To WormMail mailing list (recip. undisclosed)

WormFax NSW

This is a summary of WormTests in sheep in NSW from two major parasitology labs.

The latest issue (October 2013) is now online:



Bacteria in the gut may affect health


Motorcyclists 23% better as drivers




This message is intended for the addressee named and may contain confidential information. If you are not the intended recipient, please delete it and notify the sender. Views expressed in this message are those of the individual sender, and are not necessarily the views of their organisation.

WRML. 20131121.Kiwi parasitologist Dr Dave Leathwick gets another gong

TO: WormMail list (recip. undisclosed) cc Editor, NZFW

From NZ Farmers Weekly. http://agrihq.co.nz/article/a-champion-of-drench-resistance?p=6

Article and photograph reproduced here with permission. Please do not reproduce without permission from NZ Farmers Weekly, the journalist or editor, and the photographer.

Article heading: A champion of drench resistance

"Sheep farmers feeling powerless have a friend in parasitologist Dr Dave Leathwick. Tim Fulton reports.

"It’s almost job done for the study of parasitic resistance in sheep and science is turning to the new frontier of cattle." (See this*, for example)

"AgResearch scientist Dave Leathwick has been recognised by his peers as an R9, a lofty position that recognises the gift he has delivered to sheep farmers and the study of human health.

For the past 23 years Leathwick has been part of a team researching the epidemiology of nematode parasites of sheep, particularly the development and management of anthelmintic drench resistance.
His research director Warren McNabb said R9 status was recognition of science excellence, leadership and mentoring. Leathwick was an outstanding AgResearch citizen in that regard, he said.

Science group leader Ian Sutherland said Leathwick was also one of only a few New Zealanders widely recognised at the top of their field. Proof of this could be found in more than 50 published papers and his regular billing as a speaker at international conferences and industry forums.

In February, for example, Leathwick presented a paper to the International Sheep Veterinary Congress in Rotorua called Sustainable Control of Nematode Parasites – the New Zealand Story. The presentation was judged the best session of the conference.

His research career began with the building of computer models of drench resistance and he went on to test outputs from the models in a series of large-scale field trials. Many of today’s recommendations going to farmers about the management of anthelmintic resistance were under-pinned by research from this group.

Leathwick said the R9 status was probably a sign of getting old – and added he didn’t usually tell strangers he was a parasitologist, partly because it created too much hassle with customs at airports.

However, his contribution to animal and human health can’t be brushed off. He’s happy to acknowledge how far research has come in his field, to a point where knowledge of parasite resistance in sheep could basically be handed to farmers as ready-made extension material." (See www.wormboss.com.au for example – SL 🙂

“Essentially we think we’ve done what we were asked to do. The knowledge is there – increasingly it’s an issue of extension and adoption.” That was extremely satisfying, he said. “It’s a good feeling and there’s been a heap of people involved, but it’s a really nice place to be. And I think it’s why we’re starting to get more recognition internationally.”

Leathwick hoped the same sort of progress could eventually be made in cattle farming, provided everyone was prepared for another all-out effort.

He acknowledged his career had exposed him to all sorts of antics in animal health research and commercialisation, driven at times by short-sighted desire to get a jump on competitors. The losers would invariably be farmers left to deal with drench resistance, he said.

Animal health companies were heading in the right direction, scientifically and commercially, he said.

Novartis, for example, had identified parasite resistance as a potential threat to a new class of drench it brought to the market as Zolvix, with the active ingredient monepantel. The company had invested enormously in bringing a new drug to market and had been the first with a strong plan for how it would protect its value against parasitic resistance.“It has never happened before, so in that sense the world is changing,” he said.

Leathwick is working now on behalf of a drug company in the United States, giving advice on how drugs should be used to make them last.“Their long-term goal is to remain financially viable and if resistance kills all their products then they don’t sell them any more.”

He said some people would be surprised how closely scientists worked with drug companies, although inevitably the relationship could wax and wane as scientists tried to make objective assessments.

He remained cynical about animal health companies’ desire to make profit for shareholders but was also proud of being part of a team that had remained professional.

“I think one of the reasons over that time that we’ve got on better with the companies is that we’ve remained staunchly independent and we base everything we say on fact and evidence.”

Satisfaction also came from the contribution his peer group had made to fighting drug resistance in humans. If it wasn’t resistance to drugs for tuberculosis it might be mosquitoes that carry malaria becoming resistant to insecticides.

Leathwick began his research career in 1974 as a technician in the weed biology and control team of Ministry of Agriculture and Fisheries, before going on to complete a science degree with honours in zoology at the University of Canterbury. He followed that with a PhD in entomology at Lincoln College and with that expertise he has collaborated at times in wasp and weed research.

In 2010 he won the NZ Society of Animal Production’s McMeekan Award for his outstanding contribution to animal production in NZ."

“Essentially we think we’ve done what we were asked to do. The knowledge is there – increasingly it’s an issue of extension and adoption.”
Dr Dave Leathwick

TOP OF FIELD: AgResearch scientist Dave Leathwick has been recognised by his peers for his science excellence, leadership and mentoring. Photo: Graeme Brown"

‘A good/interesting article I thought, but I am not so sure about the (ambiguous?) heading.


* https://wormmailinthecloud.wordpress.com/2012/10/09/wrml-efficacy-of-oral-injectable-and-pour-on-formulations-of-moxidectin-against-gastrointestinal-nematodes-in-cattle-in-new-zealand/

This message is intended for the addressee named and may contain confidential information. If you are not the intended recipient, please delete it and notify the sender. Views expressed in this message are those of the individual sender, and are not necessarily the views of their organisation. 

WRML.20131119. More thoughts on (the first case of) monepantel resistance

To WormMail (WRML) list (recip. undisclosed)

More thoughts on (the first case of) monepantel resistance

Stephen Love, Veterinarian, NSW DPI, 20131119

Recently I wrote about the first confirmed case of Zolvix (monepantel) resistance (Vet Talk, The Land, September 2013; also here). This occurred on a goat farm in New Zealand and was published earlier this year, about 4 years after the world-launch of this new drench active in that country. There are reports of monepantel resistance on a further two goat farms in NZ.

Here I will discuss the matter a little further.

Goats generally metabolise (process and excrete) drenches (anthelmintics/wormers) faster than sheep, and this may be one reason why resistance often seems to develop faster on goat farms.

In the reported case, the efficacy of monepantel against small brown stomach worm and black scour worm in goats – and sheep – on the farm went to zero in less than two years. During this time the drench was used 17 times, but not on all animals on every occasion, perhaps a conscious effort on the producer’s part to maintain some worms in refugia i.e. worms not exposed to the drench and therefore not selected for resistance.

Has resistance to monepantel occurred earlier than it should have? I am not sure anyone yet knows what the mode of inheritance of monepantel resistance (e.g. dominant or recessive genes) is in worms – and it may be different in different worms – so that remains a moot point.

What has been the track record with other anthelmintics? According to parasitologist Professor Ray Kaplan, the time from initial drug release to the first reports in scientific journals of resistance to various drenches in sheep worms were as follows: thiabendazole – 3 years; levamisole – 9 years, ivermectin – 7 years (4 years in Australia); moxidectin – 4 years (8-9 years in Australia).

However it is not so much about time as about how often a drench is used and the worms in refugia. In Western Australia for example, resistance of brown stomach worm to ivermectin was found after just 4 years, possibly due to few worms in refugia. Drenching occurred on some farms only twice a year. Resistance of barber’s pole worm to ivermectin in northern NSW – where drenching is more frequent, but there are more worms in refugia? – happened just as quickly. However, the ‘ivermectin resistance’ gene in the case of barber’s pole worm is dominant, so one expects resistance to happen faster than if the gene was recessive.

Focussing now on management rather than the drench, I am wondering, for example, why the owner of the NZ goat farm did not know the drench was failing until it was completely dead. At this stage there is little publicly available detail on management aspects of this case, so I can only make educated guesses.

In particular, were drenching decisions based on results of regular worm egg count (WEC) monitoring, and was a DrenchCheck (WEC 10 days after drenching) done from time to time to check on drench efficacy?

Lest any are tempted to feel smug at this stage, a small minority of goat or sheep producers do these things, both of which are key elements of good worm control.

What about drench rotation, or better, using unrelated drenches concurrently i.e. in combination? This goat owner had exhausted all options, including ‘mectin’-based triple active drenches, but still had the new drenches Zolvix (monepantel) and Startect (derquantel + abamectin) up his (or her) sleeve(s).

Leaving aside the fact that neither are registered for use in goats, I am wondering why Zolvix was not used in rotation with Startect (not yet available in Australia, by the way) or why the two were not used concurrently each time the animals were drenched. Right now on this farm, Startect is the only drench left standing, and with the demise of ‘mectins’ on this property, the new active in Startect, derquantel, will get precious little support or protection from the abamectin component.

If used, how long will Startect last on this farm? It took about a quarter of a century – after the release of the ‘mectins’ – to get the new drench families: the AADs (aminoacetonitrile derivatives, represented by monepantel) and the spiroindoles (represented by derquantel). Will they too in typically short time go the way of all drenches?

As to other elements of integrated worm control, I have no idea whether nutrition or grazing management or animal genetics were used to help with worm control.

Regarding biosecurity, were there good quarantine procedures to keep resistant worms out? Possibly these multi – / super – resistant worms were bred entirely on this farm but who knows? And will these worms – by whatever means – end up on other farms?

The current advice in Australia is to treat sheep brought onto a farm with four unrelated drench actives, one of which is Zolvix. When and if we get Startect in Australia, I guess the recommendation will be Zolvix used concurrently with Startect along with at least one other unrelated broad-spectrum active.

Unfortunately this NZ story has a familiar ring to it. The first published report of moxidectin- resistant sheep worms in Australia was in 2003 and related to barber’s pole worm on a property in northern NSW. Ivermectin was used once only and thereafter moxidectin was used exclusively up to 4-7 times per year. Several years later there were control failures (clinical haemonchosis), after which the owner sought advice. The efficacy of moxidectin was 67 to 83% (tests on different occasions), abamectin 19% and ivermectin zero.

In case you think I am Kiwi-bashing here, I am sure we Australians – and others – are equally adept at breeding drug-resistant worms. At the very least, the dominance of the All Blacks rugby team saves me from overweening national pride.

Hope springs eternal: perhaps we can do better.

WormBoss (wormboss.com.au) has excellent information (authoritative but user-friendly) on all the points discussed in this article (apart from how to play rugby).

Reference: Kaplan RM. Trends in Parasitology. 2004.


Today’s font is Georgia.


WRML.20131114. Overview – condensed tannins and worm control – Robyn Neeson NSW DPI

To WormMail list (recip. undisclosed). Overview – condensed tannins and worm control – Robyn Neeson [NSW DPI]

Robyn Neeson has prepared an overview of condensed tannins and worm control. It is republished here with permission.

Robyn is a Development Officer, Organic Farming, based at NSW Department of Primary Industries’ district office at Berry.


This message is intended for the addressee named and may contain confidential information. If you are not the intended recipient, please delete it and notify the sender. Views expressed in this message are those of the individual sender, and are not necessarily the views of their organisation. 

Neeson R 2013 – condensed tannins- livestock nutrition and worms.Organic news Spring 2013.pdf

WRML. Resistance of important horse worms to anthelmintics. closantel tox in human. Britney repels pirates. etc

To WormMail mailing list (recip undisclosed).


UPDATE (FEBRUARY 2015):  For up-to-date information on horse worm control written by specialists in equine parasitology, consider:

  • AAEP (Equine)Parasite Control Guidelines – http://www.aaep.org/custdocs/ParasiteControlGuidelinesFinal.pdf – by various including R M Kaplan and MK Nielsen.   Undated by circa 2013.
  • Kaplan RM and Nielsen MK, 2010. An evidence-based approach to equine parasite control: It ain’t the 60s anymore. Equine vet. Educ. (2010) 22 (6) 306-316. http://onlinelibrary.wiley.com/doi/10.1111/j.2042-3292.2010.00084.x/abstract

Go to: https://wormmailinthecloud.wordpress.com/2015/03/12/wrml-2015-03-12-get-up-to-date-on-horse-worms-other/

SL, Armidale, NSW. 2015-02-24


In this issue (WRML.20131105): drug resistance of horse worms. reversible blindness in human self-administering closantel. Britney repels pirates. a plague on both houses. definition of specialist.

This WormMail is primarily about anthelmintic resistance of horse worms, which is fitting given that today is Melbourne Cup day. (The best day to invade Australia as no one will notice).

A veterinary colleague asked me about this topic and, seeing I have forgotten more about horse worms than I ever knew, I did some digging. Although by no means exhaustively accurate, following is what I have found so far.

Summary regarding resistant horse worms

For those with little time and/or short concentration spans, here are the main points:

Cyathostomins (aka cyathostomes, small strongyles, redworms): the most pathogenic group of worms for adult horses. Resistance to: benzimidazoles (BZs) (very common/widespread); pyrantel (tetrahydropyrimidines) (common); and also macrocyclic lactones (‘MLs’, avermectins-milbemycins) (’emerging resistance’ in Australia and elsewhere).

Parascaris equorum (the so-called ’roundworm’ of foals): the most pathogenic worm of foals. Resistance: commonly resistant to MLs.

What to do (based largely on Coles (see below)):

* Targeted treatment

Only treat horses with worm egg counts above a certain threshold (Coles suggests more than 200 eggs per gram of faeces).

* Know the resistance status of your horse worms

Do a faecal egg count reduction test (FECRT). Coles suggests repeating a faecal worm egg count (FWEC) after dosing to ensure that the drug used has been effective. The interval between treatment and the second test depends on the drug used:

pyrantel: the second test should be carried out one week after dosing. If the FWEC is not reduced by at least 90%, then the worms are resistant to pyrantel.

benzimidazoles: second test should be 2 weeks after treatment. Resistance is present if there is not at least 95% reduction in FWEC.

ivermectin: the second test should be 3-4 weeks after treatment. Resistance is present if there is not at least 99% reduction in FWEC.

( I am not sure why Coles applies different standards (90% c.f. 95% etc) to actives from different families. – SL).

* Don’t buy in resistance

Newly arrived horses: keep in a stable or quarantine paddock until they have been shown not to carry resistant worms. A quarantine treatment involving concurrent dosing with unrelated actives (as recommended in sheep for example) could be an option. Get expert advice.

*Use alternative methods of worm control wherever possible

According to Coles (UK): Collecting faeces twice a week in summer, or once weekly in winter, can reduce pasture contamination.

Graze the pasture with cattle or sheep. They can vacuum up the equine worm larvae. Avoid over-grazing.

* Combinations of unrelated anthelmintics

Corning (see below) refers to this.

A common recommendation for grazing ruminants (at least in Australia) is to rotate between unrelated anthelmintics (know/shown to be effective on the farm) or better, to use combinations i.e. concurrent use of unrelated anthelmintics with similar spectra of activity (and ideally with similar persistency, and optimally still highly effective on their own!).

For more on combinations: see http://www.wormboss.com.au/news/articles/drenches/understanding-drenches-mixtures-combinations-and-both.php

A note on terminology

When discussing worms of grazing livestock, mainly ruminants, ’roundworm’ is the common name for nematode. However, in horse parasitology, ’roundworm’ can have a more restricted meaning, i.e. ‘ascarid’, i.e. Parascaris equorum. P.equorum is a large round worm.

Some horse-related excerpts from Kaplan, 2004

(TRENDS in Parasitology Vol.20 No.10 October 2004, Drug resistance in nematodes of veterinary importance: a status report Ray M. Kaplan):

‘Resistance in nematodes of horses and cattle has not yet reached the levels seen in small ruminants, but evidence suggests that the problems of resistance, including MDR (multiple drench resistant) worms, are also increasing in these hosts.

‘The initial reports of anthelmintic resistance were to the drug phenothiazine in the late 1950s and early 1960s, first in Haemonchus contortus (barber pole worm) of sheep [3] and then in cyathostomins (small strongyles) of horses [4–6].

‘In 1961, thiabendazole was introduced as the first anthelmintic that combined efficacious broad-spectrum nematocide activity with low toxicity. The rapid acceptance and widespread use of thiabendazole and then other
benzimidazole anthelmintics marked the beginning of the modern chemical assault on helminth parasites. However, within a few years, resistance to thiabendazole was reported, again first in the sheep nematode H. contortus [7,8] and then in the equine cyathostomins.

… by the mid- 1970s, multiple-species nematode resistance to benzimidazole anthelmintics was common and widespread in both sheep and horses throughout the world.’

Another consideration is the fact that reversion to susceptibility does not seem to occur, meaning that resistance is essentially everlasting.

Less attention has been given to the problem of anthelmintic resistance in cyathostomin nematodes of horses (now considered the principal parasitic pathogen of adult horses), although several studies have reported a prevalence of resistance to benzimidazole drugs greater than 75% [14].

Resistance to pyrantel (tetrahydropyrimidine class) appears to be much less common, but a recent study in southern USA found that over 40% of farms demonstrated resistance to this drug [34]. Interestingly, there are still no reports of cyathostomin resistance to ivermectin, despite over 20 years of use as the most commonly administered anthelmintic drug. (No longer the case. – SL) One theory that is frequently proposed to explain the lack of resistance to ivermectin is the inability of this drug to kill mucosal larval stages of cyathostomins [14]. These mucosal larval stages tend to be much more numerous than the adult worms in the lumen, and therefore provide a large refugia*.

By contrast, there have been two recent reports of suspected ivermectin resistance in Parascaris equorum, which is the most important parasitic pathogen of foals [36,37]. These reports have not yet been confirmed with controlled efficacy studies, but P. equorum is the dose-limiting parasite for ivermectin in horses, therefore resistance might be expected to develop more quickly to this worm. The apparent excellent efficacy that avermectin–milbemycin drugs continue to have against the major strongyle nematode parasites of horses seems to have lulled this industry into a false sense of security. Considering the growing reliance upon this class of drugs, and the fact that avermectin–milbemycin resistance is becoming increasingly common in gastrointestinal nematode parasites of small ruminants and cattle, most equine parasitologists suspect that resistance in cyathostomins is inevitable [38–40].

Note: there is now evidence of ML (avermectin-milbemycins) resistance of cyathostomins. Likewise ML resistance is no longer merely ’emerging’ in worms of sheep and other ruminants (SL).

* Kaplan’s footnote on refugia: *Refugia is a term used to describe the proportion of a parasite population that is not exposed to a particular drug, thereby escaping selection for resistance. In practical terms, refugia are supplied by: (i) stages of parasites in the host that are not affected by the drug treatment; (ii) parasites residing in animals that are left untreated with a particular drug; and (iii) free-living stages in the environment at the time of treatment. Many parasitologists now consider levels of refugia as the single most important factor involved in the selection of anthelmintic-resistant parasites (see Ref. [35] for a more detailed discussion on refugia).

SL: Note that this review of Kaplan’s was published 9 years ago, in 2004. The situation has changed (worsened ) since then. The two aspects, certainly with respect to Australia, that come readily to mind are the (apparent) increase in prevalence of resistance of cattle nematodes to anthelmintics, and the increased prevalence of resistance even to moxidectin, at least in small ruminants. ML resistance of cyathostomins has been reported.

Equine cyathostomins: a review of biology, clinical significance and therapy- Susan Corning http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2751837/ Parasit Vectors. 2009; 2 (Suppl 2): S1.

‘Some (drug-related) excerpts from Corning, but don’t ignore discussion on ‘non-chemical’ control options! (Read the full article at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2751837/ ).

Here are some excerpts from this report of a talk by Dr Gerald Coles at the Thoroughbred Breeding and Racing Seminar held in Cheltenham in November 2006. (United Kingdom)

The small strongyles of horses, also known as cyathostomins, are considered the most prevalent and pathogenic parasites of horses today.

The clinical syndrome of larval cyathostominosis which occurs as a result of mass emergence of inhibited stages has a high fatality rate despite the best standard of care given to affected horses. Management of the challenge level of cyathostomins to prevent the syndrome is preferable. Many different management programmes have been tried over the past two decades, with mixed success. Programmes have relied heavily on repeated use of anthelmintic treatments throughout the life of a horse. The widespread incidence of resistance to certain anthelmintics is reducing these options. An understanding of the biology of cyathostomins, risk factors for infection and appropriate strategic use of still effective anthelmintics is essential for the future management of this parasite group. This review highlights the necessity to use currently available anthelmintics that are appropriately suited to the biology of cyathostomins, and to maintain heir efficacy through an appropriate treatment strategy.


There are three available drug classes for cyathostomin control in horses, the benzimidazoles such as fenbendazole and oxfendazole, the tetrahydropyrimidines which are the pyrantel salts, and the macrocyclic lactones (ML), ivermectin and moxidectin. All of these drugs have differing levels of efficacy, duration of activity and spectrum of stages of cyathostomins they control. The ML class of drugs has become ever more widely used due to their potency, spectrum of activity, relative safety, and as yet few reports of resistance.

In the case of fenbendazole, the recommended dose of 5 mg/Kg liveweight will control sensitive strains of adult and developing larval stages of small strongyles. For control of inhibited stages a daily dose of 10 mg/Kg liveweight for 5 consecutive days is recommended. Fenbendazole resistance has been recognised as being widespread in all major horse populations surveyed and use of this compound at either dosage regimen should be avoided where resistance occurs [4551].

Pyrantel salts are not effective against inhibited stages of small strongyles but will remove sensitive strains of adults. Resistance to pyrantel salts has been identified both in Europe and the US, but does not appear to be as widespread as resistance to benzimidazoles [5257].

As a general caution, unless sensitivity has been demonstrated by a faecal egg count reduction test, use of benzimidazole or pyrantel based anthelmintics carries the risk that treatment will be ineffective [5861].

The two compounds within the macrocyclic lactone group need to be considered separately due to significant differences in potency and spectrum. The first available ML for horse, ivermectin, is highly potent against adult stages, luminal larval stages and developing stages of larvae in mucosa, but has variable and low efficacy against inhibited stages, even when elevated doses (5×) are administered [6264].

Moxidectin, in addition to having high efficacy against all cyathostomin stages given as a single dose at a rate of 0.4 mg/Kg liveweight [6266] also provides persistent activity against re-infection by small strongyles [67], resulting in a long egg re-appearance interval. The required re-treatment interval with moxidectin is longer than that for other anthelmintics allowing less frequent treatment and less selection for resistance [44,57,6875].


We are fortunate that the last two decades have seen a large body of research, information and understanding of the complex issues surrounding the life cycle, clinical significance and control of small strongyles in horses. We have the opportunity to apply this knowledge to develop better control programmes than have been implemented in the past.

Monahan summed the situation well by stating in 2000: “Rote memorization of treatment schedules and anti-parasitic drugs without understanding the biology of the worms to be controlled concedes any intellectual advantage to the worms” [44].

These Ain’t Your Father’s Parasites: Dewormer Resistance and New Strategies for Parasite Control in Horses. Ray M. Kaplan.

Presented at Florida Equine Institute and Allied Trade Show, September 17, 2009 http://lacs.vetmed.ufl.edu/files/2011/12/kaplanparasites.pdf

Some excerpts:

From Summary:
The common practices of recommending the same treatment program for all horses despite great differences in parasite burdens, of recommending frequent preventive treatment of all horses without any indication of parasitic disease or knowing what species of parasites are present, of recommending the use of drugs without knowledge of their efficacy, of failing to perform fecal egg count surveillance and of failing to determine if treatments are effective, are all incompatible with achieving optimal and sustainable parasite control.

Anthelmintic Resistance: A growing threat to parasite control and equine health

Currently used parasite control programs are almost completely dependent upon the intensive use of dewormers. Presently there are three major chemical classes of dewormers used to control nematode parasites in horses:

benzimidazoles (fenbendazole – Safeguard®, Panacur®,oxfendazole — Benzelmin®, oxibendazole – Anthelcide EQ®),

tetrahydropyrimidines (pyrantel salts – Strongid®, others), and

avermectin/milbemycins (ivermectin — Eqvalan®, Equimetrin®,Zimectrin®, others; and moxidectin – Quest®; note that avermectin/milbemycins are also referred to as macrocyclic lactones).

Of these three drug classes, resistance to BZ is the most prevalent and widespread, with reports of resistance from over 21 countries. Reports of resistance to pyrantel are less common, but the true prevalence of resistance in most of the world is unknown.

In 2001-2002, a large multi-state study was performed to determine the prevalence of resistance on horse farms in the southern United States (Kaplan, Klei et al. 2004). 1274 horses on 44 large stables in Georgia, South Carolina, Florida, Kentucky, and Louisiana were tested in this study. The fecal egg count reduction test (FECRT) was performed on each farm using 4 different dewormers: fenbendazole, oxibendazole, pyrantel pamoate and ivermectin. Resistance testing was only done for the small strongyles. Using fairly conservative criteria that were chosen to minimize the chance that a farm would be designated as having resistant worms if resistance was not present, the percent of farms found to harbor resistant worms were as follows:

97.7% for fenbendazole,

0%, for ivermectin,

53.5% for oxibendazole and

40.5% for pyrantel pamoate.

In terms of actual reductions in fecal egg counts (FEC), the mean percent reductions for all farms were

24.8% for fenbendazole,

99.9% for ivermectin,

73.8% for oxibendazole and

78.6% for pyrantel pamoate.

With the exception of ivermectin, these values are far below the levels needed for effective worm control. Interestingly, statistical analysis between states for each treatment revealed that in almost all instances there
were no statistical differences in results between states.

The prevalences of resistance to fenbendazole, oxibendazole, and pyrantel pamoate found in this study were far greater than in any previously published report. Furthermore, results from all 5 southern states were remarkably similar despite major differences in the types of farms and in physical geography. This suggests that drug resistance in small strongyles is highly prevalent throughout the entire southern United States and probably nationwide.

These results indicate the following:

(1) that drug resistance in small strongyles is much more common than is commonly recognized,

(2) that the problem of anthelmintic resistance in small strongyles is worsening, and

(3) anthelmintic resistance may be more severe in the United States than elsewhere in the world.

It is interesting to note that the high prevalence of resistance to pyrantel pamoate found in this study has not been detected in studies performed outside the United States. Many parasitologists have suspected that low-dose daily feeding of pyrantel may lead to resistance. Because the United States and Canada are the only countries in which daily feeding of low-dose pyrantel tartrate is practiced, one must wonder whether this
mode of administration is having a major impact on the selection for resistance to pyrantel.

The results of this study indicate that a serious situation is emerging for small strongyle control in horses. More than 40% of all farms tested had small strongyle populations that are resistant to fenbendazole (FBZ), oxibendazole (OBZ) and pyrantel pamoate (PP), meaning that on almost half of all farms, only a single drug class (avermectin/milbemycin) that has been in use for 25 years remains effective.

However, in the past 3 years there have been reports out of the United Kingdom, Australia and Brazil indicating the presence of avermectin/milbemycin (AM) – resistant small strongyles.(emphasis mine-SL) AM-resistant small strongyles have not yet been reported in the US, but recently a report from the University of Kentucky indicates reduced activity of ivermectin which is suggestive of early resistance (Lyons, Tolliver et al. 2008). These reports are not surprising – what is surprising is how long it took before resistance developed.

AM resistance is extremely common and widespread in closely related parasites of sheep and goats; in the southeastern US the prevalence of ivermectin resistance in Haemonchus contortus (barber pole worm) is approximately 90%. Reports of AM resistance in parasites of cattle also are becoming increasingly common. Furthermore, numerous reports suggest that AM resistance is quite common in roundworms (Parascaris equorum) of horses, which is the most important parasite of foals.

Given the fact that resistance to ivermectin and moxidectin in the small strongyles may already be present in Kentucky, the appropriate solution to the problem of drug resistance is not to simply use more ivermectin and moxidectin because these drugs continue to be effective against small strongyles. Testing should be done on each farm to determine which drugs work and which do not. Since OBZ is considerably more effective than FBZ, OBZ should be used in all instances in place of FBZ for single dose usage.

An alternative to AM anthelmintics may be using drug combinations of OBZ and PP.

We recently completed a study to investigate whether combined use of OBZ and PP would produce clinically significant increases in efficacy as compared to the use of these drugs individually. On 10 of the 12 farms the combination treatment was highly effective in reducing FEC compared to the drugs used singly (Kaplan, Menigo et al. 2005). These data suggest that on most farms using OBZ and PP in combination results in clinically significant increases in efficacy, and produces a very high level of FECR.

Overall, based on these results, routine use of OBZ and PP in combination may be considered when the drugs are not highly effective individually. This may be especially important when treating foals since ivermectin and moxidectin resistance appears to be increasingly common in roundworms (Parascaris equorum). However, as always, the effectiveness of these drugs needs to be evaluated before relying upon them, and then monitored by periodic surveillance of FEC pre and post treatment.

These resistance issues need to be appreciated in the context of what can be expected in the future with regard to development and marketing of new dewormers (meaning completely new drug classes, not just new products of existing classes). The great cost associated with the development of new drugs has greatly reduced investment into discovery and development of new dewormers. Of promise is discovery of a new dewormer recently announced by Novartis, but it is unlikely that an equine product will be marketed in the near future, and it is possible that it may never be marketed for horses. Also of relevance is the fact that any new dewormer products are almost certain to be much more expensive that current products.

Therefore, as we diagnose AM resistance with increasing frequency, options for control with dewormers will be quite limited as there will likely be a delay of many years before any new drugs are available.
The increasingly high prevalence of anthelmintic-resistant small strongyles must therefore be taken into account when designing worm control programs for horses. It is strongly recommended that prior to using a BZ drug or pyrantel on a horse farm that a FECRT be conducted to rule out the presence of drug-resistant worms on that property. Furthermore, ivermectin resistance could appear at any time.

Equine Science Update – Resistant worms. Report by Mark Andrews. Published 2006.

Here are some excerpts from this report of a talk by Dr Gerald Coles at the Thoroughbred Breeding and Racing Seminar held in Cheltenham in November 2006. (United Kingdom)

How common is resistance? we don’t know, (Dr Coles) says but the number of reports of anthelmintic resistance has been increasing.

Most significant: the problem of resistance in the cyathostomins, or small redworms. Resistance among cyathostomins to the benzimidazole group of wormers is believed to be widespread. Resistance to pyrantel is much less common, at least in the UK, although it is common in the USA.

There are signs now (2006) of emerging resistance to the third major group of anthelmintics, the macrocyclic lactones (MLs).

2005 Conference of the World Association for the Advancement of Veterinary Parasitology in New Zealand: evidence presented suggesting cyathostomins were developing resistance to ivermectin and moxidectin.

In 1999, Dr Coles reported pyrantel-resistant large strongyles. Earlier this year (2006) he reported the death of a foal infected with ivermectin-resistant Parascaris equorum.

Parascaris equorum, the large roundworm of foals, can kill before eggs are found in the faeces. Parascaris equorum is commonly resistant to ivermectin. In foals that are infected with ivermectin-resistant Parascaris, Coles suggests using a five-day course of fenbendazole.

Coles: The macrocyclic lactones (MLs: ivermectin, moxidectin) are superb drugs but not for much longer. So how can we slow the development of resistance to the MLs?

He advises:

* Concentrate chemotherapy on wormy horses. Only treat horses with more than 200 eggs per gram of faeces.

* Know the resistance status of your horses. Do a faecal egg count reduction test (FECRT). Coles suggests repeating a faecal worm egg count (FWEC) after dosing to ensure that the drug used has been effective. The interval between treatment and the second test depends on the drug used:

pyrantel: the second test should be carried out one week after dosing. If the FWEC is not reduced by at least 90%, then the worms are resistant to pyrantel.

benzimidazoles: second test should be 2 weeks after treatment. Resistance is present if there is not at least 95% reduction in FWEC.

ivermectin: the second test should be 3-4 weeks after treatment. Resistance is present if there is not at least 99% reduction in FWEC.

(Comment: given that moxidectin appears to be longer acting (than ivermectin) in horses, as in sheep, I am not sure when the second test should be done if this drug is used. Perhaps at 3-4 weeks post-treatment, as with ivermectin, followed by a third test a number of weeks later? -SL)

* Don’t buy in resistance.

Newly arrived horses: keep in a stable or quarantine paddock until they have been shown not to carry resistant worms.

If the horse has pyrantel-resistant worms it may be possible to remove them by treating with moxidectin followed by five days of fenbendazole. However, if ivermectin-resistant worms are identified Coles recommends getting rid of the horse. At the very least the horse should remain in a separate paddock.

*Use alternative methods of worm control wherever possible.

Collecting faeces twice a week in summer, or once weekly in winter, can reduce pasture contamination. Graze the pasture with cattle or sheep. They can vacuum up the equine worm larvae. Avoid overgrazing.

ML-resistant round worm – Parascaris – Australia

Inside Racing – January 2013. Drug-resistant parasites in horses are an increasing threat. Dr Sally Church. http://www.equinecentre.unimelb.edu.au/docs/parasites.pdf

‘The prophecy is always with us: overuse of deworming drugs will lead to the development of worms resistant to those drugs. By Dr Sally Church, BVSc, FANZCVS

The latest worm to develop drug resistance in horses is the round worm (Parascaris equorum). Round worms resistant to the macrocyclic lactone drugs (MLs) are appearing with increasing frequency. The MLs are drugs such ivermectin, moxidectin or any other drug ending in ‘mectin’. The horse industry relies on this group of drugs more than any other to kill horse parasites. MLs were the drugs that rescued us decades ago when worms were developing resistance to benzimidazoles. It has taken many years for parasites to develop resistance to MLs, but the long honeymoon is starting to end. Unique in the world of horse parasites, round worms do not cause a problem for mature horses. Parascaris equorum wreaks havoc only in young horses: foals, weanlings and yearlings.’

Read the rest of the article here: http://www.equinecentre.unimelb.edu.au/docs/parasites.pdf

Macrocyclic Lactone resistance in cyathostomins and Parascaris equorumon Australian horse farms

Authors: Beasley A, Coleman G, Kotze A. WAAVP Perth August 2013

Cyathostomins are of major importance to the equine industry as agents of clinical disease (e.g. larval cyathostominosis, colic). The ascarid worm, Parascaris equorum, is common in young horses and it too is associated with disease but more commonly impacts on growth during the critical first 12 months of life.

Anthelmintic resistance is already common in the cyathostomins, particularly with regard to benzimidazole compounds. Resistance to Macrocyclic Lactone (ML) drugs has been documented in P. equorurm populations on European, Canadian and American horse farms and is likely a result of their frequent and often exclusive use in interval treatment regimes.

ML-resistance is also an emerging issue in cyathostomin control, however, data on surveillance of drug efficacy on Australian horse farms is lacking. With no new drug class expected on the equine market in the near future, it is crucial that the horse industry adopts more sustainable worm control practices in order to preserve this drug class for use into the future.

We are currently investigating the efficacy of MLs against P. equorum and cyathostomins using faecal egg count reduction tests (FECRT), on farms from several states. Preliminary results indicate that resistance is present in P. equorum. No evidence of cyathostomin resistance has been found, however, isolated cases of a shortened egg reappearance period following ML treatment have been observed.

Whilst the FECRT is the “gold standard” technique for detection of resistance, it has well-recognised limitations and consequently there is a need to develop cost-effective and reliable tools to assist early detection of resistance. This project also aims to develop and/or evaluate a number of in vitro assays to identify differences in susceptibility of equine worm populations to ML anthelmintics. Of primary importance is the development of an assay for use in determining ML-resistance within P. equorum isolates.

Pook, J.F., Power, M.L., Sangster, N.C., Hodgson, J.L., Hodgson, D.R., 2002. Evaluation of tests for anthelmintic resistance in cyathostomes Veterinary Parasitology, 106: 331–343
Boersema, J.H., Eysker, M., Nas, J.W.M., 2002. Apparent resistance of Parascaris equorum to macrocylic lactones. Veterinary Record, 150: 279–281.
Kaplan, R.M., Vidyashankar, A.N., 2012. An inconvenient truth: Global worming and anthelmintic resistance. Veterinary Parasitology 186: 70-78103.

General information on worm control in horses: NSW DPI Primefact: “Worm control in horses”- Dr Sarah Robinson http://www.dpi.nsw.gov.au/agriculture/livestock/horses/health

Reversible blindness in human self-administering closantel


Britney Spears Songs Used as a Defence Against Somali Pirates


A plague on both houses


Specialist: one who knows more and more about less and less ’till s/he knows everything about nothing. (S.Love, pers comm, 1998 (Describing himself)).





This message is intended for the addressee named and may contain confidential information. If you are not the intended recipient, please delete it and notify the sender. Views expressed in this message are those of the individual sender, and are not necessarily the views of their organisation.

WRML.20131104. Lower WECs associated with higher condition scores in ewes. Kahn Dever Bowers.

To WormMail mailing list (recip. undisclosed)

WRML.20131104. Lower WECs associated with higher condition scores in ewes

One of the interesting papers – forwarded with permission- from the WAAVP Conference at Perth recently.

I do not have the full paper/references. Questions are best directed to the authors.

Do not over-generalize the results. Consider the context of the experiment e.g the location, and other details.

For example, will one condition score unit always be equivalent to ~ 500 worm eggs per gram of faeces? I don’t think so.



This message is intended for the addressee named and may contain confidential information. If you are not the intended recipient, please delete it and notify the sender. Views expressed in this message are those of the individual sender, and are not necessarily the views of their organisation. 

Kahn Dever Bowers.2013.Lower WECS assoc with higher ewe BS.WAAVP Perth.pdf