Survey of anthelmintic resistance in Western Australian sheep flocks 2. Relationship with sheep management and parasite control practices

Abstract

Owners of 116 farms, whose flocks had been tested for anthelmintic resistance, were interviewed to determine their use of various sheep management and parasite control practices and their knowledge and adoption of recommended procedures for the prevention and control of resistance. Farmers knowledge of current recommendations related mainly to changing drenches and drench groups. Other aspects of the recommended program including reduction of drenching frequency and the use of alternative management strategies were not considered as important by farmers. For most questions a high proportion of farmers (greater than 20%) had no opinion. Associations between various strategies for nematode control and resistance of Trichostrongylus and Teladorsagia to thiabendazole and levamisole were examined. These relationships differed between anthelmintics and nematode genera. A number of factors were related to resistance of one or both nematode genera to one or both anthelmintic groups. These factors included flock size, percentage of ewes in the flock, cattle number, main sheep production activity, grazing strategy, frequency of drenching, changes in the frequency of drenching, number of summer drenches and the method of estimating dose rates. It was concluded that the methods employed to control anthelmintic resistance may vary with the nematode, its resistance status and the anthelmintic to which it is exposed. Modifications to the previously recommended program have been proposed which incorporate selection of the anthelmintic to be used following a test for anthelmintic resistance.

Gastrointestinal nematodes and anthelmintic resistance in Danish goat herds

“…Generally, the Danish treatment frequencies are low compared with countries with high AR levels, such as South Africa [48], New Zealand and Australia [10, 27], and high treatment frequencies are probably not the major cause of AR in Denmark today. A further reduction of the treatment frequency, as recommended to limit AR development [2, 15], will be hard to implement under Danish circumstances, and is not considered realistic.…”

Anthelmintic resistance: The state of play revisited

“…Underdosing is generally considered an important factor in the development of AR (Edwards et al, 1986) because sub therapeutic doses might allow the survival of heterozygous resistant worms (Smith, 1990). Several laboratory experiments have shown that underdosing contributes to the selection of resistant or tolerant strains (Egerton et al, 1988;Hoekstra et al, 1997).…”

Sheep and goat nematode resistance to anthelmintics: pro and cons among breeding management factors

“…II) are now recommended for treating goats with most anthelmintics [22]. Under-dosing caused by the under-estimation of the animal's weight is nothing less than anecdotal since epidemiological surveys mention that bodyweights of sheep and goats are estimated by the eye [8,36,67,68,83,100], and that anthelmintic doses are not calculated for the heaviest animals of the flock [32,67,68,83,100]. In French dairy goat herds, the bodyweight of goats was usually evaluated at more or less 50 kg in the 1980s [17] which grossly under-estimated the real weight of the goats.…”

“…The main factor evoked to select for resistance concerns treatment frequency and, "a large number of anthelmintic treatments with the same anthelmintic family for years" is taken as the key factor for selecting for resistance [91]. This statement is supported by surveys realised on a regional scale, in New Zealand [59,83]; in Malaysia [33]; in South Africa [95]; in the United Kingdom [53,75]; in Australia [36,72]. The "same" repeated anthelmintic treatment is a major cause, in experimental and oversimplified conditions (three-month-old lambs, synchronous infestation with only one to three gastro-intestinal nematode species...) of positive correlation between intensity of resistance and treatment frequency [5,66,71].…”