Persistent efficacy of doramectin against Haemonchus contortus in goats

THE gastric nematode Haemonchus contortus is one of the most pathogenic and economically important parasites (Vercruysse and others 1988), especially in some temperate and tropical areas. In the Canary Islands it is associated with other nematodes and contributes to the development of gastrointestinal parasitic infections in goats (Molina and others 1997). The control of these infections is usually based on the use of strategic anthelmintic treatment programmes to reduce their negative impact. Some of the most useful products for the control of nematode species of ruminants are the avermectins, which are characterised by a high degree of effectiveness and a persistent activity against a wide spectrum of helminth parasites (Jones and others 1993). The persistent efficacy of the avermectins varies between different avermectin molecules and is also affected by nematode-related factors such as the species and strain of worm and the level of infection, the breed and nutritional status of the host, and even by the design of the trial to analyse the drug’s efficacy (Armour and others 1987, Deroover and others 1997, Vercruysse and others 1998). Although there is some information on the effectiveness of doramectin against gastrointestinal nematodes in goats (Terrill and others 2001), no data are available about its persistent efficacy, an important feature for the development of anthelmintic control programmes (Fisher and Jacobs 1995). Accurate information on the efficacy of different anthelmintics in goats is essential in areas such as the Canary Islands where this ruminant species is the basis of livestock production. This short communication describes a study of the persistence of the efficacy of an injectable formulation of doramectin against H contortus in goats experimentally infected with a low burden of the parasite. The strain of H contortus used was initially isolated on Gran Canaria from a goat slaughtered at the local abattoir, and then maintained in the laboratory by routine passage through nematode-free goats. Infective larvae (L3) were obtained from 10-day faecal cultures at 22°C, by the Baermann technique, partial purification on filter paper and recovery of the cleaned larvae (MAFF 1989). Ten five-month-old male Canarian goats were used for the experimental infection. The goats had been kept under conditions excluding helminth infections from birth; when they showed a slight coccidial infection they were treated with 1 mg/kg diclazuril (Esteve Veterinaria). The animals were randomly allocated into two groups of five: a treated and infected group (A) and an untreated and infected group (B). Both groups were housed apart in covered pens and fed a commercial pelleted ration for goats. Water and mineralised salt were available ad libitum. The animals were treated with 0·2 mg/kg injectable doramectin (Dectomax; Pfizer) subcutaneously (group A) or injected with 0·02 ml/kg of a sterile saline solution (group B). From the day of the treatment for 50 days, all the goats were inoculated daily by intraruminal injection with 200 infective H contortus L3 larvae; each animal received throughout the study 10,000 infective larvae. In previous studies this infective dose and route of inoculation induced a low worm burden