Evaluation of ivermectin and milbemycin oxime efficacy against Dirofilaria immitis infections of three and four months' duration in dogs

John W. McCall From the Department of Parasitology, College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7387 (McCall); TRS Labs Inc, PO Box 5112, Athens, GA 30604 (McTier); and Merck AgVet, PO Box 2000, Rahway, NJ 07065-0912 (Ryan, Gross, Soll).

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Tom L. McTier From the Department of Parasitology, College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7387 (McCall); TRS Labs Inc, PO Box 5112, Athens, GA 30604 (McTier); and Merck AgVet, PO Box 2000, Rahway, NJ 07065-0912 (Ryan, Gross, Soll).

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William G. Ryan From the Department of Parasitology, College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7387 (McCall); TRS Labs Inc, PO Box 5112, Athens, GA 30604 (McTier); and Merck AgVet, PO Box 2000, Rahway, NJ 07065-0912 (Ryan, Gross, Soll).

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Sheila J. Gross From the Department of Parasitology, College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7387 (McCall); TRS Labs Inc, PO Box 5112, Athens, GA 30604 (McTier); and Merck AgVet, PO Box 2000, Rahway, NJ 07065-0912 (Ryan, Gross, Soll).

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Mark D. Soll From the Department of Parasitology, College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7387 (McCall); TRS Labs Inc, PO Box 5112, Athens, GA 30604 (McTier); and Merck AgVet, PO Box 2000, Rahway, NJ 07065-0912 (Ryan, Gross, Soll).

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Abstract

Objectives

To determine the efficacy of ivermectin (IVM) and milbemycin oxime (MBO) against induced heartworm infection, where monthly treatment is started 3 or 4 months after infection, and to monitor microfilaremia and antigenemia.

Animals

21 heartworm-naive Beagles.

Procedures

Each of 21 dogs was given 50 infective larvae of Dirofilaria immitis by SC inoculation. One group of 5 dogs served as nonmedicated controls, 2 groups of 5 dogs received IVM at 6 μg/kg of body weight or MBO at 500 μg/kg for 12 months beginning at postinfection (PI) month 4, and 2 groups of 3 dogs received IVM or MBO for 13 months beginning at PI month 3. Blood collected at intervals not >1 month beginning at PI month 4 was examined for microfilariae and antigen. Dogs were euthanatized at PI month 16.

Results

Adult worm counts, relative to controls, were reduced in the 4-month treatment groups by 95.1 (P < 0.01) and 41.4% for IVM and MBO, respectively. The difference between the IVM and MBO groups was significant (P < 0.01). Live worms were found in all MBO-treated (range, 8 to 27) and control dogs (range, 12 to 39) and in 3 of 5 IVM-treated dogs (range, 2 to 4). In the 3-month treatment groups, worm counts were reduced by 97.7 (P < 0.01) and 96.8% (P < 0.01) for IVM and MBO, respectively. Microfilariae were seen in all control dogs and in only 2 of the 16 treated dogs. The antigen response of MBO-treated dogs in the 4-month treatment group was only slightly weaker than that for control dogs. In all other treated dogs, this response was delayed and weaker.

Conclusion

Ivermectin is highly (≥ 95%) and significantly more effective than MBO against induced heartworm infection when 1 year of monthly prophylactic dosing is started 4 months after infection.

Clinical Relevance

In some cases of owner compliance failure, monthly administration of IVM gives a high level of protection against young adult heartworms. (Am J Vet Res1996;57:1189-1192)

Abstract

Objectives

To determine the efficacy of ivermectin (IVM) and milbemycin oxime (MBO) against induced heartworm infection, where monthly treatment is started 3 or 4 months after infection, and to monitor microfilaremia and antigenemia.

Animals

21 heartworm-naive Beagles.

Procedures

Each of 21 dogs was given 50 infective larvae of Dirofilaria immitis by SC inoculation. One group of 5 dogs served as nonmedicated controls, 2 groups of 5 dogs received IVM at 6 μg/kg of body weight or MBO at 500 μg/kg for 12 months beginning at postinfection (PI) month 4, and 2 groups of 3 dogs received IVM or MBO for 13 months beginning at PI month 3. Blood collected at intervals not >1 month beginning at PI month 4 was examined for microfilariae and antigen. Dogs were euthanatized at PI month 16.

Results

Adult worm counts, relative to controls, were reduced in the 4-month treatment groups by 95.1 (P < 0.01) and 41.4% for IVM and MBO, respectively. The difference between the IVM and MBO groups was significant (P < 0.01). Live worms were found in all MBO-treated (range, 8 to 27) and control dogs (range, 12 to 39) and in 3 of 5 IVM-treated dogs (range, 2 to 4). In the 3-month treatment groups, worm counts were reduced by 97.7 (P < 0.01) and 96.8% (P < 0.01) for IVM and MBO, respectively. Microfilariae were seen in all control dogs and in only 2 of the 16 treated dogs. The antigen response of MBO-treated dogs in the 4-month treatment group was only slightly weaker than that for control dogs. In all other treated dogs, this response was delayed and weaker.

Conclusion

Ivermectin is highly (≥ 95%) and significantly more effective than MBO against induced heartworm infection when 1 year of monthly prophylactic dosing is started 4 months after infection.

Clinical Relevance

In some cases of owner compliance failure, monthly administration of IVM gives a high level of protection against young adult heartworms. (Am J Vet Res1996;57:1189-1192)

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