Expert advice on worming horses

Spring, summer, autumn or winter, here’s everything you need to know about worming horses according to specialists from a recent UK-Vet Equine roundtable.  Dr Rosie Naylor BVetMed MVetMed DipACVIM PhD MRCVS and Technical Product Manager – Equine Virbac UK reports.
At the start of 2019, a group of horse worming experts published a consensus statement on best practices for worming horses. 
Here are their tips: 

How to tackle wormer resistance 

With increasing reports of horse wormer drug resistance, it’s important to weigh up the risk of disease and the need for treatment in an individual horse with the health of the broader equine population. 
An important way of reducing the selection pressure for resistance to wormers is to maximise the number of parasites not exposed to wormer treatments. Identify and treat only those animals with worm burdens that are potentially detrimental to their health and that are contaminating the environment of other animals. 
The need for wormer treatment will be reduced if the environmental worm challenge can be reduced. Husbandry practices have an important part to play in limiting the use of wormer products: 
  • Aim to ensure at least one acre of grazing per horse and to maintain consistent horse populations, using faecal egg counts (FECs) to monitor environmental contamination. 
  • Droppings should be removed from the pasture at least twice per week and muck heaps kept far away from the paddocks. 
  • Resting and rotating paddocks is best done in the summer months as the larvae will survive for less time in hot, dry weather. 
  • Co-grazing with sheep will help to reduce the infective burden on the pasture.
  • Quarantine protocols are an important way to prevent new horses bringing worms onto the yard. Ideally, perform an FEC on the new horse, treat it with a broad spectrum wormer, then perform another FEC two weeks after treatment to confirm treatment has been effective. In practice, treatment with moxidectin and praziquantel is usually administered, followed by stabling the horse for a minimum of three days after treatment.

When to worm your horse

Interval dosing every horse without assessing the need for treatment is an outdated approach to worming. Faecal egg counts should be performed to identify the 20% of horses that shed 80% of the worm eggs and which horses should be targeted for treatment. 
In most cases, three FECs should be performed between March and September. Such targeted worming strategies have been shown to reduce the cost of worming. Results of >200-250 eggs per gram of faeces usually warrant treatment. Repeating FECs 10-14 days after treatment and comparing pre- and post-treatment results is an important way of monitoring the effectiveness of treatments and identify if any resistance is present.
Exposure to tapeworm will not be detected by FECs and diagnosis of tapeworm exposure requires a serum antibody or saliva test. Specific testing for tapeworm is recommended every 6-12 months to determine the need for treatment. 

What to use when worming is needed

Spring and summer de-worming
The panel recommend using pyrantel or ivermectin in horses with a high faecal egg count, perhaps rotating between the two. Moxidectin is the most effective option in treating cyathostominosis, so the panel said it should be preserved and its prophylactic use reduced. 
On well-managed properties, dosing and moving horses after treatment may reduce the number of worms in refugia and actually increase the selection pressure for resistance, therefore such practice is not advised. Where the risk of reinfection is high, this approach may still be necessary. 
Autumn de-worming
At the end of the grazing season, worming is recommended to reduce the risk of larval cyathostominosis. In moderate- to high-risk animals, treatment with moxidectin is prudent. In low-risk animals that have had repeatedly negative faecal egg counts throughout the grazing season, this may be unnecessary or ivermectin may be a suitable alternative. If the level of risk is unknown, moxidectin should be administered as a precaution. 
If treatment of tapeworm is required, praziquantel should be used to target tapeworms specifically. In the absence of a product containing only praziquantel, the panel suggested that ivermectin combinations should be used where possible to reduce unnecessary use of moxidectin. If moxidectin treatment is also required, a combination product is available. If treatment of strongyles is needed, a double dose of pyrantel also targets both strongyles and tapeworm and is an alternative to combination products.