When to worm? Low egg counts

Q: I do faecal counts both spring and autumn, and my horse routinely shows low counts. Should I deworm once a year anyway? Is it safe after not doing so in such a long time? My horse does not share paddocks with any other horse, and I muck him out twice a week.

Amber, via email

 Vet Dave replies:

An excellent question, and the short answer is yes, you should still deworm once a year – either in winter or spring – with a product containing Moxidectin and Praziquantal. 

Faecal worm egg counts are excellent at giving an indication of the adult roundworm population in the gut – this is because adult roundworms secrete eggs at a relatively steady speed – thus in any one sample of faeces the concentration of eggs is proportional to the number of adult worms. 

This is not the case, however, for tapeworms. Tapeworms are intermittent shedders, and therefore it is entirely possible to have a tapeworm burden and not see eggs in the sample of faeces tested. There is a blood or saliva test available for tapeworms; however, sadly it has yet to become commercially available in New Zealand. 

The roundworms that encyst and hibernate in the gut wall are also hidden from us in our ability to test – although there are several promising threads being chased to be able to test for the encysted worm burden of a horse, so far none are commercially available. 

As a result of this inability to predict both the tapeworm burden and also the encysted roundworm burden of a horse, we are unfortunately left only with the option of a treatment. 

There is a good reason that this treatment is required to be once a year and in spring or winter.

With regards to tapeworms, recent studies have shown that horses that have been treated within the last 12 months for tapeworms have a reduced incidence for colic compared to those who don’t. 

With regards to encysted roundworms, the main risk is a mass emergence of all of the hibernating worms in spring. The logic behind the hibernation of these worms is that they are waiting for improving conditions in the gut before reaching adulthood – this either means less competition in the intestine or an increased availability of food. The most common time we see mass emergence of these encysted roundworms is in spring, when there is a sudden increase in the quality and quantity of food available. 

A mass emergence of encysted roundworms is a serious veterinary concern, as each worm creates a pin-prick in the intestinal wall. This pin-prick pushes blood and protein into the intestine – if only one worm emerges then there is no problem; however, when this problem is multiplied out by the mass emergence of millions of worms, then you end up with a situation where the intestine is facing death by a million small cuts. 

The mass protein loss into the intestine causes a diarrhoea that in more severe cases can end up being fatal. 

Although the majority of anthelmintic (de-worming) products have good effectiveness against adult parasites, only Moxidectin has some efficacy against the encysted roundworms. Facing a possible serious event, worming with Moxidectin in winter or early spring is the only available option to reduce the risk of fatal mass emergence of encysted roundworms.

Putting together the need to worm with  a tape-wormer once a year to reduce the risk of a colic, and the need to prevent a mass emergence of encysted roundworms, then the only option to horse owners is a mandatory treatment with a Moxidectin-Praziquantal combination wormer in winter or early spring. 

I have no brand affiliation with regards to wormers – the two that we as a clinic has sold that contain Moxidectin and Praziquantal are Ultramox and Equest-plus tape. To be honest, it is more important that the wormer contains the correct ingredients rather than being of one particular brand or another. 

If I had the option of skipping this mandatory treatment and instead testing for both tapeworms and the encysted roundworms, then this would be my preference as it would then mean that only horses that actually required treatment received it. This would reduce the risk of over-treatment, which obviously occurs when you are required to treat blindly and thus the risk of resistance developing.