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Although the same species of tick can pass on bacteria that cause Lyme disease and human granulocytic ehrlichiosis, the animal that provides the blood meal that infects the tick may not be the same.
ATLANTA—Despite being passed on by the same kind of tick, bacteria species causing two diseases in humans—Lyme disease and human granulocytic ehrlichiosis (HGE)—may have different natural cycles, says an article in the upcoming issue of Emerging Infectious Diseases, CDC's peer-reviewed journal, which tracks new and reemerging infections worldwide.
Lyme disease is already the most common disease passed on by ticks in the United States, and HGE may not be far behind. Both diseases are highly endemic in the Northeast and Midwest. Although the bacteria causing each disease are unrelated, their passage by the same deer tick has led scientists to suspect that their life cycles might also be the same. Ticks get these bacteria from naturally infected wildlife before they can pass them on to humans. In nature, ticks become infected when they feed on wildlife as immature larvae and then later pass on the bacteria to other wildlife when they feed again as nymphs. Continuous passage from wildlife to ticks ensures that these bacteria survive in nature and remain a constant threat to public health.
While the white-footed mouse is the main source of Lyme disease infection for ticks, researchers do not know which animal species is the main source of ticks carrying the bacteria causing HGE. To determine if the white-footed mouse is also responsible for infecting ticks with HGE, the article's authors examined both deer ticks and white-footed mice at a field site in Connecticut. But the numbers of infected ticks and infected mice found at this site did not add up. The differences they found suggest that the source of infection for both pathogens is not the same. HGE must be maintained by some animal species other than the white-footed mouse, and the natural maintenance cycles for both Lyme disease and HGE are not the same.
One piece of evidence leading to this conclusion was that the prevalence of ehrlichial infection in nymphal versus adult ticks increased 5 to 10 times, even though only about 20% of the white-footed mice were infected with ehrlichial bacteria. Thus, some other host species was probably involved in infecting the adult ticks with the bacteria. They also found that infection with Lyme disease bacteria was more common in white-footed mouse-fed ticks than in ticks collected from vegetation, while the bacteria causing HGE was half as common in mouse-fed ticks as in ticks collected from vegetation. This finding suggests that the mouse is important for infecting the ticks with bacteria that cause Lyme disease but less important for infecting ticks with bacteria causing HGE.
Dr. Michael Levin concludes: "Although the white-footed mouse is susceptible to infection with both bacterial species, this mammal alone cannot account for the observed prevalence of the agent of HGE in adult ticks. Our data suggest that most nymphal ticks acquire the agent of HGE from animal species other than white-footed mice."
For more information, contact Dr. Michael Levin, Yale School of Medicine, at 203-785-3223 (tel), 203-785-3604 (fax), or Michael.Levin@yale.edu. Access the full article at http://www.cdc.gov/ncidod/EID/vol5no2/levin.htm. All material in Emerging Infectious Diseases is in the public domain and may be used without special permission; proper citation, however, is appreciated.
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