Anthelmintic resistance refers to worm populations surviving a correctly-applied, standard dose of anthelmintic (anti-worm treatment).
Anthelmintic resistance is:
Genetic: Resistant worms produce resistant offspring
A pre-adaptive phenomenon: Genes that lead to resistance are already present in the worm population
Irreversible: No effective return to susceptibility even after removal of selecting anthelmintic.
Resistance to all the main anthelmintic classes has been exhibited by nematode populations in most sheep-rearing countries over the last twenty years.
It is important to note:
Benzimidazole resistance has been reported in Teladorsagia circumcincta, Haemonchus contortus, Cooperia curticei and Trichostrongylus spp. Additionally 1-BZ resistant Nematodirus battus has been detected in a small number of flocks across the UK. A survey showed that the genes for resistance were found in around 1 in 4 flocks but generally at very low levels (on 2% of the individual parasites examined but at higher prevalence on some farms). See our Nematodirus page for more information.
Levamisole resistance has been reported in T circumcincta, C curticei and Trichostrongylus spp.
IVM resistance has been reported in T circumcincta and Trichostrongylus species in a number of sheep flocks in parts of Great Britain. There have also been a reports of moxidectin (MOX) resistance. In the early stages of selection, this usually manifests as a reduced period of persistency.
Monepantel resistance has been reported in T circumcincta, Trichostrongylus vitrinus and Oesophagostomum from a single flock in the UK.
Resistance has not currently been reported to 5-SI.
The emergence of ‘triple resistance’ is becoming greater, with the majority of farms examined in surveys containing parasite populations that are resistant to more than one anthelmintic class.
These populations are a cause for concern and presents a challenge in terms of correct advice and management.
Benzimidazole resistance has been reported in Ostertagia ostertagi in UK herds.
IVM resistance has been reported in Cooperia oncophora in a number of cattle herds.
Testing for resistance has a number of benefits:
Consideration should also be given to the species of nematode concerned, since efficacy results will vary according to the species and hence also the time of year the test is carried out. It may be that drug efficacy changes throughout the season as different roundworm species become more prominent.
Early detection of reduced efficacy means that efforts can be concentrated on reducing selection pressures to help maintain efficacy for longer. However, under field conditions anthelmintics will apparently continue to give clinical responses in parasitised sheep when the reduction in faecal egg count (FEC) is substantially less than 95%. Consequently, sheep farmers remain unaware that resistance to an anthelmintic is present until the reduction reaches a high prevalence.
Resistance has been shown to result in significant production losses in affected animals, particularly young stock but also in older stock with concurrent infections.
1) Post-Drench Efficacy Check (PDEC)
Simply, are there any eggs left following treatment?
FEC is carried out post-treatment (timings for assessment in Table 1).
Any eggs present suggest that the drug is not working effectively.
2) Faecal Egg Count Reduction Test (FECRT)
True efficacy testing FEC carried out before AND after treatment Timing of FEC post-treatment depends on class of chemical used (see Table 1).
.If treatment has been effective, there should be a >95% reduction in FEC.
Anthelmintic resistance is not the only reason that anthelmintics sometimes appear to fail to control worm parasites.
Other reasons include:
-- underestimation of the animal's weight
-- poorly maintained dosing equipment
-- poor administration technique
-- incorrect storage of products
-- using products beyond their use-by date
-- mixing anthelmintics with other products
-- incorrect formulation used for your host species
Research being undertaken at Moredun is focussed on improving our understanding of a variety of parasitic diseases along with the mechanisms of resistance and the way in which resistance develops.
The aims of this research is to provide improved means of diagnosis and develop effective management strategies that can be used to conserve the efficacy of our current anthelmintic families.
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