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WHY DO WE WORM HORSES?

Posted by Dr Danielle Plummer on 2 June 2020
WHY DO WE WORM HORSES?

In Australia, seasonal weather encourages parasitic worms to grow throughout most of the year, particularly in wet, warm months. However, each different type of worm has a different lifecycle, which means that they become active at different times of the year. This means that it is essential for a worm control program to be in place all year round.

Parasitic worms have the potential to effect the health and well-being of horses and ponies of all ages, and cause a variety of problems, which may include:

  • Irreversible damage to the gut and other organs
  • Poor body condition, weight loss or poor growth
  • Colic
  • Scouring
  • Death

Worm control and resistance

Previously, blanket administration of oral wormers ('drenching') was common practice. However, we now know this method of worm control may lead to the development of drug resistance. Drug resistance means that some populations of worms are no longer susceptible to a particular worming compound that previously would have killed them, allowing the worms to continue to reproduce. These worms then continue to compromise the horse's health and contaminate paddocks.  Drug resistance may also arise from not administering the correct dose (under-dosing) of worming compound to a particular horse.


Types of intestinal parasitic worms
1. Small Redworms (Cyathostomes)

    

This internal parasite hibernates within the gut wall in small cysts. When all of these encysted larvae emerge from the gut wall at the same time (usually during late winter), significant damage to the lining of the gut occurs which causes loss of condition, diarrhoea, colic and sometimes death. 


2. Roundworms (Ascarids)

                   

These large worms can reach up to 50cm in length. They typically affect foals and young horses as immunity towards them develops with age. Ill thrift and digestive problems may be seen in affected horses, and a cough may develop as these worms migrate from the lungs to the digestive tract as a part of their lifecycle.

3. Large Redworms (Strongyles)

         

This type of worm is not as common as it used to be due to improved worming treatments. During their larval stage, Large Redworms migrate through blood vessels and then develop into adults within a major artery responsible for supplying blood to the intestines of the horse. This migration causes weakening and damage to the blood vessels, blood clots, colic and occasionally death.


4. Pinworms

                                         

This worm is highly irritating to the horse as it lays its eggs on the skin around the outside of the anus, which causes the horse to scratch and rub the tail area. This leads to loss of hair and wounds in this region.

 

5. Tapeworms

                                       

Tapeworm can affect horses from as early as 4 months of age. They are a wide and flat, and reside at a narrow junction between the small and large intestine.  They have been associated with intestinal blockage, condition loss, colic and even death.

 

6. Threadworms

                                    

Threadworm mainly affects foals, and can do so within the first few days of their life. It may cause diarrhoea, inappetence, anaemia and dullness. They also damage the lungs as they migrate through them before reaching the intestine. Natural immunity develops at around 6 months of age.

7. Lungworms

                                       

Lungworm usually resides in donkeys without causing many clinical signs, but may cause respiratory signs including a persistent cough in horses sharing paddocks with donkeys.  An appropriate worming program with prevent cross-infestation where horses and donkeys are paddocked together.


WORM CONTROL PROGRAM
An effective worm control program includes the following:

  • Monitoring for worm burdens
  • Strategic drenching or interval drenching program
  • Pasture management

1. Monitoring worm burdens
Worm burdens and drench efficacy is easily monitored by performing faecal egg counts (FECs) which can be performed by your veterinarian. These counts can identify if the worm burden is low, moderate or high within a paddock, which individual horses are at greater risk of infestation (i.e. young, old, or immune-compromised), and whether or not a particular wormer is effectively treating the worm burden present.  To perform a FEC, collect a small sample of fresh manure from each horse into a plastic bag or container and bring it to our veterinary clinic. Results should be available in 24 hours. FECs should be performed every 8-10 weeks post-drenching. If monitoring drench efficacy, a FEC should be performed 10-14 days post drenching. Please note FECs do not detect the presence of tapeworm or encysted Small Redworms, so treating these is based on suspicion and should be done annually regardless.

                                                                                                            

2. Drenching

Strategic drenching
This is the gold standard for all horses and pony owners, which involves minimising drug resistance by accumulating less drug residue in the paddock, and preventing worm infestation in the paddock. It is also a more cost effective method of treating worms, as only the horses that require worming are treated. Which wormer to use (and when) is based on several factors including the lifecycle of the target parasite, the risk of disease and the resistance status of worms present.

Targeting horses that most require treatment will reduce over-usage of chemicals in other horses. Strategic drenching involves preforming a faecal egg count (FEC) before drenching, and if the egg count is greater than 200 eggs per gram (epg), a worming treatment is given. FECs are performed every 8-10 weeks or earlier if the horse is showing signs of worm infestation.

It is essential to dose on an accurate weight, and check the effects of a deworming compound before use. The FEC should drop below 95% if checked 10-14 days post-drenching. If this drop is not seen, a resistant worm population may be present in your paddock or under-dosing may have occurred. 

However, it is important to know that it is not possible to guarantee that a horse is worm free from the result of a FEC, as only adult worms shed eggs into the manure. FECs detect the presence of adult worms, rather than the entire population that may be present. This may include immature worms and possibly encysted Small Redworms. Tapeworm are also not detected on FECs.  To manage Tapeworm and encysted Small Redworms that are undetectable on FEC, it is important to use a larvicidal worming with praziquantal (i.e., Equest Plus Tape) once a year during early winter (June to July). A second dose of praziquantal may be needed for tapeworm prevention in horses that are high shedders.

Interval drenching
Interval or seasonal drenching is the next best option If a targeted strategic drenching program can not be implemented. Prior to starting an interval-drenching program, an initial FEC should be performed to determine whether your horse/horses are low shedders (<200 epg), moderate shedders (200-500 epg) or high shedders (500epg). The optimum time to perform a FEC is 3-4 weeks after the effect of the previous wormer has warn off.  Studies have suggested that the number of eggs shed by an individual horse remains fairly consistent throughout their life, particularly low shedding horses. No single worming protocol will suit each individual horse or property, so if in doubt call us at Knox Veterinary and we can advise which product will be suit your horses circumstance.

 

Pregnant Mares

Perform targeted strategic worming or interval worming program, but include a wormer with moxidectin and praziquantel (Equest Plus Tape) when administering vaccinations 4-6 weeks prior to foaling.

Foals

  • 2 months of age ivermectin (Equimec)
  • 4 months oxenfendazole (Kelato Revolve)
  • 5 months pyrantel pamoate (Equimax Elevation)
  • 6 months moxidectin with praziquantel (Equest Plus Tape)
  • 8 months - pyrantel pamoate (Equimax Elevation)
  • 9 months fenbendazole (Panacur)
  • 10 months ivermectin (Equimec)
  • 12 months - fenbendazole (Panacur)

3. Pasture Management
A pasture management program will complement a horse worm control protocol and should include:
Managing manure - manure should be removed from the paddock 2-3 times per week to remove the source of new worm eggs. Horse manure should never be used as fertiliser for equine pastures.
Rotating grazing paddocks - where possible, sub-divide the grazing area into smaller paddocks and rotate horses between paddocks at regular intervals. Horses grazing together should be on the same worm control strategy (this does not necessarily mean that all horses are treated at the same time with the same product).
Spelling paddocks - harrowing paddocks and spelling them during hot dry conditions for 6-8 weeks will expose and kill worm larvae, reducing the worm burden and potentially removing resistant worms.
Do not overstock paddocks - paddocks should be stocked at a ratio of 2 horses per hectare (1-1.5acres per horse). Overstocking will concentrate manure and force horses to graze closer to droppings and increase the risk of worm ingestion.
Cross-species grazing - you can graze the pasture with cattle or sheep as livestock generally eat grass that horses find unpalatable (i.e. near manure). Livestock grazing near manure leads to ingestion of a high number of worms, which will not survive in cattle or sheep. Remember to institute a worming program for livestock also.
Do not feed on the ground - avoid feeding horses on the ground and use feed bins, hay nets or racks instead to reduce worm burden.
Isolate new arrivals - all new horses should be isolated for 7 days in a separate paddock or stable to prevent new contamination of paddocks. A FEC should be performed on arrival and the horse should be treated with a wormer if necessary.

 

Reference:

Horse Products: Wormers / Worming. (n.d.). Retrieved February 16, 2018, from https://au.virbac.com/home/horse-owner/products/wormers-worm-control.html
                              
Pasture management for worm control. (n.d.). Retrieved February 16, 2018, from https://au.virbac.com/home/horse-owner/3dworming/pasture-management-worm-control.html

Understanding worm egg counts. (n.d.). Retrieved February 16, 2018, from https://au.virbac.com/home/horse-owner/3dworming/direction/understanding-worm-egg-counts.html

Your Guide to Sustainable and Effective Equine Worming. (n.d.). Retrieved February 16, 2018, from https://au.virbac.com/home/horse-owner/3dworming/horse-worming-guide.html
Dr Danielle PlummerAuthor: Dr Danielle Plummer
About: Danielle "Bean" Plummer BVBio/BVSc, graduated from Charles Sturt University, Wagga Wagga in 2017. She grew up on a sheep, cattle and cropping property in Central New South Wales. Her love of the land and understanding of primary production, reflects well in her role as Veterinarian with Knox Veterinary. Danielle enjoys large animal work and is always keen to get out in the paddock. She is a rugby fanatic and is captain of the local Wheatchix rugby team, she was awarded highest point scorer and best all round player for 2019.
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