16 – Employing the right worming strategies

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How do you know whether the dewormers you're using are proving effective?
How do you know whether the dewormers you’re using are proving effective? © Horsetalk.co.nz

There are drawbacks in having a drenching programme that simply runs on a pre-set six- or eight-weekly cycle, together with some rotation of the active ingredient.

Certainly, any drenching programme is better than no drenching programme at all.

The label on the drench will no doubt spell out the active ingredients and which parasites they are effective against, together with recommended dosing intervals.

However, it’s important to understand the weak spots in this game plan.

Ideally, you want to be using drenches to get the best control, using no more than is necessary to achieve that aim.

A standard de-worming programme – let’s say a drenching every eight weeks for adult horses – leaves a couple of important questions unanswered.

Firstly, are the drenches you’re employing proving effective against the parasites you’re targeting or has worm resistance rendered some of them ineffective?

Secondly, are you potentially drenching horses for worm burdens they simply don’t have?

It is equally possible that, at any given time, some of your horses require a drenching and others don’t. It has long been proven that some horses quickly develop heavy worm burdens while others enjoy long periods with low burdens, perhaps because of a more effective immune system.

Worming at set intervals has another drawback: some drenches are effective for longer periods than others.

A whole raft of other factors – from your pasture management practices to the local climate – may impact substantially on the infective risk to which your horses are exposed.

Worm resistance is well and truly here. It has been a problem for decades.

That is further incentive to take your parasite control programme to a new level; to monitor worm burdens and apply some smarts to the way you use drenches, and when.

Central to all this is faecal egg counts. They will help you determine which horses are most susceptible to parasite infestation and which drenches are proving effective.

Egg counts will also provide you with the information needed to determine whether the time is right to drench.

Perhaps you are using a natural deworming agent without any particular evidence as to whether it’s effective against all or any of the worm varieties affecting your horses. A faecal egg count can ensure your natural worming programme is working.

The prudent horse owner will work with the local climate and take advantage of vulnerable spots in parasite life cycles to get the best result.

Before we turn to egg counts, let’s look at climatic factors.

One important factor is whether summer temperatures reach a level capable of killing eggs and larvae in pasture.

In a country such as New Zealand, crossing temperature and sub-tropical zones, this is unlikely.

It is sensible to consult your equine vet, who will be familiar with local conditions and provide some advice.

The programme outlined below should not be considered set in stone, but it demonstrates how you can work with the climate and faecal eggs counts to get maximum effectiveness out of your de-worming programme without using drenches unnecessarily.

Let’s start in autumn and aim for a good clean-out. Let’s assume you haven’t drenched your horses in a while. It’s possible some might be carrying a heavy burden.

First, either carry out a faecal egg count on your horses yourself or collect the samples and send them away to be done. This will hopefully provide some indication of each horse’s susceptibility to worm infestation and a broad measure of their natural resistance.

Some of your horses may return a low egg count – that’s anything under 100 eggs per gram of faeces (epg). Others may be moderate (100 epg to 500 epg) and some may be high (above 500 epg).

If all your horses return low counts, you should be pleased. Either your drenching and pasture management programme is working, or perhaps your horses all have good natural immunity.

If the eggs counts are variable across your horses, it is likely this pattern will repeat. In other words, horses with a heavy burden will tend to return to that state if your drenching programme is inadequate.

Let’s send the biggest star into bat for the first round. Moxidectin is a great broad-spectrum performer and will deal with encysted small strongyles, which are otherwise difficult to kill. Moxidectin will result in a good clean-out across all your horses.

Ivermectin is a good alternative, although it won’t deliver a killer blow to encysted strongyles. However, a low egg count would certainly make ivermectin a good option.

Don’t forget to keep a detailed written record of which drench you’ve used and when.

Regular faecal egg counts will not only tell you whether your deworming programme is effective, but whether your horses require drenching. © Horsetalk.co.nz
Regular faecal egg counts will not only tell you whether your deworming programme is effective, but whether your horses require drenching. © Horsetalk.co.nz

Heading into winter, some eight weeks later, perform an egg count and see what the results show. Moxidectin is effective for up to a month longer than ivermectin, so those treated with the former should be returning low counts.

Initially at least, you might want to perform an egg count in as little as four weeks, as research in Kentucky by Gene Lyons has unearthed several farms where the egg reappearance period has dropped to four weeks for ivermectin and five weeks for moxidectin. Drench resistance is here, and you need to establish just how resilient the parasites are on your property to the biggest hitters in the worming arsenal.

Those horses returning a moderate count (more than 150 epg) should be drenched again.

Part of your worming strategy is to minimise drench resistance, so it wouldn’t be unreasonable to use a different drench with an active ingredient such as oxibendazole or pyrantel, or even both.

A couple of weeks later you would be wise to do a faecal egg count on the recently drenched horses as resistance to both these drenches is possible.

It’s important you measure whether your drenching programme and the drenches you use are proving effective.

Next up will be a winter drench, when we want to deal with tapeworms. Remember, faecal egg counts do not give a reliable indication of tapeworm infection.

Mature tapeworms drop eggs off in packets from their tail so it’s entirely possible you won’t find any evidence of tapeworms in the tiny sample measured in an egg count.

Tapeworms are most likely to infect horses in autumn so this is the drenching cycle where we are going to assume all horses are infected. The drench you need to do this must contain praziquantel and you’re only likely to find it in combination with either ivermectin or moxidectin.

This will deliver another good, broad-spectrum hit and will also deal with any bot larvae which infected the horses before the egg-laying insects found conditions too cold. This should allow horses to remain bot-free zones until the botflies find conditions warm enough again.

Again, you should perform an egg count on your horses to see how effective you have been in keeping worm burdens low. It is all part of building up a picture on how each of your horses cope with parasites, and the effectiveness of the drenches you are using.

Don’t think that winter necessarily provides a holiday from the burden of worm control.

Winter conditions can allow infective larvae to remain viable for quite long periods in pasture. That said, the cold conditions may stop or eggs or larvae developing to an infective stage – at least until the weather warms up. For strongyles, freezing conditions can easily kill early-stage larvae but the later infective stage are well protected against such trying conditions and can lay in wait for months waiting to infect a horse.

Let’s look at early-spring drenching. Perform a count and see how egg numbers are looking. Horses with high egg counts may well be an indication of encysted strongyles that haven’t been killed (did you use ivermectin last time?), so moxidectin is the best course.

Bear in mind that if you used moxidectin in your drench in winter as opposed to ivermectin, you’ll be getting four weeks of added protection, so you can afford to wait a little longer.

Horses with a low count – below 150 epg – could potentially skip this drench cycle.

Going into summer, you should continue to monitor egg counts and aim for a further drenching with a drench containing praziquantel to have another crack at tapeworms, especially if your property has any history of a tapeworm problem.

The rise in egg counts during the summer months will, to a degree, depend upon the fierceness of your summer and whether strongyle eggs can survive the conditions.

Strongyles are your big target during the warmer months and their consistent – and persistent – shedding of eggs means your regular faecal egg counts will enable you to monitor each of your horse’ worm burden.

If your summer is hot enough, you may be well pleased at the slow rate of increase in the worm burden of your horses over summer. If conditions are to the liking of strongyles, additional summer drenchings will prove necessary.

If you’re concerned about the potential of resistance developing to moxidectin (which you’re using primarily because it performs well against encysted strongyles), your vet might be able to recommend a course of fenbendazole at a higher rate as an alternative.

By the end of the first year, a horse owner should have a clear picture of how each of their horses handle parasites. Some will be destined to require more drenching than others to keep their worm burden within acceptable limits.

It is important not to lose sight of the purpose of your drenching programme. Your aim is to maintain a consistently low worm burden across all your horses by using drenches to their maximum effectiveness.

You will not achieve an egg count of zero. A “good cleanout” of your horse’s gut will not be a complete one. Hence the problem of drench resistance.

Aside from keeping your horses healthy, your programme needs to minimise the shedding of eggs into the pasture. If this can be kept to a minimum, it means the rate of re-infection will be lower and you will use less drench.

It is also vital that you recognize how long each drench type is likely to keep eggs out of the faeces. Drenches in the benzimidazole family – that’s fenbendazole, oxfendazole and oxibendazole – will typically give four weeks, as will the pyrantel salts. Ivermectin will give eight weeks and moxidectin hits 12 weeks. If you waited another four weeks beyond each of these periods, it’s likely that a horse prone to higher worm burdens will be shedding significant numbers of eggs, perhaps millions a day.

These time periods are important, especially in the management of small strongyles. Most drenches will do little more than give encysted strongyles.

Thus, for surviving encysted strongyles, it is only a matter of time before they mature and begin producing eggs in industrial quantities. Your worm control programme will be most effective if you can get back and hit them again before they get that opportunity.

The above-mentioned programme is a general guide only and is no substitute for advice from a veterinarian familiar with local conditions.

There are other drench families and combinations that can prove effective for at least some of your drenchings and these should not be ignored.

The above drenching programme would suit more intensively grazed horse properties. However, if you have the opportunity to rotate stock, rest paddocks, and are religious in picking up manure, it’s possible you may be able to drench even less.

The next article discusses such strategies and explains how drenching at certain times of the year can deliver long-lasting benefits.

It’s important to realise that no battle was ever won by chemical warfare alone. There is much we can do in the field to help with the fight.

» Next: Employing the right farming strategies

 

First published on Horsetalk.co.nz in February, 2009

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