12 Sept 2016
Jacqueline Matthews explains the importance of, and processes involved in, establishing an effective control programme for intestinal helminths in horses in the fifth of six reports.
Jacqueline Matthews
Job Title
Rotational grazing with sheep can be beneficial – they can act as biological hoovers.
With intestinal worm burdens commonplace in grazing horses, establishing an effective control programme is probably second only to providing horses with access to water and the appropriate nutrition.
Almost all grazing horses become infected, the most common culprit being the strongyles. However, most horses will have only low burdens, with higher burdens affecting a relatively small minority of individuals.
When it comes to diagnostics, knowing what information to obtain and how to interpret it is key to providing horse owners with the correct worm control advice.
Within horse populations the distribution of strongyle infections varies, with the general rule of thumb being about 20% of horses harbour about 80% of the worms. This is reflected in the levels of faecal egg shedding in populations of horses and ponies.
When using faecal egg counts, a key question the author asks is: “What worms are we looking for and how do we interpret the information?” Following the introduction of widely available faecal egg counts, horse owners have taken this on board to the point where worm egg shedding testing is quite commonplace.
However, when it comes to measuring the effectiveness of the dewormers used in control programmes, it would appear not everyone is on the same page.
A question the author often asks equine practitioners is: “Do you assess dewormer sensitivity in the horse populations you are working with?” – that is, the process where a horse is tested, treated and then tested again 14 to 17 days later to ascertain the efficacy of the wormer reducing worm (in this case, strongyle) egg shedding in dung.
To get the best value out of your faecal egg count analysis, it is important to handle and process dung samples correctly. Worm eggs are not usually spread evenly in dung, so owners should be encouraged to take their samples from several balls within one dropping.
This will give the optimum chance of a representative sample being analysed.
How the sample is managed from collection to testing in the lab is also important. Prompt posting (or in-house testing) and/or refrigeration is crucial to preserve the sample – as is exclusion of air from the sample vessel or bag (Panel 1).
Because cyathostomin encysted and large strongyle larvae cannot be detected by faecal egg count analysis, the recommendation in the UK is all horses should receive an annual treatment with moxidectin in autumn/winter. This can be combined with a tapeworm treatment (such as praziquantel) if necessary.
Best practice worm control should involve several management measures to reduce levels of infection in the environment to try to minimise the use of dewormers so drug resistance is not selected. These include:
Rotational grazing with sheep can be beneficial – they can act as biological hoovers. Awareness of liver fluke in sheep, especially on boggy/wet land, is important, as this can be passed on to co-grazing horses.
Advise testing here; vets need to be involved.
In a nutshell, when advising clients on worm management, ask yourself and the owner these key questions:
