J. Vet. Med. B 40,287-290 (1993) 0 1993 Paul Parey Scientific Publishers, Berlin and Hamburg ISSN 0931 - 1793

Instituto Nacional de Temologia Agropecuaria, Estacidn Experimental Agropecuaria Rafaela, Santa Fe, Argentina

Prevention of Cutaneous Myiasis Caused by Screw Worm Larvae (Cochliomyia hominivorax)

Using Ivermectin

0. S. ANZIANI and C. LOREFICCE

Address of authors: Area de Investigaciones en Producci6n Animal, EEA INTA Rafaela, C.C. 22,2300 Rafaela, Pcia de Santa Fe, Argentina

With 2 tables

(Received for publication September 24, 1992)

Summary During March of 1991 and 1992, four trials were carried out in the central area of Argentina to

evaluate the efficacy of ivermectin in the prevention of myiasis caused by Cochlzomyu hominivorux larvae. In the first trial 24 steers were artificially wounded, in the second and third trial, 36 and 20 bull calves were castrated while in the fourth trial, 30 newborn calves were used. On day 0, half of the animals of each trial was treated with ivermectin (200 pg/kg) and the rest remained as untreated controls. None of the treated animals sustained a screw-worm larva infestation, but 3 of 12 (25 YO), 8 of 18 (44 %), 5 of 10 (50 %) and 8 of 15 (53 Yo) of the controls developed active myiasis in trials 1 to 4 respectively.

Introduction The New World screw-worm fly, Cochliomyia horninivorax, is the cause of cuta-

neous myiasis producing infestation of all warm-blooded animals including livestock, wildlife, pets and occasionally humans. It is considered one of the most important insect pest of livestock in the Americas and is still an obstacle to increased cattle production in many of the Central and South America countries (VARGAS TERAN, 1991).

In endemic screw-worm areas, at the present time, prevention of C. hominivorux infestations requires routine animal inspection and insecticide treatment of susceptible wounds. Prophylaxis of susceptible wounds, especially when they are inaccessible, o r after massive ranch operations as for example castration, would be facilitated if a syste- mic insecticide with long-residual action was available.

Ivermectin, a fermentation product of an actinomycete, Streptomyces avermitilis, is highly effective against several economically important nematodes and arthropods (CAMPBELL et al., 1983). The injectable formulation of ivermectin (1.0 % wlv solution), is active against a group of fly larvae (MILLER et al., 1981; CAMPBELL and BENZ, 1984; RONCALLI, 1984 a, b) including the Old World screw-worm Chrysomya bezziuna (SPRADBERY et al., 1985). This suggests that the drug might also be active against the N e w World screw-worm fly, C. horninivorax.

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288 ANZIANI and LOREFICCE

This paper describes the results of 4 tests designed to determine the prophylactic effects of ivermectin in the prevention of myiasis caused by C. hominivorux larvae under field conditions. No attempts were made to test the susceptibility of different aged larvae to this drug in infested wounds.

Material and Methods To evaluate the efficacy of ivermectin against C. hominivorax larvae, four field trials were con-

ducted during March of 1991 and 1992 in the central area of Argentina (Province of Santa Fe) where this pest is enzootic. This time of the year is considered as the peak of screwworm outbreaks. Trials numbers 1, 2, and 4 were undertaken at the Rafaela Experimental Station of the National Institute of Agriculture Technology with purebreed Holstein steers and bull calves. Trial number 3 was conducted in a commercial beef farm in the same area with Brahman cross bull calves. In trials number 1 to 3, the steers and bull calves were grazed, at approximately a stocking rate of 2 animals/ha, on lucerne pastu- res (Medicago sativa) and provided with a supplemental concentrate mixture. In trial number 4 the newborn calves were kept on a 2.6 ha paddock; all animals had received colostrum within the first few hours of birth and then they were maintained on milk replacer until the end of the experience.

Trial 1: Twenty-four Holstein steers aged 14 to 18 months were weighted and alloted to two groups of 12 each (A and B), so that the steers in each group were of similar weight. O n day 0, all ani- mals received a 20 mm incision through thc dermis of the rump, between the tuber ischii and the sacrum vertebrae, after local intramuscular injection of 2 % lidocaine.

Trial 2: Thirty-six Holstein bull calves, aged 2 to 4 months were divided in two groups in the same way as in trial 1. O n day 0, surgical castration of all calves was performed.

Trial 3: Twenty Brahman cross bull calves, approximately 6 months of age, were allotted to two similar groups according to size and castrated on day 0.

Trial 4: Thirty male and female Holstein newborn calves were used within the first 24 hours of birth. First born calves were treated and secondly born calves remained as a control until all calves were allotted to two groups.

Treatments andprocedures: In each trial, half of the animals were treated with ivermectin ("Ivo- mec", M.S.D. Agvet, Division of Merck Sharp and Dohme Argentina Inc, Buenos Aires, Argentina) on day 0 and the other half remained as untreated controls. The drug was administered by subcutaneous injection in the prescapular region of the neck at a dose of 200 pg/kg. In all trials experimental and con- trol animals were run together and inspections for screw-worm larvae infestation were carried out on days 4, 7, 1 1 and 14 post treatment in trials 1 to 3, and days 3, 6, 9 and 12 in trial 4. A minimum of 10 larvae were collected for laboratory identification from each animal with infested wound.

Results The results of the four trials (summarized in Tables I and 2 ) showed that ivermectin

was highly effective to prevent the development of active myiasis. None of the 55 treated animals showed signs of active myiasis while 25 to 53 % of the 55 controls were affected

Table 1 . Trials 1 to 3 . Myiasis of rump and scrota1 wounds in treated and control animals (Summer of 1991)

Number of animals with active myiasis

Group/n" of animals Day of inspection post treatment

Treated 12'' Control 12" Treated 18** Control 18**.*

Control 10"":' Treated 10"*::.

4 7 1 1 14 Totals 0 0 0 0 0 2 1 0 0 3 (25 %) 0 0 0 0 0 4 4 0 0 8 (44 Yo) 0 0 0 0 0 4 1 0 0 5 (50 Yo)

:'. Holstein steers (rump wounds); :.,* Holstein bull calves (castration wounds); *"* Cross-breed Brahman bull calves (castration wounds).

Prevention of Cutaneous Myiasis 289

Table 2. Trial 4. Myiasis of navel wounds in male and female newborn Holstein calves (Summer of 1992)

Number of calves with active myiasis

Grouph" of calves Day of inspection post treatment

Treated 15 Control 15

3 6 9 12 Totals 0 0 0 0 0 4 3 1 0 8 (53 %)

by screw worm fly. All larvae collected from'untreated animals were classified as C. hominivorax. In trials number 2 to 4 some treated animals showed masses of eggs on the castration and navel wounds, but none of these developed to active myiasis.

Discussion The lowest infestation level (25 %) was recorded in the control steers of trial num-

ber I . Probably, the small and sharp incisions performed by the surgical blades enhanced the very quick cicatrization process observed and these wounds were not very attractive to ovipositing female flies. O n trials number 2, 3 and 4, the higher infestation level re- corded in the untreated cattle confirmed more conclusively the prophylactic effects under field conditions of a single 200 pg/kg of ivermectin on very susceptible hosts.

There are few reports about prophylactic application of systemic insecticides against C. hominivorax larvae. In experimental work, chlorinated hydrocarbon insecti- cides showed larvicidal activity for up to one month post treatment, but extensive resi- dues of these insecticides in tissues of treated animals precluded their use as practical pro- phylactic treatment (DRUMMOMD et al., 1988). The salicylanide compound, closantel, has also shown systemic activity against C. hominivorax larvae infestation in sheep (GUERRERO, 1984).

SPRADBERY et al. (1985) reported high efficacy of ivermectin in the prevention of myiasis caused by Chrysomya bezziana (the Old World screw worm) larvae whose life cycle, habits and parasitic niche is remarkably similar to C. hominivorax. In this study, a single 200 pg/kg dosage of ivermectin provided 16 to 20 day protection against C. bezzia- na infestation. These results are comparable with our evaluation of the same dosage of the drug in the prevention, for at least 14 days, against myiasis caused by C. hominivorax. Under field conditions, this residual protection period seems to be sufficient for castra- tion and navel wounds to cicatrize and cease to be attractive to ovipositing female flies. This is also supported by the fact that 95 % of the larvae infestation in control animals were sustained during the first weak after birth, artificial wound or castration.

A 200 Fg per kg dose rate of ivermectin has also been used in Argentina to control gastrointestinal nematodes (STEFFAN et al., 1987). O n the vast rangeland of this country, the common practices of castration and deworming at weaning time could be simplified by a single injection of this drug.

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