Horses are kept in a wide range of conditions. They might share pasture with other farm livestock or might be living in a single paddock they have grazed for years.
They might be grazing on their own or have a few other equine paddock mates. They might come indoors at night, be stabled throughout the winter, or spend all year round at pasture.
If plenty of land is available, the opportunities for pasture rotation exist in a way they don’t in more intensive operations, such as racing stables.
In short, the way in which your horse or horses are kept will have a major bearing on your worm management programme.
If you have only a few horses and enough grazing for some rotation, it’s likely you can manage with only limited use of drenches, provided you monitor faecal egg counts to ensure the worm burden is within acceptable limits.
With pasture rotation and dung collection every two or three days, a horse owner could get down to using chemical drenches two or three times a year, but egg counts must be undertaken.
More intensively grazed properties will likely involve more use of drenches to keep the worm burden low.
It’s essential to understand precisely what you’re trying to achieve in your worm control programme.
Worm eggs and larvae are not an occasional visitor to your pasture. A horse pasture will likely have parasites in their various stages of development waiting to infect a horse, unless the pasture has been shut up and free of horses for years.
In the case of ascarids, infective larvae can typically survive within their protective coating for two years and, if conditions are to their liking, up to 10 years.
The aim of your management programme is therefore to knock your horse’s worm burden on the head and then do everything you can to minimise the rate of re-infection – that is, the rate at which it picks up infective larvae or eggs from the environment.
The single most important strategy is dung collection. Key parasites all make their exit from horses in their dung, so by collecting the manure you reduce the worm larvae in the pasture waiting to be ingested by grazing horses.
It has been demonstrated that this strategy alone will reduce the larvae count in pasture by 80% or more. There is also the added benefit that horses will graze more of the paddock, given their preference to eat grass well away from horse droppings.
Unfortunately, you can’t leave the dung in the paddock for too long. Some eggs turn to infective larvae in under a week during warm weather. There is the further risk that rain or horses knocking the dung on the way past will help the infective larvae either into the soil or on to nearby grass to re-infect horses.
Dung should be routinely collected every two or three days and composted. If composting is done properly, the heat generated should be enough to kill eggs and larvae – and provide you with a lovely end-product for use on your garden.
Dung collection will not provide instant results as millions of infective larvae or eggs are likely to infest an established horse pasture. However, through regular dung collection and a regular drenching programme, over a period of months the numbers in the pasture will dwindle and you should be able to reduce the frequency of your drenching.
The results of your dung collection should eventually be apparent in faecal egg counts, but you will need to be patient.
The use of harrows to break up the dung and distribute it around the paddock has been discussed earlier, but it’s worth revisiting the pitfalls.
Certainly, if done in hot and dry conditions, a good number are larvae and eggs are likely to die. However, a good many more will survive and still be able to infect your horses.
Generally, in temperate climates, harrowing is unlikely to be effective in controlling parasites, and is likely to distribute the infective larvae to all corners of the paddock. This is far from desirable.
Those with bigger acreages and other stock have a powerful advantage.
One of the biggest enemies of parasites is time. Once a parasite reaches an infective stage in a paddock, it must then play a waiting game to find a new host in the form of a grazing horse.
While ascarids are able to lie in wait for two years or more, your major nemesis, the small strongyle, can hang on for only a few weeks if conditions are warm and dry.
The benefits of rotating other livestock through pastures is discussed here.
Drenching will prove more effective at certain times of the year. For example, a drench in early spring is crucial as you can deal with small strongyles that have wintered over in your horse. If you can take out the adult strongyles and those encysted in the intestine wall, you will go a long way towards minimising the number of eggs being dropped on the pasture during the warmer months.
Two agents are effective against the toughest nut – encysted strongyles. They are moxidectin at 0.4 milligrams per kilogram of horse, and the five-day course of fenbendazole at the rate of 10 millgrams per kilogram of body weight each day, although the latter is likely to prove effective only if resistance is not present.
A late-summer drench will knock back the burden picked up over summer and an autumn drench, just on the cusp of winter, should keep levels within check over the winter months, again targeting small strongyles.
A wise horse owner might look for a broad spectrum drench for the late-autumn and early-spring drenches containing praziquantel, which will deal with tapeworms.
The way in which tapeworms release their eggs means you may not see them during an egg count. It is best to assume a horse is infected and drench accordingly.
The timing of these same two drenching cycles will also prove particularly effective against bots.
It’s essential to realise that reducing drench use must be based not on a whim, but on evidence through faecal eggs counts that your programme is effective.
If paddocks are heavily infested through inadequate management practices in the past, it may take a couple of years of dung collection and egg-count monitoring before a reduced drenching programme is effective in keeping parasites under control in your horses.
Remember, too, that your drenching programme is aimed not only at reducing the burden in your horses, but reducing the re-infection rate as much as possible. That is why timing is important.
First published on Horsetalk.co.nz in February, 2009