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

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.

A Snapshot of 2019 for Worm-Ex Lab


(That got your attention!  But don’t panic … you won’t be wanting to drink these gins!)


Gauteng is an area of very low GIN levels (gastro intestinal nematodes – a fancy acronym for worms and being used in more and more scientific papers … maybe to brighten up the lives of parasitologists and make them seem erudite?).  This phenomenon is probably a reflection of intense deworming practices as well as attention to maintaining clean, and thus creating low-risk, paddocks.  We also feel that the hot spring months in this area are  very hostile to free living parasites in the environment, acting as a deterrent to GIN survival thus further reducing the need to deworm if pasture / paddock management is efficient.




It appears that the prevalence of Gastrodiscus is higher than most other worms in the Gauteng area especially where a seasonally moist habitat is accessible to horses.  We have encountered far more Gastrodiscus positive cases than say Ascarids and / or tape worm cases.  In fact, tape worm eggs seem particularly scarce, but as the intermediate host is a nocturnal pasture mite (Oribatid sp) and horses are usually stabled at night due to fears of AHS, this may be one of the only “upside” of AHS if one has a perverse sense of reasoning and investigation!   Gastrodiscus parasites are being exported to areas, such as George on the Garden Route, that are not the usual breeding grounds for the intermediate snail host.  Body score may or may not be affected by this parasite, so knowing the history of the horse may be the only reliable way to deciding which horses need surveillance testing.  Worm-Ex Lab is plotting the prevalence of this parasite by the river network of Gauteng.  So far we have found positive cases in Kyalami, Hartebeespoort, Meyerton and in George, W.Cape. Bear in mind that we only test for gastrodiscus on specific request, as this parasite cannot be identified on a standard worm egg count.




Over deworming remains a concern as this is the practice that results in resistance.  However, no resistant worms have actually been diagnosed so far in adult pleasure horses in the Gauteng area.  This is however not a message that we should be complacent; it is rather that we have been lucky so far (see below on the totally different anthelmintic scenario happening in youngsters).  Once resistance sets in we will be on the back foot and our behavior today will determine if and when this sets in.  Let’s learn from the sheep and goat farmers. They have huge resistance in their flocks and they have 9 families of dewormers to choose from.  Horse owners only have 4 families of dewormers to choose from and there are populations of worms out there that are resistant to every one of these families. 


It is Worm-Ex Lab’s pledge to look for what works and what is failing on your stud/yard/individual horse.




Young racehorses with resistant worms are being identified.  Very little surveillance testing has been performed on these horses however.  Trainers are “leaving money on the table” by not testing their new arrivals.   Recently, one newly arrived racehorse needed colic surgery due to a small intestinal impaction secondary to a large mass of pencil sized Ascarid parasites.  A worm egg count at the cost of ZAR120 could have saved a colic fee at the cost of almost ZAR120 000!  So why scoff at a return of 1000 fold on testing poo by adopting an enquiring mind! 


Ask for ADVICE on whether deworming is really needed, and if so with what and finally, how effectively the dewormer has worked .  Remember that Worm-Ex Lab’s commitment and firm belief in our service is based on doing free of charge retests on all low counts and all horses that are dewormed to determine efficacy.  Why do anything that’s not really effective?  We are committed to ensuring that efficacy rules!  Also, very little surveillance work is being carried out by veterinary practices to see if the hypothesis that worms contribute a risk factor to colic in RSA exists.


Thoroughbred yearlings being shown to harbor multidrug resistance GINs.  Macrocyclic lactone (ML) dewormers like Doramectin, Abamectin, Ivermectin and Moxidectin are failing to obtain 95 to 99% clearance rates in young horses.  Pyrantel pamoate is failing, as is Fenbendazole.  It remains a confounding mystery why this anthelmintic resistance (AR) appears to be primarily a problem in young horses rather than in older horses?   We have diagnosed this problem in an 8 YO too but horses under 3 years of age are the main protagonists of the problem.  

Of interest is that we have seen three older horses (6,8 & 16 years of age) with multidrug Ascarid resistance.  Usually this parasite undergoes age resistance by the time a horse reaches 18 months of age.  But if a horse has grown up without exposure to Ascarids they should be able to establish themselves in any age group before immunity kicks in and overcomes the infection.  If this immunity kicks in, in the face of a heavy infection this rapid die off can cause a blockage and colic as outlined in the case above.


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Horse owners need to apply a modicum of CRITICAL THINKING when deciding on deworming and what better way than thinking like an EVIDENCE BASED STATISTICIAN!

By doing this, a commonly held and seriously erroneous myth may be laid to its grave, permanently!

Lets help you through a fictitious discussion between Worm-Ex Lab and a client based on critical thinking!

Worm-Ex Lab : So you prefer deworming 2 or 3 times annually (some owners even more often) with different dewormers, rather than testing?

Horse owner : Correct. It’s in the best interest of the yard.

Worm-Ex Lab : OK, I can see where you are coming from. Clearly you are concerned, but can I ask you one more question?


Horse owner : Sure, go ahead Doc

Worm-Ex Lab : What do you consider to be the likely probability of worms in your yard? You are in fact basing your decision on what outwardly seems to be a good "insurance policy” of regular deworming all horses.

To a casual thinker, the answer to this question has to be that MOST horses must have worms therefore this warrants deworming an ENTIRE yard of horses! Surely that has to be the correct and logical conclusion to this deworming strategy?

The converse argument also applies.

Most managers pride themselves in having a clean yard with no dung build up on the paddocks. Critical thinking thus implies that you then have low risk pastures. Remember that by your horse sniffing or even eating another horses fresh dung at a show, there is absolutely zero chance of contracting worms from this seemingly disgusting activity. The eggs need to hatch and molt three times to before they become the infected third stage larvae. Only these third stage larvae, that take on average 4 to 10 days to arrive on the scene, can infect your horse, not eggs. So there is a biological safety window between depositing dung on the pastures and worm danger arriving from this dung.

This is a very important building block in understanding and managing worms.

So let’s examine this myth of regularly deworming all horses like a critical thinker who is unwilling to allow a deceptive mind to make erroneous conclusions.

Did you know that an assessment of most stable yards has shown that less than 4 % of horses actually require deworming intervention? Deeper analysis is showing that the youngsters are more likely to be carriers of worms too (see important caveat below). Our horses are definitely not awash with malevolent worms. But without testing each and every single horse in one's care one cannot possibly determine if there are individuals with a parasite problem. No single-shoe-fits-all policy applies to worm management, unfortunately. All horses are born individuals and parasite susceptibility is equally, highly individualistic!

So, in a yard with a "deworm all” policy in place, 96% of horses with either a zero or a very low worm burden are receiving a costly dewormer, and only high worm burdens carry any potential harmful effects. Nobody would decide to take an antibiotic because they have an innocuous bacterial population residing in their intestinal tract, yet many horse owners insist on deworming regardless of the zero to low needs that actually exist.

Apply critical thinking and refrain from guessing when your horses next due for routine deworming. Surely, now that our deceptive minds and the law of probabilities are being applied, a simple assessment of the dung is the correct way forward?

Make one of your New Year's resolutions to become a critical thinker on matters deworming.

Apply hard science to your decision making.

Dispel guess work in 2020.

(PLEASE NOTE: This does not apply to young horse under three years of age)



In summary, 2019 was our formative year but some of the revelations and the remedies we have been able to institute at relatively low cost to the owners have been very uplifting to NJ and myself.  We feel like crusaders for a new belief, a new following.  Rejection is always present and is part of any crusade!  However, the victories far outweigh the setbacks and we are convinced that this trend will continue into the forthcoming year for your horses and your yard.


We at Worm-Ex Lab thank you for your stance to test, for your support, your belief and your trust in us and we wish you and your family a joyous Christmas and a health filled New Year for you and the horses in your care.

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