Tickborne Disease


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.

Human Monocytic Ehrlichiosis

Figure 1. Reported cases of Human Monocytic Ehrlichiosis

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.

Amblyomma americanum

Figure 2. Lone Star Tick

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. Rhipicephalus Sanguineus

Figure 3. Rhipicephalus Sanguineus

Canine ehrlichiosis has been known from dogs and other canids since the 1930's. It is worldwide in distribution, and the brown dog tick Rhipicephalussanguineus 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.