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Dedicated to effective

parasite control


With the advent of widespread worm resistance to the commonly used dewormers, the term BROAD SPECTRUM DEWORMER is now a myth.  We are in real danger of landing up with worms that are multi drug resistant.  Just as medical doctors are concerned about antibiotic resistance bringing back old long forgotten diseases into our midst, so too we as horse owners need to be concerned about parasites that are flourishing in spite of our efforts at eradicating them. 


The minute brain of the parasite is beginning to outsmart the shrewdest horse owner and his veterinarian! 


Our team at Worm-Ex Lab will put you, your horses and your regular vet in an informed position to manage the increasing complexity of equine parasites better and cheaper than regular across the board deworming. 


It really does pay to count your eggs before they hatch!

About us
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The Worm-Ex Lab exists to assist the owner and their regular veterinarian to cost effectively implement modern evidence based preventative veterinary medical guidelines to enhance equine (and livestock and wildlife) health.


The Lab was founded on the passion and wide experience of Dr Karl van Laeren and his veterinary practice partner, Dr Jane Howes, and the widely recognized Danish veterinary deworming system.

Global worming.*    What are the consequences for our horses and the South African horse breeding industry?

The majority of horse owners the world over are still deworming horses as they did 40 years ago - by the calendar regardless of need or species affecting the horse.

Four major groups of parasites need to be considered : 


1. Small Strongyles or Cyathostomins;

These worms live in the gut wall of the cecum (the very large equivalent of a human’s appendix but designed to digest grass) before maturing and migrating into gut lumen or cavityThey often occur in numbers exceeding 100,000 worms in the gut wall. As adults they migrate out of the wall into the intestinal cavity where they live on bacteria and protozoa and cause little harm.This is where reproduction and egg production occurs and is the principle egg seen in horse’s dung.If a rare mass emergence of larvae from the gut wall occurs then this could be fatal in young horses.

2. Large Strongyles; 

These worms are the real “bad boys” of yesteryear.They migrate beyond the gut wall and into the arteries of the abdomen, or in the liver and other abnormal sites like the testicular arteries in stallions or the aorta, before it splits into the leg arteries.Left untreated, they form masses, called thrombosis, which can break looseleading to blockage of the arterial tree, starving the intestine of blood supply, which causes areas of the intestine to die off, resulting in colics and other complications.Fortunately, nowadays Large Strongyles are all but extinct due to the deworming efficacy on many studs.

3. Non strongylid Gastrointestinal nematodes (parasites) or GIN’s

These include (Ascarids), . and (Summer sore worms), (pinworms), (bankrupt stomach worms contracted from cattle and sheep especially in winter rainfall areas.This is the only parasite horses and ruminants share in common) and (transmitted in mare’s milk to foals).On studs and racing stables the Ascarid worm that develops after passing through the lungs of horses is the most spectacular due to its large pencil-like size.The eggs of this GIN are thick walled and very well designed to survive in the environment for years overcoming freezing cold or hot dry spells.As these parasites pass through the horse’s respiratory system, left uncontrolled, they can result in respiratory issues like “the snots” as well as intestinal blockages, leading to colics and associated complications.Ascarids developed resistance 20 years ago to the newest generation of dewormers.

4. Tapeworms that undergo an indirect lifecycle needing a forage mite to complete the life cycle.


Parasite control is effected by the administration of anthelmintics (a group of antiparasitic drugs that expel parasitic worms (helminths) and other internal parasites from the body by either stunning or killing them and without causing significant damage to the host.


Two to four to even eight times a year, all horses, regardless of age or immunity, get dewormed willy-nilly.   


The more we, as humans,get exposed to antibiotics the more likely a bacterium will mutate into a super bug.The same principle applies to our horse’s parasites or GIN’s.The more dewormers used per year the greater the likelihood of resistance occurring and the faster it is likely to happen.


Anthelmintic resistance (termed AR in the scientific literature) occurs when worms get exposed to an effective dewormer, but fail to respond satisfactorily.A satisfactory response is a reduction of at least 95% on the pre-worming worm egg count.So, if a horse shows 1000 eggs per gram of dung pre-deworming, they should have 50 or less eggs two weeks after deworming to meet the 95% acceptance level.If not, they are showing signs of AR.


Small Strongyles are the most common parasites in horses and also the most commonly susceptible to AR.However, there are over 40 species of small Strongyles and only one or two may have mutated into a resistant species.So, acount may only drop from 1000 eggs per gram of dung to say 150, which indicates only 85% reduction.This is an indication that some worm species are not being effectively destroyed and it may be worth trying another dewormer to rid the horse of the remaining worms. This war against parasites is won or lost by the accumulation of small victories and/or oversights.


As young horses carry the highest worm burdens and may be exposed to up to eight dewormings per year, this is the portion of the horse population that has shown the greatest tendency to develop resistance.


In essence, resistance is occurring in the Performance Horse and Thoroughbred studs and has the potential to spread to in contact horses.To date we have encountered no less than ten such young Thoroughbreds that fail to clear fully after ivermectin dewormers.In such cases, we then advocate moxidectin dewormers which may reduce the burden some more.However, these horses never drop to the mandatory 95-99% reduction levels one would expect with macrocyclic lactone dewormers (the family of drugs containing ivermectin, abamectin, the cattle dewormer, doramectin, and moxidectin).


There is evidence of Macrocyclic lactone dewormer resistance in sheep, cattle and in horses.We diagnosed our first case in a horse from the Eastern Cape in 2016. A new molecule, monopantel, developed in New Zealand is now available for sheep and goats but alas, the first cases of resistance reported in 2013 in NZ. Horses, however, have no new “get out of jail solution” if resistance is encountered, as monopantel’s safety in horses and in pregnant mares is not assured.We are having to revert to older families of drugs or a combination of deworming drugs to clear resistant parasites.Stomach tubing with up to three drugs has been required in some Ascarid infections.


As long as horse owners and even their veterinarians remain confused or unaware of the risk factors of repeated deworming with, moxidectin in particular,  GIN’s, especially in young horses, are prone to encourage and select for resistant genes to evolve and disseminate to other horses sharing the same paddock space.  Stud farms are breeding good equine genetics (bloodstock) through selecting strong compatible pedigrees, but inadvertently, excessive and unnecessary dewormings are breeding bad persisting parasite genes with a propensity to show resistance to drugs that previously proved highly effective.


Anthelmintic resistance is now a worldwide phenomenon.  Just think of the old days were one syringe of dewormer was truly BROAD SPECTRUM.  This term is now a dream of the past as Ascarids, pinworms and now Small Strongyles all show signs of AR.  


The Southern African evidence pertaining to AR has mainly come from the Thoroughbred studs producing stock for racing purposes.  Usually this trend to AR is only noticed when these horses arrive at the few training establishments practicing  selective deworming.  Selective deworming is an evidence-based deworming protocol where only horses with over a predetermined threshold level are dewormed.  Studs and racing stables should do worm egg counts (WEC) and especially feacal egg count reduction tests  (FECRT) to establish the efficacy of their deworming drug of choice and whether AR is present in their herd.  “Is our deworming achieving a 95% plus outcome post worming?” should be on every forward-thinking stud personnel’s mind.


If doing these tests on the entire draft of yearlings and two year olds in one’s care is initially unattractive, one should consider testing a sector of the stud to establish whether the drugs one is employing Is producing  the desired outcome.  A reduction of 95% plus should be achievable on an individual basis and certainly on a stud basis to ensure that one’s deworming protocol is effective. 


The one clear oddity about AR is that it never shows up in an entire draft of horses.  One or more individuals may be affected while other members of the herd may show no signs.  So, throw one’s net out as far as possible as the problem is still very selective in distribution.  Only by looking critically at ones deworming methods are we likely to avert a looming disaster.


Trying new technologies, we tend to compare this in our minds with what we perceive to be a perfect solution, global across-the-board deworming.   Unfortunately, the latter is far from a perfect solution and it beholds stud managers, racehorse trainers and veterinarians to realize that performing WEC is the gold standard in parasite diagnosis.  Not doing this is leading to an already occurring dilemma of up to 10 % of 2 Year olds presenting with resistant worms arriving at the training yards.  


Acknowledgements. This term “Global Worming” was coined by  Dr Rose Nolen-Walston  and sited in an article written by Ray Kaplan the Equine Parasitology scientist from the University of Georgia, USA .

An Inconvenient Truth: Global Worming and Anthelmintic Resistance. 

Veterinary Parasitology, 2012:  Vol 186 Issue 1 & 2 pgs. 70-78.  Kaplan R et al. 

Latest News 

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.