Anaplasma phagocytophilum in White-tailed Deer

We examined the reservoir potential of white-tailed deer for Anaplasma phagocytophilum. Results suggest that white-tailed deer harbor a variant strain not associated with human infection, but contrary to published reports, white-tailed deer are not a reservoir for strains that cause human disease. These results will affect surveillance studies of vector and reservoir populations.

A naplasma phagocytophilum is an obligate intracellular bacterium and the etiologic agent of human granulocytic anaplasmosis (formerly known as human granulocytic ehrlichiosis). From 1999 to 2003, a total of 1,686 cases of human anaplasmosis were reported in the United States, and >95% of these cases occurred in northeastern or upper midwestern states. Transmission within and between reservoir populations in these regions occurs by Ixodes scapularis ticks (1,2). Infections occur in humans who have been fed upon by infected nymphal or adult ticks. No evidence shows that A. phagocytophilum is transmitted transovarially within the tick population; thus, both infected reservoirs and ticks that can transmit the infection must be available to maintain the agent in nature. Three mammalian species are reservoir competent: the white-footed mouse (Peromyscus leucopus), raccoon (Procyon lotor), and gray squirrel (Sciurus carolinensis), although serologic and molecular evidence has suggested that numerous other small, medium, and large mammals may also be reservoirs (1,3).
Every examined A. phagocytophilum sample from a patient with a confirmed case of human granulocytic anaplasmosis from the northeastern or upper midwestern United States has shown identical 16S rRNA sequences. This sequence, referred to as the A. phagocytophilum human anaplasmosis (AP-ha) signature sequence, differs by 2 bp from the sequence of the 16S rRNA gene of a variant strain, AP-Variant 1. Recent studies that compared the prevalence of AP-ha to AP-Variant 1 in tick populations showed the variant to be the predominant strain at 2 of 3 sites and suggest that AP-Variant 1 is common in nature (4,5).
The white-footed mouse serves as a natural reservoir for AP-ha, and laboratory studies have shown that numerous inbred strains of mice (e.g., Balb/C, C3H, DBA/2) are also highly susceptible to infection (1,6,7). In contrast, AP-Variant 1 does not infect the white-footed mouse, DBA/2, and severely immunocompromised (SCID) mice (8). These results suggest that rodents are not a natural reservoir for AP-Variant 1 and that alternative reservoir species exist in nature. Previous reports have identified 3 whitetailed deer (Odocoileus virginiansus) from Maryland and 2 white-tailed deer in Wisconsin that harbored an agent with a 16S rRNA gene sequence identical to that of AP-Variant 1 (4,9,10). Several previous studies have also suggested that white-tailed deer are a reservoir for the human agent AP-ha (10)(11)(12). These results led to the current study, which was conducted to investigate the relative potential for white-tailed deer to be a reservoir for AP-ha, AP-Variant 1, or both strains. We examined blood samples from white-tailed deer to determine the strains of A. phagocytophilum with which these deer were infected and the ticks feeding on these deer to identify strains to which they were exposed.

The Study
I. scapularis ticks were collected from white-tailed deer during controlled hunts at Ridley  The collection of samples in December 2002 resulted in very few ticks (n = 6) because low temperatures inhibited tick activity. PCR amplification and DNA sequencing of A. phagocytophilum from female ticks (n = 4) showed that 1 tick was positive for AP-Variant 1 and 1 tick was positive for AP-ha (Table). Only 1 of the 2 males that were collected was positive for A. phagocytophilum, which sequencing showed to be AP-ha. Blood samples were collected from 24 white-tailed deer in 2002, and 5 (20.8%) of these were positive for A. phagocytophilum. Each of these 5 positive samples was AP-Variant 1.

Conclusions
The differential between the high percentage of AP-Variant 1-positive female ticks (range 25%-72.6%) and the relatively low number of male ticks positive for the variant (<15.4%), combined with the fact that deer were positive for only the variant, suggest that these partially and fully engorged female ticks were acquiring AP-Variant 1 as they fed on the deer. The presence of AP-ha in both male and female ticks that were feeding on deer showed that these deer were frequently exposed to AP-ha. Despite this demonstrated exposure to AP-ha, infections by AP-ha in white-tailed deer were not found. These results suggest that white-tailed deer are a natural reservoir for the variant, and further, that white-tailed deer are not a reservoir for AP-ha strains. These results also suggest that previous serologic studies that identified white-tailed deer as a reservoir for AP-ha strains were likely the result of the cross-reactivity of the anti-AP-Variant 1 serum from deer with the AP-ha antigens used for detection (10)(11)(12). In fact, in a study in which serologic testing showed that 8% of white-tailed deer in Wisconsin were positive for A. phagocytophilum, the only 2 samples that were PCR amplified and sequenced were identical to AP-Variant 1 (10). Strong antigenic cross-reactivity of the AP-ha and AP-Variant 1 strains would not be surprising, considering their 16S rRNA genes are >99% identical.
Seroprevalence studies for infectious agents using animal reservoir and host populations and PCR amplifications from vector species are commonly used to assess the disease risk for humans in a particular region, particularly for viral and bacterial zoonotic agents. Our results show that animal seroprevalence studies for A. phagocytophilum must be carefully evaluated to determine whether the agent inducing the immune response is truly infectious in humans. Our results further show that while PCR studies of ticks may identify A. phagocytophilum, DNA sequencing of the PCR products is necessary to differentiate AP-ha and AP-Variant 1 and therefore to assess the potential for human infections. These issues were not addressed in earlier studies and likely resulted in overestimation of the prevalence of AP-ha in nature and in the implied risk for human anaplasmosis. Therefore, future studies of host or vector populations must be evaluated and interpreted carefully, with the knowledge that non-disease-causing variant strains may influence results.
While our results suggest that white-tailed deer are a reservoir for AP-Variant 1, additional studies that examine the interaction of AP-Variant 1 with white-tailed deer populations in other parts of the United States are needed to determine if they correspond to our results from Pennsylvania. Differences in AP-Variant 1 strain composition or local white-tailed deer or I. scapularis tick populations may alter the interaction of the bacterial agent, vector, and reservoir. AP-ha strains cause a transient, relatively mild febrile illness with no overt signs of disease in immunocompetent mouse species, including the natural reservoir, Peromyscus leucopus (13). Inbred laboratory mice infected with AP-ha may remain infected for up to 55 days (6), and previous infections induce an immune response that is only partially protective, since mice may be reinfected (14). We have not determined whether AP-Variant 1 produces any disease manifestation in whitetailed deer, although the high number of positive deer in the current study suggests that persistent infections, reinfections from feeding ticks, or both mechanisms may be involved in maintenance of AP-Variant 1 in white-tailed deer populations.