I happen to think the subject of equine worms is important to EVERY owner, whether they have a single horse or a herd, are a hobby owner or a professional, then you will know now.
I've seen too many horses, cattle and sheep over my lifetime that have suffered the tragic consequences of worm burden and ineffective management. "Ineffective" is "too much" as well as "too little".
Too much is just wasteful and expensive and IN THE LONG TERM, responsible for anthelmintic resistance.
When the USA and Canada opted out of the global initiative to tackle that problem and went on to licence and permit the likes of daily wormers I was VERY vocal via the organisations I am a professional member of and also at a more personal level face to face with people I meet and even on forums.
So appreciate that I don't endorse OVER worming. Or doing it too frequently. EVER. Never have done and never will do.
I've also allowed my premises and stock to be used as part of extensive research undertaken by The Royal (Dick) School of Veterinary Studies at Edinburgh. As a benefit of that to me and my horses, I've had the opportunity to obtain high quality leading edge research information and been able to apply considered and detailed recommendations to my horses, cattle and sheep over an extended period of time.
The studies have been extensive not only in duration but also in the breadth of subject matters. Basically what's happened is that as the years have rolled on there's been a variety of vets who are specialising in gastroenterology, immunology, immunobiology, geneology who have particular subject specialist areas.
So they've investigated the likes of nematodes at the DNA level, looked at optimal timings of administration of wormer in relation to lifecycles, investigated how anthelmintic resistance develops, looked at the prevalence of equine worm resistence, studied cyathostomins (small strongyles) and their ability to cause disease and particularly the link to colic, considered the effect of different sorts of pasture management, studied faecal sampling.
What is VERY well known with wormers is that it's vital to worm effectively at the right time of year with the right doseage and to ensure that you never permit large infestation.
It's known that drug resistance develops within worm populations because these populations are extremely large and because, at the DNA level, pathogens are diverse. This means that worms respond and adapt, under ‘selection pressure’ . When powerful drug treatments are applied, these act as a very powerful trigger for adaptation so that even if one in 1,000,000 worms initially has a mutation that allows it to ‘resist’ the drug, it can survive – this effect builds up over time so that if the same drug type is administered frequently over time, a significant part of the population will consist of resistant types of worms. At this stage, the drug becomes ineffective – this is either seen as an inability to reduce faecal egg counts after treatment (to levels described when the drug was first licensed) or as parasite-associated clinical disease in a group of horses which have a ‘good worming history’.
It’s therefore important to use anthelmintic drugs by treating horses at the appropriate time of year with the correct time of wormer. Also, at certain times of the year, to target horses carrying high burdens of worms and leave those with lower burdens untreated. It is important to remember that not all horses carry similar sizes of burdens: a reasonable rule of thumb is that 20% of horses harbour 80% of the burden. This means that worm control strategies can based on targeted treatments of individual animals taking into account level of burden. Faecal egg counts at an appropriate time of year (in the UK, spring and summer) can be used to identify which horses to treat to reduce contamination onto pasture. Remember that encysted cyathostomins in the gut wall, usually at highest levels in autumn and winter, cannot be detected via faecal egg counts, and these should be treated at these times of year using an appropriate treatment that kills encysted larvae.
Again because I've seen and heard of too many horses, cattle and sheep over my lifetime that have suffered the tragic consequences of worm burden and ineffective management, I get VERY concerned when folks aren't ensuring they're keeping the burden in check.
Over here there's a relatively high level of awareness of the risks of wormer tolerance.
I use a rotational worming programme using faecal sampling to help to inform and also taking into consideration the time of year.
New horses to the premises: I always quarantine horses on arrival for 21 days and no matter what the owner tells me about worming its a condition of horses coming here that they are wormed on arrival with moxidectin and kept in a restricted paddock for two days with collection and disposal of droppings prior to any introduction to communal pasture. I then follow that with a 5 Fenbendazole course for small redworms. Then they're either blood tested or treated anyway for tapeworms and treated with "double-dose" Pyrantel P or Strongid P depending on the results.
For my own horses I have faecal egg counts every 4 months and dependent on those use ivermectin or moxidectin based and normally equalan or quest.
Spring and autumn is the time to also worm for tapeworm so then is the time to dose with Benzimidazoles or Pyrantel Embonate and for that I use Pancur, pyratape, or strongid P as well as an Invermectin which will protect against other worm infestation and including bots.
I use a weight tape and dose appropriately - don't mind being slightly over NEVER slightly under though. Again because of the risks of wormer tolerance.