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Tackling the re-emerging menace of worms in horses

For many years the horse industry in general has been casual in their approach to worm control and trusting that regular worming will be  perfectly adequate.  However, there is growing evidence that this approach has led to a significant build up of worms resistant  to nearly all wormer types.  There are no new generation wormers coming onto the market, nor are there likely to be so.  Therefore unless our approach changes we are facing a serious emerging problem.  We may have increased incidence of colic, ill thrift, diarrhoea as well as liver and lung disease.


Some of the main mistakes made:


1:  Treating all horses the same:  Different ages and types of horses should have different programmes.


2:  Treating too frequently:  Ironically over worming is the single biggest factor in building up a resistant worm population.  It is a good thing to leave some horses untreated to leave a percentage of non-resistant  worms in the environment.  These worms will out compete and suppress the development of resistant worms.  Over-treatment is also a huge waste of money.


3:  Linking treatments to suit other management events.


4:  Moving horses to clean pasture after worming:  This is a sure way of contaminating clean pasture only with eggs from resistant worms.


Therefore a new approach is needed and is based on the following principles:-


The types of worms affecting horses

Although 150 different species of worms have been identified in horses there are really 5 types that are of main concern.  Large red worm, which luckily at present are not very common, yet!  Small redworm which is currently the most important.  They can encyst in the lining of the large intestine for long periods and can suddenly re-emerge in the spring time or sometimes after a worm dose witch can have devastating consequences.  Round Worm (Parascaris equorom) which effects young animals under a year old, but whose eggs are very resilient and can last for years.  Tapeworms are becoming more prevalent and reside in one area of the intestine where they can cause blockages.  They are not eliminated by some wormer types and also infestation can only be identified by a blood test.  Thread worms (Strongyloides) affects foals and can cause fatalaties. The foal ingests larvae through mares milk or the larvae can burrow through the skin in a foals mouth.


Types  of Wormers available

Know the major type of wormers and what they are effective against. Remember not all wormers are equal:


Benzimidazoles  eg Panacur.  Will kill only adult redworm and not immature or encysted worms. Widespread resistance reported.  Worm eggs will appear again 4 weeks after worming.  A five day course will kill some of encysted larvae in gut wall.


Pyrimidines   eg Strongid-P,  Embotape, Pyratape.  Will kill adult redworm but resistance reported. A double dose will kill tapeworms. Eggs will reappear 4 weeks after dosing.


Macrocyclic lactones

Ivermectin   eg Eqvalan, Bimectan.  Will kill adult redworms and round worms but will not kill encysted larvae or tapeworms.  Resistance  developing and eggs will reappear 6-8 weeks after dosing.


Moxidectin   eg Equest.  Very effective against  adult and larval encysted redworm but not as effective against round worms and no effect against tapeworms.  Eggs will reappear after 12 weeks from dosing.

Praziquantel   eg Equitape.  Only effective against tapeworms but with no activity against any other worms.

*Equest Pramox and Equimax are dual active ingredient dose and is effective against both Tapeworm and redworm.



Worm control programme DO'S


Base treatments on needs of individual animal:

Foals and younger horse under age of three are more susceptible to developing high worm burdens and consequently require a more rigorous programme.  80% of older horses have excellent natural immunity and require a minimum of worming during the year. However there are 20% that are potentially high shedders and therefore require more frequent monitoring and worming. Mares should be wormed just after foaling to protect foals from threadworms. Foals  should be treated at two, three and four months of age with an Pyrantel Embonate dose and at six and nine months with an Ivermectin dose  against round worm.


Only give treatments from Spring through Autumn:

There is no need to worm between  end  October and End of March unless there is a very mild winter.


Administer wormers at optimal intervals determined by   worm  egg counts

Taking Worm egg counts is absolutely vital to the control of worms in horses.  All horses should have a worm egg count done at end of march to establish which horses have a high( above 500 epg) , medium(200-500 epg) or low( less than 200epg) worm burden. Horses with a low burden would be best not treated . The high and medium burden horses should be treated then.  If there is a concern regarding resistance to wormers on a farm then a repeat worm egg count should be done 10-14 days after worming to establish effectiveness of wormers used. Otherwise it would be sufficient to repeat worm egg count in 6-8 weeks to see which horses need worming again. The Moxidectins suppress  egg re emergence for longer than the other types of wormer. The high burden horses should be wormed every 4-12 weeks (depending on type of wormer used)  during spring and summer. The moderate burden horses  may  just be wormed again in  Autumn. It is impossible to over stress the importance of doing regular worm egg counts. It will save money in the long run and will help prevent the development of worm resistance to anthelmintics. It is possible to do pooled worm egg counts on groups of up to 10 horses which will further cut down on costs.


Leave as many horses as possible untreated apart from an anti tape worm treatment in the Autumn.


Good management policies:

Removing droppings from pasture regularly  ( preferably weekly) during summer months is  a factor of five times more effective at worm control than the use of anthelmintics. Harrowing fields during a hot dry spell and leaving field un-grazed  for at least two weeks is also effective. Letting sheep and cattle graze pastures reduces worm egg burdens.


Check all new horses with  worm egg count before turn out onto pasture.



Worming Programme DONT'S


DON’T routinely worm your horse at regular intervals  without knowing their worm burden status. 


DON’T swap wormers randomly or settle on the cheapest wormer. 


If a wormer is required , DON’T use  too small a dose in adults. 


DON’T give  too large a dose to foals especially with moxidectin which can be toxic.


DON’T let out new arrivals with unknown history for three days after worming onto fresh pasture.




Proper worm control needs to be tailored to each individual farm and group of horses.  Younger animals have lower immunity as well as 20% of adult horses, which are the most important to target for treatment.  Regular worm egg counts are vital.  Drawing up a worm control progamme with your Veterinary team is an extremely helpful tool. At Troytown GreyAbbey we have invested in laboratory equipment and staff training to ensure that we can advise you on your horses worm control. 


For further details please call 045 521686 to speak to a member of our team.

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