Vet Services Hawkes Bay Ltd

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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  This was a traditional approach which is outdated and possibly detrimental in terms of developing resistance to drenches.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. 


Read more >Thursday 27th of July 2017: The first five years of a horse's life are critical in regards to dental development. Like us, horses have two sets of teeth, the deciduous being shed from about two and a half years until the permanent teeth have erupted at around five years.

Kennel Cough

Read more >Tuesday 11th of July 2017: Kennel Cough (also known as canine infectious tracheobronchitis) is a highly contagious respiratory disease. Dogs commonly contract kennel cough at places where large amounts of dogs accumulate, such as boarding and daycare facilities and dog parks.

Dental Disease

Read more >Tuesday 11th of July 2017: As our pets age dental disease can start to set in. We often start noticing signs from around the age of 5 (which equates to around 35 in human years). Dental disease can affect the internal organs and the overall health of your pet. There are some measures you can take to help slow or prolong the effects of dental disease at home.

Is your pet visiting our clinic soon for a procedure?

Read more >Friday 23rd of June 2017: It is important to Vet Services that you understand what happens to your pet when they come to us for surgery. We appreciate that you may feel anxious leaving your pet and we hope the following will help ease any concern you may have.

Let it Flow

Read more >Friday 23rd of June 2017: A common emergency condition that we see at a vet clinic is the cat with a 'blocked bladder' (urethral obstruction). They often present to us as a cat that is in pain due to an unknown cause. The owner may find them hiding in the garden or under a bed and suspect that the cat has had some sort of trauma.

Pet rabbits

Read more >Friday 23rd of June 2017: In recent weeks we have been presented with several cases of rabbit haemorrhagic diseases. This is a viral condition which is unfortunately deadly to pet bunnies.

Pet Insurance

Read more >Friday 23rd of June 2017: Many people know the importance of insuring their items, their house or car, even their own health. Fortunately we are also able to insure pets, for not only medical and surgical care but in some cases routine visits can be covered (including vaccinations and wellness checks/blood tests).


Read more >Tuesday 28th of March 2017: General stiffness, slowing down, difficulty rising... Is it just old age? Our senior pets may show subtle signs or be quite obvious in their attempts to tell us about their problems. One of these problems that we commonly see is arthritis.

The Duke of Tamatea

Read more >Tuesday 28th of March 2017: A few months ago I was presented with a 7 year old male cat Beau who had been weak, lethargic and drinking more than usual for about 3 weeks at home. Closer examination revealed very weak and floppy muscles with an almost distended abdomen. Blood and urine tests then showed elevated blood glucose confirming my suspicion that this boy had developed diabetes mellitus.

Severe Anaemia in a Cat

Read more >Tuesday 28th of March 2017: Being from South Africa it was relatively easy diagnosing the cause of severe anaemia in dogs as almost always it was due to a blood parasite called Babesia (which we don't have in New Zealand), and if it wasn't that then the chances were good that it was due to an auto immune disease called Auto Immune Haemolytic Anaemia or IMHA for short.

Wellness Testing in the Geriatric Pet

Read more >Tuesday 28th of March 2017: What is wellness testing?

Wellness testing is the term given to a group of tests that is performed specifically to detect signs of early disease in a pet that is apparently healthy.

The Three-Host-Tick (Haemophysalis longicornis)

Read more >Friday 20th of January 2017: The 'New Zealand Cattle Tick' or 'Bush Tick', as an adult, is a red-brown, 8-legged tick visible with the naked eye from 3x2mm to around 9x7mm (whne it’s full of feed). Larval and nymph (juvenile) stages are much smaller (but still visible) with 6 legs and a dark to black colour. It is known as the three-host-tick as it transitions through three stages from larvae to nymph to adult by attaching to a host, engorging by sucking blood, then dropping back onto the ground and repeating through the stages.

Ideal Weight Benefits

Read more >Tuesday 13th of December 2016: The case of an overweight pet visiting the vet clinic is an all too common theme. Furthermore, the majority are visiting for health reasons that could be prevented if these animals were at their ideal weight.

Heat Stroke

Read more >Thursday 8th of December 2016: With summer just around the corner we thought it appropriate to give our clients some information on heat stroke, what to look out for and what to do should it happen.

Heat stroke occurs when your pet's internal temperature rises abnormally high above 39 C. because he/she is unable to lose excess heat through normal processes: mainly panting and radiation of heat into the surrounding environment.

Barley Grass

Read more >Thursday 8th of December 2016: Summer is here and so are those nasty seeds that stick into anything, anywhere, any time.

Apart from it being painful to our four footed companions when these seeds burrow into them, it can pose important health risks too. In clinic we have experienced their migration into different areas of the animal, each with their own complications.

Cushing’s in dogs

Read more >Thursday 8th of December 2016: Cushing's is a hormonal disease state caused by the excessive production of cortisol, one of the "fight or flight" hormones produced by the adrenal glands.

Normally, when the cortisol level of blood is low, a gland in the brain (called the pituitary gland) secretes a stimulating hormone (ACTH) to tell the adrenal glands to produce and release more cortisol. Once blood cortisol levels are high enough again, it inhibits further secretion of ACTH by the brain. In this way the body keeps blood cortisol levels balanced.

Fleas - How do we prevent the little blighters from biting?

Read more >Tuesday 1st of November 2016: With the warmer months just around the corner it's time to once again consider the dreaded flea. Fleas can be a real problem over the warmer months, but it all starts now!

Skin Allergies in Dogs

Read more >Monday 12th of September 2016: With spring arriving we are starting to see more itchy dogs through the clinic doors. The main cause of this seasonal itch is allergy. Skin allergies can be divided into a number of causes including contact allergy, flea allergy, atopy and food allergy/intolerance.

Preventing Unwanted Kittens

Read more >Monday 12th of September 2016: Desexing our pets is an important part of responsible pet ownership. As the days get longer and the nights get warmer, our pet cats will start to venture away from the fire and off the bed. More cats out and about means there is greater chance of unwanted pregnancies. Without any control in place, a single un-speyed female cat can produce up to 3 litters of kittens per year, with approximately 3-4 kittens per litter.

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