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For a wormer to go to market it needs to be more than 95% effective against the worms it is licenced to kill.

Resistance is now a real threat and with overuse of certain chemicals, resistance on the pasture grows and egg reappearance times (as dictated in the drug’s data sheet) diminish over time.

When testing shows that treatment is necessary, especially for medium and high risk horses, we should measure the efficacy of the worming chemical used with a simple follow-up test known as a resistance or reduction test.

 

HAS THE WORMER WORKED?

To work out how affective your worming has been use the reduction test result and divide it by your original worm egg count result and then multiply by 100.  This tells you the percentage of worms that have not been killed by the product used. Anything over 15% would warrant further treatment with a different drug but please get in touch if you need more clarification.

Reduction test result/original worm egg count result x 100 = % of worms not killed by product used

For a wormer to go to market it needs to be more than 95% effective against the worms it is licenced to kill –  but with regular treatment resistance on the pasture grows and egg reappearance times (as dictated in the drug’s data sheet) begin to diminish over time. This means the usefulness of a drug against a specific parasite species can be measured:

  • After a positive worm egg count for redworm or roundworm that requires treatment, conduct a second worm egg count 10-14 days after worming.
  • After a positive EquiSal saliva test for tapeworm treat with a recommended drug and test again 12 weeks later.

If sufficient dose has been administered correctly for the weight of the animal and the follow-up test is showing a reduction of less than 95% efficacy to the targeted parasite(s) then some resistant worms are present.  For example a medium worm egg count between 200 and 1150 e.p.g. should reduce to <50 epg and a high worm egg count of 1200 e.p.g .should reduce to less than 100 e.p.g. on a follow-up test to not be classed as resistant.