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Internal parasite anthelmintic resistance and control
Gastrointestinal nematodes are major pathogens of small ruminants and control has relied almost exclusively on the frequent use of anthelmintics. Nematodes, especially Haemonchus contortus, have developed resistance to all approved anthelmintics. H. Contortus is a problem in warm/wet environments like the Southeastern US (year round) and is now a seasonal (summer) problem in the rest of US. For the past 50+ years recommended control strategies maximized benefits of treatment and ignored resistance issues. Resistance is the ability of nematodes to survive treatments that are generally effective at the recommended dose. Treatment eliminates those whose genotype renders them susceptible and those that are resistant survive and pass on their "resistant" genes. Resistant nematodes accumulate and finally treatment failure occurs. For anthelmintics to remain effective, refugia must be preserved. Refugia is the proportion of the population that is not selected by drug treatment and provides a pool of susceptible genes to dilute resistant genes in that population. Targeted selective treatment is a concept based on the unequal distribution of nematodes in the animal population. A small proportion of animals in a population harbor most of the nematodes and are responsible for most of the egg output and thus pasture contamination. Targeting only those animals for treatment will provide refugia in the untreated animals. Monitoring with FAMACHA and/or fecal egg counts are proven concepts to identify those animals needing treatment. Once identified, smart use of anthelmintics and incorporating alternative control measures into an integrated control program are essential for sustainability.
Keywords: Small Ruminant Nematodes, Anthelmintic Resistance, Control