WEB Vetnews May 2015

Regulars I Merial pages

Parasite control programs - Part 2 (Part 1 was published in the April 2015 issue of VetNews)

Dr Sheelagh Lloyd, Dr Byron Blagburn and Dr Michel Levy

When starting this program remember that the proportion of low egg shedders may depend on the previous control program used within the herd. A very efficient control program would have produced low pasture challenge that will manifest as low egg counts even in susceptible horses for several months to a year or more after that program has ceased. Advantages • Fewer anthelmintic doses required: • Reported to be cost effective in adult horses; faecal egg count costs paid for by reduced anthelmintic usage. • Should reduce rate of development of anthelmintic resistance. • Parasites in untreated horses are in refugia, unselected for anthelmintic resistance and available to dilute emerging resistant genes. Disadvantages • Anthelmintics are used in suscept­ ible horses, so promoting worm resistant genes. • Bots will not be controlled – preferably cut off or pluck out hairs with eggs while grooming or drug treat in the late autumn/early winter. • Anoplocephala tapeworms will not be controlled – anticestode treatments may be targeted after blood samples (taken late autumn/ early winter) have identified individuals using the ELISA antibody test. However, serologic

tests may only confirm exposure. • Time consuming – requires regular quantitative faecal egg counts by knowledgeable, trained personnel. ANNUAL ROTATION WITH INTERVAL TREATMENTS Annual rotation using the same class of drug for a year • The same class of anthelmintic is used regularly throughout a year, and then substituted with a different class in the following year, i.e. alternate year’s macrocyclic lactone and pyrantel, or a 3 year rotation with macrocyclic lactone, pyrantel and benzimidazole if benzimidazole resistance is absent. • The treatment interval will vary with the class of drug (4-5 weeks for benzimidazoles and pyrantel; 6-8 weeks for ivermectin; 10-13 weeks for 5-day fenbendazole and moxidectin). • Advantages: slows development of multiple drug-resistance but not as much as other methods; easy to implement; user-friendly. • Disadvantages: doesn’t take into account efficacy of drugs against different worm populations. May have to include additional drugs for parasites not removed by selected drug, e.g. bots and benzimidazoles/pyrantel – may have to add macrocyclic lactone that year – preferably manually >>> 33

TARGETED DOSING WITH ANTHELMINTICS Worm the wormy horses • Based on the principle that some horses carry few worms and are consistent low egg shedders (200 epg). • Preferably begin in late winter. • Initially consider faecal egg counting every 2-3 weeks. • Low egg shedders: 200 epg treat (carry out a faecal egg count reduction test particularly for benzimidazoles at the first treatment). - Perform faecal egg count at egg reappearance period for the drug (4-5 weeks benzimidazoles and pyrantel; 6-8 weeks ivermectin; 10-13 weeks after 5-day fenbendazole and moxidectin). - If <200 epg, re-examine at 2-3 weeks intervals (some horses, even though susceptible, will have a longer egg reappearance period, young horses usually have a shorter egg reappearance period.

vet nuus • news

32

Mei/May 2015