WORMS AND HORSES - This Article Not For The Squeamish!

Worms.... no, we're not talking about the regular earthworm that inhabits the garden, but the internal parasites which our horses host. Most horses that are carrying large worm burdens will show signs such as a poor, dull coat, weight loss or inability to gain weight and, in more severe circumstances if left untreated, stomach upsets, illness and serious colic.
Correct and regular worming of your horse will never completely remove all worms or parasites, but will help to control them from potentially harming your horse. Here we look at some of the different major internal parasites of horses, see what effect they can have, and discover which active ingredients found in common wormers will work best.

Redworms
These are split into two types, Large Redworms and Small Redworms. Large Redworms (Strongyle) are now relatively uncommon thanks to modern wormers which effectively control them, however, left untreated, the Large Redworm larvae can multiply and block the arteries into the gut which can cause life threatening colic.
Small Redworms (Cyathostomes) are one of the most common horse parasites and can remain seemingly dormant in the the gut by enclosing themselves in a protective membrane for months, or even years, before all suddenly emerging at the same time, causing severe illness that can include diarrhoea, colic and weight loss. If these worms are not prevented from reaching this encysted state, successful treatment of them becomes much harder, making regular worming treatment (with the right active ingredient) the best solution.

Threadworm
The presence of threadworms can be seen by coat dullness and horses that dont hold their weight. Foals are very susceptible to threadworms which can be passed to them via the mares milk and decrease the natural weight gain of the foal. By worming the mare while she is pregnant this can be prevented.

Large Roundworm
Large roundworms can grow up to 40 centimetres long! They are mostly found in young horses up to a year old and can migrate to the lungs and liver sometimes causing respiratory disease in foals. If lots of large roundworms become present in the small intestine of the foal, they can also cause a partial or complete blockage, so it is important to treat your youngstock for these.

Tapeworm
There are several species of tapeworm which can infect horses, and they can cause serious colic when present in large numbers. These worms are ingested by the horse as small mites in pasture which contain the developing tapeworm.

Lungworm
Not so common in horses, but are a common worm in donkeys (although horses can become infected with lungworm if grazed with donkeys). They live in the animals airways.

Pinworms
Pinworms glue their eggs to the skin underneath the horses tail which causes discomfort and makes the horse want to rub its hindquarters and tail, so although these worms are not a serious health threat, they are still best avoided.

Bots
The bot fly is well known to most horse owners, and their annoyance to horses during summer months is common around New Zealand. These big dozy flies lay small yellow eggs usually around the horses legs, stomach and flank areas. They enter the horses mouth when they rub their face or scratch on these parts, before turning into small maggots inside the stomach, staying in place until they are passed out in the dung the next spring (and the cycle starts all over again). Worming treatment for bots is best timed in the autumn after all of the adult flies have died.

There are a number of worming (or drench) treatments available for sale, however when purchasing you need to bear in mind the active ingredients to ensure that you are worming your horses for the different types of worms, as not all wormers available cover all worm types.

Ivermectin
This is good overall wormer ingredient which treats most roundworms and bots. Does not have any effect against tapeworms or small encysted redworms.

Fenbendazole
Effective against small redworms, although there is resistance developing to this wormer in some parts of the world (such as the UK). Does not have any effect against tapeworms or bots.

Pyrantel
Effective againsts tapeworms, although it does not kill larval worms of most species, and has no effect against small encysted redworms or bots.

Moxidectin
This is the longest lasting wormer currently available and kills around 80% of developing encysted redworms in one dose. It is effective against most roundworms as well as bots, but has no effect on tapeworms.

Praziquantel
This is specifically for tapeworm and is suggested to be used once or twice yearly for tapeworm treatment, best times are autumn and spring.

To ensure you are protecting your horse from worms most effectively, your worming programme needs to include a combination of the above mentioned ingredients at the most appropriate time of year. It is important to start young foals on a regular worming programme early in their life as they are very susceptible to worms and long term damage at this age is best avoided. It is wise to worm all of your horses at the same time with the same wormer. Good management of your horse pasture, such as picking up dung, cross grazing with other animals, and allowing the paddocks time to rest, all help towards better worm control.

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