Evidence that moxidectin is a greater risk factor than ivermectin in the development of resistance to macrocyclic lactones by Ostertagia spp in sheep in south eastern Australia.
OBJECTIVE
To determine associations between resistance of Ostertagia (=Teladorsagia) spp to macrocyclic lactone (ML) anthelmintics and history of use of anthelmintics, by type, on commercial sheep farms in temperate regions of southern South Australia and Victoria, Australia.


METHODS
Faecal egg count reduction tests (FECRTs) were conducted during a 2.5-year period (from August 2001 to January 2004) and records of the type of anthelmintic used in the 5 years preceding the FECRTs were collected from commercial sheep farms (n=103) in southern South Australia and Victoria, and data analysed retrospectively. ML resistance was defined as <95% reduction of Ostertagia spp 10-14 days after treatment with ivermectin (IVM), orally, at half the manufacturer's recommended dose rate. Use of anthelmintics in the preceding 5 and 10 years on each property was classified according to the nett number of years each of the following classes of drug had been used: IVM oral liquid (IVO), IVM controlled-release capsules (CRCs), abamectin (ABA), moxidectin (MOX) or a non-ML anthelmintic. The prevalence of ML resistance, by property, was analysed for associations with prior use of anthelmintics.


RESULTS
Resistance by Ostertagia spp to ML anthelmintics was evident on 51/103 (49.5%) properties. The prevalence of resistance was lowest (23%) on properties on which MOX had not been used, and was significantly higher (64-77%) on properties on which MOX had been used for > or =2 of the preceding 5 years (p<0.001). In contrast, the prevalence of resistance was highest (70-74%) on the properties on which IVM, or IVM and/ or ABA, had not been used in the previous 5 years (on which the use of MOX was predominant), and was markedly lower (20- 42%) on properties that had used IVM or IVM and/or ABA for at least one of the preceding 5 years. Prevalence of resistance was higher for properties on which the only ML anthelmintic used was MOX (19/29=66%) than for those on which the only ML used was IVO (2/19=11%; p<0.001). Properties on which the only ML used was MOX were 2.72 times more likely to have resistance than properties on which the only ML used was IVO (95% confidence interval (CI) = 1.01-5.08).


CONCLUSIONS
Use of MOX for > or =2 of the preceding 5 years was associated with a higher prevalence of resistance to ML by Ostertagia spp on sheep farms in south eastern Australia than the use of IVO.
Rendell
D K
DK
David Rendell & Associates, 170 Mt Baimbridge Rd, Hamilton, Victoria 3300, Australia. drendell@bigpond.net.au
Rentsch
T E
TE
Smith
J M
JM
Chandler
D S
DS
Callinan
A P L
AP
D000284:Administration, Oral; D000822:Animal Husbandry / Q000379:methods; D000818:Animals; D000871:Anthelmintics / Q000627:therapeutic use*; D001315:Australia; D004351:Drug Resistance; D004359:Drug Therapy, Combination; D005243:Feces / Q000469:parasitology; D005260:Female; D007559:Ivermectin / Q000627:therapeutic use*; D018942:Macrolides / Q000627:therapeutic use; D008297:Male; D010028:Ostertagia / Q000187:drug effects*; D010029:Ostertagiasis / Q000188:drug therapy / Q000453:epidemiology / Q000662:veterinary*; D010270:Parasite Egg Count / Q000662:veterinary; D021261:Parasitic Sensitivity Tests / Q000662:veterinary; D015995:Prevalence; D011897:Random Allocation; D012307:Risk Factors; D012756:Sheep; D012757:Sheep Diseases / Q000188:drug therapy* / Q000453:epidemiology / Q000469:parasitology; D016896:Treatment Outcome; D014739:Victoria
D000871:Anthelmintics; D018942:Macrolides; D007559:Ivermectin; C027837:moxidectin