Epidemiology and Spatial Emergence of Anaplasmosis, New York, USA, 2010–2018

Alexis Russell; Melissa Prusinski; Jamie Sommer; Collin O'Connor; Jennifer White; Richard Falco; John Kokas; Vanessa Vinci; Wayne Gall; Keith Tober; Jamie Haight; JoAnne Oliver; Lisa Meehan; Lee Ann Sporn; Dustin Brisson; P. Bryon Backenson

Disclosures

Emerging Infectious Diseases. 2021;27(8):2154-2162. 

In This Article

Discussion

This 9-year study characterizes the emergence of anaplasmosis in NYS through the analysis of trends in human case and vector surveillance data over time and geography. Anaplasmosis poses an increasingly substantial public health risk in NYS, and the 2010–2018 time frame captures a dramatic increase in the burden of this newly emergent disease. A closer look at the changing epidemiology and exposure risk for this disease helps to elucidate when, where, and why anaplasmosis is rapidly expanding in NYS.

The basic epidemiologic characteristics of anaplasmosis cases in NYS are consistent with previous reports at the national level and are comparable with those of other tickborne diseases transmitted by I. scapularis ticks in NYS.[7,10,25,26] Anaplasmosis, similar to Lyme disease and babesiosis, disproportionately affects White men, possibly because of residential and behavioral factors that increase the risk for tick bites in this group.[27,28] The age distribution of patients shows a unimodal peak in the range of 60–69 years, similar to babesiosis but unlike Lyme disease, which shows bimodal peaks in the 5–9 and 50–54 year ranges.[27,28] This finding might be related to the increased susceptibility for severe anaplasmosis infections with age, and the greater likelihood of subclinical infections in young patients.[7,29] Anaplasmosis infection causes a constellation of nonspecific symptoms that mimic those of other tickborne diseases, but without a characteristic rash, such as erythema migrans, seen in Lyme disease.[27,28] The hospitalization rate for anaplasmosis is higher than that for Lyme disease but lower than that of babesiosis, and the case-fatality rate of 0.5% is much lower than the 6.5% found in babesiosis patients in NYS.[26,27,29]

The summertime peak in anaplasmosis incidence implicates I. scapularis nymphs, which are most active during summer months, as the developmental stage responsible for most cases, even though nymphs are approximately half as likely as adult I. scapularis ticks to carry A. phagocytophilum. This finding is consistent with other tickborne diseases and might be attributed to NYS residents spending more time outdoors during summer months and the increased difficulty of finding and removing the much smaller nymphs during the 12–24-hour window before A. phagocytophilum transmission occurs.[9,25,27,30]

Spatial assessment of the emergence of anaplasmosis indicates that the increase in cases is not occurring diffusely across NYS but is instead originating primarily within the Capital Region. The dramatic 8.4-fold increase in incidence in the Capital Region during the 9-year study period indicates a rapidly intensifying focal area of disease emergence. Hot spot analysis pinpoints an expanding focal area centered around Columbia and Rensselaer Counties, bordering the Hudson River to the west and Vermont and Massachusetts to the east. This area might be located within a local epicenter of anaplasmosis emergence in the northeastern states because case data for neighboring states indicate increasing anaplasmosis incidence in NYS-adjacent counties over the time frame of our study.[31,32] The geographic expansion of anaplasmosis generally mimics that of Lyme disease in NYS decades earlier, with initial emergence northward along the Hudson River.[33,34] However, the spread of Lyme disease more closely followed the apparent range expansion of I. scapularis ticks from coastal areas northward and westward across NYS, whereas anaplasmosis is less common in coastal NYS and shows a more radial expansion further inland.[35] A similar inland radial expansion pattern was seen in the emergence of anaplasmosis in Minnesota during 1996–2011.[36]

The hot spot defined by this study coincides with a map of seroprevalence of Anaplasma species in a large sample of domestic dogs across the contiguous United States during 2011–2015.[37] Dogs, which are affected by the same pathogenic variant of A. phagocytophilum as humans, might be an excellent sentinel species in forecasting the spread of anaplasmosis, as they have been for other tickborne diseases.[38] Many potential driving forces, including changes in land use, host density, and climate, have been implicated in the geographic spread of I. scapularis ticks and associated pathogens, and it is probable that a multitude of factors are shaping the spread of anaplasmosis in NYS. The rapid, geographically focused pattern of anaplasmosis emergence might also indicate recent changes in risk factors that are unique for this disease.

This study describes the changing prevalence of A. phagocytophilum in a large sample of host-seeking I. scapularis ticks collected across NYS. The overall statewide increase in pathogen prevalence over the study period, and in particular the large increase within adult I. scapularis ticks in the Capital Region, parallels the focal increase in human anaplasmosis incidence. The correlation of anaplasmosis ERI, which accounts for pathogen prevalence and tick population density, to yearly human incidence at the ZCTA-level, further supports the hypothesis that localized changes in exposure risk are driving emergence of this disease. However, the trends found in our tick surveillance data cannot fully explain the dramatic increase in anaplasmosis cases. Relatively high prevalence rates of A. phagocytophilum have been documented in I. scapularis ticks from the Metro Region of NYS as early as 1996.[39] A previous NYSDOH study found A. phagocytophilum in 6.5% of nymphs and 12.3% of adult I. scapularis ticks collected during 2003–2006 in the Hudson Valley, a region that overlaps most of the Metro Region and the southernmost part of the Capital Region as defined by this study, with no noted increase in pathogen prevalence over the study period.[17] Clearly, A. phagocytophilum has been present at appreciable levels in the Metro Region tick population well before the recent increase in anaplasmosis cases, and although other tickborne diseases are endemic to this region, the Metro Region has not experienced a major emergence of anaplasmosis as seen in the Capital Region. The presence of multiple high-ERI tick surveillance sites, especially within the Metro Region, which were located well outside the anaplasmosis hot spot each year, underlines this discrepancy.

Some notable limitations of our host-seeking tick sampling might partially contribute to this incongruity, including greater vector surveillance coverage in some regions than others, increasing level of sampling effort over the study period, and repeated sampling of some locations but not others. Another explanation might be the distribution of pathogenic versus nonpathogenic genetic variants of A. phagocytophilum. The PCR used in this study does not distinguish between Ap-v1, a nonpathogenic variant that has a major reservoir in white-tailed deer (Odocoileus virginianus), and Ap-ha, a human pathogen that has white-footed mice (Peromyscus leucopus) and Eastern chipmunks (Tamias striatus) as major competent reservoirs.[40,41] Studies of I. scapularis ticks in Ontario, Canada, which borders NYS, indicate an increase in the proportion of the pathogenic Ap-ha variant relative to the Ap-v1 variant in ticks collected after versus before 2010.[42–44] A similar shift in variant prevalence might be occurring in NYS and could be a driving force for human disease emergence. A follow-up study using genotyping to differentiate between variants of A. phagocytophilum in ticks collected across NYS, coupled with spatial analysis to examine changes in variant distribution over time and geography, is in progress and will hopefully further elucidate factors contributing to the emergence pattern of anaplasmosis in NYS.

The true burden of anaplasmosis in NYS is probably greater than that captured by our analysis of mandated case reporting. Anaplasmosis cases that are subclinical, self-limiting, misdiagnosed, or co-infections with other tickborne pathogens might be unreported or do not meet the strict case definition. In addition, the level of awareness of tickborne diseases among healthcare providers and the general public probably varies across NYS because tickborne diseases are hyperendemic in some regions and newly emergent in others. Resulting differences in patient behavior, provider diagnosis, and local health department reporting make estimating the true incidence of anaplasmosis a challenge, similar to what has been documented for Lyme disease in NYS.[45] Lack of awareness can increase the likelihood of undiagnosed and untreated cases, which is especially relevant for a new and rapidly expanding disease such as anaplasmosis. Assessing disease epidemiology and clusters over time and geography enables us to pinpoint the populations at highest risk and anticipate when and where the disease will spread in the future so that public health efforts can be targeted toward populations who might benefit the most.

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