We evaluated a comprehensive national database that documents canine infection with,
or exposure to, four vector-borne disease agents, Dirofilaria immitis, Borrelia burgdorferi,
Ehrlichia canis, and Anaplasma phagocytophilum in order to assess geographic trends
in rates of positive tests. While the percent positive test results varied by agent
in different regions of the United States, with D. immitis antigen and antibodies
to E. canis more commonly identified in dogs from the South (3.9% and 1.3%, respectively),
and antibody to B. burgdorferi and A. phagocytophilum found more frequently in dogs
from the upper Midwest and Northeast (4.0-6.7% and 5.5-11.6%, respectively), evidence
of at least one agent was found in dogs from every state considered. Furthermore,
each organism also appeared to occur in endemic foci within larger areas of relatively
low prevalence. Relocation of infected or previously exposed dogs from endemic regions
likely accounts for some of the unexpected geographic distribution seen, although
local transmission in previously under-recognized areas of endemicity could also be
occurring. Although data were only available from the 48 contiguous states (Alaska
and Hawaii were not included), taken together, our results suggest that these disease
agents may be present over a wider geographic area, and thus pose greater animal and
public health risks, than is currently recognized. Dogs can serve as sentinels to
identify the presence of vector-borne disease agents of both veterinary and public
health significance.