehrlichiosis
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Since 1986, approximately 400 cases (nine fatal) of HME have been confirmed
in 30 states (see figure 1), mostly in the southeastern and south
central United States. The distribution of cases resembles that of
the Lone Star tick (see figure 2), Amblyomma americanum.
Hence, the Lone Star tick was considered a likely vector for the infection,
and subsequent studies have verified this conjecture.
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Figure 2. Lone Star Tick
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Approximately 100 cases (four fatal) of HGE have been confirmed in
Connecticut, Massachusetts, Minnesota, New York, Rhode Island, and Wisconsin.
The true incidence, geograhic distribution, and vector of HGE is largely
undescribed because of the novelty of the illness. However, HGE is believed
to be transmitted by
blacklegged ticks
Ixodes
scapularis. HGE may frequently infect residents where blacklegged deer
ticks are abundant but may be misdiagnosed as "rashless" Lyme disease.
Fortunately, both HGE and Lyme disease can be
treated with tetracyclines if caught early in the infection.
Another human ehrlichiosis, Sennetsu fever, is found in western
coastal regions of Japan and Malaysia. The causative
agent,E. sennetsu, has been described since 1950. However, the vector
is still unknown. The occurrence of the illness in late summer and fall
suggests that the infection is tick-borne, but this has not yet been
proven. The disease is characterized by fever, generalized lymphadenopathy,
and atypical lymphocytes in the peripheral blood. Fatigue, anorexia, chills,
headache are also common. A skin rash is not normally seen. These symptoms
may last for 2 weeks. Treatment with tetracyclines can bring a prompt
recovery.
Other than Human ehrlichiosis, there are several species of Ehrlichia
that cause other forms of disease.
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Figure 3. Rhipicephalus Sanguineus
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Canine ehrlichiosis has been known from dogs and other canids
since the 1930's. It is worldwide in distribution, and the brown dog
tick Rhipicephalus
sanguineus vectors the causative agent - E. canis.
In northern Europe and the United Kingdom the sheep tick, Ixodes
ricinus, transmits E. phagocytophila. The resulting
infection is known as Tick-borne fever. Sheep, cattle, and deer
are considered the natural hosts for the infection.
Horses are susceptible to both Equine ehrlichiosis and Potomac
horse fever - caused by E. equi and E. risticii,
respectively. Both infections occur in the eastern United States and no
vectors have been identified.