GEOSPATIAL PATTERNS OF RESISTANCE OF CANINE HEARTWORM (DIROFILARIA IMMITIS) TO MACROCYCLIC LACTONE PREVENTIVE DRUGS

Strains of Dirofilaria immitis resistant to currently used preventive drugs are reported to be emerging in spotty locations in dogs by practicing veterinarians in the Lower Mississippi Delta. If not controlled in the early stages, resistance threatens to become a widespread problem in the US that may limit the effectiveness of current preventive drug treatment methods. To validate these practice reports, a statewide survey of veterinarians was done to define the extent of the problem and identify focal ‘hotspots’ of reported macrocyclic lactone (ML) lack of efficacy (LOE). Geographic information systems (GIS) methods were used to develop statewide and community-level risk maps for one parish (Avoyelles) based on practitioner-reported cases of preventive drug LOE and to define clustering, time-space patterns or other geospatial evidence of focal resistance of D. immitis to ML drugs. Statewide survey results (25.8% response rate; 221/855) indicated there are focal locations with high rates of reported LOE of ML drugs against heartworms and that numbers increased from 2005-2010. Geospatial analysis using kernel density and space-time permutation modeling software revealed focal ‘hotspots’ of population-adjusted cases of preventive drug failure in Avoyelles Parish in 2007, 2008 and 2009. Buffers zones of 0.5 km radius were created centered on case locations (to simulate flight range of vector mosquitos). Between 40% and 53% of buffers intersected, suggesting potential dog-to-dog transmission of resistant heartworms between neighboring dogs. Results were used as a criterion for identifying and establishing two resistant strains in the laboratory by in vivo experimental infection of dogs with D. immitis isolates from focal areas with high LOE rates. Further studies are in progress using geospatial analysis and genomics methods to characterize the dynamics of emerging ML resistance in Louisiana.