Riverina Equine Vet



Equine De-worming


There are a lot of different (and outdated) practices and theories about how to de-worm your horse. 

Internal parasites = a significant threat to the health of horses

  • Can cause extensive internal damage without you even realizing your animals are heavily infected
  • Effects of internal parasites 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, 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.


Types of worms in horses

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.


  • There are large strongyles and small strongyles
  • Large strongyles:
    • Adult large strongyles are found in large intestines
    • Females deposit large numbers of eggs that are excreted in the manure à eggs hatch and larvae develop and climb blades of grass à horses then consume larvae while grazing à ingested larvae penetrate the intestinal walls and migrate to various organs and arteries
    • Very susceptible to most drench classes
    • Small strongyles:
      • Similar life cycle to large strongyles BUT do not migrate outside of the gut to other organs
      • Have the ability to become “encysted” in the gut wall during the larval stages and can undergo “arrested development” whereby they can delay their larval development and become dormant for a period of time
      • Whilst encysted, small strongyles are largely protected from the majority of drenches commonly used in horses
      • 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

  • Adult stage of the large roundworm is found in the small intestine
  • Female lays large numbers of eggs in the intestines which are passed out in the manure à horse ingests eggs while grazing à eggs hatch in the stomach and intestines and migrate into the blood circulation where they are carried to the liver and lungs
  • Can 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 mature in the large intestine and rectum of the horse
  • Worms are irritating and cause the horse to rub its tail and rump àeggs are smeared onto any surface the horse touches and some are passed in manure à eggs are picked up by horses from contaminated feed, water, bedding, stall walls, fences etc.
  • Can affect all ages but young animals are most susceptible
  • Symptoms including: digestive disturbance, slow growth, irritation and tail rubbing
  • Environmental contamination must be addressed when managing heavy infections
  • Drenches containing pyrantel or morantel (AMMO RED) are useful when treating for pinworms


  • Bots are the immature stage of the bot fly
  • Female bot fly lays yellow eggs on the horse’s coat à eggs are licked/chewed by the horse and mature into larvae in the mouth à larvae then migrate in the mouth and attach to the lips, tongue, gums and burrow into the tissue where they stay for about 3 weeks à larvae emerge and then progress down the throat, attaching to the stomach lining where they remain for many many months à they are eventually passed in manure and then develop into adult bot flies in the soil
  • Bots can cause damage and irritation in the mouth as they migrate as larvae, and damage in the stomach due to obstruction and irritation of the stomach lining
  • Bots are best treated with an ivermectin drench in Autumn, after the first frost which should have killed most of the active adult bot flies


  • Tapeworms attach to the horses intestinal lining
  • Horse tapeworms require a host (forage mite)
  • Eggs are passed in manure onto pasture à forage mites ingest them à immature tapeworm develops within the mite and is ingested by the grazing horse à the tapeworm is released and within develops into an adult that attaches to the horse’s intestine and continues the cycle
  • Susceptible to drenches containing praziquantal


Parasite Control 

Internal parasite control = pasture/environment management + FEC + drenching


Pasture management

The control of internal parasite of the horse is based on cleanliness, management and deworming 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 practiced. 

  • Manure Management:
    • Regularly vacuuming or collecting manure can be very effective
    • Mowing and spreading manure aids in drying out and destroying larvae in the pasture. It is best done in the morning on hot sunny days
  • Grouping horses in pastures according to age:
    • Different age-groups of horses required different control programs
    • Grouping horses according to age can help to minimise more susceptible young horses coming in to contact with heavy larval infestations and can enhance the effectiveness of drenching programs
  • Pasture Rotation:
    • Spell pastures for a minimum of 6 to 8 weeks in pasture rotation
    • Where possible add sheep or cattle to your rotation as this aids in disrupting the worm’s life cycle and can help to cut down on parasite numbers
    • 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/treated 


A new approach to internal parasite management – Faecal Egg Count

Traditional parasite control programs: - NOW NOT RECOMMENDED

  • Involved rotational treatment with drenches at regular intervals
  • This approach was based on concepts and strategies developed more than 40 years ago when large strongyles were the most common and damaging internal parasite in horses
  • This approach was very successful in controlling large strongyles, and they are now relatively uncommon in managed horse groups
  • THE PROBLEM: Decades of frequent drench use has selected for high levels of drug resistance in small strongyles and roundworm populations, which emphasizes that the traditional approaches for parasite control are not sustainable and that new strategies are needed.

 Faecal Egg Count

  • Periodic faecal examinations assessing faecal egg count (FEC) are the most accurate way to determine drenching needs.
  • A faecal egg count measures the number of strongyle or roundworm eggs your horse is passing in each gram of manure (epg = eggs per gram).
  • Drenching should be based on faecal egg count examination and may vary from farm to farm.


Table 2. Faecal egg count contamination levels

Faecal Egg Count (EPG)

Low contamination

Less than 200

Moderate contamination

200 to 500

High contamination

More than 500


Where and how do I get a faecal egg count done on my horses?Faecal egg count test kits can be acquired from your veterinarian or nearest veterinary laboratory.


  • Collect fresh manure from each individual horse (less than 12 hours old)
  • Take enough manure to fill the sample pot (minimise air)
  • Ensure you label each sample pot for each individual horse
  • Keep cool/refridgerated until submitted to vet/lab

How often should I do an FEC on my horses?

  • All new horses should have an initial FEC test conducted
  • Biannual to annual tests (in the spring) should be conducted in mature horses depending on their exposure to other horses.
  • Horses under the age of 2 may need more frequent FEC test conducted (every 6 months).
  • Initial FEC will determine worm burden and a follow-up FEC 14 days after de-worming will evaluate medication effectiveness.
  • If used correctly, a FEC can decrease your reliance on drenches reducing the risk for drug resistance.
  • Consult with your veterinarian to assist you in conducting a FEC on your farm and developing a specific de-worming program.


De-Worming Programme

What do I look for when selecting a de-wormer?

  • The four main de-worming chemical groups are:
    • Benzimidazoles
    • Tetrahydropyrimidines
    • Macrocyclic lactones
    • Pyrozines
    • 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.


Chemical Groups and their active ingredients against types of worms:

DRENCH ROTATION MYTH: “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 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.
  • 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 deworming treatments.
    • First deworming should be carried out at about 2-3 months of age, and a benzimidazole drug is recommended to ensure efficacy against large roundworms.
    • Second deworming 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.
      • Recently weaned foals should be turned out onto the “cleanest” pastures with the lowest parasite burdens.
  • 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.