De-worming: Part 1 - Worm Egg Counts
Worm Egg Counts are now recommended as part of a targeted worming programme. However, it is important to understand that they have limitations. Information on commercial websites can be mis-leading and, for various reasons, no test is 100% accurate. Therefore, WEC’s should be used as guidance rather than taken as absolute fact. The tests do not detect all types of endoparasite; they may be inaccurate; and more research is needed. However, they can be very important when formulating a de-worming programme, to reduce anthelmintic resistance, and to treat individual horses at risk.
Anthelmintics = drugs effective against internal parasites (especially worms).
Endoparasites = includes the horse worms (parasites) that live on the inside of the body (in comparison to ectoparasites such as lice, which live on the body surface). Also includes parasites that are not worms e.g. bot fly larvae.
Anthelmintic resistance in horses = the worms become resistant to the effect of the drugs used to de-worm horses, so ‘wormers’ no longer remove enough worms to ensure health.
What are Worm Egg Counts –WEC’s (also known as Faecal Egg Counts- FEC’s)?
Many species of internal parasites (but not all) lay eggs which pass out in the horse’s droppings. By looking at a sample of droppings (under a microscope) and counting the eggs we can make an estimate of how much of a worm burden the horse has. I.e. it is a guide to how large a burden of parasites the horse has.
Results are given as “Number of eggs per gram” = epg. Horses with 0-200 epg are usually considered to have a low worm burden; those with 200-1000 epg have a moderate burden; and those over 1000 epg have a high burden.
Why do a WEC?
WEC’s are used to help judge if a horse has a worm burden that could affect its health, to reduce anthelmintic resistance, to reduce de-worming costs (by allowing strategic de-worming when necessary, rather than de-worming every horse every time), and to detect whether worms are already resistant to the drugs being used.
Are WEC’s accurate?
Egg counts are only an ESTIMATE of the burden i.e. they are not proof of how many worms the horse is carrying.
There are many reasons why a sample may be inaccurate, including:
- Worm eggs are not distributed evenly through a dropping heap, so if a sample is taken from a part with no or few eggs then it may give a false idea of how many worms the horse has. Ideally, take several samples from different parts of the dropping then mix them together.
- Natural variation in egg numbers throughout the season. These can be affected by numerous factors, including when the horse was last de-wormed and random variation.
- Freshness of the sample: samples need to be less than 12 hours old – ‘fresh is best’.
- Handling of the sample: samples need to be kept in an airtight container and refrigerated as soon as possible. This helps reduce the chance of the eggs hatching into larvae. If you are going to post a sample off for testing and leave it in a hot car while you go to the shops, then the results are less likely to be accurate.
- Type of test the laboratory carries out, and number/size of samples tested. Some tests are more accurate than others, so very cheap tests may not always be the best option. The larger the faecal sample tested, and the more samples that are tested, then the more accurate the results are.
- Individual horses tend to have fairly consistent egg output, so repeat testing of a horse throughout the grazing season will give more accurate results than a one-off test. However, if the one-off test indicates high numbers of worms are present then there is a strong chance that it is correct.
WEC’s are not 100% accurate, and can give false positives (the test says the horse has a high worm burden but it has a low one) or false negatives (the test says the horse has a low burden but it has a high one). In general, they are more likely to give a false negative result than a false positive.
Research has shown that the epg from a WEC is NOT linear with the actual worm burden inside the horse i.e. if a horse with 200 epg actually had 300,000 worms then it does not mean that a horse with 400 epg has 600,000 worms. This is particularly true for worm types such as cyathostomes (small stronglyes/redworm) and ascarids (which are a significant problem in young horses).
As well as these factors, WEC’s don’t detect all types of endoparasites, or all of the worms at all stages e.g. the horse could have many immature worms that are not yet laying eggs, or the parasite itself may not lay eggs while in the horse (e.g. bots).
Because of all of these points, WEC’s are only ever a guide to worm burdens and one individual test may or may not be accurate.
If my horse has a zero WEC, does that mean he has no worms?
All horses have worms. As mentioned above, there are many factors that can affect WEC’s and make them less accurate. The sample of faeces that is tested is only small, so if the horse has a low burden of worms that are producing eggs then the sample tested may not find an egg.
NB: only part of the sample you send will be tested, as it needs to be diluted to fit onto a microscope slide; so, each single egg that is actually found under the microscope may represent 25-50 eggs in the whole sample. I.e. if your horse is given a WEC of 200 epg (eggs per gram) then in reality the tester only found 4 eggs on the microscope slide (depending on the type of test).
However, if your horse has consistently low or zero WEC’s over a season, then it is highly probably that it's worm burden is low. BUT SEE BELOW…
Which endoparasites are NOT tested for by WEC’s?
Unfortunately, many people believe that WECs test for everything and that if their horse is given a low count then they don’t need to de-worm their horse at all. However, this may not be true. There are four common endoparasites that WEC’s do not test for.
- Tapeworms: Tapeworm eggs are sometimes found in WEC’s, but eggs are shed intermittently so it is not a reliable way of detecting them or assessing the actual tapeworm burden inside the horse. However, they can be assessed using a separate test on the horse’s saliva, or a test on the horse’s blood (Elisa test). The newer saliva test is believed to be better for assessing an individual horse’s burden. Testing is usually done over the autumn/early winter period (and again in late winter/early spring if needed) and has a good degree of accuracy in most horses but, as with other tests, has limitations i.e. just because a horse has one negative result doesn’t guarantee a horse always has a low tapeworm burden.
- Encysted cyathostomes: these are immature larvae of small redworms, which spend the winter in a cyst within the horses’ intestinal wall. If many larvae ‘hatch’ out at once (e.g. spring) then they can cause serious colic. There is no test for encysted cyathostomes, so horses can have a high burden even with a very low WEC. This also applies to other immature worms e.g. a horse may incur damage from Strongylus vulgaris larvae before the worms become adults and start laying eggs.
- Whipworm: these live inside the horse’s rectum and lay eggs around the horse’s anus, therefore the eggs don’t show up in the faeces. They cause intense itching of the tail area, and the eggs can be identified by placing a piece of sellotape onto the skin round the anus then looking for the eggs under the microscope.
- Bots: Theses are not worms, but are part of the bot-fly lifecycle. Eggs are laid on the skin and it is the larvae that live inside the horse and are passed out in the faeces (after the horse has chewed eggs of its skin). Removing the eggs as soon as they appear will help reduce the burden inside the horse.
Is more research needed?
The cut-off point for when a horse has a ‘significant’ burden is not based on much specific evidence i.e. scientists don’t know for sure what epg would be best to treat at for ideal long-term results (both in terms of horse health and preventing resistance to drugs). This is partly because the number of epg does not exactly represent the number of worms inside the horse. However, it is usually agreed that a figure of 200-500 epg is regarded as a suitable cut-off point for treating a horse.
Commonly used tests vary in accuracy between worm types e.g. a negative WEC in a horse may be only 35-65% accurate and the horse may actually have a significant worm burden. Tests that check for both eggs AND larvae will be more accurate, particularly if the individual worm species are identified, but these tend to be more expensive and take longer. There are currently tests being developed for encysted cyathostomes and larvae of Strongylus vulgaris, although it will be some time before these are commercially available. A cheap test that accurately assesses a horse’s true worm burden, with all species of paraistes included, would be ideal!