A Bayesian spatio-temporal model for forecasting Anaplasma species seroprevalence in domestic dogs within the contiguous United States
July
A Bayesian spatio-temporal model for forecasting Anaplasma species seroprevalence in domestic dogs within the contiguous United States
Funding: IDEXX Laboratories 2
. CSM was partially supported by Grant R 2
from the National 2
Yan Liu 1 2
Stella C. Watson 1 2
Jenna R. Gettings 1 2
Robert B. Lund 1 2
Shila K. Nordone 0 2
Michael J. Yabsley 2
Christopher S. McMahan 1 2
0 Department of Molecular and Biomedical Sciences, Comparative Medicine Institute, North Carolina State University, College of Veterinary Medicine , Raleigh, NC , United States of America, 3 Southeastern Cooperative Wildlife Disease Study, Department of Population Health, College of Veterinary Medicine and the Warnell School of Forestry and Natural Resources, The University of Georgia , Athens, GA , United States of America
1 Department of Mathematical Sciences, Clemson University , Clemson, SC , United States of America
2 Editor: J. Stephen Dumler, Johns Hopkins University , UNITED STATES
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This paper forecasts the 2016 canine Anaplasma spp. seroprevalence in the United States
from eight climate, geographic and societal factors. The forecast's construction and an
assessment of its performance are described. The forecast is based on a spatial-temporal
conditional autoregressive model fitted to over 11 million Anaplasma spp. seroprevalence
test results for dogs conducted in the 48 contiguous United States during 2011±2015. The
forecast uses county-level data on eight predictive factors, including annual temperature,
precipitation, relative humidity, county elevation, forestation coverage, surface water
coverage, population density and median household income. Non-static factors are extrapolated
into the forthcoming year with various statistical methods. The fitted model and factor
extrapolations are used to estimate next year's regional prevalence. The correlation
between the observed and model-estimated county-by-county Anaplasma spp.
seroprevalence for the five-year period 2011±2015 is 0.902, demonstrating reasonable model
accuracy. The weighted correlation (accounting for different sample sizes) between 2015
observed and forecasted county-by-county Anaplasma spp. seroprevalence is 0.987,
exhibiting that the proposed approach can be used to accurately forecast Anaplasma spp.
seroprevalence. The forecast presented herein can a priori alert veterinarians to areas expected
to see Anaplasma spp. seroprevalence beyond the accepted endemic range. The proposed
methods may prove useful for forecasting other diseases.
Institutes of Health. JRG is supported by The
Boehringer Ingelheim Vetmedica-CAPC Infectious
Disease Postdoctoral Fellowship.
Introduction
Canine anaplasmosis is caused by gram-negative intracellular bacteria of the family
Anaplasmataceae within the order Rickettsiales [
1
]. Anaplasma spp. bacteria are transmitted through
the bite of infected ticks, with different tick species transmitting distinct types of Anaplasma
bacteria in different regions of the country. A. phagocytophilum is transmitted by Ixodes
scapularis and I. pacificus and maintained in a vector-reservoir-host system similar to that of Borrelia
burgdorferi (the causative agent of Lyme disease), with the highest canine A. phagocytophilum
seroprevalence reported in the Northeast, upper Midwest and along the west coast of
California [
2
]. Although an important canine pathogen, A. phagocytophilum is also zoonotic and
causes human disease in the same regions where Lyme disease occurs. In contrast, A. platys is
presumed to be transmitted by Rhipicephalus sanguineus, and has relatively low prevalence
across the contiguous United States with a slightly higher prevalence seen in the southern
states [
2
]. Dogs in the southern U.S. (Florida, Georgia, North and South Carolina, Tennessee
and Texas) show equivalent seroconversion to both A phagocytophilum and A. platys [
2
],
suggesting exposure to multiple tick vectors. Veterinary wellness exams commonly include annual
screening for exposure to Anaplasma spp., as well as Ehrlichia spp., Borrelia burdgorferi and
Dirofilaria immitis (heartworm disease agent) using a rapid, in-house enzyme-linked
immunosorbent assay (ELISA) platform (SNAP14Dx1 and SNAP14Dx1 Plus Test, IDEXX
Laboratories, Inc., Westbrook, ME, USA) [
3, 4
]. These tests detect antibodies to both A.
phagocytophilum and A. platys on a single spot and therefore no in-house speciation is possible.
Of four million dogs tested for exposure to Anaplasma spp. in 2015, over 100,000 dogs were
seropositive. Seroreactivity on these tests are interpreted by veterinary clinicians to indicate
tick exposure and a history of transmission of Anaplasma spp.
Many, if not most, dogs remain asymptomatic following exposure to Anaplasma spp.. For
example, in areas such as the northeastern US where disease is endemic, as many as 60% of
dogs may have antibodies specific for Anaplasma spp. and the majority of these dogs do not
have overt evidence of clinical disease [
5, 6
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