Vet Services Hawkes Bay Ltd

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Drenching your horse

Dave Kruger

Some horse owners are still drenching their animals on a six to eight weekly basis regardless of age or worm burden.  This was a traditional approach which is outdated and possibly detrimental in terms of developing resistance to drenches. 

The main internal parasites we see in horses in New Zealand include the following:

Large roundworm (Parascaris equorum) is aptly named and can grow up to around forty centimeters in length.  This worm is of major concern in young foals and if present in large enough numbers may lead to intestinal obstruction and perforation.  Its life cycle includes migration though the liver and lungs and can cause widespread health consequences, with affected foals appearing stunted and listless.

Large strongyles including Strongylus vulgaris are amongst the more dangerous of the equine roundworms.  These parasites can cause colic, anaemia, diarrhoea and weight loss.  They have a complex life cycle including a larval stage which migrates in the walls of the major abdominal arteries.  This can lead to weakening of the vessel wall with resultant aneurisms or blood clots.  This may lead to severe and sometimes fatal colic, even later in life.  Whilst large strongyles are uncommon nowadays, we still see occasional and sometimes fatal cases.

Small strongyles include a number of species, the main ones being the Cyathostome group.  These are very small parasites which may be present in large numbers.  They live in the large intestine and can cause diarrhoea, weight loss, colic and anorexia.   This parasite also has the ability to wall off immature stages in the intestinal lining over winter or if large numbers of adults are already present in the gut.  Large numbers of these worms can emerge simultaneously in spring, pre-foaling or if the horse is stressed.  This leads to associated health issues as well as potential massive pasture contamination.

Pinworm (Oxyuris equi) is not a serious threat to horses, but can cause severe irritation of the perineal area, resulting in rubbed tails and self-trauma to the dock and rump.  This is the white tapered worm horse owners may see in the manure.  The adult females lay their eggs on the perianal skin and then die.  Although very little trial work has been done on this parasite anecdotal reports and our own observations would suggest that they might have developed resistance to ivermectin and related drenches. 

Tapeworm (Anoplocephala) has a complex life cycle involving a stage in pasture mites.  The adult tapeworms may lead to intestinal irritation in young horses and have been implicated in colic caused by interssussception (telescoping of the bowel into itself).

Worm control encompasses various strategies including the use of drench to kill worms present in the animal and various methods of controlling larvae on the pasture.

Epidemiological work done at Massey University studying the worm burden in adult horses has revealed some interesting findings.  When faecal egg counts were done on a group of mares most of them had very low or zero egg counts, suggesting a good immune response to parasites in these animals.  A few however had consistently very high counts and consequently contributed the bulk of egg contamination of the pasture.  The take-home message is that there is little if any point in treating the zero egg count horses with anthelminthics whilst the high egg producing mares definitely required regular treatment.  In this particular study ninety percent of the eggs were being produced by ten percent of the mares!  By only treating the mares that needed it, huge savings could be made, whilst also providing a low level of egg production without adverse effect on these mares.  The theory behind allowing a low level of pasture contamination to continue from untreated animals is that any worms surviving the drench in treated mares (resistant worms) would be likely to breed with non-resistant worms, delaying the onset of widespread drug resistance. This is known as “Refugia” and is the favorite catch-word among parasitologists at present.

It is important to differentiate between young horses and adults.  As described above, most adults have an intrinsic ability to mount an effective immune response to internal parasites, suppressing the number in the gut and the number of eggs these produce.  Youngsters have yet to develop this ability and are hence far more susceptible to the adverse effects of worms.  In terms of drench interval, each property is unique, but generally young horses should be regularly treated with an effective drench.

Faecal egg counts should be done on ALL adult horses once or twice annually, and only those showing significant egg counts treated.  On most properties this will mean a far lower number of treatments than is currently used.

In terms of which anthelminthic to use, in general terms a combination drench containing both an ivermectin family and white drench family is better than using single actives.  This may also contain praziquantal for tapeworms.  A drench test can be done by performing a faecal egg count prior to drenching and identifying horses with significant egg levels, then repeating ten days later.  The post-drench sample should have zero eggs – if any are present this suggests survival of resistant adult worms and a larval culture can be performed to identify the species involved.

Control of the infective worm larva on pasture is an integral part of effective parasite management.  This is best achieved by removal of manure on a regular basis.  Where this is not practical cross-grazing with sheep or cattle works well.  The worm species of concern are all highly host specific so use of ruminants as “vacuum cleaners” is effective.  In our climate harrowing only spreads the eggs and infective larvae over a larger area of pasture – probably increasing the chances of ingestion by horses.  Harrowing prior to cross grazing is thus recommended.

We now have an equine faecal egg counting service available.  Packs can be collected from any of our clinics.  The cost per horse is $15-00.  As mentioned in the article above, identifying which horses actually require drenching will not only save on anthelminthics but increase “Refugia” on your property.

NOTE: All drenches, especially the “ML” family, including abamectin, moxidectin and ivermectin are potentially extremely toxic to dogs.  The biggest risk of access to these drugs is where dogs chew used or partially used tubes of drench.  Please ensure that these are stored and disposed of safely.  If using the liquid multi-dose packs ensure no spillage occurs which could be ingested by inquisitive dogs.

Feel free to talk to one of our equine vets if you require more information. 

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