Short answer: don't rotate your wormers.
Long answer: A survey once showed that over 80% of horse owners rotate through anthelmintics when worming. This is most likely due to the advertising of pharmaceutical companies who recommend changing wormers throughout the year. The hypothesis is that by changing drugs any worms with resistance to the previous wormer will be removed, and the development of resistance will be slowed. Unfortunately, there is no scientific evidence to back this hypothesis up. In fact, studies have shown that rotation of wormers (across many livestock species) do NOT slow down the development of resistance. So what does rotation of wormers do then? 1. Gives livestock owners a false sense of security: “I rotated this time, so of course it worked.” 2. Mask the development of resistance 3. Potentially treat animals with a wormer that doesn’t work, and cause harm to the animals through inefficient treatment
Why are livestock owners encouraged to rotate wormers then? Back in the 1960’s anthelmintics were narrow spectrum (i.e. only killed a small number of worm species) and so it was recommended that drugs be rotated through to make sure everything was treated for. When the broad spectrum drugs (the ones we use today; benzimidazoles, ivermectin) were introduced, the recommendation was kept based on the above hypothesis. The cynics amongst us may also say that it is a good marketing tactic for the pharmaceutical companies.
All of this is not to say that rotation is bad – it is only bad if you use rotation and only rotation to prevent drug resistance. The use of any drug without a follow up test to prove efficacy is pointless, and only provides a false sense of security. This is why annual FECRT (faecal egg count reduction tests) are recommended following worming to ensure the drug is effective. Swapping between effective and ineffective drugs will only mask the development (not prevent) of resistance, and mean you may be treating animals with ineffective drugs without knowledge, which can have consequences for animal welfare.
The development of resistance is complex – too complex to get into for this post, however resistance develops due to the overuse of drugs. Overuse of drugs, even with rotation, will still cause resistance to occur. Modelling has shown that if treating animals once a year, with the same drug, resistance would take well over 20 years to develop.
If you feel the need to rotate drugs due to long held ideas about treating animals, rather than rotate drugs treat animals with a combination drug. By combination I do not mean a combo of a broad spectrum (BS) and a narrow spectrum (NS) (e.g. ivermectin(BS)/praziquantel(NS)), rather ones that contain two or more Broad Spectrum drugs (brands like Trifecta, Q Drench for ruminants, Equitak for horses). Combinations of BS drugs increases the efficacy of both drugs, and a single dose of a combination drug may drive resistance development less than multiple doses of different drugs.
So instead of following the rotation calendar this spring, the best way to prevent resistance and have effective control of worms is: - Only worm animals that need it (FEC). - Make sure you do a FECRT once a year per drug you use to ensure they are effective.
Your best defence against resistance is to reduce your worming as much as possible. Simple as that.
Raza et al Raza, A., Qamar, A. G., Hayat, K., Ashraf, S., & Williams, A. R. (2019). Anthelmintic resistance and novel control options in equine gastrointestinal nematodes. Parasitology, 146(4), 425-437. Leathwick, D. M., Sauermann, C. W., Geurden, T., & Nielsen, M. K. (2017). Managing anthelmintic resistance in Parascaris spp.: A modelling exercise. Veterinary parasitology, 240, 75-81 O'Hara, C. R. (2016). A Survey Of Anthelmintic Control Practices Of Horses Used For Trail Riding And Horse Show Competitions. Kaplan, R. M., & Nielsen, M. K. (2010). An evidence‐based approach to equine parasite control: It ain't the 60s anymore. Equine Veterinary Education, 22(6), 306-316