equine health

Pasture, Soil & Grasses

Horse Worms and Worming
When to Deworm Horses
Controlling Horse Worms on Pasture
Worm Resistance to Wormers

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Controlling Horse Worms on Pasture

The life cycles of most horse worms start with the larvae or eggs being ingested from the grass. They mature through several larvael stages into adult egg laying worms in the horse. The eggs are passed out of the horse in the faeces onto the pasture. The eggs then hatch into fresh larvae. This revolving cycle means new infestations constantly occur. It is almost impossible to completely clear pasture of all types of worm. Re-infestation can be drastically reduced by adopting the following sensible control measures.

Horse Manure

The removal of all horse manure is an essential chore, it should be done at least twice weekly. Preferably, the faeces will be completely removed from the pasture to be composted. Horse manure left in a heap in a corner of the field provides a breeding nursery for worms. Care should be taken to add layers of agricultural lime betwen the layers of horse faeces to hasten decompostion and destroy the worm larvae. Otherwise, you will be providing a perfect breeding ground for worm and fly larvae, especially pesky stable flies. Stable flies are the intermediate hosts to small stomach worms Habronema spp.. Horses are infected by stomach worm larvae that emerge from the flies, whilst the flies feed around the horse's lips. The infective larvae are swallowed, taking up residence in the stomach mucosa where they cause lesions and ulceration. Horse flies are often the cause of persistent summer sores and blisters round the muzzle due to the bacteria they carry.


Harrowing is only useful during the dry spells with hot temperatures, in mid summer (July & August over 25 degrees Celsuis). This will expose the worm larvae to the hot sun, temperatures over 25 degrees Celsuis are fatal to worm larvae. Warm, wet spells in Spring and Autumn favour worm eggs hatching. Worm larvae survive low temperatures in winter by hibernating until Spring when they become infective. Harrowing at any other time of year, is a futile waste of time as you will be spreading the infective larvae across the land.

Past History & Companion Animals

Your pasture may have been contaminated by other animals? Consideration should be given to other grazers past and present of your pasture. Ruminants such as cattle, sheep and goats are infected by the same small stomach worm of horses - Trichostrongylus axei. These worms cause a chronic catarrhal gastritis of the stomach, diminishing the horse's ability to digest and absorb feed. Repeated infestations can cause painful lesions leading to stomach ulceration. Horses become afraid to eat anticipating the discomfort. Foals are particularly susceptible to stomach worms.

Donkeys are host to lungworms Dictyocaulus arnfield. Up to 70% of donkeys carry lungworms often showing no clinical signs of infestation. Lungworms can over winter in pasture unaffected by low temperatures. Grazing horses ingest infective larvae that migrate to the lungs via the lymphatic system, and the pulmonary arterial blood supply. Larvae travel from the alveoli to the bronchi and bronchioles where they mature. The eggs are coughed up then swallowed and expelled in the faeces. Once a pasture has been contaminated with lungworm, they emerge every summer. Many horses who are hypersensitive to dust from hay and straw have suffered lung damage as foals due to lungworm. Foals are particularly susceptible to this pest with long lasting lung damage.

All companion animals should be treated for worms. New animals bring added risks of further infestations. They should be dewormed before introduction to the pasture.

Rotational Grazing & Pasture Contamination

A high density of horses per acre increases the worm count. Over grazing forces horses to graze on rough patches of grass that they usually leave for dunging. Inevitably, this leads to reinfestation and high worm counts.

Rotational grazing will help to control the build up of parasites. The amount of acreage per horse/pony is dependent on the amount of supplementary fed provided and whether winter grazing is provided. A pony would need one and a half to four acres, an average adult horse three and a half to five acres, split into three or four separate paddocks. Many horses particularly those in livery stables have considerably less than three acres. As a result fields become contaminated with high levels of parasite infestation.

Horse Worming Schedule

Regular de-worming is necessary to keep the worm population of your pasture under control. A regular schedule starts with the first flush of Spring grass within 3 weeks of horses being put out to grass. Followed by regular applications every 8 weeks throughout the summer and autumn months. This regime can be relaxed over the winter months November to late February if the horse is stabled, in a clean environment. The re-infestation rate from pasture is virtually nil once the temperatures drop below 10 degrees Celsuis, early December through to early March. If your horse is left out all year round then the last deworming will be in late November for encysted red worm, then left to over winter until early Spring when temperatures start to rise. Contrary to belief, frost and low temperatures do not kill worm larvae. They hibernate until Spring then moult into infective larvae.

Faecal Worm Counts

These can prove a most useful aid in worm control. Regular worm counts taken 14 days after worming show the efficacy of the wormer used. Worm counts done before worming can show the level of worm infestation on your pasture. If your worming schedule and control measures are working, the results should be low to medium less than 250 epg, high counts over 500 epg are cause for concern.

Moon Cycles

Traditional de-worming of farm animals followed the moon cycles. The worming being started 2 days before a full moon (the moon waxing) and finishing 2 days after a full moon (the moon waning), 5 days in total. This practice is still followed in many parts of rural Europe. We have had consistently good results using the moon cycles every 3rd to 4th month. Adult red worms for unexplained reasons are attracted by the light and pull of gravity. Red worms become active over the full moon phase. Could this possibly be explained by some programming in their genes? What rural peasants have always known, science has yet to discover!


A blend of traditional herbs added to the feed to maintain the overall health of coat and gut whilst repelling unwanted internal parasites. Added to the feed over 5 consecutive days. One litre is sufficient for an average sized (500kgs) horse for 2 parasite repel applications 8 to 12 weeks apart. Two litres is enough for one average sized horse, to cover the entire year's parasite repel program, 4 seasonal applications in all.

N.B. Not suitable for pregnant/lactating mares or foals under 6 months old.

Contains: Extracts of traditional vermicide & demulcent herbs. One litre self dispensing bottle

Free Faecal Worm Count Kit & Moon Calendar is included and full directions on using Zilch Verm

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