Worming: A targeted programme

Dr Michelle Logan takes a look at the latest developments and recommendations for worming

There have been a lot of studies on worms, the diseases they cause and the drugs we use against them. Most horse owners will be aware that some worms are developing resistance to some of these drugs. 

Due to this resistance, there have been many recent developments in the recommendations on how horses are wormed – the traditional six-week worming plan, in most cases, has given way to a targeted programme.

But this doesn’t mean you only worm when there is a worm problem. The damage has been done by then.

A  targeted plan means that we use wormers more selectively throughout the year – but we still give preventative treatment for ALL horses at certain times of the year, and autumn, is one of those key times. At other times of the year, we only worm the horses who need it. 

The 20/80 rule

In normal healthy, non-breeding adult horses, several different studies have shown that 20% of these horses will shed 80% of the worm eggs on to the pasture, and obviously the more eggs that are passed into the manure, the more worm larvae will hatch to begin the life cycle again when they are ingested with the grass. 

Fortunately, for most types of worms, it’s fairly simple to work out which horses fall into the 20% category, by way of a worm faecal egg count (FEC). You need to collect a fresh ball of manure (keep it cool once collected) and take it to your veterinarian as soon as possible to get the FEC done. If you have a few horses, and get this done every three months, recording the counts, you will soon see a pattern of which horses are passing the highest number of eggs, and which horses have low numbers.

But keep in mind that FECs don’t work for every type of worm. For tapeworms and encysted cyathostome larvae, egg counts are not accurate. 

So, all adult horses should be treated in the autumn with a wormer that treats tapeworm (Praziquantel, Morantel or Pyrantel) and a wormer that treats encysted larvae – Moxidectin or five days of Fenbendazole at the correct dose.

Autumn is the end of the grazing season in a lot of parts of the world. However, this is often not the case in New Zealand as particularly in northern areas, where the winter is mild, as worm eggs will be able to hatch and continue the lifecycle all through the year. 

When you think about the differences in climate from the Far North to Southland, it is easy to understand that there are always going to be variations to a set worming programme. There may also be resistance to certain wormers in some areas, and not in other. Below is an example of a programme which you can use as a basic plan to discuss with your vet and modify to suit your location, situation and environment.

In the tables in this article, ML stands for the macrocyclic lactone family of drenches which includes: ivermectin, moxidectin, abamectin etc. BZ stands for the benzimidazole family of drenches e.g. fenbendazole.

Review your plan after one year, or sooner if there is any evidence of resistance or parasite-related problems.

It is very important that all the information from the FEC pre- and post-drenching are kept, together with any evidence of parasite-related problems.

Adult horses (non-breeding) initial plan example:

Autumn– All adults wormed.
– Targeting encysted cyathostomes and  tapeworms.
– Moxidectin and Praziquantel combination.
– Or 5 days of Fenbendazole at larvicidal dose together with a Praziquantel, Morantel or Pyrantel wormer for tapeworms
Care with Moxidectin. Do not overdose and take care in thin or sick animals.
Winter– Faecal egg count (FEC) taken.
– Worm only those with higher counts (>200-300 eggs per gram (epg))
– Use an ML drench but not Moxidectin
Record the FEC results for each horse. Do a drench check FEC 10 days after treatment
Spring– FEC taken.
– Worm only those with higher counts. (>200-300epg)
– In some situations, a repeat of the autumn worming may be advisable here
Record the FEC results for each horse. Do a drench check FEC 10 days after treatment
Summer– Faecal egg count (FEC) taken.
– Worm only those with higher counts (>200-300epg)
– Use an ML drench but not Moxidectin
Record the FEC results for each horse. Do a drench check FEC 10 days after treatment

Mares and foals

Broodmares will generally follow the adult worming programme; however, around two to three weeks before foaling they should be drenched with a combination drench (a macrocyclic lactone (ML) but not moxidectin with a benzimidazole (BZ)).

Foals are susceptible to more worms and different types of worms compared to adults. They haven’t developed any natural resistance, so do still need worming regularly. Ascarids are the large roundworms that foals can suffer from (these can be up to 50cm long!) and there has been resistance of this worm to the ML family of drenches. Combining one of the ML family, Abamectin, with another drug Morantel, has been shown to be effective even against ascarids that have some resistance to MLs alone, so is a very good option. 

Another good option is to use a combination of a ML and a BZ. Below is an outline of a plan which will need to be adapted according to the exact situation, again, and especially with foals which are at higher risk of problems, it isn’t one size fits all!

Foals’ worming plan

SpringSummer AutumnWinter
First drench at 60-70 days of age, then treated every 60 days. Drench to be a combination drench either including a BZ and ML (but not Moxidectin) or a combination of Abamectin and Morantel.Treated every 60 days with a combination including a BZ and ML (not Moxidectin) or a combination of Abamectin and Morantel. Treat regularly until six months of age.Once over six months of age can be given a Moxidectin and Praziquantel drench at the end of autumn/beginning of winter. Care to dose accurately for weight and only once old enough.FEC every three months. Treated with a ML (not Moxidectin) if over 100 epg. 
A lower cut-off is usually used for youngsters as they are more susceptible.

FEC taken before and 10 days after treatment and recorded to monitor for resistance
FEC taken before and 10 days after treatment and recorded to monitor for resistance

Again; review after the first year, or sooner if evidence of resistance or parasite-related problems.

Pasture management

In addition to the targeted drench plan, pasture management is very important:

  • Removing manure, either the hi-tech or the low-tech way! This is effective in reducing the number of eggs on pasture if it is performed at least twice a week. The manure must them be composted so it reached a high temperature before being used. Removing manure is only practical in certain circumstances
  • Mixed grazing. This can work well, as hardly any worm species can infect both horses and either cattle or sheep. The best method is to leave the paddock empty for a few weeks in between, and then have either cattle or sheep on it for at least a month.
  • Spell paddocks. This works for some worms but not others. The large roundworm eggs can survive for several years in the soil. 
  • Harrowing. The idea is that this breaks up the manure, exposing the eggs and larvae to the sun (or frost) to kill them. Often, all it does is spread viable larvae around the pasture. If enough of a manure ball is left to shelter a larvae then it can survive for a few weeks at >40 degrees C. Larvae outside of manure will die at this temperature, but they can survive outside manure for a few weeks at 25-33 degrees C, which is a more realistic temperature for most of New Zealand in the summer.


The end of autumn or beginning of winter is an ideal time to get on track with your worming programme and make sure all adult horses and ponies are given a drench effective against both tapeworms and encysted cyathostome larvae. Do take care if using Moxidectin – dose according to the correct weight and be extra careful with any thin horses. This is a very important drench to be given. 

Following this, the monitoring of worm egg counts in manure samples will help your vet tailor an individual programme for your horses. This will often mean less drenching is required, which is always good news! There will be some situations (eg. intensive grazing with a lot of horses coming and going) when the traditional set time regular worming plan is the best solution, but in most cases a targeted programme can be adapted to the individuals and is the best solution, both for now and the future. 

Roundworms in a pile of horse manure