In this issue:
Inheritance of resistance of gastrointestinal nematodes (GINs) of sheep and cattle
Vaccination needle lengths
Recently revised ‘wormy’ PrimeFacts
Smoking Young Guns – LambEx 2016
Footrot in Central West – first time in decades
Inheritance of resistance of gastrointestinal nematodes (GINs) of sheep and cattle
The following is largely from Sutherland and Scott, 2010. (Pages 123 ff). (In fact, the following is virtually verbatim, unless indicated otherwise. The incorrect bits are possibly all mine. -Ed).
I have invited CSIRO scientist, Dr Peter Hunt, to make comments, given his work in this area.
As always, check the source material for yourself.
“The likelihood of and speed at which resistance of GINs to anthelmintics occurs may be due to:
- presence of genes in the treated population
- drug efficacy insufficient to remove resistant genes (g. under-dosing/suboptimal administration; resistance -Ed)
- selection for resistance is correlated with the proportion of the population exposed to the treatment (the ‘refugia’ story)-Ed)
- susceptible worms are unable to establish and to dilute or replace resistance genes (e.g. immunity of the host, use of long-acting drenches, frequent drenching -Ed)
- resistant worms establish and multiply in naïve hosts
- inheritance of the resistance (This point added by P Hunt).
How does resistance develop?
“Simplistically put, observable resistance will develop when individual worms are able to survive drug treatment, then pass on their resistant genes to subsequent generations; assuming no positive effects of the resistant mutations other than the ability to survive treatment, these subsequent generations also need to be subjected to drug selection. Ultimately, there are sufficient numbers of resistant worms to successfully breed with each other.
“There are a number of factors which influence how likely and how quickly this process can occur:
(a) presence of resistance genes in the treated population,
(b) drug efficacy is insufficient to remove worms with resistance genes,
(c) selection for resistance is correlated with the proportion of the population exposed to treatment,
(d) susceptible worms are unable to establish and to dilute or replace resistance genes and
(e) resistant worms establish and multiply in naïve hosts.
“There are two obvious routes via which GIN become resistant to anthelmintics.
“First, drug treatment selects for those individuals in a population with a necessary mutation. Certainly the extremely high genetic variability of GIN suggests this to be a reasonable theory; this would include the situation in which resistant individuals were already present before an anthelmintic was developed and also where mutations occur subsequent to drug development.
“Alternatively, resistance could develop by selecting those individuals which are resistant to sub-optimal drug exposure i.e. by under-dosing. There is good evidence available to suggest that both of these routes have resulted in the selection of resistant populations of GIN. Furthermore, evidence suggests that selection via these routes results in two distinct types of resistance.
“Selection of resistance to ‘full-dose’ therapy in the field generally produces worms with monogenic resistance, whereby a single mutation is involved, often conferring a ‘high’ level of resistance. Conversely, selection of resistant worms by exposure to low levels of drug in the laboratory often results in worms with polygenic resistance, either involving more than one gene conferring resistance against a single drug or more than one gene conferring resistance to more than one drug.
“It is noted that observations of macrocyclic lactone (ML) resistance mechanisms in Cooperia spp. (small intestinal worm) from cattle appear to involve polygenic resistance; this may have arisen due to the regular use of topical applications such as pour-on and injectable formulations leading to effective under-dosing. Other indirect evidence for the likelihood of under-dosing selecting for resistance is the number of resistant populations which have arisen in goats.
“Anthelmintics have markedly different pharmacokinetic profiles in these animals compared to the situation in sheep, which may significantly reduce the peak of exposure to treatment.
“It should be noted, however, that actually determining what quantum of in vivo drug exposure will provide significant selection advantages to worms in the early stages of developing resistance is impossible.
Inheritance of resistance
“The mechanisms by which resistance is inherited will significantly influence the ability of resistant genes to survive and spread through worm populations” Peter Hunt adds: “ The speed with which a population becomes highly resistant to the point of drench failure is highly influenced by the inheritance mechanism as is the (theoretical) chance of resistance gene frequency falling again once the use of the drench is discontinued”.
“Simplistically, a heterozygote-resistant worm in which the resistance is inherited as a dominant trait may survive treatment, which would effectively remove a heterozygote-resistant worm in which the resistance is inherited as a recessive trait. Assume that a worm population within a host consists of a mixture of 90% fully susceptible worms, 9% heterozygote-resistant worms and 1% homozygous-resistant ones. Further assume that a treatment removes all of the susceptible worms but none of the homozygous-resistant worms. Depending on whether the resistance is inherited as a dominant or recessive trait, either 10% or 1% of the resident worms may survive treatment and go on to produce progeny with resistant genes”.
“The actual situation in resistant GIN is of course much more complex, particularly with polygenic resistance. Furthermore, there can be significant variability in the inheritance of resistance to the same anthelmintic between the various GIN species”.
Peter Hunt adds, “Variation in the mode of inheritance between different isolates of the same species has also been observed. For example ivermectin resistance in the Wallangra (NSW) 2003 isolate of H. contortus (barbers’ pole worm) cannot be a single gene, and resistance conferred by two recessive genes is consistent with the observations (Hunt et al., 2010). In contrast, in the CAVR (Chiswick Avermectin Resistant) isolate, ivermectin resistance is clearly conferred by a single dominant gene (Le Jambre et al., 1999, Redman et al., 2012)”.
“Benzimidazole resistance inherited in T. colubriformis (black scour worm) is incompletely recessive and is maternally influenced. However, in H. contortus, resistance is inherited through more than one gene, was semi-dominant and autosomal, and has further been described as multigenic, autosomal but recessive. Subsequent studies established that resistance was inherited similarly between these two species. At least two loci are involved in the inheritance of high levels of benzimidazole resistance.
“Resistance to levamisole in H. contortus is inherited as a recessive autosomal trait, while the resistance in T. colubriformis is inherited via a single sex-linked gene. Nematodes have an XX (female) and XO (male) system of sex determination, and sex-linked recessive traits are therefore recessive in females but dominant in males.
“Interestingly, following in vitro treatment of H. contortus to benzimidazole or levamisole, no evidence was found to suggest sex linkage in either inheritance. As stated above, this may reflect differences in the methods of selection of resistant isolates between field/monogenic and laboratory/polygenic.
“Resistance to ivermectin in H. contortus is inherited as a completely dominant trait (Dobson et al., 1996; Le Jambre et al., 2000), while in T. circumcincta (small brown stomach worm), ivermectin resistance is inherited as a dominant trait (Sutherland et al., 2002) but was effectively incompletely recessive to moxidectin (Sutherland et al., 2002). However, a subsequent study demonstrated that the resistance was dominant during the period of declining drug activity, which is responsible for the persistent efficacy of moxidectin (Sutherland et al., 2003). Interestingly, the same study established that resistance was partially dominant/recessive under treatment with controlled-release capsules releasing ivermectin. In T. colubriformis, ivermectin resistance is inherited as an incompletely dominant/recessive trait which appears to be multigenic (Gill & Lacey, 1998; Gopal et al., 2001)”.
“No information is available on the inheritance of resistance to any of the important GIN in cattle”.
“Detection of resistance
“Faecal egg count reduction test
“The gold standard method of determining the resistance status of a parasite population, or all parasite species/populations on a property, is the faecal egg count reduction test (FECRT). In the FECRT, animals are allocated to groups of 10 based on pre-treatment FEC, with one group of 10 for each anthelmintic treatment tested and a further untreated control group. In many regions, this requires the use of 40 animals, given there are three active families available. In some regions where combinations containing benzimidazoles and levamisole have been administered to sheep for long periods, a further group may be included to test for resistance to the dual combination. In addition to sampling for FEC, a bulked faecal sample should be cultured for larval differentiation to species or (at least) genus. The appropriate treatments should be calculated for individual liveweight, then administered by syringe for accuracy. Thereafter, 7–14 days later, each individual is sampled for FEC, while bulk samples of faeces are cultured for larval differentiation. A full FECRT is understandably expensive and takes a significant length of time before farmers are presented with the results of their resistance profile. Accurate larval differentiation also demands a high degree of skill.
Comment from Ed.(SL): “Currently in Australia, often 15 sheep are allocated to each group, to ensure that faeces can be obtained from not least then 10 per group. Also, standard protocols tend to stipulate a minimum faecal (worm) egg count (WEC) – for example 150 eggs per gram – per genus or species of worms of interest. If WECs are thought to be lower than desirable, and/or there is considerable variation between individual WECS, then investigators may increase the number of sampled animals (e.g. to 20) in each group. This could often be the case when doing a FECRT on cattle, especially when worm species somewhat less fecund than say Haemonchus or Cooperia are present. To reduce the number of treatment groups necessary, there is also a trend in Australia to testing different single actives in sheep, and then calculating the likely efficacy of combinations from the results for the single actives that are tested. For more information:
- Checking for drench resistance with a DrenchCheck-Day10
- Testing drench effectiveness with a DrenchTest
- http://www.wormboss.com.au/tests-tools/management-tools/combination-drench-efficacy-calculator.php “
“Care must be taken when conducting an FECRT to delay sampling animals until at least 7–20 days post-treatment; some drench products, particularly the MLS, can cause a temporary suppression on egg production from resistant female survivors (Sutherland et al., 1999), and there is a danger of a misdiagnosis should samples be taken too early”.
Comment from Ed.(SL): But keep in mind the prepatent period for the worms of interest, e.g., as short as 18 days for H contortus, around 21 days for most other important sheep roundworms, a bit longer for large bowel worms, and as short as 12 days for some Cooperia in cattle. If you do the post-treatment WEC after the pre-patent period, you are left wondering if positive WECs are due to surviving resistant worms /and or new infections”.
Comment from Ed. (SL): “In Australia, when testing drench efficacy against roundworms we do worm egg counts (WECs aka FECs) ~ 10 days (7-14 days) after treatment in sheep, 14 days in cattle. The 14 day figure for cattle aims to account for, one the one hand, the observation that some MLs may suppress egg production for 16 days or so –perhaps more? – but, on the other hand, some Cooperia sp in cattle can have pre-patent periods as short as 12 days. When testing for the efficacy of triclabendazole against liver fluke using the faecal egg count reduction test, the post-treatment WEC is done at 21 days (e.g. Kelley and others, 2016) to 28 days (JC Boray, pers comm). Doing the fluke egg count at 14 days may not allow enough time for all eggs to pass out of the biliary tract, including gall bladder, and gut.”
“Any species for which the reduction in egg count is less than 95% is then considered to be resistant. This unfortunately highlights two major deficiencies of the FECRT and of resistance testing in general. The first of these concerns seasonal variability in nematode populations, e.g., in New Zealand, T. circumcincta, the parasite of most concern in surveys of ML resistance (in NZ- Ed.), is more prevalent in early summer. Resistance testing at other times of the year, even if egg counts may be higher, may not detect resistant populations of this species. The second caveat inherent in the FECRT is the use of the 95% threshold for reduction in FEC. While this may have some practical benefit in simplifying outputs, it most certainly does not present a firm line in the sand in which a 95% reduction means no resistance is present or is likely to arise”
Peter Hunt adds: “For each property a decision will need to be made regarding how often a FECRT is conducted. We cannot advise this on the basis of our understanding of resistance because there are too many unknowns. In particular, the mechanism of inheritance of resistance genes differs between drench groups, drench compounds within groups, worm species, and also worm isolates from different properties. All these unknowns suggest that more frequent testing is the safest option, however economic considerations will come into play”.
Comment from Ed (SL): “But, despite all these complexities and unknowns, we generally suggest a FECRT every 2-3 years, largely based on the observation that the resistance profile of a farm can change substantially in that time. We also advocate periodic DrenchChecks in between FECRTs. These are simple to do and very cost-effective. For more information on DrenchChecks (with specific reference to roundworms in small rumminats): Checking for drench resistance with a DrenchCheck-Day10 “
(In the above, emphases, ie, bolding, are mine, – Ed. (SL)). ‘Chiswick’ is the name of CSIRO’s property (farm) at Armidale , NSW.
Kelley JM, Elliott, TP, Beddoe T, Anderson G, Skuce P and Spithill T (2016). Review: Current threat of triclabendazole resistance in Fasciola hepatica. Trends in Parasitology. http://dx.doi.org/10.1016/j.pt.2016.03.002
Le Jambre, L.F., Dobson, R.J., Lenane, I.J., Barnes, E.H., 1999. Selection for anthelmintic resistance by macrocyclic lactones in Haemonchus contortus. Int. J. Parasitol. 29, 1101-1111. (Reference provided by P Hunt).
Redman, E., Sargison, N., Whitelaw, F., Jackson, F., Morrison, A., Bartle,y D. J., Gilleard, J. S., 2012. Introgression of ivermectin resistance genes into a susceptible Haemonchus contortus strain by multiple backcrossing. PLoS Pathog. 2, e1002534. (Reference provided by P Hunt).
Sutherland I and Scott I, 2010. Gastrointestinal nematodes of sheep and cattle. ISBN: 978-1-4051-8582-0. 256 pages. October 2009, Wiley-Blackwell.
Vaccination needle lengths
From SheepConnect Tasmania newsletter ~ 22 August 2016 https://sheepconnecttas.com.au/
“Research findings: Dr Tristan Jubb of Bendigo Sheep Vets recently undertook a study into best practice vaccination technique for sheep.
The key findings were as follows:
- lambs and ewes in short wool– use ¼ inch needles at a 45° angle to the skin
- ewes with significant wool growth – use ¼ inch needles at a 90° angle to the skin.
Whilst most vaccines for sheep in Australia are designed to be injected subcutaneously (under the skin), even experienced sheep producers and contractors may be unintentionally administering vaccines incorrectly, into muscle tissue.
Vaccinating on the side of the neck, approximately 5cm from the base of the ear, helps to avoid hitting structures including bone, ear cartilage and glands in the head/neck region, whilst also minimising the risk of injecting into relatively valuable meat cuts.
By vaccinating at the correct site, using the right equipment and approach, sheep producers can maximise vaccine efficacy and minimise the risk of adverse reactions.
Click here for more information.”
Recently revised ‘wormy’ Primefacts from NSW DPI
The 2nd edition of Primefact 477 Quarantine drenching – don’t import resistant sheep worms.
Smoking Young Guns – LambEx 2016
From “DPI Active”, an in-house NSW DPI blog.
by Ashley White, Leader, Sheepmeat Performance, Cowra | 22 Aug, 2016 |
LambEx 2016 Young Guns was coordinated by Edwina Toohey NSW DPI and sponsored by the Australian White Suffolk Association. The competition aims to support and celebrate the next generation in the red meat industry. All finalists displayed a high level of skill and knowledge of their industry and it was highly competitive to make the top 12.
A professional development program was held for the finalists at LambEx to discuss possible roles within the industry, some insights on writing and publishing and options available after post grad study in the industry.
Finalists were assessed on how they interacted and participated throughout the day and then were judged by a panel of 6 during a 3 minute power point presentation on their involvement in the lamb industry and then their ability to answer industry questions from peers and panellists.
LambEx is the major biennial lamb industry event and this year was sponsored and organised by NSW DPI.
It was a huge success this year at Albury NSW with one thousand producers, processors and service providers from all over Australia attending from Aug 10-12.
The 12 finalists include Early Career Professionals Jordan Hoban, NSW DPI, Cowra; James Preston, Preston Livestock Solutions, Bendigo,Vic; Elise Bowden, Sheep Data Management (winner), and Mary Chirgwin, Zoetis Australia, SA. In the Honours, Masters or PhD student section, finalists include Cassius Coombs, NSW DPI/Graham Centre, Cowra; Maddison Corlett, Murdoch Uni, WA; Steve Connaughton, Murdoch Uni (winner) and Octavia Kelly, Adelaide Uni, SA. In the High School or Undergraduate student section finalists include Josephine Webb, Marcus Oldham College, Vic; Laura Wishart, Marcus Oldham College (winner), Bianca Agenbag, Adelaide Uni, and Charlie Shadwell, Year 10, Farrer Memorial Agricultural High School (highly commended).
Footrot in Central West – first time in decades
24 August 2016
“Central West Local Land Services District Veterinarians have diagnosed virulent footrot in sheep in the north west of the region.
Areas such as Coonamble, Narromine, Warren, Nyngan and Gilgandra have not seen the notifiable disease in decades and may come as a nasty surprise to some graziers.
Signs of footrot include lame sheep, inflammation between the digits and underrunning of the sole and heel of the foot. In some severe cases sheep will lie down, walk on their knees and lose weight.
Dr Jillian Kelly, Team Leader Animal Biosecurity and Welfare, said Local Land Services has taken immediate action to limit its spread and find its origins.”
More here: centralwest.lls.nsw.gov.au/resource-hub/media-releases/2016/footrot-strikesfirst-time-in-decades
SL, Armidale 2016-08-26