A guide to the most common internal parasites affecting horses
Worms are a common occurrence in horses and ponies, and they are present in the vast majority of equines at varying levels of infestation. There are varying methods of transmission of worms, depending on the type of worm and the stage of its lifecycle.
When present in low numbers, worms cause minimal problems for horses. However, when the worms are present in moderate or larger numbers, they can severely affect your horses health and can result in weight loss, ill health and possibly even colic. If infestation is severe they can damage your horses intestines and other internal organs, often causing irreversible harm and potentially resulting in death.
Older horses can often be susceptible to larger worm burdens.
It is often said that 20% of the equine population carry 80% of the worm burden, for a variety of reason including age or compromised immune system. Usually worm eggs are ingested from infected pasture, and the worm eggs develop inside the horse’s gut or lungs. Eggs produced by the adult worm will then be shed in the faeces to increase existing worm burdens on the pasture and to potentially infect new horses, and they cycle continues.
We now have the added risk of worms becoming resistant to wormers (anthelmintics) so it is vital that we carry out a proper worming programme, at the correct dosage to minimise the risk of resistance to wormers.
Ideally we should be carrying out regular Fecal Egg Counts to ensure that we are using the correct anthelmintics, and only using them if actually necessary rather than just routinely worming by calendar. Fecal Egg Counts won’t include encysted redworm or tapeworm and these are targeted seasonally.
Please note: This is a generalised guide and is not intended to replace veterinary advice. If you have any health concerns about your horse, please consult your vet.
Types of Worm and the wormers that will target them:
Large Redworms:
Large redworms are one of the most dangerous internal parasites but fortunately are much less common now due to the use of chemical wormers. They migrate through the blood vessels of the intestine causing significant bleeding and damage. They can cause rapid weight loss, diarrhoea and surgical colic. Severe cases of infection can lead to death. Large redworm is detected in Fecal Egg Counts so ideally you should carry out a FEC before treating for Large Redworm. If you need to target this then using a wormer containing Fenbendazole (such as Panacur or FenBen10), Pyrantel (such as Embotape), Ivermectin (Bimectin, Eqvalan or Equimax) or Moxidectin (Equest) will target large redworm.
Small Redworms:
Small redworms are the most common internal parasite in horses. The ingested larvae burrow into the gut lining. Here they will either continue to develop or, in the winter months, they will hibernate in the gut wall. This hibernation is particularly dangerous to the horse as in the spring the larvae emerge in large numbers. This mass emergence can cause severe damage to the gut wall leading to weight loss, diarrhoea and colic with potentially fatal consequences.
Encysted small redworm hibernate in the gut wall and emerge in spring.
Encysted small redworm will not show up in a Fecal Egg Count, and ideally a small redworm blood test is required to identify this. In the Winter between November and February you can target encysted small redworm with a wormer containing Moxidectin such as Equest, or Equest Pramox (if you also wish to treat tapeworm at the same time)
Alternatively you can target encysted small redworm with an elevated dose of Fenbendazole, such as a 5 day course of Panacur, or FenBen10.
Redworm that is not encysted can be detected on a Fecal Egg Count.
All other adult stages of small redworm will be detected in a Fecal Egg Count, and is treated the same as large redworm, so using a wormer containing Fenbendazole (such as Panacur or FenBen10), Pyrantel (such as Embotape), Ivermectin (Bimectin, Eqvalan or Equimax) or Moxidectin (Equest) will target small redworm.
Roundworms:
Adult roundworms or ascarids can grow to 50cm in length and are particularly dangerous to foals and young horses as older horses develop immunity. When ingested from the pasture the larvae transfer through the gut wall, to the liver and then to the lungs. The larvae are then coughed up and swallowed where they mature to egg laying adults within the intestine.
Heavy infestation can cause respiratory signs, such as a cough and nasal discharge, as the larvae journey through the lungs, or it can cause intestinal signs such as weight loss, a pot-bellied appearance and diarrhoea. These are the large white worms that can be seen easily with the naked eye in droppings, causing some concern for owners.
Ascarids are most commonly found in young horses.
The best wormers to target roundworm is a wormer containing Fenbendazole (such as Panacur or FenBen10) or Pyrantel (such as Embotape). There has been some resistance documented with Ivermectin and Moxidectin so although these will also treat roundworms, Fenbendazole and Pyrantel are preferred.
Pinworms:
Pinworms lay their eggs around the outside of the anus causing intense itching and irritation. Persistent scratching will result in hair loss and open sores, around the tail head which can become infected. If pinworm is detected then the best wormer to target them is Pyrantel (such as Embotape) or a 5 day course of Fenbendazole (Panacur or FebBen10)
Tapeworms:
Tapeworms can grow up to 20cm in length and have a width of 1.5cm. The tapeworm form into clusters at the junction between the small and large intestines (ileocaecal junction) where they can cause digestive disturbances, loss of condition, colic and fatal blockages.
Horses become infected through eating the intermediate host, the oribatid mite, found on grass and forage. The lifecycle of a tapeworm is six months.
Ideally you should carry out a saliva test to test for tapeworm, as this is not covered by a Fecal Egg Count. Between September and October , and again between March and April, you can target tapeworm with a wormer containing Praziquantel or an elevated dose of Pyrantel. Eqvalan Duo, Equest Pramox and Equimax are wormers that contain Praziquantel and will cover tapeworm in a single dose.
Lungworms:
Lungworms prevail in pastures shared with donkeys – the lungworm’s natural host. These worms cause persistent coughing in horses as respiratory problems develop. Donkeys can tolerate very large worm burdens without showing any clinical signs. If lungworm is indicated then the best treatment is a wormer containing Ivermectin (such as Bimectin, Equimax or Eqvalan) or Moxidectin (Equest).
Bots:
Bot flies are an irritant to horses during the grazing season. They lay sticky yellow eggs on the horse’s coat which are ingested as the horse grooms itself or another horse. You should take care to remove these with a bot knife to minimise risk of infestation.
On entering the mouth the eggs hatch out into larvae, which migrate to the stomach. Here they attach themselves to the stomach lining and continue to develop. Once developed, they will detach and be passed out in the horse's faeces where they will pupate into flies. Ingested larvae aren’t treatable until they reach the horses stomach. It is recommended to treat them with a single treatment after the first frost of winter, using a wormer containing Ivermectin (such as Bimectin, Equimax or Eqvalan) or Moxidectin (Equest).
Types of wormer:
Equest - Moxedectin
Equest Pramox - Moxedectin and Praziquantel
Bimectin - Ivermectin
Equimax - Ivermectin and Praziquantel
Eqvalan - Ivermectin
Eqvalan Duo - Ivermectin and Praziquantel
Panacur - Fenbendazole
Embotape - Pyrantel Embonate
FenBen 10 - Fenbendazole