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    Egg reappearance periods associated with anthelmintic treatments given to horses in winter and summer over two years
    (Elsevier B.V., 2024-12-18) Scott I; Lawrence KE; Gee EK
    Shortened egg reappearance periods (ERP) have been seen as an early warning of anthelmintic resistance development in cyathostomins in horses. The hypothesis has been that in these instances, efficacy against egg laying adult cyathostomins remains high, but a decline in activity against later larval stages leads to the earlier resumption of egg shedding after treatment. In this study using a single herd of horses we investigated the ERP of a number of commonly used equine anthelmintics and examined whether ERP might show seasonal variation between winter and summer. Four main Faecal egg count reduction tests (FECRT1–4) were conducted respectively in Winter (Jun/Jul) 2019, Summer (Jan/Feb) 2020, Winter 2020 and Summer 2021. The tests examined the efficacy and ERP of ivermectin, moxidectin, abamectin and fenbendazole. Egg counts of two groups of horses were monitored before and for 6–7 weeks after treatment - however long it took for counts to return to at least 10 % of what they had been before treatment. One additional FECRT was also conducted, using a second abamectin-containing product (FECRT5 - Spring 2020). Treatment with ivermectin (FECRT1–4), moxidectin (FECRT 1–2) and the first abamectin product tested (FECRT3) all reduced egg counts by >99 % for 4 weeks after treatment, with ERP of 5–7 weeks and with minimal differences between the 3 treatments. There was a tendency for counts to rise more rapidly in summer, and in the second year of testing as opposed to the first. Both the second abamectin product (FECRT5) and the fenbendazole (FECRT4) were found to be ineffective, reducing egg counts immediately after treatment by 68 and 52 % respectively.
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    Clinical parameters of hypervirulent Klebsiella pneumoniae disease and ivermectin treatment in New Zealand sea lion (Phocarctos hookeri) pups
    (PLOS, 2022-03-03) Michael SA; Hayman DTS; Gray R; Roe WD; Raverty S
    Hypervirulent Klebsiella pneumoniae infection causes significant mortality of endangered New Zealand sea lion pups at Enderby Island, Auckland Islands. Gross necropsy and histopathology findings are well reported, but little is known about the clinical course of disease in affected pups. To determine factors feasible as clinical screening tools for hypervirulent K. pneumoniae in live pups, 150 pups over two field seasons (2016-18) were recruited shortly after birth for a prospective cohort study. A randomised controlled clinical treatment trial with the anthelmintic ivermectin was conducted concurrently and risk factor data and biological samples were collected approximately fortnightly. Treatment with ivermectin has been demonstrated to reduce the risk of hypervirulent K. pneumoniae mortality in pups, so effects on clinical parameters between the treated and control cohorts were also investigated. A broader sample of pups were monitored for clinical signs to investigate the course of disease in affected pups. Clinical signs, haematology and oral and rectal swabs to detect gastrointestinal carriage of hypervirulent K. pneumoniae were not useful for detection of disease prior to death. Of those pups that died due to hypervirulent K. pneumoniae, only 26.1% (18/69) had any clinical signs prior, likely a reflection of the peracute course of disease. On comparison of haematological parameters between ivermectin-treated and control pups, significantly lower total plasma protein and higher eosinophil counts were seen in control versus treated pups, however standard length as a surrogate for age was a more important influence on parameters overall than ivermectin treatment. This study also highlighted a cohort of pups with severe clinical signs suggestive of hypervirulent K. pneumoniae infection were lost to follow up at the end of the monitored season, which could be contributing to cryptic juvenile mortality.
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    Risk factor epidemiological studies of ivermectin resistant Ostertagia circumcinta on Western Australian sheep farms : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Studies in Epidemiology at Massey University, Palmerston North, New Zealand
    (Massey University, 2004) Suter, Robert John
    This study was designed to investigate the farm level epidemiology of ivermectin resistant Ostertagia circumcincta on Western Australian sheep farms. The study involved a postal survey and the results of that survey were used to develop statistical models to identify risk factors associated with ivermectin resistance. The survey was mailed to farmers in July 2001 who had conducted faecal egg count reduction tests on their properties in 1999 and 2000. The questionnaire contained questions about farm management practices, particularly those pertaining to worm control. Some mail and telephone follow-up was conducted. The response rate to the survey was 54%. The period prevalence for ivermectin resistance in Western Australia 1999 – 2000 as defined in this study was 38% (95% CI 29%, 46%) and for the period 1999 – 2001 was 44% (95% CI 39%, 58%) as some farms were diagnosed with ivermectin resistance in 2001. Two main effects models of anthelmintic resistance at the farm level were developed: a logistic regression model for risk factors for a farm having been diagnosed with ivermectin resistant Ostertagia circumcincta by 2000, and a Weibull parametric survival model studying the effective life defined as time to onset of resistance, for those farms using ivermectin. The logistic regression model contained three main effects variables: selling 10% more sheep in 2000 than is the usual policy (OR = 4.00), farm purchased since 1975 (OR = 2.34), and number of winter flock anthelmintic treatments in the previous 5 years (OR = 1.04). A secondary logistic-regression model assessed risk factors for farms selling 10% more sheep than usual in 2000; these farmers appeared less committed to their sheep enterprises than other farmers. The survival analysis model contained four main effects variables: winter drenching frequency, 0-2 vs. 3+ flock treatments in 5 years (RH 0.52); availability of alternative effective anthelmintic classes on the farm (RH 0.30); always using safe pastures (RH 0.23); and veterinarians as the primary source of worm control advice (RH 0.58). A major outcome of the study has been to identify that the farmer’s management of worm control in the sheep flock has an important influence on whether or not the farm develops anthelmintic (ivermectin) resistance.
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    Some aspects of ivermectin resistance in gastrointestinal nematodes of goats and sheep : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University
    (Massey University, 2000) Gopal, Rajesh Mohan
    Experiments were conducted to determine whether sheep are at risk from grazing pasture previously grazed by goats harbouring ivermectin-resistant Ostertagia spp. by monitoring the level of resistance with faecal egg count reduction tests and larval development assays. Ivermectin resistance emerged rapidly in goats grazed on the paddocks previously grazed by goats or sheep. In contrast, resistance was never consistently identified in sheep grazed on paddocks previously grazed by sheep although ivermectin resistance emerged after about 5 months in sheep grazed on paddocks previously grazed by goats. Ivermectin resistance was suspected in Trichostrongylus colubriformis and Ostertagia species on a goat farm in Northland, New Zealand. A controlled efficacy study was conducted in lambs (n=12) and young Angora goats (n=10) with this isolate. The establishment rate of T. colubriformis and Ostertagia spp. was similar (p>0.05) in the sheep and goats. Following ivermectin treatment (0.2 mg/kg b.wt. per oral) to half of the lambs and goats, the burdens of T. colubriformis were reduced by 39% and 13% in lambs and goats respectively whereas Ostertagia spp. burdens were reduced by 33% and 0% in lambs and goats respectively. In series of larval development assays with ivermectin aglycone, ivermectin and avermectin B2, this isolate of T. colubriformis had a resistance ratio of 37, 4-5 and 3-4 respectively indicating ivermectin aglycone is the drug of choice for detecting ivermectin resistance in T colubriformis. However, both ivermectin and avermectin B2 were still able to discriminate between resistant and susceptible T. colubriformis under controlled experimental conditions. The LC50 values of ivermectin were found to be influenced by the age of the infection of T. colubriformis in sheep. These LC50 values were relatively constant at 23 to 37 days after infection, then rose about four fold to peak between 51 to 58 days post infection, followed by a decline close to the original starting values by 72 days post infection. The resistance ratios recorded with thiabendazole were also high (14 to 15) suggesting that the ivermectin-resistant strain of T. colubriformis was also resistant to benzimidazoles. The LC50 values of ivermectin for T. colubriformis in a larval development assay for the two reciprocal first generation (F1) crosses of ivermectin-resistant and -susceptible parents were slightly lower than the LC50 values for ivermectin-resistant parents but greater than the ivermectin-susceptibie parents suggesting that ivermectin resistance in T colubriformis is inherited as an incompletely dominant trait. The fitness of the ivermectin-resistant strain of T. colubriformis was investigated and there was no significant differences (p>0.05) in infectivity, longevity of infection, fecundity and development of eggs to larvae under natural or laboratory conditions between the ivermectin-resistant strain and two susceptible field isolates. The survival of ivermectin-resistant strain larvae was intermediate between the two susceptible field isolates at 10°C, 20°C, 25°C and 30°C under laboratory conditions. An efficacy study in sheep indicated that the moxidectin oral formulation (0.2 mg/kg b.wt.), moxidectin injectable formulation (0.2 mg/kg b.wt.) and ivermectin oral formulation were 98%, 4% and 62% effective against this ivermectin-resistant strain of T. colubriformis respectively. These findings indicate that formulation of an anthelmintic plays an important role in the efficacy against resistant nematodes.