Efficacy of long-term monthly administration of ivermectin on the progress of naturally acquired heartworm infections in dogs
Abstract
This study was designed to evaluate the efficacy of prolonged monthly ivermectin treatment against Dirofilaria immitis in client-owned dogs with naturally acquired infections and to clinically monitor the animal's response to the slow killing of heartworms, with death of the worms distributed over a period of up to 2 years. A total of 17 male and female dogs of different breeds and ages were used. Prior to treatment, all of the dogs tested positive for heartworm antigen (Ag) and all but two had microfilariae (mf). The dogs were randomly allocated to one group of seven dogs which received a commercial formulation of ivermectin (minimum, 6 mcg IVM/kg) plus pyrantel (minimum, 5 mg PP/kg) (Heartgard Plus Chewables, Merial, Ltd.), another group of seven dogs which received a commercial formulation of IVM (min, 6 mcg/kg) (Heartgard Chewables, Merial Ltd.), and a group of three dogs which served as an untreated controls. All dogs were evaluated prior to initiation of treatment and thereafter at 3- to 5-month-intervals for mf, Ag, and radiographic and echocardiographic findings. All of the 17 dogs, with the exception of two dogs in the IVM group, had circulating mf of D. immitis prior to the 1st monthly dose, and a few also had mf of Dirofilaria repens. After 4 monthly doses, only one dog in the IVM/PP group and two dogs in the IVM group had a patent heartworm infection, and no heartworm mf were seen in the 14 treated dogs thereafter. After 10 monthly doses, the number of Ag-positive dogs in both of the treated groups decreased gradually. Efficacy, based on the reduction in number of Ag-positive dogs, was similar for the IVM/PP and IVM groups, with overall efficacy scores for the 14 dogs of 21, 21, 43, and 71% after 10, 14, 19, and 24 monthly doses, respectively. Two of the seven dogs treated with IVM/PP, one of the seven treated with IVM, and two of the three untreated controls showed echocardiographic evidence of a parasitic burden prior to treatment, and all of these scores had decreased by the end of the study. Only one dog (IVM/PP group) had a cardiovascular pattern of heartworm disease by echocardiography prior to treatment, but this dog's score increased to two and the scores of two additional dogs increased from zero to two (IVM group) or three (IVM/PP group) by the end of the study. Only 1 (IVM/PP group) of the 17 dogs showed a pulmonary pattern of heartworm disease by radiography prior to treatment, but this dog's score increased to three by the end of the study. The radiographic scores of two additional dogs in the treated groups increased from zero to three (IVM/PP) or two (IVM) by the end of the study. Thus, monthly administration of IVM to dogs with clinical, radiographic or echocardiographic evidence of heartworm disease is ill-advised and such treatment of even the asymptomatic dog should be done only with much caution and frequent monitoring by the veterinarian.
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Some researchers have observed that health of animals treated with ivermectin for 24 months can worsen.
“Indeed, some researchers have observed that the health of animals treated with ivermectin for 24 months can worsen (438).”
From Human and animal dirofilariasis: The emergence of a zoonotic mosaic (2012) · cites this paper as [438]
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- Authors
- Venco, L., McCall, J. W., Guerrero, J., & Genchi, C.
- Affiliations
- University of Georgia (US)
- University of Pennsylvania (US)
- University of Milan (IT)
- Corresponding author
- Genchi, C.
- Venue
- Veterinary Parasitology
- Publisher
- Elsevier BV
- Published
- 2004-10
- Volume
- 124
- Issue
- 3-4
- Pages
- 259-268
- ISSN
- 0304-4017 (ISSN-L)
- OpenAlex
- W2131159581
- Cited by
- 65
- References
- 8
- Citations by year
- 2026: 4 · 2025: 2 · 2024: 2 · 2023: 4 · 2022: 4 · 2021: 1 · 2020: 7 · 2019: 1 · 2018: 3 · 2017: 3 · 2016: 1 · 2015: 6 · 2014: 4 · 2013: 2 · 2012: 1
- FWCI
- 0.21
- Type
- journal article
- Language
- en
- Open access
- closed
- Locations
- Veterinary Parasitology (publishedVersion)
- PubMed (publishedVersion)
- Topics
- Health Sciences › Medicine › Infectious Diseases › Parasitic Diseases Research and Treatment
- Health Sciences › Veterinary › Small Animals › Helminth infection and control
- Health Sciences › Medicine › Public Health, Environmental and Occupational Health › Research on Leishmaniasis Studies
- Keywords
- canine heartworm disease, Dirofilaria immitis, ivermectin, pyrantel, microfilariae, Dirofilaria repens, treatment efficacy, dogs
- MeSH terms
- Animals, Dirofilaria immitis, Dirofilariasis, Dog Diseases, Dogs, Dose-Response Relationship, Drug, Female, Filaricides, Ivermectin, Male, Pyrantel, Random Allocation, Time Factors, Treatment Outcome
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