nitrophenols and closantel

This paper describes an exceptional spectrum of multiple anthelmintic resistance in two strains of Haemonchus contortus in South Africa, one from Howick in KwaZulu-Natal, and the other from Badplaas, in Mpumalanga. Apparently for the first time, a helminth strain is described with resistance to compounds from all five of the modern anthelmintic groups used for nematode control in sheep; also, two strains of H. contortus show resistance to the two substituted phenols, dinitrophenol and nitroxynil. Only closantel at 5 mg kg-1 of nine compounds tested appears to have undiminished efficacy against the Howick strain, but even in the case of closantel, the residual activity and minimal effective level need to be tested before it can be concluded that its efficiency is unaffected. The exceptional resistance of the Howick strain is demonstrated by the fact that sequential daily drenching of sheep infected with the strain, with levamisole at 18 mg kg-1, oxfendazole at 20 mg kg-1, levamisole at 20 mg kg-1 and a mixture of fenbendazole at 10 mg kg-1 plus trichlorfon at 132 mg kg-1 on the fourth day, failed to clear sheep of the infection. There are strong indications that side-resistance occurs between dinitrophenol and nitroxynil, on the one hand, and the salicylanilides, on the other, and it is suggested that, before long, strains of H. contortus will be found with high levels of resistance to all the currently available anthelmintics.

Topics: Animals; Antinematodal Agents; Drug Resistance, Multiple; Female; Haemonchiasis; Haemonchus; Male; Nitrophenols; Nitroxinil; Salicylanilides; Sheep; Sheep Diseases; South Africa

Topics: Albendazole; Animals; Anthelmintics; Brazil; Drug Resistance, Multiple; Feces; Fenbendazole; Haemonchiasis; Haemonchus; Ivermectin; Levamisole; Nematoda; Nematode Infections; Nitrophenols; Parasite Egg Count; Salicylanilides; Sheep; Sheep Diseases; Trichostrongylosis; Trichostrongylus

Topics: Administration, Oral; Animals; Anthelmintics; Benzimidazoles; Drug Resistance; Feces; Fenbendazole; Haemonchiasis; Haemonchus; Injections, Subcutaneous; Ivermectin; Nitrophenols; Nitroxinil; Parasite Egg Count; Salicylamides; Salicylanilides; Sheep; Sheep Diseases; Time Factors; Trichostrongyloidea; Trichostrongyloidiasis

Two long-acting anthelmintics, closantel and disophenol, were tested by treatment 90, 60 and 30 days before challenge with a thiabendazole selected laboratory strain of Haemonchus contortus. The sheep were slaughtered 28 days after infection for total worm counts. A significant reduction in the number of adults remaining at autopsy was recorded after treatment with both anthelmintics 30 and 60 days before challenge but there was zero efficacy when the animals were treated 90 days before challenge. No significant difference was exhibited between the two compounds. Variability in response was recorded between animals on the 60 day period of treatment, which for disophenol may be dose related because animals receiving the lower volumes showed higher worm counts. For both anthelmintics a variability in the rate of metabolism may be responsible. The possibility offered by both anthelmintics for treating benzimidazole resistant strains of H contortus is discussed.

Topics: Animals; Anthelmintics; Drug Resistance; Haemonchiasis; Haemonchus; Nitrophenols; Salicylamides; Salicylanilides; Sheep; Sheep Diseases; Thiabendazole; Trichostrongyloidiasis