Doctor of Philosophy (PhD)


Pathobiological Sciences

Document Type



Strains of Dirofilaria immitis suspected of lack of efficacy (LOE) to macrocyclic lactone (ML) preventive drugs have been increasingly reported in dogs by practicing veterinarians since 2005 in the Lower Mississippi Delta region. If proven, and not controlled in the early stages, the emergence of ML drug resistance threatens to become a widespread problem in the US that may limit the effectiveness of current preventive drug treatment methods. To validate practice reports, a statewide survey of Louisiana veterinarians was done to define the extent of the problem and identify focal ‘hotspots’ of reported ML LOEs using Geographic Information Systems (GIS) methods. The present study utilized microfilariae (Mf) from two canine field cases from different state locations that fit criteria for a high index of suspicion of LOE against heartworms by ML drugs. Blood containing Mf from the field cases was used to infect and produce infective larvae (L3) in Aedes aegypti for experimental infection of two groups of dogs, each of which contained two laboratory dogs, one treated with prophylactic ivermectin (12 μg/kg) monthly for 6 months, and one untreated control. Both treated and untreated dogs from Group I and Group II developed patent D. immitis infections by 218 DPI and 189 DPI, respectively, as evidenced by a positive occult heartworm antigen test and Mf by the Knott’s test. Mf counts gradually increased post-patency in test and control dogs. L3 raised from Mf from the treated Group I dog were used to successfully establish a second generation isolate, confirming heritability of resistance in the face of a monthly ivermectin challenge dose of 24 μg/kg, given monthly for 3 months. These experimental infection studies provide in vivo evidence of the existence of ML drug resistance in dogs infected by D. immitis L3 from suspect field LOE cases in the Lower Mississippi Delta. Results encourage further work on mechanisms underlying the emergence of ML resistance in D. immitis and development of evidence-based resistance management strategies for heartworm preventives in order to extend the useful life of current drugs.



Committee Chair

Malone, John B