24. Anthelmintic resistance Flashcards
What is the definition of anthelmintic resistance
a heritable (genetically passed to offspring) reduction in efficacy of a drug against a particular species and stage of a parasite
where efficacy of a drug should be >99% resistance is confirmed where efficacy is <95%
What is a overdispersed distribution?
within a host population; a lot of hosts have zero or a few parasites and a few hosts have lots of parasites
How do we classify anthelmintics?
Benzimadazoles
Tetrahydropyrimidines + imidazothiazoles
Macrocyclic lactions
in livestock
Any resistance to one drug in a group means resistance to all drugs in that group
What are some factors determining rate of resistance development?
underdosing, length of time used (BZ, IVM-30 years)
frequency of treatment, pasture management
Size of in-refugia population
What is refugia
proportion of the parasite population not exposed to drug treatment - parasites in untreated hosts, in ext enviro (free-living stages)
the larger the in-refugia population the less the selection pressure - best way to reduce # of resistant worms the survivors prod is to make sure they breed with susceptible worms because this will dilute their resistant genes
What are some available diagnostic tests to test for anthelmintic resistance
usually suspected when poor response to tx
Fecal egg count reduction test - very insensitive
lab tests (genetic or other) not commonly availablw
What are some strategies to reduce the development of anthelmintic resistance?
quarantine and anthelmintic treatment of purchased livestock - tx all incoming animals - a combo of drugs is appropriate here, isolate 12-24hrs before going into herd, test before into herd
Avoid underdosing - calculate dose based on heaviest in the group
minimize anthelmintic use - inc dosing interval, mixed grazing, good nutrition, wean lambs early and move
