WRML: Lachlan LHPA NSW drench resistance (sheep worms) survey-preliminaryresults

TO: WormMail mailing list (recipients undisclosed).   WRML.20120726.Lachlan LHPA NSW drench resistance (sheep worms) survey-preliminary results.

The District Veterinarians in the Lachlan Livestock Health and Pest Authorities (LHPA)  are doing a drench resistance survey (sheep worms).

If you don’t know about LHPAs, or where the Lachlan LHPA is, see here:  http://www.lhpa.org.au/districts/lachlan

The vets involved are Drs Elizabeth Braddon (Young), Belinda Edmonstone (Forbes), and Katharine Marsh (Condobolin).

Here, with permission, are the preliminary results ( to May 2012) by way of a summary (press release)  from Liz:

"Preliminary drench trial results are in

Written by Elizabeth Braddon, Lachlan LHPA senior district veterinarian

The Lachlan LHPA has been undertaking a drench resistance trial to determine, firstly, if we have any worm resistance issues; and secondly, how much and in which species. This trial involves 64 properties from all over the Lachlan LHPA. So far we have done 43 properties and hope to complete the rest by the end of April.

We are testing three drenches: naphthalophos/BZ/LEV (e.g., Rametin combo, Combat etc); abamectin (Virbamec, Noromectin etc); and abamectin/BZ/LEV (e.g., Hat-trick, Tri Guard, Pyrimide etc).

We have all the “bad” worm species represented (barber’s pole worm, black scour worm, small brown stomach worm). The results so far have been somewhat surprising.

Barber’s pole worm (Haemonchus):

·        43% of properties to date are showing significant resistance to abamectin drench when it comes to barber’s pole worm.

In the current season, this is very concerning as we are finding losses from barber’s pole after drenching with mectin drenches as producers consider these generally a very effective drench for this area. This may no longer be the case where barber’s pole is concerned.

·        The good news is that we do still have effective drenches for these worms. Naphthalophos drenches, closantel based and mectin/BZ/Lev products are still working well.

Small brown stomach worm (Teladorsagia):

·        20% of farms so far are showing resistance to abamectin drench and this worm. This is the worm that we felt was most likely to be causing some problems. This is one to watch and indicates that our mectin single active drenches are struggling and will need looking after from now on.

·        This worm has varying resistance to naphthalophos/BZ/Lev combination. This is not a great surprise as naphthalophos based drenches are more specifically aimed at barber’s pole with the BZ/LEV components aimed at the scour worms. BZ/LEV combination or singly have shown anywhere from 60-80% of properties having resistance from past studies.

Black scour worm (Trichostrongylus):

·        10% of farms are showing resistance to abamectin and black scour worms. Again this indicates that single active products may be struggling against our normal worm species and we will have to look after them.

·        Naphthalophos/BZ/LEV combination had a similar profile as for the small brown stomach worm.


·        Worm testing pre- and 10 to 14 days post-drenching is a quick and easy way to check if your drench is effective. You can do a resistance study all at once or you can build up a profile for your farm by doing post-drench checking each year at the annual drench rotation.

·        With the high activity of barber’s pole worm this year, a triple Mectin drench, monepantel or specific barber’s pole active in combination would be a prudent choice.

Please contact any of the LHPA veterinarians if you need advice on worm control."



BZ=benzimidazole group of broad-spectrum drenches

LEV=levamisole-morantel group of broad-spectrum drenches

Naphthalophos: an organophosphate (OP) drench. Generally classed as mid-spectrum (highly effective against adult Haemonchus but only moderately effective (e.g. 70-90%) against Teladorsagia and Trichostrongylus spp., and immature Haemonchus).

The other OP sheep drench on the market in Australia is pyraclofos (marketed in combination with a BZ (‘Colleague’)).

‘Mectin’= the macrocyclic lactone group of broad-spectrum drenches. Includes (in ascending order of potency with respect to sheep worms): ivermectin and abamectin (both avermectins and both short-acting) and moxidectin (a milbemycin).

Monepantel: a member of the novel (new) drench group, the AADs (aminoacetonitrile derivatives). Derquantel, representing another novel group, the spiroindoles, has not yet been released in Australia.

ML-resistant Trichostrongylus in sheep: there appears to be mounting field evidence for this in various parts of Australia (Love S, Australian Sheep Veterinarians’ Conference, 2011). To confirm these however requires identification of adult worms to confirm the findings of larval differentiation. It is well-accepted however that resistance of Haemonchus and Teladorsagia to the MLs is common/widespread in the Australian sheep industry, especially in summer rainfall areas such as north eastern NSW / south-eastern Queensland (Haemonchus) and Western Australia and other winter dominant rainfall areas (Teladorsagia).

Closantel-resistant Haemonchus are very common (>80% of farms) in NE NSW / SE Qld.



WRML. why lambing ewes are susceptible to worm infection (periparturient relaxation of resistance) – REDUX 2 (Kahn, Leathwick and Karlsson)

TO WormMail mailing list (recip. undisclosed)

WRML.20120726.why lambing ewes are susceptible to worm infection (periparturient relaxation of resistance) – REDUX 2 (Kahn, Leathwick and Karlsson)

‘Some more information, this time from John Karlsson (Western Australia). This wraps up these WormMails (three) on this topic for now.

For information on Rylington Merinos and selection for worm resistance, see:   http://www.agric.wa.gov.au/PC_91868.html?s=0

Rylington Park is located in the Western Australian Shire of Boyup Brook.  – SL.

"Hi Steve

  A few comments from a winter rainfall environment based on ‘Rylington Merino’ (Australia’s most worm resistant Merino flock).

  1.     A. R. Williams, J. C. Greeff, P. E. Vercoe, R. J. Dobson and L. J. E. Karlsson (2010) Merino ewes bred for parasite resistance reduce larval
contamination onto pasture during the peri-parturient period. Animal 4:1, pp 122–127.

            This reports on; host and parasite genotype effect, fecundity effects, in a set stocking environment.
2.     J. Greeff, L Karlsson and Noreen Underwood (2006) BREEDING MERINO SHEEP FOR WORM RESISTANCE INCREASES
PROFIT IN A MEDITERRANEAN ENVIRONMENT. 8th World Congress on Genetics Applied to Livestock Production, August 13-18, 2006, Belo Horizonte, MG, Brasil

This paper follows the progeny of the ewes reported on in Williams et al. above through to hogget age and is one of the first effort to derive an economic value of resistance to worms and a lower periparturient rise.

  3.     K.E. Kemper, D.G. Palmer, S.M. Liu, J.C. Greeff, S.C. Bishop, L.J.E. Karlsson (2010)  Reduction of faecal worm egg count, worm numbers and worm fecundity in sheep selected for worm resistance following artificial infection with Teladorsagia circumcincta and Trichostrongylus colubriformis. Veterinary Parasitology 171  238–246

     This paper is also based on the Rylington sheep but using males and artificial challenge in a pen trial. Again shows both host and parasite genotype effects. It also shows important parasite species differences in their ability in coping with the host immune response with Teladorsagia in the best position to exploit a suppression of the host immune response.

4.     S. M. Liu, T. L. Smith, D. G. Palmer, L. J. E. Karlsson, R. B. Besier and J. C. Greeff (2005)  Biochemical differences in Merino sheep selected for resistance against gastro-intestinal nematodes and genetic and nutritional effects on faecal worm egg output.  Animal Science  81: 149-157.

This paper as the title describes looks at biochemical differences in the sheep referred to by Kemper et al above. The relevance to this discussion is that under immune challenge there is a strong demand from the immune system for the sulphur-containing amino acids and they are also in high demand from wool production. This may explain some of the apparent differences in the periparturient rise comparing the Merino and NZ sheep.

John K            jkarlsson[AT]agric[dot]wa[dot]gov[dot]au   "




Previous two WormMails on this topic follow:

  From: stephen.love….
Monday, 16 July 2012 1:42 PM
WRML. why lambing ewes are susceptible to worm infection (periparturient relaxation of resistance) – REDUX (Kahn and Leathwick)

  To WormMail mailing list (recip. undisclosed).   WRML.20120716.why lambing ewes are susceptible to worm infection (periparturient relaxation of resistance) – REDUX (Kahn and Leathwick)

Further to the WormMail of 6 July [ https://wormmailinthecloud.wordpress.com/2012/07/06/wrml-why-lambing-ewes-are-susceptible-to-worm-infection/  ], with Dr Lewis Kahn’s article on periparturient relaxation of resistance (P(P)RR), here are some further comments from Dr David Leathwick of Palmerston North, New Zealand and Dr Lewis Kahn of Armidale, NSW, Australia.  

Comments from
D Leathwick (Agresearch, Palmerston North, NZ)

Steve, I would like to point out that the situation below (see article of 6 July  below)only applies (I believe) to Merinos.

This issue was raised some years ago when we proposed, based on modelling, that anthelmintic treatments to ewes around lambing would significantly increase selection for resistance.  

When the equivalent drenching scenarios were run through the Australian Wormworld (computer)model by Ian Barger, the answers were quite different.  Ian’s conclusion was that the different outputs from the models was a direct reflection of the assumptions regarding the relaxation of immunity around lambing – the Australian model (for Merinos) used a large and protracted loss of immunity until late in lactation, while the New Zealand model (Romneys) allowed for only a very limited relaxation of immunity in the weeks either side of lambing.  

This story can be found in:

Barger I.A.  Models as a guide to sustainable worm control. Chapter 16 in Barrell G.K. (Ed.) Sustainable control of internal parasites of ruminants.  Animal Industries Workshop, June 1997, Lincoln University, Canterbury NZ, 203-213.

We subsequently measured the establishment of challenge infection in Romney ewes after lambing and found that establishment rates, although relaxed for a short interval, returned to low levels (<1%) within a few weeks of lambing – this can be found in:

Leathwick D.M., Miller C.M., Brown A.E. & Sutherland I.A.  1999.  The establishment rate of Ostertagia circumcincta and Trichostrongylus colubriformis in lactating Romney ewes.  International Journal for Parasitology 29: 315-320.

Not surprisingly this results in a different picture of worm dynamics than that described below (L Kahn’s article).

We subsequently tested the model hypothesis that drenching ewes pre- or post-lambing would accelerate the development of resistance in a 5 year field trial, which showed that the hypothesis was true for Ostertagia, T. colubriformis and H. contortus, thereby supporting the structure of the model. See:

Leathwick DM, Miller CM, Atkinson DS, Haack NA, Alexander RA, Oliver A-M, Waghorn TS, Potter JF, Sutherland IA. 2006 Drenching adult ewes: Implications of anthelmintic treatments pre- and post-lambing on the development of anthelmintic resistance.  New Zealand Veterinary Journal 54 (6): 297-304

Anyway, I think you should clarify that the description of immunity relaxation outlined below is unlikely to represent all countries and/or breeds of sheep; it is certainly not the case in New Zealand.

Cheers Dave

Response from
L Kahn (UNE, Armidale, Australia)

Hi Steve and Dave,

Thanks Dave for your email which is very timely as we are just embarking on some new work with crossbred  (Border Leicester (BL) x Merino (Mer)) ewes and so I have been thinking about breed differences in worm resistance.  

When writing the article for WormBoss I did indeed have Merino ewes in mind and I should have made comment that the Merino represents the most extreme example of the loss of immunity at parturition and during lactation.  Having said that, loss of immunity associated with reproduction has also been reported in other species (pig, cow, mouse, etc) and so the periparturient effect has some general basis.  I did not have a copy of your research note and this very nicely contrasts the situation with your Romney ewes and what has been reported for Merinos.  It is interesting that an early report on the PPR came from Romney ewes:

Brunsdon R.V. (1964) The seasonal variations in the nematode egg counts of sheep: a comparison of the spring rise phenomenon in breeding and unmated ewes. New Zealand Veterinary Journal 12:75-80.

We are expecting our BL x Mer ewes to be more worm resistant than Merino ewes (Donald A.D., Morley F.H.W., Waller P.J., Axelsen A., Dobson R.J., Donnelly J.R. (1982) Effects of reproduction, genotype and anthelmintic treatment of ewes on Ostertagia spp. populations. International Journal for Parasitology 12:403-411), but as few BL studs in Australia place any emphasis on worm resistance, and because there is still 50% Merino, we are also expecting considerable within-cross variation.  We are only just starting with the new project but already we have recorded worm egg count values in BL x Mer ewes comparable to the range that we would expect for Merino sheep.

On the matter of linkage between host resistance and drench resistance: even though I was writing principally about susceptible Merino ewes, we too are concerned about the potential for pre-lambing treatments to select more strongly for drench resistance (whereas Barger et al were not).  In our case, where lambing typically comes in Spring and at a time of year where L3 on pasture are at a low point (and made lower through deliberate management in the previous Autumn) I suspect that the effect of the pre-lambing drench may be more similar to the situation of a resistant breed, where there are few incoming L3 to address the selective bias in favour of resistant worms.  In our case we look to manage this potential through within-farm quarantine practice at weaning (see more below-Ed.) as part of the drench choice annual strategy.

I also have an obvious eye to management of H. contortus evident in the WormBoss article and Steve has already (thank you) clarified this with readers.



Additional comments

D Leathwick:
 As an aside, one of the things I find intriguing from Brunsdon’s work is that the relaxation of immunity seems to be specific to some species. The best example for us (in NZ) would be Nematodirus which is extremely rare in the PPR (periparturient relaxation of resistance). Also the intestinal Trichostrongylus sp occur but are less dominant than Trichostrongylus axei, yet in lambs it is the reverse.  

L Kahn:
 I agree that the species specific nature of the PPR (periparturient relaxation of resistance) is intriguing.  For example, there are reports that the rise in Haemonchus contortus WEC with lambing is due principally to increased fecundity but we have demonstrated high establishment rates of incoming Haemonchus contortus 3rd stage larvae (L3) – in Merino sheep.


Some extra comments and notes:

"within-farm quarantine practice at weaning": this has been included in the revamp of WormBoss, of which Lewis Kahn of course is a part.   Essentially this entails treating lambs and ewes when they leave the lambing paddock (usually at weaning) with an effective short acting drench or combination of drenches unrelated to that which was used as a pre-lambing drench (if one was used). So, if a macrocyclic lactone drench was used pre-lambing, the ‘quarantine’ drench at weaning to ewes and lambs (weaners) might be, for example, an organophosphate-based combination (if highly effective on the property).

The other part of the strategy is to then dilute the ‘resistant’ worms’ left behind on the lambing paddock post-weaning. Most people assume this means putting cattle into the lambing paddock post-weaning to reduce numbers of sheep worm larvae. But, while this will reduce overall numbers, the proportions of resistant and susceptible worms will remain the same. The better option then is to put into the lambing paddock relatively wormy sheep that preferably were not last/recently drenched with the same or related drench active that was given to ewes  pre-lambing.   After a period during which resistant worms on pasture are ‘diluted’, then cattle can be put into the paddock to reduce numbers of sheep worm larvae.

‘lambing times’   Spring is indeed a common lambing time in much of Australia, not least in the summer rainfall zone of north eastern NSW and SE Queensland where Haemonchus (and haemonchosis) is endemic.   However autumn and early winter lambing is also frequently encountered in other sheep raising areas for various reasons. In some districts, spring and autumn lambing can both be common.

Haemonchus contortus: barber’s pole worm

Trichostrongylus spp. (intestinal species): black scour worm.   Trichostrongylus axei: stomach hair worm

Ostertagia (Teladorsagia) circumcincta: (small) brown stomach worm.

‘Low worm-risk paddocks’: clearly this is relative.  You might also call them ‘clean paddocks’ which could be misleading, especially as a ‘clean’ paddock in say the New England region of NSW in a wet summer has many more worm larvae on it than say a cereal stubble in December in parts of Western Australia or the Riverina of NSW.

Can we infer that Australians are generally more relaxed than their uptight cousins across the Tasman ?  🙂

John Cleese and Palmerston North



Somehow, I don’t think Cleese is smarting at all.

The WormMail of 6 July:

Subject: WRML: why lambing ewes are susceptible to worm infection

To: WormMail list (recip. undisclosed)        WRML.20120706. why lambing ewes are susceptible to worm infection

immunity, Kahn, lambing ewes, periparturient relaxation of resistance (PPRR), sheep, worms, McClure, WormBoss redevelopment

WormMails have been a bit thin on the ground of late as I have been tied up with other things.

By way of recompense, here is a nice article on ‘PPRR’ by Lewis Kahn, a nice follow on from Dr Sue McClure’s article on immunity.(See here: https://wormmailinthecloud.wordpress.com/2012/06/06/wrml-sheep-immunity-to-worms-mcclure/)

Some of you have had the good sense to subscribe to WormBoss Monthly News, and would have seen this article there in the June issue:


The article is republished here with permission.

"Why are lambing ewes susceptible to worm infection?

Dr Lewis Kahn, Associate Professor, Animal Science, University of New England, Armidale NSW, Australia

Adult sheep typically have reasonable levels of immunity to most types of round worm with their greatest struggle often being immunity against barber’s pole worm. A reasonable level of immunity will mean that no more than 5% of infective larvae are able to establish as adult worms in the sheep. For the lambing ewe, this situation starts to change as early as three weeks before lambing when ewes start to lose immunity. Loss of immunity gets continually worse over the next 8-12 weeks (until lamb marking or a little after) which makes lambing ewes highly susceptible to worm infection. Ewes that rear twin lambs suffer an even greater loss of immunity.

During this time 30-40% of infective larvae can establish as adult worms causing high worm egg counts in the ewes, lower milk production and higher worm challenge for lambs.

Over the years a number of reasons have been put forward to account for the loss of immunity in the lambing ewe. Research has demonstrated that rather than hormonal changes associated with lambing and lactation it is the supply of protein and energy that is most important for maintaining immunity. The energy and protein requirement of the lambing ewes increases two and threefold respectively in the 4 week period between lambing and peak milk production. It is common for increased requirements to outstrip the supply of energy and protein from pasture and the gap contributes to a loss of immunity. The larger the gap the bigger the loss of immunity and this explains why twin-rearing ewes are much more susceptible to worm infection.

The most effective ways to manage worm infection in lambing ewes are to ensure ewes have:

1.low worm egg count (by conducting a WormTest) prior to lambing or receive an effective drench;

2.access to low worm-risk paddocks which have not been contaminated with infected faeces for 2-5 months (shorter for warmer months and longer for colder months) prior to lambing;

3.increased genetic worm resistance by using worm resistant sires in a breeding program;

4.achieved target condition score 3 for lambing."

The times mentioned above (2-5 months) relate particularly to barber’s pole worm (Haemonchus contortus) and the New England region of NSW.

‘WormTest’ is faecal worm egg counts (with or without larval culture/differentiation (‘worm type’)).

‘Infected faeces’ in point 2 above refers of course to faeces containing sheep worm eggs. i.e. grazing with cattle, for example, is not excluded.

By the way, WormBoss is undergoing redevelopment, part of which involves a new and better WormBoss website. ‘Shouldn’t be too long now…..