Natural History

Studies throughout the world have shown that many cases of human illness are caused by zoonotic pathogens that are maintained by animal hosts in their natural cycles. Many zoonoses require a vector (e.g., mosquito, tick, mite) to be transmitted from the animal host to the human host. The ehrlichial disease cycle includes four components: the pathogen, the animal reservoir species, the vectors, and the human host. For the types of human ehrlichiosis in the United States, the arthropod vectors are ticks.

Only one of the three tick families, Ixodidae (hard ticks), is associated with ehrlichiae.  These ticks have four stages in their life cycle: egg, larva, nymph, and adult.  After the eggs hatch, each stage must feed once to develop into the next stage.  Larvae are uninfected with ehrlichiae when they begin to look for a bloodmeal.  Ticks become infected with ehrlichiae while feeding on blood from the host in either the larval or nymphal stage.  After the tick develops into the next stage, the ehrlichiae may be transmitted to the following host during the feeding process.  Both male and female ticks may bite humans but it is the females that are responsible for most transmission.  In the United States, it appears that both the nymphal and adult stages are responsible for transmission of ehrlichiae, but one stage may be more important for each Ehrlichia species.

Ehrlichia chaffeensis is principally transmitted by the lone star tick (Amblyomma americanum) (Figure 6). White-tailed deer are a major host of lone star ticks and appear to represent one natural reservoir for E. chaffeensis. Antibody to E. chaffeensis has been found throughout deer populations in the southeastern and midwestern United States, and the organism has been cultured from deer blood. (natural history diagram)

Figure 6.  Approximate distribution of the lone star tick 

    
           Lone star tick
 (Amblyomma americanum)           

The HGE agent has been associated with the blacklegged tick (Ixodes scapularis) in the northeastern and upper midwestern United States (Figure 7). The western blacklegged tick (Ixodes pacificus) is a vector in northern California (Figure 8). Ixodes ricinus has been shown to be a vector of E. phagocytophila in Europe. Deer, elk, and wild rodents are likely reservoirs.   (natural history diagram)

Figure 7.  Approximate distribution of the blacklegged tick

        
             Blacklegged tick
           (Ixodes scapularis)                          

Figure 8.  Approximate distribution of the western blacklegged tick

       
     Western blacklegged tick
           (Ixodes pacificus)             

Distribution of Ixodes scapularis and Ixodes pacificus by county

The natural history of Ehrlichia ewingii is not completely known. However, dogs may be a reservoir host and the lone star tick (Amblyomma americanum) may be an important vector (Figure 6). Canine granulocytic ehrlichiosis caused by E. ewingii has been described in south central and southeastern states, including Arkansas, Georgia, Mississippi, Missouri, North Carolina, Oklahoma, Tennessee, and Virginia. To date, human cases have been observed in Missouri, Oklahoma, and Tennessee.  It has been demonstrated experimentally that the lone star tick is able to transmit the disease among dogs. Other potential reservoirs and vectors remain to be identified.

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This page last reviewed April 5, 2000

Centers for Disease Control and Prevention
National Center for Infectious Diseases
Division of Viral and Rickettsial Diseases
Viral and Rickettsial Zoonoses Branch
1600 Clifton Road, NE
MS G-13
Atlanta, Georgia 30333
(404)639-1075