Frequently Asked Questions

Why do Worm Egg Counts (WEC) when just deworming every 8 to 12 weeks is an easier option?

This is the main reason for the diagnostic services that Worm-Ex lab has created.   Firstly, some background detail on what we are proposing, so bear with me.  


Until now most vets and owners alike have been “managing” horse parasites regularly with broad spectrum dewormers that used to kill most classes of worms very effectively.  Deworming was carried out at intervals between every 8 weeks and twice yearly. This was probably based on size of paddocks, intensity of grazing pressure,  performance demands on the horses, value of the horses, etc. the reasons are endless.  This is called STATEGIC deworming as you have a game plan based on the calendar, rainfall, frosts, and the seasons of the year that you employ to deworm all your horses. 


In human medicine it has been shown that whenever a “best practice policy” is established it takes on average 12 years or so for general practitioners to adapt it as standard practice.  It’s quite scary how slowly medicine actually embraces change and improves itself.  However Veterinary Medicine probably changes even more slowly, as 15 years ago equine parasitologists were beginning to advocate only deworming horses known to have worms at a level above a certain threshold. This is called SELECTIVE deworming.   


In 1999 Denmark passed a law stating that all dewormers could only be purchased with a veterinary prescription stating that the horse had been examined by means of a fecal test and this had revealed a high worm egg count.  By 2009 Sweden, Holland and Finland had passed similar laws.  However, Kentucky, USA and indeed the rest of the world still buy in to deworming all horses (Strategic deworming) periodically, sadly. 


Fortunately, many individual horse owners and veterinary practices are adapting this more progressive deworming policy (Selective deworming).  Leading equine veterinary practices like Rood and Riddle, and Haggard, both in Kentucky and Rossdales in Newmarket in the UK have adopted this deworming policy as their  “gold practice standard”. 


Ok, now back to the question.  Equine parasitism is a function of three elements, namely your horse, its parasites and its environment.  All three play an important role and the environment is the weak link in the chain of control.  There is a law called Posset’s law that states that 80% of the worms live in 20% of the horses.  This has been our lab’s experience almost exactly.   Eighty percent of the horses in general  have very few or zero worms but this percentage is even higher in Racehorses and in well managed yards where horses get short turn out times into cleaned paddocks. 


One Macrocyclic dewormer which contains either ivermectin, abamectin or moxidectin are the most commonly used dewormers nowadays, after dewormers like Fenbendazole and Pyrantel pamoate which have been around for 50 years started to show massive resistance problems.  However, since 1999 our lab has been diagnosing Ascarid (Large round worm) resistance in young horses, this has been followed by small Strongyle resistance since 2004 and since 2011 pinworm (Oxyuris equi) resistance has been documented in various countries around the world.  So, our GO TO broad spectrum dewormer is being dethroned by the worms.  They are becoming resistant!  Resistant worms are shouting at us horse owners. “Give it all you’ve got!  Is that the best you can do?!”  Well we at the Worm Ex- lab will help steer you through the conundrum of horse parasites that are now showing resistance.


The goal is not to eliminate parasites.  This is both unrealistic and unsafe. We wish to introduce the concept to our clients that worms are in fact our FRIEN-EMIES.  We need to make that paradigm change in our approach to parasitism in horses or our horses are doomed.  We wish to keep parasite burdens below levels that cause disease, are responsible for some forms of colic and loss of performance,  while striving to preserve the effectiveness of the current deworming agents.


In the past worming advice was gleamed from tack shops and feed merchants all of which have no training on equine parasites.  Would you take your medical issues to a fast food outlet for answers?


At the Worm-Ex Lab we strive to turn this around by giving you veterinary advice from our parasite knowledgeable veterinary team or we can pass on your results to your personal veterinarian to advise you on the best action to pursue as they are well positioned in this regard.


There are about 60 species of equine Strongyles that horses are susceptible to and nowadays no single anthelmintic is effective against all of these worms.  Without checking the prevalence and intensity and the family grouping of worms infecting your horse, reaching for one specific dewormer may be falling short of the mark.  No broad spectrum dewormer developed in 1983 is still a true broad spectrum agent.  Sadly, this is a myth of the past.   Strategic therapy goals and those of the Worm-Ex Lab are four fold;

A)     To reduce the Strongyle egg count in your horse/s to acceptable levels

B)     To reduce pasture shedding and contamination

C)     To evaluate the efficacy of one’s chosen dewormer by carrying out post deworming testing  called Fecal Egg Count Reduction Tests or FECRT’s (at no extra charge other than shipping costs)

D)     To make testing cheaper than routine deworming


So, testing prior to deworming will need some extra effort on an owner’s part but unless we change our thinking about deworming only the needy, we stand to be encouraging a rapid increase in the trend  towards worm resistance.  This has already commenced and it is now a matter of trying to identify if you horse has this resistant strain and how to overcome this problem.

How susceptible are my horses to liver fluke as we live on a farm and our cattle and sheep are frequently infected and our horses share the same pastures ?  

Good question.  Liver fluke is caused by either Fasciola gigantic or Fasciola hepatica. I have been told of populations of horses in the Western Cape that are all infected with liver fluke!  However, I find this hard to accept at present. 


In a paper coming out of Denmark the authors infected horses with hound Liver fluke and only one of 10 horses drenched with liver fluke parasites became infected.  The infection rate was very low as only about 3% of infective larva established themselves to adult egg producing fluke.  A very experienced  cattle liver fluke parasitologist once told me that in his entire working career he has only seen Fasciola infection in a single horse but in thousands of cattle   In Ireland there has been a reported increased incidence in liver fluke infections in horses. 


So BOTTOM LINE is that the chances of an infection even if your horses are sharing infected pastures with cattle and sheep is very low but not zero.  I advise testing to be sure   (note GASTRODISCUS is a totally different kettle of fish as this is common in any places of South Africa.  But as it does not migrate to the liver of horses - calling this close relative of Fasciola a liver fluke is technically incorrect). See the horse seen in Malawi below with weight loss and Gastrodiscus.

My horse rubs his tail incessantly but a recent Worm Egg Count came back negative for worm eggs.  Please explain.

If your horses rubs its tail and its mane then suspect Sweet itch (also called Queensland itch or Culicoides hypersensitivity).  This may be an allergy to the saliva of biting midges and is quite difficult to suppress unfortunately as needs a big commitment in time and effort.  


However, if your horse rubs its tail and not the mane and especially if you are referring to a young horse, suspect pin worms. (Oxyuris equi). The female pin worm lays her eggs on the skin around the anus of your horse and this creates a severe itch.  I have never been fortunate enough to see these eggs that are easy to diagnose definitively in a routine worm egg count.   I suggest deworming this horse with an appropriate pinworm treatment.   Unfortunately, the regularly used remedies containing ivermectin, Abamectin, and moxidectin are now being proven in various European countries to be showing signs of resistance.  


(Footnote:  Speak to us about a Cellophane test where one samples the debri around the anus of a horse to perform a definitive diagnosis for pinworms in horses).  In our experience these horses should be dewormed with either Fenbendazole or Pyrantel pamoate regardless of their WEC results.  This is why our submission form has a box to tick if you are noticing tail rubbing (without mane of face rubbing) in any horses

My young horse has just passed an almost 20 cm long worm the size of a pencil and a recent WEC was negative.   Explain this to me!

I have had similar experiences too.  It is in fact quite easy to explain.  Like us, worms are either male or female in gender and clearly only females lay eggs.  So,  if you horse is a youngster say under 5 years of age it will be getting to a stage in its life where it will be developing age related solid resistance to these worms called ASCARIDS.   Your horse may in fact be holding on to only a single Ascarid and if this happens to be a male then the recently performed WEC will sadly have missed this non egg laying individual ! 


Anyhow biology is a very  dynamic field and while one parasite may be repulsive enough to a horse owner in the big scheme of things it will do no harm to your horse.  Aim to keep the egg laying female ASCARIDS at zero as their eggs are very resistant once shed into the stable or the paddock.  Even a foot of snow covering an Ascarid egg will have no effect on their viability or life cycle.  Colleagues of mine who have to manage Ascarids in commercial piggeries use a flame throwing blow torch to sanitize and sterilize their pig sties of these thick shelled resistant eggs!  Not something we can apply on our wooden horse stable walls unfortunately but regular WEC are a feasible solution.

How accurate are these worm egg counts as my mare recently changed stables and the new yard manager had her dung tested?  She looks “like a million dollars” but her WEC was over 1000 Strongyle eggs!  We dewormed her and saw no dead worms being passed.

Horses can be fed enough that even high worm burdens make little difference to their conditions.  Not all horses show poor body scores due to worms so the eye of the horseman as a diagnostic test falls far short of a dung test unfortunately. 


(Footnote:  In sheep looking at body weight and mucous membrane colour is very helpful in diagnosis of Heamonchis contortis infection that is a rapid killer.  But these tell-tale signs don’t apply to the horse).  Also, as many worms are very small and get killed by the dewormer they then get digested on their way out of the intestinal tract so one doesn’t always encounter obvious entire worms in the dung after deworming. 


High Ascarid burdens or pin worms may however not be able to hide that easily after an effective dewormer.  But remember, these two families of worms are both showing resistance to the commonly used Macrocyclic Lactone class of dewormers (these include drugs such as Moxidectin, Ivermectin, Abamectin and Doramectin – the latter is a cattle dewormer not approved for use in horses!)


Another important aspect of parasites is that it is the non-egg laying larvae that do the damage and not the egg laying adults.  So sometimes repeating a WEC a few weeks after a bad bout of diarrhea  or colic may be helpful in hindsight to diagnose a suspected parasitic involvement.

My horse suffers regularly from colic.  Should we do a WEC or a tapeworm test on this horse even though he is regularly dewormed? 

A horse that is frequently down with colic is a very worrisome case.  I would not be brave enough  to give you a reason without doing some horse side examinations and even then, the cause may remain a mystery unfortunately. 


Clearly worms and tape worms and Gastrodiscus can all predispose to colic symptoms and one can and should test for at least two of these parasites.   (Tapeworms are occasionally seen on WEC examinations but as they are periodic shedders some labs prefer to do either a blood test or recently a saliva test has been validated in the UK.  However, these UK labs are not allowed to receive samples from Africa due to veterinary concerns about other diseases that they may be importing inadvertently.  These tests are looking at worm exposure by testing for Antibodies rather than current infection by testing for Antigens.)  


A horse that has had a heavy exposure to Large Strongyles with its associated damage in the arteries of the intestine may suffer for the rest of its life from this damage!     Ascarids may have resulted in adhesions forming outside of the gut and these adhesions are permanent so they can affect gut motility for the life of a horse once established, sadly.  However, if your horse is currently infected say with a resistant group of worms then testing may be hugely helpful in getting to the cause of the problem rather than just managing the signs of colic. 


Our lab has been doing research on this topic for many years and the incidence of worms in colic horses appears to be almost twice as high as in non-colic-ing horses. Stated differently, the chances of a bout of colic is nearly 100% higher for a horse with worms than for a horse without worms.    As an aside, I feel that your veterinarian should not only be asked to manage and treat your horse’s colic but that every colic should have a WEC performed to establish if worms may be a possible cause of the colic.  After all, prevention is both better and cheaper than only trying to cure a colic.   


The cause of the colic needs to be pursued at all costs.   So besides high worm counts at present or heavy burdens in the past one needs to consider aspects such as too much concentrates per feeding and at irregular intervals, bad teeth and oral health such as periodontal disease, diastemas between teeth (food traps) caries on the occlusal or periphery surface of teeth, come to mind   Course hay and straw bedding, lack of water, irregular exercise all come to mind that your veterinarian can advise you on. 

My horse is losing weight ever since being imported from Zimbabwe to Blantyre in Malawi.  The vet  says that the liver enzymes are slightly elevated.  Can you advise what I can use to help my horse’s liver ?

This was a very interesting case as the new owners were so worried about the weight loss that they were concerned that their newly purchased horse with the suspected liver failure was going to die!  Fortunately for me I knew where the horse had been stabled before being vetted for

export to Malawi.  


Some places in Zimbabwe have a high incidence of Gastrodiscus which is a large fluke like parasite that needs an aquatic snail to act as an intermediate host.   Before leaving Zimbabwe, this horse was being stabled at a newly established yard that had never had horses before as the paddocks were water logged in the rainy season.  I suspected that Gastrodiscus rather than liver damage was at work. 


A dung sample was sent in on ice and examined.  Hey Presto!  This horse was very heavily infected with Gastrodiscus!  (Had this horse been negative for Gastrodiscus then I would have been concerned about an occult form of pleuropneumonia also called travel sickness.   However, blood tests were normal for White cell counts and for fibrinogen making this an unlikely cause in this horse). 


I put the horse on a low dose of Gastro-Rid dewormer (our home packaged dewormer as no drug is actually registered to treat this parasite in horses).  The low dose got rid of most of the burden.  


Gastrodiscus is a round parasite that look like pinkish grey turtles about the size of a fingernail with a conical head to one side.  This head like structure is a large sucker that removes small plugs of intestinal lining creating small bleeding ulcers in the large colon and cecum (appendix) of horses, zebras, warthogs and apparently elephants in the wild. 


As a heavily infected horse treated with a high but recommended dose of dewormer can result in a mass die off of parasites and these horses can have significant illness from the numerous bleeding ulcers that have been created, we always treat our highly infected horses with a trickle dose of dewormer and retest.  As the count drops we increase the deworming dose to a higher more recommended dose to prevent our patients reacted badly to the dewormer! 


This is another reason to test horses as not all parasite burdens can be managed with a heavy-handed  approach.  Tailoring the treatment and the dosage employed to the problem is advisable but this does mean that retesting is imperative to establish efficacy and if a residual infection is still present.  


I am glad to report that I have seen this horse regularly hereafter and I am overjoyed to see just how well this horse now looks and how close a disaster was avoided.  I was glad to have been in the lab when this lady described her dilemma.  I was glad that my sixth sense was saying, “Hang on! I can smell a rat here” .

Why does my horse now have 7400 EPG when he has been zero for the last two times and your lab does all my testing!

My knowledge on this horse and this yard is both intensive and extending back over 10 months plus.   


These horses have all been tested at least 6 times in 10 months. 


The horse in question is one of 11 horses in the yard and none of the other stable companions has needed more than one deworming in the 10-month period.  In fact, 6 of the eleven horses (55%) have never needed deworming in the past 10 months and the other three have needed only a single deworming over this 10-month window. 


The horse involved shares a paddock with one other horse that was recently dewormed due to a WEC of 500 EPG. This horse has been dewormed three times in the last 9 months and the highest count was 5600 suggesting total lack of resistance to internal parasites. 


The owner hoped that putting her horse on Maringa powder would make a difference in the worm susceptibility, however, after adding this to the diet the WEC came back borderline at 350 and the horse then was dewormed with ivermectin.*   An initial retest was at 29 days post deworming and was zero.  A second test was performed at 52 days post deworming and was also zero.  Then at 88 days after deworming the count was now 7400!


Understandably a big concern to the horse’s hard trying and totally confused owner.  So ivermectin should cover most horses with susceptible worms for 8 weeks or 56 days.**  Only after 56 days should one expect eggs to be reappearing.  So the initial two tests were both done within the 56 day clear window thus resulting in zero counts.   


It takes 5.5 to 6 weeks for small strongyles to reach sexual maturity.  It takes 8 weeks for Triodontophores to reach sexual maturity and 5 to 7 months for large strongyles to reach sexual maturity.  Some worms undergo a hypobiotic stage where they rest up in the gut wall for upto 2.5 to 3 years before reaching sexual maturity.   


So, predicting when to expect worms is like reading tea leaves accurately.    An ivermectin dewormer seems to have a post deworming inhibitory period of 14 days so if one adds this 14 days to the 6 weeks for small Strongyles to reach sexual maturity then a non-resistant population of small Strongyles will take 8 weeks to show their presence and Triodontophores will take 10 weeks.  


But small Strongyles and Triopontophores and large Strongyle eggs look identical so predicting which species one is dealing with and when to expect worm eggs to reappear is virtually impossible.  However as small Strongyles represent the great majority of the parasites in horses nowadays, one can safely say that one can expect eggs to appear after 8 weeks or 56 days and that this is the interval one should be using on horses that have recently been dewormed, and then every 30 to 45 days thereafter or unless the results indicate otherwise.  

Herd resistance also comes into play.   Remember this is a complex science but once you know your horses and the stable’s performance it gets a lot easier to manage and advise appropriately. 


But remember the longer you stretch your testing interval the greater the risk of getting shocked by unexpectedly high counts as worms are very fecund individuals indeed!    Then remember that 90% of the worms live in the horse’s environment.   And that a count of 7400 eggs can hatch into infected larvae in 2,5 to 3 days and that  this count of 7400 is per gram of faeces and thus a horse producing 8 kg of dung per day (8000gms) is producing 59,200,000 potential parasites every 3 days when conditions like cool wet summer days present themselves!     


WEC is a very specific diagnostic test.  This means that if you see worm eggs it is beyond doubt a definitive diagnosis that mature egg laying parasites are present.  However WEC lack sensitivity as they only diagnose mature worms that are sexually active and thus producing eggs.


Remember you are counting eggs to make a diagnosis of parasites.    Worms are like people, they occur as both males and females, immatures that are not sexually active and then mature worms that are producing eggs and finally an older generation of parasites that have stopped producing eggs but still reside in the gut and will ultimately die out and be shed in the dung.   


In medicine we always talk to the specificity and the sensitivity of any test and in the case of WEC there is a difference that one needs to understand.  Understanding one’s enemy, its life cycle and its prepatent period (from ingestion to egg laying) is paramount.


*I have not read of any natural plant based dewormer that has been both effective and safe in eradicating worms in horses.  Having said this we would welcome hearing from anyone who feels they know of any effective natural remedy as there is no reason something like this does not exist   Note DIatomous earth has been shown to be ineffective too.


** we are seeing horses dewormed with ivermectin become infected and need more deworming within 28 days already.  These are resistant worms and this phenomenon is being seen in the USA and in the UK too.  So, we are already failing to manage worms which is only likely to snow ball out of control unless we understand our horses, the environment, the parasites and the drugs we are using.

Deworming strategies where worms are rarely encountered


In adult Sport-horses in Gauteng,  we are seeing less than 4% of horses needing deworming.  This means horses with 350 or more worm eggs noticed per gram of dung tested by McMaster slide examination. 


So about 96% of horses appear to not need deworming! 


Does this mean it’s safe to break away from regular deworming as has been drummed into us for years?  Indeed it does!  We have a small yard that has not had to deworm any of their horses in eight years with no ill effects.


However, owners are getting jittery.  They feel they need to deworm even if a single worm egg is encountered! 


What is a good policy and what is a safe, scientific, well thought out approach to deworming? 

It is hard to get owners to be comfortable with low counts of 50 to 300 and doing nothing at all.   It is very easy indeed to create a zero WEC sheet.  Just deworm repeatedly till the horse is zero.  Simple!  

Wrong answer! 

Parasite resistance in our horses is what we should be aiming for AS WELL AS minimizing paddock contamination.  Achieving the latter is labour intensive but achievable, especially in yards with small paddocks.  Achieving the former, resistance to parasites, may be manageable in most horses over time by low parasite exposure. 


Resistance is more likely in older horse than young stock.  We are thinking 10 to 12 years of age as there are a lot of species of worms that horses are exposed to.  Eighty three species in 29 genera on last count! Imagine if horses had to develop immunity to 83 strains of African Horse sickness!


That won’t happen overnight.   


This is a big ask and will take time, but many horses’ wonderful immune systems are up to this challenge!   They do develop a solid immunity and ultimately can cope without deworming.    Well, there are a few exceptions.  There are always outliers and exceptions to every scenario.  Nothing is cast in concrete in biology.  I see a few horses with presumably a weak immune system that are always wormy but I can count the number I am aware of on one hand (out of tens of thousands tested over the years!).  This is especially true where less than meticulous paddock management is also at work.


So back to the main concern.  Should one deworm horses with ivermectin if they are worm free?


In essence, a strong No!


You are simply wasting money deworming such a horse with, for example, ivermectin, as you can’t expect any gain from such a practice, especially if no immature worms are present either. 


However, there is a slight amount of merit in doing this seemingly senseless act once every two or three years!   To make this point clearer I wish to take you back to the findings of the research groups in Denmark and in Kentucky, the home of the flat racehorse breeding industry and where most of the first class equine parasite research is coming from.


So, as you may be aware, Denmark has started to make deworming of horses illegal unless a competent lab has found worms to be present in sufficient numbers.  This is called the Selective  or Targeted approach  and was adopted into law in Denmark 20 years ago.  Since then a few forward thinking individuals, like you reading this article, and in countries like Holland , Sweden and Finland have followed suit.  Hooray, these people are thinking about the consequences of excessive, uncalled for, deworming.


Kentucky by contrast deworm by calendar month.  This is called Strategic or Seasonal deworming.  However, this is largely a system based on the needs of foals and young horses and this should not apply to adult horses. 


There is a new type of test called PCR testing where one can look for a genetic finger print of a specific pathogen, bacteria, virus or, in this case, worms.  This specific PCR test has been set up to look for the Strongylus vulgaris (large strongyle) parasite egg in particular.  What has been uncovered is that this “bad boy” parasite, whose lifecycle includes time inside the arteries of horses, is not present in Kentucky, but is present in small numbers (14.1%) in Denmark.  So, while Kentucky may have a dangerous deworming policy, they can show some definite positive effects of this system, whereas Denmark may have the gold standard ideal in mind but there is a draw back.  The bad boy parasite S vulgaris is raising its ugly head here!


So, WORM-EX LAB has come out with a largely unique solution.


We have married the two systems into one unit to get the best of both worlds.  We don’t want to over deworm and neither do we want the bad boy parasite to get a toe hold in our clients’ horses.  Also, current testing methods for stomach bots and tapeworm diagnosis are mildly flawed as they under-represent the actual incidence in horses, and perhaps gives a false sense of security.  We need therefore to counter this by taking precautions against possible infection.


So, we are saying that clients who feel uncomfortable not deworming at least once in a while may want to consider meeting on neutral ground.   We are suggesting that, once every two years, all the horses in your yard get dewormed with an ivermectin dewormer plus praziquantel.   This will cover tapeworms, stomach bots (Gasterophilus) and Strongulus vulgaris but will not destroy the small strongyle refugia  (dormant worms in the gut lining), so immunity development to these more common parasites will continue, unabated.    


And what is the best time to do this?  Autumn is supposedly the best time to get rid of tapeworms which generally have a single cycle per year with the highest prevalence at the tail end of summer.   However, winter is the best time to clear horses of bots when the free flying bot fly is at its least prevalent.  So, doing one broad spectrum dewormer in winter will cover all these bases adequately. 


As most Gauteng horses have been experiencing regular unnecessary deworming we feel that one should deworm only based on WEC results for now.    Then, come Winter 2020, that one embarks on an “entire yard” deworming spree with ivermectin plus praziquantel that will last till  2022 for large strongyle, tapeworm and bots.


Regular WEC testing between these dates remains important to pick up the individuals with counts of 350 and above.