State and territorial health departments report Lyme disease cases to CDC as part of the National Notifiable Diseases Surveillance System (1
). Data on canine seroprevalence of B. burgdorferi
antibodies were obtained from a 2009 publication by Bowman et al. that reported results for 982,336 dogs tested throughout the United States by using a commercial C6-based assay during 2001–2006 (8
). We obtained state-specific seroprevalence from Table 1 of this publication and county-specific seroprevalence as categorical values (0%, 0.1%–0.5%, 0.51%–1%, 1.1%–5%, >
5.1%) from Figure 2 of this publication after digital enlargement. We excluded counties too small for the value to be determined reliably. We calculated average annual human Lyme disease incidence for 2001–2006 and 2007–2009 using US Census Bureau population estimates for 2004 and 2008, respectively. To evaluate county-level emergence of Lyme disease among humans, we stratified counties by the mean observed annual incidence for all counties during 2001–2006 of 4.7 cases per 100,000 population. We defined an emergent county as a county in which incidence was below this value during 2001–2006 and above this value during 2007–2009.
Detailed canine seroprevalence data were available for 46 US states. In linear regression analysis, state canine seroprevalence and human Lyme disease incidence were positively correlated (; r2 0.75, p<0.001). On the basis of this relationship, human Lyme disease incidence was effectively zero when the canine seroprevalence was <1.3%. States generally fell into 2 distinct categories according to canine seroprevalence (). Median Lyme disease incidence was uniformly low (median 0.3 cases/100,000 population) and not correlated with canine seroprevalence (r2 0.0, p>0.4) among 32 states with canine seroprevalence <5%. Among 14 states with canine seroprevalence >5%, median annual human Lyme disease incidence was ≈100-fold higher (24.1 cases/100,000 population) and positively correlated with canine seroprevalence (r2 0.33, p = 0.03).
Borrelia burgdorferi antibody seroprevalence in dogs and reported Lyme disease incidence in humans, counties in 46 US states, 2001–2006.
Categorical canine serologic data were available for 866 (28%) of 3,141 counties in the 46 states (8
). Median population in 2004 was 85,699 for counties for which data were available, compared with 25,505 for all counties in the 46 states. As in the state-level analysis, human incidence and canine seroprevalence were positively associated at the county level. Median annual reported Lyme disease incidence for humans was 0.2 per 100,000 population in counties with canine seroprevalence <
1%, 1.4 in counties with canine seroprevalence 1.1%–5%, and 25.9 in counties with canine seroprevalence >5% (p<0.001; ). Five (1%) of 520 counties with canine seroprevalence <
1% had rates of human illness above the overall county mean of 4.7 cases per 100,000 population annually, compared with 171 (85%) of 201 counties with canine seroprevalence >5%.
Median Lyme disease incidence in humans and Borrelia burgdorferi antibody seroprevalence in dogs in counties in 46 US states. Error bars represent 25th and 75th percentiles.
Overall, 153 (5%) of 2,830 counties with average annual human incidence <4.7 per 100,000 population during 2001–2006 met the criteria for emergence during 2007–2009. Emergence was more common in counties with higher canine seroprevalence (). Eighteen (56%) of 32 counties with canine seroprevalence >5% met the criteria for emergence, compared with 6 (1%) of 519 counties with seropositivity <1% (p<0.001). Among the 32 counties with canine seroprevalence >5%, a total of 12 (67%) of the 18 counties with emergent Lyme disease were immediately adjacent to a county with seroprevalence >5%, compared with 4 (29%) of the 14 counties with nonemergent Lyme disease.
Counties meeting criteria for emergence of human Lyme disease during 2007–2009, by canine seroprevalence of Borrelia burgdorferi antibodies during 2001–2006, 46 US states*