Why Do We Worm Our Horses?
The answer to this is simple; to break the worms’ life cycle. The worms that live within the horse have one aim and that is to reproduce. By breaking their lifecycle, using wormers or other pasture management techniques, we can prevent them fulfilling this aim and therefore act to keep our horses and ponies free from disease. However, the aim of any worming program should not be to remove all the worms that a horse or pony is exposed to because a small worm burden actually acts to boost their immune system and thus their general health. It must be remembered that no parasite wishes to kill its host as this is detrimental to its own survival.
Worms
There are 7 species of worms that we aim to control. These are:
Large redworms (Strongyles) need worming for twice a year. These are the most dangerous of all horse worms as they migrate around the body as part of their life cycle. This migration can cause scarring in the arterial walls and even blockage of blood vessels that can lead to colic and death. Less of a problem now as in previous years.

Small redworms (Strongyles/Cyathostomes) are the most problematic worm we have to deal with now. These worms reproduce and grow from egg to adult in 6 weeks. The issue with these worms is their ability to burrow into the large intestine wall and hibernate there for up to 2 years. It is this hibernation that must be dealt with in the winter when worming. The mass emergence of these larvae in the spring can cause serious problems and maybe even death. Up to 90% of a horse’s small redworm burden can be found hidden in the gut wall and therefore if all these worms emerge at once the damage they cause to the gut wall can be very serious.
Large roundworms (Ascarids) are a serious problem in horses and ponies up to the age of 18 months
Bots are the larvae of the Bot fly that live in the stomach of the horse. These larvae do not appear to cause any disease, but it can’t be a good thing to have these attached to the stomach lining. Treat a horse for these once a year.

Tapeworm attach themselves to the join between the small and large intestines. A link between Tapeworm and colic has been identified. A horse should be wormed for Tapeworm at least twice a year.
Threadworm – problematic in foals up to 6 months old. Can be controlled partially by worming the mare before foaling

Pinworm – annoying not harmful

Pasture Management
The aim of pasture management is to keep the worm challenge presented to the horse as low as possible. To do this there are a number of things that can be done:

Faecal egg counts (FEC)- identify the horse or horses that contribute the largest number of eggs to the pasture making it ‘dirty’. Worm these animals only to reduce the risk of infection.  Generally we would worm those horses with a burden of 200 epg or more.
Poo pick – prevent larvae hatching and being ingested whilst grazing. Also helps to reduce the number of irritating flies in the summer.
Cross grazing – sheep are best. Act as biological hoovers as they can safely eat horse worm larvae and break the life cycle.
Reduce paddock size – this increases the likelihood of poo picking and allows for paddock rotation and resting
Paddock rotation and resting, for 3-6 months, means that the majority of larvae are killed by hard frost or hot dry weather.
Don’t over stock and maintain good quality grazing

Testing For Worms
There are 2 tests that can be carried out to assess worm burden, the faecal worm egg count (fec) and the tapeworm antibody blood test. Both these have advantages and disadvantages to use.
The fec measures the number of eggs per gram (epg) of faeces, a base line for worming is usually given as more than 200 eggs per gram, a count under this is considered to be ok. Unfortunately however, the fec correlates badly with the total internal worm count because only adult worms lay eggs – the fec cannot give us an indication of larval burden. The fec is not much good when used between November and March due to the hibernation of the red worm larvae.

Using the fec to identify and only worm horses with an egg count of over 200 epg, however, means that we can start to dilute the population of worms that are resistant or resilient to the wormers we have available to us.  Also on the plus side using fecs properly can be cheaper than worming in the long term and is probably better for the horse.
The tapeworm antibody test requires a blood test to be taken by a vet to give an indication of tapeworm burden. The problem with this test is, not only the cost, but the fact that even after the tapeworm have been killed the antibody test will still show a positive result for tapeworm for up to 6 months. It is cheaper and easier to worm for tapeworm twice a year.

Resistance to Wormers
Resistance begins to develop when the level of drug required to kill the worms is not reached within the body and therefore a sub-lethal dose of the drug is given. This sub lethal dose can be given when either, the horse or pony is not weighed correctly and is not then given enough wormer, the wormer is not administered correctly or when the wormer given breaks down too slowly within the body. The sub lethal dose given will only kill the worms that are least resilient to the drug, therefore leaving the ‘stronger’ worms alive to breed and reproduce. Due to genetics these more resilient worms pass on their resilience to their offspring meaning that eventually the whole population will become resistant to the wormers. This is natural selection.
Resistance can also develop when using high frequency dosing using the same drug group on a regular basis. This method of worming continually allows only the most resilient worms to reach maturity and breed. Eventually, through natural selection, the population of worms becomes resistant to the drugs.
By using accurate weight measurements or slightly over worming and a selective worming program we can slow down the onset of resistance. The selective worming programs acts to reduce the onset of resistance by maintaining a population of worms that is diluted by ‘weak’ worms that have not been exposed to the drugs.  It is important to note that most weightapes are underweighing horses and ponies by about 10%. For worming purposes, therefore it is important to add bit extra to ensure we do not under dose our animals.

 
Wormers and Worms
There are 5 different active ingredients available within the current wormers that kill different worms. As a result we need to select the most appropriate wormer based on the horse or pony’s worming history, the way in which it is kept, pasture management and the results from any fecs or tapeworm blood tests.
1. Pyrantel – found in Stongid P etc. Kills only adult roundworms, redworms and tapeworm (with a double dose).
2. Macrocyclic Lactones – 2 variants:
a. Ivermectin – e.g Eraquell etc. Kills larvae and adult redworms, roundworms and bots
b. Moxidectin – Equest. Kills larvae, encysted larvae and adult redworms, roundworms and bots
3. Benzimidazoles – Panacur Guard etc. Kills adult redworms (5 day course kills encysted larvae) and roundworms

4. Praziquantal – Equitape. Kills tapeworm only.

What To Do Next?

With all the relevant information in terms of test results and pasture management techniques, your local Animal Health Advisor (the person who is legally allowed to sell you wormers) can work with you to devise an effective program for worming your horse or pony.  Initially with the cost of FECs and the necessary wormer it may be a little more expensive, but over the long term you will be saving money and your horse will be healthier and happier.  On top of this you may find your feed bill decreases slightly as you are no longer feeding a high number of worms too.