Worms in horses

In terms of equine health and management priorities, creating an effective parasite control program is probably second only to supplying the horse with clean water and high quality feed and roughage. While there are numerous parasites that can infect horses the most common internal parasites seen in horses are strongyles (large and small), large roundworms (ascarids), pinworms, bots and tapeworms.

Strongyles– Adult large strongyles (sometimes called bloodworms) are found in the large intestines either attached to the walls or in the intestinal contents. There the females deposit large numbers of eggs that are excreted in the manure. These eggs then hatch and the larvae develop and climb blades of grass. Horses then consume these larvae while grazing and ingested larvae penetrate the intestinal walls and migrate to various organs and arteries. Large strongyles mature about 6 to 11 months after larvae are ingested. Small strongyles larvae migrate into the intestinal wall but not into other organs or blood vessels. Small strongyles develop to maturity about 6 to 10 weeks after larvae is consumed. All horses can be affected by strongyles but young horses are most vulnerable. Signs of strongyle infection are loss of appetite, weight loss, fever, depression, weakness, anaemia, diarrhoea and death.

Large Roundworms– The adult stage of the large roundworm (ascarid) is found in the small intestine of the horse. The female lays large numbers of eggs in the intestines which are passed out in the manure. It takes approximately 2 weeks for these eggs to be infective and the horse picks them up while grazing. The eggs hatch in the stomach and intestines and migrate into the blood circulation where they are carried to the liver and lungs. A complete life cycle takes 3 months and eggs can remain infective in the environment for several years if not properly removed. Large roundworms cause digestive upset and damage the liver and lungs, if the burden is large enough, rupture can occur in the small intestine leading to death.

Pinworms– Pinworms mature in the large intestine and rectum of the horse. The eggs of pinworms are picked up by horses from contaminated feed, water, bedding, stall walls, fences etc. These worms are irritating and cause the horse to rub its tail, resulting in hair loss. The eggs are smeared onto any surface the horse touches and some are passed in manure. Pinworms can affect all ages but young animals are most susceptible, with symptoms including digestive disturbance, slow growth, irritation and tail rubbing. The life cycle is completed in five months.

Bots– Bots are the immature stage of the bot fly. The female bot fly lays their yellow eggs on the hairs of the horse, typically on the throat, front legs and underline. The eggs containing first stage larvae hatch after a 2- to 5-day incubation period either freely or after being licked by the horse. The larvae then migrate to or enter the mouth and attach to the lips, tongue, bums or other parts of the mouth and burrow into the tissue where they about 3 weeks. About three weeks later they emerge as second stage larvae and progress down the throat, attaching to the stomach lining. In the stomach they develop to third stage larvae and remain for up to 7 months. Damage in the stomach occurs due to obstruction of food flow and irritation of the stomach lining. In severe case stomach rupture can occur. These larvae finally pass through the horse and hatch from manure by entering the soil below the manure pile and pupate in approximately 2 weeks to 2 months depending on the season (from spring to autumn).

Tapeworms– Tapeworms attach to the horses intestinal lining where they rob the horse of nutrients and damage tissues. Horse tapeworms require a host (forage mite). Tapeworm eggs are passed with the manure of infected horses onto pasture, where forage mites ingest them. The immature tapeworm develops within the body cavity of the mite and is ingested by the grazing horse. When the horse digests the infested forage mite, the tapeworm is released and within 6-10 weeks develops into an adult that attaches to the horse’s intestine and continues the cycle.

Worming horses: The basics

There are a lot of different practices and theories about control worms in your horse.
Internal Parasites are a significant threat to the health of horses. The horse is susceptible to numerous internal parasites and may harbour several species of worms at any given time. The effects of internal parasites are more evident in young and malnourished horses. They can cause extensive internal damage without you even realising your animals are heavily infected. The effects of internal parasites on a horse range from not putting on weight, dull coat, reduced appetite, mild colic or anxiety and an itchy tail head. If left untreated symptoms can progress to include diarrhoea, anaemia, lowered ability to exercise (e.g. lethargy), susceptibility to infections, non-healing sores (“summer sores”), coughing and significant or recurrent colic. If still untreated parasites can cause pneumonia, emaciation, severe and debilitating diarrhoea, permanent organ damage and colic, it is possible that some of these could lead to death.

Parasite Control
Pasture management: The control of internal parasite of the horse is based on cleanliness, management and de-worming drug treatment. Appropriate removal of manure from stalls and pastures is paramount to parasite management. In small pastures (less than 3 acres) manure should be removed from the paddock at least twice a week and placed in a compost pile. The larvae in composted manure will be destroyed if sufficient heat is built up. In large pastures frequent mowing, chain harrowing (dragging), and rotation of pastures along with separating age classes of horses and avoiding overcrowding should be practised.

  • Manure Management: Mowing and spreading manure by dragging pastures will decrease incidence of infective larvae if the climate allows for drying of manure. Try to drag the pastures in the morning and on hot sunny days to allow a full day of drying. Vacuuming or collecting manure in pasture is expensive, but it can be very effective.
  • Grouping horses in pastures according to age: This will help minimise young horses coming in contact with heavy larval infestations. For example, pasture mares and foals away from other horses less than 2 years of age. Yearling horses often need a different control program than a broodmare. It can be more difficult to control parasites in a herd if all ages and classes of horses are in a pasture together.
  • Pasture Rotation: Horses should be removed from pastures for a minimum of 6 to 8 weeks in pasture rotation. Where possible add sheep or cattle to your rotation to cut down on parasite numbers. Alternative grazing with ruminants and pasture rotation schemes will aid in disrupting the parasite life cycle. Grazing ruminants in rotation with horses will reduce parasite infestation, because most internal parasites are host specific. Pasture rotation may also help by decreasing incidence of overgrazing, thus decreasing ingestion of parasites.
  • Feeding: Always feed out of tubs and put hay in racks of hay feeder to avoid contamination with manure.
  • Introducing new horses: New or transient horses should be kept separate from existing horses until parasite burden is assessed.
  • Faecal Egg Count–Treatment for worms with de-worming medications should be based on faecal egg count examination and may vary from farm to farm. Periodic faecal examinations assessing faecal egg count (FEC) are the most accurate way to determine de-worming medication needs. A faecal egg count measures the number of strongyle or ascarid eggs your horse is passing in each gram of his manure. All that is required is a fresh faecal sample to be submitted to your veterinarian for assessment.
    • De-Worming Horses
      What do I look for when selecting a de-wormer?– The four main de-worming chemical groups are benzimidazoles (white drench), Tetrahydropyrimidines (clear drench), macrocyclic lactones, and pyrozine. It is recommended that chemical groups be rotated every 12 months to delay a build-up of resistance in worms to a particular drench chemical. Praziquantel is the only de-wormer effective at killing tapeworms.

      There is a well know myth surrounding parasite control in horses – “Change your de-wormer every time you de-worm”. This used to be common advice, but is now known not to be the best practice as it increases parasitic resistance. While you should change the chemical group used, it should not be done every time but every 3rd time or based on your veterinarians assessment of the type of worm infestation your horse has. Be aware that changing the brand of de-wormer does not mean you are changing the de-wormer, make sure to check the active ingredients.

      Which active ingredient should I use and when?– In mature horses focus on control of small strongyles. Depending on climatic conditions, one or two yearly treatments are sufficient to prevent occurrence of strongyles. Treatment is best done towards the end of the grazing season when internal parasite burden is at its peak (spring in tropical and subtropical climates). Include treatments against bots (along with removal of bot eggs from the horses’ hair coat), ivermectin & moxidectin are the only available medications for horses with activity against bots. Include a tapeworm treatment (Praziquantel) at least annually if they are a problem in your region.

      De-worming foals and weanlings– In young horses during the first year of life foals should receive a minimum of four de-worming treatments. First de-worming should be carried out at about 2-3 months of age, and a benzimidazole drug is recommended to ensure efficacy against large roundworms. Second de-worming is recommended just before weaning (approximately six months of age). An extra treatment can be justified before weaning if the time period between the two treatments exceeds 3 months. At weaning FEC are recommended to determine whether worm burdens are primarily strongyles or large roundworms, to facilitate the right choice of drug class. Third and fourth treatments should be considered at about 9 and 12 months of age, respectively, and treatment should primarily be targeting strongyles. Tapeworm treatment should be included on one of these latter treatment occasions. Recently weaned foals should be turned out onto the “cleanest” pastures with the lowest parasite burdens.

      Pasture Management – Mowing pastures
      The main goal in pasture management is to maintain or to enhance grass quality. The intake of pasture grass can be a significant source of nutrition for the grazing horse if the pasture is properly managed. Mowing is one tool that can be used to better manage pasture. Some horse owners mistakenly feel that mowing pastures is simply done to make the pastures look nice. However, there are several valid reasons to consider for mowing pastures. Some potential benefits of mowing include: weed management, enhancing forage quality and reducing grazing patterns.

      Controlling weeds – Repeated mowing of pasture decreases the competitive ability of a weed to survive in a grass paddock. By keeping weeds the same height as grass, the grass has the advantage and prevents weeds from shading and restricting grass growth. Mowing also serves to prevent weeds from establishing seed heads. Eliminating seed heads prevents weeds from reproducing and spreading in the pasture. The control of weeds in a pasture does not occur with a single mowing, but instead is facilitated with multiple mowing.

      Enhance pasture quality – A grass plant that is actively growing is constantly producing nutrients that horses can utilise. The mowing process keeps grass plants in a vegetative or growing state. Mowing prevents the plants from reaching a reproductive state when they develop a seed head and ultimately cease growing. Mowing also keeps plants at a shortened height which increases digestibility and palatability. As grass plants grow tall they become fibrous and less digestible. When mowing grass pastures, it is important to not cut grass plants too short since cutting too short will reduce the leaf area which is needed to stimulate growth. A grass plant that is cut too short is also prone to stress and may die. The optimum height for a cool-season grass is approximately four inches (10 cm), while the optimum height for a warm-season grass is approximately eight inches (20 cm).

      Reduce grazing patterns – Horses tend to graze in certain areas of a pasture and utilise other areas of the pastures to pass manure. The grazed areas are very short and known as “lawns”. The non-grazed areas consist of taller grass and they are known as “ruffs”. Mowing serves to shorten the taller grass and enhance its palatability. Over time this will help to eliminate the grazing patterns that can exist in horse pastures and provide better utilisation of the pasture.

      Potential Risks for Horses Grazing Mowed Pasture

      What are the risks, if any, for horses grazing mowed pasture? The single biggest risk associated with mowed pasture is the horse consuming mowed grass that has mould on it. Once grass is mowed, the portion of grass that is clipped from the plant contains a high moisture content. These clippings are prone to mould. If horses eat grass that has mould, it can cause a variety of symptoms including coughing and nasal discharge and in extreme instances result in death due to mould toxins. Another potential risk of clipped forage is choking. If horses take in large mouthfuls of short grass clippings they can potentially choke. Both the risk for ingestion of mould and for choking can be virtually eliminated if the pasture is harrowed following mowing. The harrowing process spreads the grass clippings evenly throughout the field and dramatically decreases the likelihood of any problems.

Comments are closed