Larvicidal Activity of Abamectin against Natural Cochliomyia hominivorax Larva Infestation

Larvicidal Activity of Abamectin against Natural Cochliomyia hominivorax Larva Infestation 0. S. ANZIANI," A. A. GUGLIELMONE," AND D. H. AGUIRREb "Estacidn Experimental Agropecuaria Rafaela Instituto Nacional de Tecnologi'e Agropecuaria CC 22, 2300 Rafaela, Santa Fe, Argentina bEstacidn Experimental Agropecuaria Rafaela Instituto Nacional de Tecnologia Agropecuaria CC 228, 4400 Salta, Argentina

The prophylactic activity of some members of the avemectin family against cutaneous myiasis caused by Cochliomyia hominivirax larvae has been recently reported.'-3 This paper describes the results of a field trial carried out during December 1994 in a beef farm of the central area of Argentina (31" 27' South 60" 56' West) to evaluate the efficacy of abamectin in the prevention of myiasis caused by Cochliomyia horninivorax larvae. Fifty-six bull calves (Bos taurus x Bos indicus),4 to 6 months old, and with a minimum body weight of 100 kg were allocated by ranked pairs to a control (CG) or to a treated group (TG) of the same number of animals on the basis of bodyweight. On day 0, all experimental animals were surgically castrated, and calves of TG were treated subcutaneously with abamectin ("Duotin" Merck Sharp & Dohme AGVET) at a minimum dose of 200 pgkg. Calves of CG remained as control animals. After castration, the calves of both groups were run together on the same pasture, and inspections for screwworm larva infestation were carried out on days 3, 5, 7, 10, 12, and 14. A minimum of 10 larvae were collected for laboratory identification from each infested animal. During the observation period, C. hominivorax egg masses were found on the scrotal wounds of most of the calves of both groups, but 27 animals of the CG (96.4%) and 1 of the TG (3.5%) developed active myiasis (p < 0.01, chi-square). All collected larvae were classified as C. hominivorax. The results seem to indicate that abamectin did not interfere with the oviposition of gravid C. horninivorax females, and this drug exerted a larvicidal action after egg hatching. All scrotal strikes in the CG occurred during the first 5 days postcastration, while in the TG, calf myiasis was observed on day 10 (TABLE1).

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ANNALS NEW YORK ACADEMY OF SCIENCES

Postcastration Myiasis in Calves Treated with Abamectin and Controls. Number of Calves with Active Myiasis

TABLE 1.

Days of Inspection Posttreatment

Treatmenmo. of Calves

3

5

7

10

12

14

Total

Abamec tid28" ControlsR8

0 9

0 18

0 0

1 0

0 0

0 0

I* 27

" Minimum dose = 200 pg per kg of body weight. b p < 0.001.

REFERENCES 1. ANZIANI, 0. S. & C. LOREFICCE. 1993. Prevention of cutaneous myiasis caused by screw worm larvae (Cochliomyiu hominivorax) using ivermectin. J. Vet. Med. B. 40: 287-290. A. BRIDI,j. EAGLESON, D.FARRINGTON & 2. BENiTEZ USHER,c., J. CRUZ, L. CARVALLO, R. A. BARRICK. 1993. Efficacy of ivermectin against cattle myiasis caused by Cochliomyiu hominivorar. In 14th International Conference of the World Association for the Advancement of Veterinary Parasitology: 198. Cambridge, United Kingdom. 3. MUNIZ,R. A,, 0. S. ANZIAM,J. ORDONEZ, J. 0. ERRECALDE, J. MORENO& R. S. REW. Efficacy of doramectin in the protection of neonatal calves and post-parturient cows against field strikes of Cochliomyia hominivorax. Vet. Parasitol. (In press.)