In this issue of WormMail (WRML):
- Resistance of Oxyuris equi?
- Off-label use of anthelmintics in horses – a NZ case study
- Real men don’t need instructions
- Pinworm and Kemo Sabe
- Using sheep as wi-fi hotspots
Get up-to-date on horse worm control
On Melbourne Cup Day in 2013, I cobbled together a relatively non-expert post on resistance of horse worms to anthelmintics.
Here it is: https://wormmailinthecloud.wordpress.com/2013/11/05/wrml-resistance-of-important-horse-worms-to-anthelmintics-closantel-tox-in-human-britney-repels-pirates-etc/
I have recently updated that post, suggesting that those interested in horse worm control could do worse than consulting more expert and, in some cases, more recent overviews. I suggest you look at these:
- AAEP Parasite Control Guidelines – http://www.aaep.org/custdocs/ParasiteControlGuidelinesFinal.pdf – by various including R M Kaplan and MK Nielsen. Undated as far as I can see, but probably ~ 2012 – 2013 (SL). AAEP = American Association of Equine Practitioners.
- 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
- Nielsen MK et al, 2014. Anthelmintic resistance in equine parasites – Current evidence and knowledge gaps. Vet. Parasitol. (2014), http://dx.doi.org/10.1016/j.vetpar.2013.11.030
Some notes from the AAEP guidelines
Here are some notes I made, an interpretive summary. (My comments are italicised). However, read the original documents (above) for yourself. These notes are just a starter. The guidelines may have wide application but keep in mind regional variations (climate, management practices, available anthelmintics etc.)
Horse worm control practices are commonly based on knowledge that is more than 40 years old.
These points underpin the recommendations in the guidelines:
- Strongylus vulgaris and other large strongyles are now rare (in managed horses in the USA and perhaps most countries). Cyathostomins (‘small strongyles’) are now most important in adults. Parascaris equorum remains the most important parasite in foals and weanlings.
- Anthelmintic resistance of cyathostomins and P equorum is very common.
- Innate susceptibility of adults to infection with cyathostomins – and strongyle egg shedding as a result – is highly variable. Individualized management is required.
- Horses less than about 3 years old are more susceptible and require special attention.
The old approach to horse worm control was to kill S. vulgaris worms before they could produce eggs and contaminate pasture. This required treatment every two months (it took ~ 2 months for eggs to reappear after treatment). The importance of the ‘small strongyles’ (‘small redworms’) was overshadowed by the ‘large strongyles’ (‘large redworms’) which were considered to be more pathogenic.
(Aside: Most “small strongyles” are appreciably smaller than the “large strongyles,” but Triodontophorus spp (sometimes classified as nonmigratory large strongyles) are almost as long as Strongylus vulgaris. Source: http://www.merckmanuals.com/vet/digestive_system/gastrointestinal_parasites_of_horses/small_strongyles_in_horses.html#v3264928)
The tapeworm Anoplocephala perfoliata has been recognised as a potential cause of colic.
The epidemiology of cyathostomins, P equorum and A perfoliata differs from that of S vulgaris and thus they require different control strategies.
Cyathostomins are ubiquitous, infecting all grazing horses. But they are relatively mild pathogens, causing disease only with heavy burdens.
After decades of frequent anthelmintic (‘drench’) use, drench resistance – an inherited trait – is common (ranging from ‘early indications’ to ‘common’ to ‘widespread’, depending on drench class/group) in cyathostomins and P equorum.
Table 1, page 3 summarizes the situation in south eastern USA (benzimidazoles (BZs)), pyrimidines (notably, pyrantel salts), and macrocyclic lactones (MLs) vs cyathostomins, large strongyles (apparently no resistance) and P equorum.
Table 1 (modified). (SE USA)
||Cyathostomins (‘small’ strongyles)
|Pyrimidines. e.g. pyrantel
(As of December 2012, resistance of strongyles to IVM or MOX had not been diagnosed in the US –although (page 18) signs of emerging resistance reported (~ 2012) from Central Kentucky)).
Parasite ‘refugia’ is discussed (page 3.) Eggs and larvae on pasture are in refugia, as are stages in the horse against which a drench has no efficacy, e.g. pyrantel has no effect on worms outside the gut lumen, and ivermectin (IVM) has no documented efficacy against encysted cyathostomin larvae.
Minimise treatments when pasture refugia is low. The old practice of ‘drench and move’ (to a ‘clean’ paddock) selects more strongly for resistance. Leaving some horses untreated utilises refugia. Identify moderate and high egg shedders by faecal worm egg count (WEC). In one study (Kaplan and Nielsen, 2011): just treating horses with WEC>200 worm eggs per gram of faeces(epg), resulted in 50% of horses being treated but reduced egg shedding by ~ 95%.
Faecal or worm egg count reduction test (WECRT or FECRT) – described, page 4ff.
Table 2, page 5. Suggested cut-off values (mean reduction in strongyle WEC) for the categories ‘susceptible’, ‘suspected resistant’ or ‘resistant’, for BZs, pyrantel and MLs.
Practical pointers include: at least 6 horses in a WECRT on a farm; aim for highest possible pre-treatment WEC; use egg counting technique with limit of detection of 25 epg or less; no prior drench within 8 weeks or more (12 weeks if moxidectin (MOX) used). Interpret results with care.
ERP – egg reappearance period (page 5) – a simple definition: the time from the last effective treatment until significant strongyle egg shedding resumes. Another definition: the week post-treatment when the reduction in WEC falls below 80% (BZs, pyrantel) or 90% (IVM, MOX). This requires weekly WECs until eggs reappear. A shortening ERP may indicate emerging resistance.
Table 3, page 6. Cyathostomin ERPs for different horse drenches.
Strongyle shedding / pasture contamination (page 6). ‘Over-dispersion’ is common in all species including horses. i.e., for horses, about 20-30% of adult horses (i.e. >3 yo) shed about 80% of the eggs. (Yet another example / corollary of the Pareto or 80/20 principle). Healthy low or high shedding horses on pasture tend to remain low or high shedders. Test shedding status of adult horses using WEC at least 4 weeks after the ERP for the last drench used.
Table 4, page 7. Currently suggested guidelines/ best guesstimates for categorising shedders (of strongyle eggs)/contaminators, preferably based on more than one test (because some horses – not usually the ‘high shedders’ – switch categories):
Low shedders: 0-200 epg (50-70% of adult horses); medium: 200-500 epg (10-20%); high: >500 epg (20-30% of adult horses).
(No exact guidelines have been published re acceptable numbers of P equorum eggs in young horses).
Parasites (apparently horse ones at least?) shed less eggs when conditions are less favourable for free-living stages (on pasture), so WECs may be less reliable in cold winter months (northern USA) or hot dry summers (southern USA).
Goals of parasite control, page 7ff.
Eradication is impossible and attempts to do so result in accelerated drench resistance. Also strongyles, small and large, cause most damage during the larval stages, against which most drenches are ineffective. But, killing adult worms, although of limited direct benefit to the horse, is of indirect benefit because of reduced pasture contamination.
Goals of worm control:
- minimise risk of disease
- control egg shedding
- maintain drench efficacy as much as possible
Achieve this by knowing who the high shedders are (this requires periodic worm egg counting) and treating them with the right drench (effective) and at the right time (i.e. before they start shedding large numbers of eggs; and not when conditions are adverse for free-living stages (few worms in refugia), unless clinical signs indicate a horse is parasitized and needs de-worming).
Reasons to WEC
- to test drench efficacy (by WECRT)
- to monitor the ERP after drenching
- to ascertain shedding status of a horse
- to see if foals / weanlings have P equorum and / or strongyles
Limitations of WEC
- not an accurate indicator of size of burden of adult P equorum or strongyles
- does not detect immature / larval incl migrating larval stages of large strongyles or ascarids, and / or encysted cyathostomins.
- tapeworm infections can be missed or underestimated, because of intermittent egg shedding
- pinworm infections often missed: egg packets are glued to the perianal skin
Faecal sampling, storage, worm egg counting, microscope maintenance – page 9
Interpreting WECs, page 9ff. Ninety nine percent of strongyle eggs in managed horses are from cyathostomins; this is 90-95% in feral and severely neglected horses (the rest being from large strongyle species) (USA-centric information? but seemingly applies similarly elsewhere). Differentiating large from small strongyles requires larval culture and ID (not apparently offered in commercial labs in the USA (I was surprised by that), but it is in Australia – but, some Australian labs may not routinely offer larval culture of strongyle dominated WECs because they are usually almost all cyathostomins). A PCR for S vulgaris eggs in faeces has been developed – currently (~ 2012 – 2013?) only for research purposes.
Other gastrointestinal parasites – pages 10 – 12.
- Anoplocephala perfoliata (tapeworm). Common in many areas. Intermediate host: oribatid (pasture) mites (as with Moniezia spp in sheep). Possibly higher numbers of mites under moist conditions. So, less tapeworms in arid areas. Evidence for tapeworms as an important cause of colic is still scant. They cause small mucosal erosions at attachment points and when present in high numbers have been associated with ileocaecal impaction and spasmodic colic. Most horses have relatively few tapeworms – unlikely to cause significant pathology. Seeing eggs in standard egg counts is a chance event, unless burdens are heavy – eggs are shed intermittently. Therefore report result as positive or negative; eggs are clustered in faeces. Other test methods can improve test sensitivity. Also greatly improved sensitivity if egg counting 24 hours after treatment with a cestocide (praziquantel (PRAZ), or cestocidal dose (2x the nematode dose) of pyrantel (PYR)). Validated serological test (Proudman – Univ Liverpool) is commercially available. (A perfoliata specific antibodies; titres correlate with burden). Annual treatment (PRAZ or PYR, as above) when transmission ends (cold weather) is suggested. More frequent treatment of dubious value. Treatment may be unnecessary in dry areas.
- Parascaris equorum (roundworm; ascarid), page 11 ff. Most important in foals, causing ill-thrift. Airway inflammation, cough, nasal discharge (migrating larvae); higher risk of intestinal impaction. A possible link, evidence suggests: after treating with effective drugs with paralysis as mode of action (unlike BZs), sometimes complicated further by intestinal rupture. P equorum is very common in breeding operations; eggs can be viable / infectious for several years if organic material in soil. Adult horses occasionally infected (possibly becoming more common – usually on farms with foals), but rarely with clinical disease. Resistance: high levels world-wide to IVM and MOX (fenbendazole at 10 mg/kg daily for 5 days may on some of these farms be the only larvacidal option); early indications of PYR resistance; no confirmed cases (~ 2013) of BZ resistance.
- Oxyuris equi (pinworms). Previously mainly caused clinical signs in young horses; now becoming more common in adults. Infections often sporadic; usually just a few horses affected – perianal itching (due to material secreted by female when depositing eggs). Eggs are hardy and rubbing spreads eggs through the horse’s environment. Resistance: anecdotal reports of resistance to MLs; none yet documented (2012). (But, see below…). BZ or PYR are alternatives. Use good hygiene when clearing rear end of affected horses (eggs are hardy).
- Bots (Gasterophilus spp.) Unsightly, but rarely associated with measurable disease. Boticide annually at onset of cold weather often recommended. IVM and MOX currently the only effective drugs available.
Methods of parasite control. p 12ff
- Environment-based approaches. Removal of faeces from pasture. One study: twice weekly vacuuming controlled pasture infectivity better than regular de-worming, but very expensive and only worked well on level fairly dry pasture.
- Environmental control. p.14ff. Table 5, page 14: Effects of temp. on development and survival of free-living stages of strongyles. (Broadly similar to important nematodes of sheep – see WormBoss.com au here and here). Composting (manure, soiled bedding) – a good practice; can kill even ascarids. Most strongyle larvae in manure die after 2+ weeks at >40 degrees C. Never spread non-composted horse manure onto pasture. Spelling pasture. Depends on temp: L3s may only survive a few weeks in hot weather but can last 6-9 months in colder weather. Drench at right time: focus on treating when conditions optimal for larval development. Avoid treatments in times of low refugia – very cold winters or very hot summers. Other: haymaking; cross-grazing with ruminants, camelids; nematophagous fungi (not yet available commercially (~ 2013)).
(My understanding is that, in Australia, a nematophagous fungi-based product is still proceeding towards commercialisation – and for a number of livestock species. SL, 2015-03-11).
Table 5 (somewhat abbreviated; see paper). Effects of temperature (degrees C) on free-living horse strongyles (eggs, larvae (L1-L3)). (Nielsen et al., 2007).
|No development above this
||Free-living stages outside faecal balls die rapidly
|Optimal for devel. of eggs, larvae, getting to L3 in as little as 4 days
||Larvae survive a few weeks; too warm for long survival
|No hatch, no devel.
||Eggs, L3 survive many weeks, even months just above zero (but repeated freeze-thaw is detrimental)
- Alternative remedies. p.14ff. Efficacy has never been demonstrated in formal, controlled evaluations.
- Anthelmintics available. p.15ff. Benzimidazoles (BZs): Affect energy metabolism; prevents creation of mictrotubules. Tetrahydropyrimidines: pyrantel (PYR). Affects intra-luminal adults only. Sometimes used as a preventative – regular admin. in feed. Heterocyclic compounds: piperazine; depolarizes muscle membranes, causing resistance to acetylcholine. -> rigid, irreversible paralysis. Adulticide only. Infrequently used now. (In the olden days, we used to give it by stomach tube: e.g. piperazine, thiabendazole, trichlorfon. I never got a nose-bleed – neither did the horses). Macrocyclic lactones. Act on glutamate-gated chloride channels in nematode nerve, muscle cells -> flaccid paralysis. Also kill ectoparasites, and cutaneous larvae of Onchocerca, Habronema, and Draschia. Isoquinoline-pyrozines: praziquantel – tapewormer. No effect on nematodes.
- Parasite control programs- points to ponder. p.15ff
Mature horses. Focus on cyathostomins (small strongyles). Test drench efficacy (FECRT) at least every 3 years. Basic program for all horses: 1-2 treatments per year for large strongyles, tapeworms, bots, and spirurid nematodes (Habronema, Draschia) causing summer sores. Extra treatments are for high shedders. Focus treatments during peak transmission periods – usually spring/autumn when pasture refugia highest.
Foals, weanlings, yearlings. Don’t do targeted (selective) treatments, i.e. based on FECs, in this group. First year of life: at least 4 drenches, first at 2-3 months old, with BZ (for efficacy against ascarids); second just before weaning (~ 6 months old); maybe an extra before weaning if the time between treatments is > 3 months. WEC (FEC) at weaning to see if mainly strongyles or ascarids (to ensure right drug choice). Third and fourth at 9 and 12 months, mainly targeting strongyles. Tapewormer also at 9 or 12 months. Annually test (WECRT) drug efficacy against ascarids, strongyles. Strongyloides westeri: rare cause of diarrhoea in young foals since advent of BZs, MLs. MLs often used on mares pre-foaling to prevent lactogenic spread of S westeri, but not justified if recently (same spring) treated with an ML. Move recently weaned foals to ‘cleanest’ pastures. Treat yearlings and 2 year olds as high shedders: 3-4 effective drenches p.a.
General considerations. Correct dose – use weight tapes or scale. Sources of infection: cyathostomins, large strongyles, tapeworms – from pasture; ascarids, pinworms – in confinement as well as on pasture. Consider using tapeworm serology (ELISA). Do most of the drenching when conditions favour worm transmission. Drench less when conditions are adverse (very hot, or freezing). Fine-tune worm control program for each farm – consider stocking density, time spent on pasture, age of horses, open or closed herd? – new arrivals: WEC and larvacidal deworming before release; non-chemical options – manure removal / composting, pasture rotation, cross-grazing with other species.
Other (included in summary): individual horses, with overt endoparasitism, may need treating at any time, with MOX being best for strongyles (e.g. efficacy against encysted mucosal larvae), or, if WECRT shows high efficacy, ‘can use fenbendazole larvacidal regimen – 10mg/kg for 5 consecutive days.
Oxyuris equi – drug resistance? and/or innate lack of efficacy?
There have been a few recent papers which suggest at least apparent lack of drench efficacy against pinworm in horses. Some examples:
Reinemeyer CR, 2012.Anthelmintic resistance in non-strongylid parasites of horses.Vet. Parasitol. 2012 Apr 19;185(1):9-15. “.. Some specimens of Oxyuris equi regularly survive treatment with macrocyclic lactones, but it is uncertain whether this constitutes resistance or merely confirms the incomplete oxyuricidal efficacy of virtually all broad spectrum equine anthelmintics”.
Schankova et al.,2013. Research Note: Treatment failure of ivermectin for Oxyuris equi in naturally infected ponies in Czech Republic. HELMINTHOLOGIA, 50, 3: 232 – 234, “Case report and overview of anthelmintic resistance of horse worms. “This is the first European study to demonstrate anthelmintic resistance in Oxyuris equi to macrocyclic lactones in naturally infected ponies.”
Wolf D, Hermosilla C and Taubert A, 2014. Short Communication. Oxyuris equi: Lack of efficacy in treatment with macrocyclic lactones. Veterinary Parasitology http://www.elsevier.com/locate/vetpar Short Communication. “This is the first report in Europe showing inefficacy of commercial ivermectin compounds and furthermore the first report at all documenting ineffectiveness of moxidectin compounds in the treatment of O. equi infections in horses indicating a possible development of resistance or confirming an existing incomplete oxyuricidal efficacy.”
(So, two ‘first reports’ from Europe…?).
Rock, C., Pomroy, W., Gee, E., Scott, I., 2013. Macrocyclic lactone resistant Oxyuris equi in New Zealand. In: Proc. 24th. Int. Conf. of the WAAVP, 25–29 August, p. 520. “The studies on these two horses provide some evidence to support the anecdotal reports of ML resistance in Oxyuris equi in New Zealand.”
Off-label use of anthelmintics in horses – a NZ case study
Scott and others in 2014 discussed a case of off-label use – which is frequently done – of a ‘drench’ (anthelmintic) in horses.
In this case it was injectable moxidectin for sheep given orally to horses. Three weeks after dosing a group of horses with this product at 400 µg/kg, worm egg counts (WECs) had gone done by only 72%. To determine whether sub-optimal pharmacokinetics and / or drench resistance was involved, the horses were later treated with an ivermectin paste formulation registered for horses, at 200 µg/kg, and sampled 7 and 22 days later. The WEC reduction was 100% and 99.5 % respectively.
Conclusion: inappropriate use of drenches may result in reduced efficacy and possibly higher selection for drench resistance.
Scott I, Pomroy W, Gee E, Toombs-Ruane L, Adlington B, Moss A, Reilly M, and Sparrow G, 2014. Off-label use of anthelmintics in equines – a case study. New Zealand Society for Parasitology – 42nd Annual Conference, 19-212 October, 2014.
Real ‘men’ don’t need instructions
I recently got a call from a producer (NSW South Coast, cattle producer) who bought a couple of goats from someone.
The original owner used the oral sheep drench, ‘Q-Drench®’. Method of application: mix some of it with water and spray it onto the goats….
Pin worm and Kemosabe
Allegedly The Lone Ranger in retirement discovered Kemo Sabe means ‘rear end of a horse’. If so, hopefully that related to a horse without pin worm.
But Tonto was the butt (sic) of someone’s joke as well: Tonto is Spanish for ‘fool’.
Using sheep as wifi hotspots
“Bulverism is a logical fallacy in which, rather than proving that an argument in favour of an opinion is wrong, a person instead assumes that the opinion is wrong, and then goes on to explain why the other person held it. It is essentially a circumstantial ad hominem argument. The term “Bulverism” was coined by C. S. Lewis.”
“Lewis wrote about this in a 1941 essay which was later expanded and published in The Socratic Digest under the title “Bulverism”. This was reprinted both in Undeceptions and the more recent anthology God in the Dock. He explains the origin of this term:
“You must show that a man is wrong before you start explaining why he is wrong. The modern method is to assume without discussion that he is wrong and then distract his attention from this (the only real issue) by busily explaining how he became so silly.
In the course of the last fifteen years I have found this vice so common that I have had to invent a name for it. I call it “Bulverism”. Some day I am going to write the biography of its imaginary inventor, Ezekiel Bulver, whose destiny was determined at the age of five when he heard his mother say to his father — who had been maintaining that two sides of a triangle were together greater than a third — “Oh you say that because you are a man.” “At that moment”, E. Bulver assures us, “there flashed across my opening mind the great truth that refutation is no necessary part of argument. Assume that your opponent is wrong, and explain his error, and the world will be at your feet. Attempt to prove that he is wrong or (worse still) try to find out whether he is wrong or right, and the national dynamism of our age will thrust you to the wall.” That is how Bulver became one of the makers of the Twentieth Century.” “