Worm control, season by season

Spring – roundworm (small redworm), tapeworm

• Getting your worming right now could reduce the number of times you need to worm during the summer.

• Spring is when a lot of us turn our horses out onto fresh pasture. For many people, this is the time to treat for tapeworm, but roundworm control is equally important.

• Small redworm (a type of roundworm) is the main worm to watch out for. They can be present on the pasture when spring arrives, harboured from the previous year. Any worm eggs shed from horses as soon as they are turned out will also add to the initial pasture contamination.

• Targeted use of wormers will break the worm’s lifecycle by killing the worms in the horse. However, timing of dosing may positively affect the pasture as well as the horse.

• During the summer months, the worm burden on the pasture may increase hugely as the worms continue to cycle through the grazing horses. If the horse is carrying a minimum worm burden from the winter months, one approach to help avoid an early redworm burden involves delaying treating the horse until two weeks after turnout, then using a product licensed to control all stages of small redworm.

• This ensures that any initial worms picked up by the horse from the pasture are treated before they reach maturity. In addition, using a product with licensed persistence, such as one containing moxidectin, will continue to kill any larvae picked up from the pasture for two weeks.

• If fewer worms are cycling through the horse at the start of the season, this may reduce the initial worm burden on the pasture and in your horses, reducing the build-up of worms over the grazing season and potentially reducing the number of worming doses needed in the later summer months.

Summer – roundworm (small redworm)

• The key in summer is to keep an eye on your horse’s worm burden by carrying out regular faecal worm egg counts (FWECs) and then treat as necessary.

• FWECs will help you to detect which horses are shedding the most worm eggs and larvae onto your pasture. Don’t be surprised to have a diversity of FWEC results from horses who are sharing a field.   Each one should be wormed according to his need, rather than  adopting a ‘one size fits all’ approach to the horses.

• This will reduce further contamination of the pasture by horses with high egg counts ?– helping to keep all the horses on your yard free of significant worm burdens and any worm-related disease. It will also have long-term benefits in helping to slow resistance to the wormers available to us.

Autumn – encysted small redworm, tapeworm, bots

• Your focus should be on the management of tapeworm, and in late autumn, encysted small redworm and bots – none of which will show up in a standard faecal worm egg count.

• Test for tapeworm using a tapeworm antibody test carried out by your vet. To control tapeworm break the cycle by using a suitable wormer on a strategic basis. Treatment is carried out every six months, usually in the spring and autumn, and involves either a double-dose of a pyrantel-based wormer or a wormer containing praziquantel.

• Small redworms are the most common and harmful worms found in our horses. The larvae can remain dormant inside a horse for up to two years as encysted small redworms. They usually ‘wake-up’ in the late winter or early spring, developing and emerging from the gut wall all at the same time. In severe infestations, this mass emergence can lead to ‘larval cyathostominosis’, causing diarrhoea and colic with up to a 50% mortality rate.It’s vital to worm for encysted small redworm in the late autumn or early winter. Moxidectin is the only licensed single dose treatment for encysted small redworm.

• Bots are the insect larvae of the bot fly, a common adult parasite found within the horse’s stomach. The presence of bots is most easily identified by the presence of yellow eggs on the legs of animals, but this is an unreliable indicator of infection. A wormer containing ivermectin or moxidectin is recommended for the control of bots, administered in the late autumn or early winter, after the first frost when the adult flies have died and before the bots mature.

Winter – tapeworm, pinworm

• During the winter, not only will many horses spend much less time grazing – the main route to a parasite burden – but also worms don’t cycle in such great numbers when the temperature drops.

• In theory, low exposure to worms means that there is less need to give a routine dose unless the weather is mild and you are particularly concerned about the level of worm burden on the pasture being grazed.

• For horses on winter turnout, it is best to assess the overall risk from the pasture and worm accordingly, but stabled horses treated appropriately in late autumn should require minimal treatment at least until spring. However, if you treated your horse earlier in the autumn for encysted small redworm, it may be advisable to re-dose for them over the winter.

• The only complicating factors are tapeworm and in some cases pinworm, as neither are exclusively transmitted during grazing and are not reliably detected in FWECs. Following this approach, you will have treated for tapeworm in the autumn and will again in the spring, and assessed overall risk with a tapeworm antibody test if needed.

• Pinworm won’t show up reliably in any tests. They lay their eggs around the anus, causing itching, and are rubbed off by the horse. They can be transmitted from horse to horse via things like stables, buckets, hay, bedding and grooming brushes. Prevention includes good stable management, but treatment will be necessary in the case of an established burden.

Keep your fields in the best condition to avoid contamination

• Poo pick regularly, idealy every day.
• Make paddocks smaller so that they can be alternatively grazed and rested.
• If harrowing, leave for three months and a significant amount of worm larvae could be destroyed (especially in extreme weather conditions).
• Cross-graze with sheep and cattle as they hoover up the worms without being affected by them.

Ask the experts…

Once aware of the general parasite threats to your horse it is bect to consult your vet to ifdentify the right treatment.