The seminar conducted by our vet last evening was excellent and very well-attended. We turned in our (equine) fecal balls for testing beforehand and got our tickets for the door prize drawing - there were close to 100 people there (and no, I didn't win any of the door prizes). The title of the talk was "Parasite Challenges in the 21st Century". The big takeaway was that deworming strategies need to change, but also that there's no one-size-fits-all solution and that each barn and/or owner has to develop their own deworming strategy in consultation with their vet, and that the right strategy will vary depending on the circumstances. So neither deworming every two months using a rotation of dewormers, nor just doing two fecals a year and only treating what shows up (or not treating or continuing to test at all if fecals are negative), are necessarily the answer. Hang in there - this is a long post with lots of information - so here's the poop (couldn't resist!).
The biggest challenge for the 21st century in equine deworming will be to prevent parasite resistance. Small ruminants - sheep and goats - worldwide already have a huge problem with parasite resistance - most of the dewormers are ineffective. All equine dewormers are started to produce some resistance as well. One striking example is in Germany, where resistance of strongyles to dewormers is high - lytha, this is the reason you can't get dewormers there except by veterinary prescription. The last new dewormer for equines was developed almost 25 years ago, and there's nothing new in the pipeline, so we have to preserve the efficacy of the dewormers we have for as long as possible by delaying the development of parasite resistance. The old days, before effective dewormers, weren't the good old days - horses died and were very sick - worms were a major cause of colic and failure of horses to thrive and roundworms caused pneumonia in foals. We don't want to go back there, but if our current dewormers become ineffective due to the development of parasite resistance, that's where we and our horses will be.
The answer is to be smarter about how we de-worm - 80% of the parasites are transmitted by 20% of the horses, and stable management practices can also make a big difference. Also, our vet pointed out that it isn't necessarily a bad thing for horses to have a few parasites - the objective doesn't have to be zero parasites - carrying a few parasites actually strengthens the horse's immune system.
The strategy involves using effective products properly - the right dosage administered properly at the right time, monitoring fecal egg counts if possible and deworming only when appropriate. One big issue our vet has found is that people administering dewormers often don't get it all down the horse due to poor technique and also often underdose. He said 80% of the horses he sees are overweight and that owners often seem to dose at the ideal rather than the actual weight - some larger horses might need more than just one tube of de-wormer and weight-taping isn't all that accurate. What happens with underdosing is not that you only kill x% of the worms, but that you selectively kill the most susceptible of the worms, leaving the stronger ones to reproduce - this is one of the primary ways parasite resistance develops. Our vet also said that one reason some people underdose is from a fear of toxicity - he said almost all dewormers in use today are very safe for horses, although he said care must be used when deworming with Quest in skinny, debilitated/immune-compromised or very young (foals) horses as toxicity can result - this has to do with the fat levels in the horse's body and how Quest is metabolized.
But he did make the very important point that even if your fecal egg counts are negative, you likely will have to still deworm - he didn't want the take-away to be that you can stop using dewormers completely. The two culprits are bots and the three species of tapeworms that affect horses. In both cases, only a couple of the dewormers are effective against these, and they aren't easily detectable using fecal egg counts. Encysted strongyles that don't show up in fecal egg counts also are an issue. Interestingly enough, exposure to tapeworms varies greatly in different parts of the United States - in some parts of the desert Southwest the exposure rate is as low as 19% but in our part of the world - upper Midwest - the exposure rate is 96%. (No one's entirely sure why this is, but it's likely due to moisture/temperature/humidity conditions. Unrelated factoid - over 70% of horses in our part of the world have also been exposed to EPM, which occurs only through exposure to possum manure.)
To develop an effective deworming strategy, a number of factors should be taken into account. Particular products administered with particular timing should be recommended depending on a number of factors: time of year, humidity, temperature, how are horses housed/turned out (stalls, paddocks, dry lots, pastures - strongyles is primarily a worm of pastures - degree of mixing of the horses), age (foals and immune compromised horses such as Cushings horses may need more treatment as their immune systems don't cope as well with worms), health status of the horse and the prepotency period of the parasites.
A brief digression about prepotency period. The prepotency period of a particular species of worm is the time from the hatching of the egg until the adult is sexually mature and produces eggs itself - the full life cycle. This period can be 3 to 4 months or in some species up to a year. So effectively your horse has "rolling exposure" to worm species in its environment, meaning that any fecal egg count you do is just a snapshot from a moment in time. This is another reason why not deworming at all after one or a few negative or low fecal egg counts may not be wise.
Three very important factors in establishing a worming program for your own situation are sanitation, identification of high shedders (more on this below) and the concentration of horses at your facility. Although the vet said (and I agree) that horses on pasture are the happiest, horses on pasture are at highest risk for worm exposure. Removing manure from paddocks and dry lots, pasture rotation and dragging pastures at least several times a year can significantly reduce worm transmission - most are sensitive to heat, drying and UV radiation so the more air and sunlight the manure particles can be exposed to, the better. Heat and moisture promote worms. Horses that are in stalls or paddocks or dry lots (provided manure is picked up) are at lower risk. Are horses in separate groups or are they mixed together? The medical history, body condition, age and occupation/travel of each horse is also important. Horses with Cushings present special issues as they are often high shedders due to having compromised immune systems. Foals and broodmares also require special regimens.
What you decide to do may be very different if you have a few horses at home or if you manage a large boarding stable where horses frequently move in and out. Some barns may decide to continue with every two months deworming, and some horse owners may be able to monitor, deworm and manage in such a way that only deworming for tapes and bots is required once a year - our vet believes that not deworming at all isn't a good option.
Here's our vet's recommendation for year one - develop a specific program based on discussions with your own vet, that takes account of your circumstances and needs and the specifics of your own horses. Do a minimum of two fecal egg counts for each horse in year one - spring and fall - to establish a baseline. (Our vet says it is possible to learn to do this yourself using an ordinary microscope, and several people in the audience were doing it themselves.) Determine which horses have some degree of resistance to parasites - which horses are high, medium or low shedders of parasite eggs - this cannot be done just by looking at a horse's health and condition as some apparently very healthy horses are high shedders and a source of worms for other horses. Also, horses can change status - to a higher load in case of persistent exposure to high shedders, or lower due to treatment. In some cases, doing a mixed sample for a pen or paddock may be OK, although if the results show there is a high shedder present, it will probably be necessary to determine which horse it is in order to treat appropriately. Depending on how each horse is classified, treat appropriately - when to do fecal egg counts for each horse, which products to use and when, the correct dosage for each horse (this is really critical to prevent parasite resistance) - the best thing is to end up with a written chart summarizing the recommended protocol for each horse in the barn, specifying in which months testing and/or treatment should be done. In year two and thereafter, continue to sample and modify the program as needed. Our vet emphasized that it isn't just a case of worming less, or not at all - it is necessary to monitor over time as well and take account of parasites that don't show up in fecal egg counts.
Our vet defines a low shedder as a horse with an egg count from zero (a negative horse) of up to 200 eggs/gram of stool. This may seem like a lot of eggs, but it really isn't. A low shedder should have a fecal egg count twice a year or perhaps once if the horse is in a low-risk environment. These horses should be dewormed three times a year - spring, summer and late fall (some vets will recommend twice a year). Specific products should be recommended by the vet for each time of year.
A medium shedder has eggs counts of 201-400 eggs/gram of stool. These horses should have fecal egg counts two to three times a year, and should be dewormed four times annually - spring, summer, fall and late fall.
High shedders have egg counts over 400 eggs/gram of stool. These horses should be a focus of testing and treatment as they are the vector for transmission to other horses - the objective is to break the cycle of transmission. They should have a fecal egg count prior to and two to three weeks after each worming - to determine their resistance levels - and should be dewormed five to six times a year until their status changes.
None of this is costless, of course. Our vet currently charges $18 for a fecal egg count - the one last night was free (Dawn was negative and also was negative on her last count in the fall). Our vet will not be charging for the consultation necessary to set up the program for each horse. Each barn and owner will, in consultation with their vet, have to come up with a program that works for them and their horses and practical considerations will enter in. If you have less control over your barn's management practices or how other horses in your barn are dewormed, your choices may be different. If there are multiple vets involved, some coordination will be required by the barn management and the barn management may have to decide what practices are required rather than leaving it up to individual owners.
I found this information very valuable. Our barn has three different vets, but I expect we can work out a program that meets the needs of all horses and owners.