Parasite Control

As veterinarians, we are all too familiar with the adverse effects of internal parasites on horses’ health and condition. The importance of implementing a suitable de-worming protocol for your horse should not be underestimated, as high burdens can do irreversible damage to the gut, causing weight loss, poor condition, and in some cases, colic.

It is important to recognise that poor parasite control in one horse will have an effect on the other horses in the paddock, as worm eggs are shed in faeces on to the pasture. In this way, the lifecycle of the parasite can continue between horses, so we should be working as a team to maximise control and minimise spread where possible.

The aim of an effective worm control programme is to prevent the parasites from completing their lifecycle and therefore prevent further contamination of the pasture. In New Zealand, our ongoing threat to achieving this aim is that of resistance. Resistance is defined as the ability of individual parasites in a population to survive a treatment that was once effective.

Several factors have contributed to the development of resistance in our equine population – most notably the use of blanket dewormers (that is, dewormers with combination products to treat several classes of parasite) given at too high a frequency (when they are not necessarily required) and/or at too low a dose (meaning that the compounds do not reach a concentration to kill the parasites). This has encouraged individual parasites to be able to withstand our therapy and complete life cycles within the horses we treat. With no new deworming products on the horizon, it is important that we work to preserve the efficacy of these products by modifying our approach to parasite control.

How can we prevent parasite resistance?

The answer is to be strategic and target parasites when we identify them, as opposed to continuing with the previously widely used protocols of blindly deworming our horses every 8 weeks.

At Veterinary Associates, we recommend that a very simple faecal egg count test be performed on your horse to identify which parasites are present and in what numbers. All this test requires is a small sample of fresh faeces from your horse which is then prepared and examined under the microscope. If using this test for screening purposes, it should be performed at least eight weeks after the most recent dewormer administration.

As well as being able to select the most effective and appropriate product based on the results of this test, we can also use the information to identify high and low shedding horses in a herd and therefore tailor our treatments to minimise pasture contamination. High shedders will inevitably require more doses through the year than low shedders in order to limit spread on the grazing and protect other individuals from higher parasite burdens.

Low shedding horses will not require as frequent treatment, saving you the expense of a dewormer and reducing the number of unnecessary doses given, thus helping to limit the development of resistance.

The faecal egg count can also be used to test the efficacy of our dewormers and allow us to identify the level of resistance within a population. This is achieved by carrying out the same process 2-4 weeks after deworming and identifying whether or not the number of parasite eggs has been reduced by the expected amount (at least 95%). If this is not the case, we can then select a different product for your horse that will control the burden to a safer, more satisfactory level.

Good husbandry and pasture management also play a key role in parasite control. Removing faeces from your grazing is of proven benefit when it comes to reducing the number of worms on the pasture and will help break the parasite life cycle. It is also important not to overstock your pasture and where possible, allow it to rest between grazing seasons.

Strategy Summary

  • Do not deworm too frequently/blanket deworm – use faecal egg counts to determine when your horse needs to be dewormed. You can drop a sample to us at the hospital, or we can always collect one if we are on a visit to you.
  • Be sure you are dosing to the correct weight – underdosing will mean the active ingredients in the wormer do not reach a concentration that will kill the parasites, and those left behind are likely to develop resistance. Use a weight tape to estimate your horse’s weight more accurately and dose appropriately.
  • Keep pastures clean and do not overstock (we recommend no less than 1.5 acres per horse).
  • Consider co-grazing with other species such as cattle or sheep which interrupt the parasite life cycle.

If you would like to learn more about deworming and how to perfect a deworming protocol for your horse, please do not hesitate to contact us at the clinic. Any of our vets would be happy to talk you through a plan to responsibly protect your horse from the effects of parasites.