18 March 2012

8 minute read

Studies in Europe

Therefore, results of a survey to (1) screen for reduced anthelmintic efficacy in Belgian and German cattle farms; (2) evaluate the usefulness of a simplified faecal egg count reduction test (FECRT), where efficacies are based on the mean FECs of 10 at random collected faecal samples pre- and post-treatment per farm and (3) identify possible risk factors for reduced anthelmintic efficacy. Of 88 farms included in this study, 84 farms used macrocyclic lactones (MLs). A FECR <95% was observed on 39% of these 84 farms.

However, using a Markov chain Monte Carlo simulation analysis, to correct for the used McMaster FEC technique with a detection limit of 50 epg, reduced efficacy could only be confirmed in 25% of the farms (21/84). Only Cooperia spp. were found in significant numbers in the coprocultures post-treatment. Reduced efficacy was significantly associated with farm type and with a lower efficacy in beef herds compared to dairy herds. Four farms were revisited and a standardized FECRT was performed to confirm anthelmintic resistance (AR).

Surprisingly, macrocyclic lactone resistance against Cooperia oncophora was only confirmed in one of four farms. In conclusion, the results showed that a reduced efficacy observed in a FECRT are not only caused by AR but that the detection limit of the FEC technique used and the (in)correct administration of the anthelmintic drugs are confounding factors of major importance (El-Abdellati et al., 2010a).

Faecal egg count reduction test (FECRT) was used to monitor the evolution of ivermectin resistance on a Belgian cattle farm between 2006 and 2009. The presence of ivermectin resistant Cooperia oncophora worms on this farm was first detected in 2006. During the following years, the FECRs on day 21 post-treatment decreased from 73% in 2006, over 40% in 2007, to 0% in 2008. The ivermectin resistant C. oncophora showed side-resistance against moxidectin, indicated by a FECR of 83%, suggesting that the use of any type of ML is discouraged once ivermectin resistance has been detected. Benzimidazoles on the other hand were still fully effective on this farm.

The resistant C. oncophora larvae collected in 2007 (CoIVR07) and 2008 (CoIVR08) were also used to evaluate a modified version of the larval migration inhibition assay (LMIA). The results indicated that it is possible with the LMIA to differentiate susceptible from ivermectin resistant C. oncophora isolates. The EC50 values of CoIVR07 and CoIVR08 were, respectively, 4.5- and 5.8-fold higher than the value of a susceptible isolate. Furthermore, the results of the LMIA reflected the outcome of the FECRT, with the C. oncophora isolate collected in 2008 being more resistant than the isolate collected in 2007. However, the test should be further optimized, e.g. more isolates with different susceptibility to ivermectin and mixtures of species, in order to use the test in field conditions.(El-Abdellati et al., 2010b)

Studies in rest of world

Faecal Egg Count Reduction Tests (FECRTs) for macrocyclic lactone (ML) and levamisole (LEV) drenches were conducted on two dairy farms in the subtropical, summer rainfall region of eastern Australia to determine if anthelmintic failure contributed to severe gastrointestinal nematode infections observed in weaner calves. Subtropical Cooperia spp, were the dominant nematodes on both farms although significant numbers of Haemonchus placei were also present on Farm 2. The results show the first report in Australia of the failure of macrocyclic lactone treatments to control subtropical Cooperia spp and suspected failure to control H placei in cattle. (Lyndal-Murphy et al., 2010)

Further studies on the level of anthelmintic resistance on 13 commercial cattle properties in south-west Victoria, Australia were reported. Between 2006 and 2009 worm egg count reduction tests were conducted on calves on the 13 properties. Samples were collected 10-14 days post anthelmintic treatment and worm egg counts and larval differentiation tests were conducted. Resistance was defined if there was less than 95% reduction (lower confidence limit <90%) in the faecal worm egg count for the particular genus.

The percentage of properties with anthelmintic resistance in at least one species was 54% for benzimidazole (BZ), 100% for levamisole (LEV) and for ivermectin (IVM) it was 100% for the half-dose (0.1 mg/kg) and 62% for the full dose (0.2 mg/kg). A substantial frequency of resistance was detected in Ostertagia ostertagi to BZ (5/11), LEV (3/3) and IVM (5/11), in Trichostrongylus spp. to BZ (4/7) and in Cooperia spp. to IVM (6/11). No resistance to LEV was detected in Trichostrongylus or Cooperia spp. Suspected IVM-resistant Trichostrongylus spp. and BZ-resistant Cooperia spp. were only detected on one property each. This is the first Australian report of macrocyclic lactone-resistant O. ostertagi in the refereed literature. The frequency of resistance in O. ostertagi to BZ, LEV and IVM and in Trichostrongylus spp. to BZ in the present study appears higher than levels detected in the 2004-05 New Zealand survey, whereas the resistance frequency in Cooperia spp. to IVM and BZ was less (Rendell, 2010).

Studies in USA were also reported. In June 2008, 122 yearling heifers with a history of anthelmintic resistance were obtained from pastures in northern California and transported to a dry lot facility in southwestern Idaho, USA. Fifty heifers with the highest faecal egg counts were selected for study enrolment. Candidates were equally randomized to treatment with either injectable ivermectin, injectable moxidectin, oral fenbendazole, oral oxfendazole, or saline. At 14 days post-treatment, nematodes were recovered from the abomasum, small intestine, and large intestine.

Parasitism was confirmed in the control group when 10/10 animals were infected with adult Ostertagia ostertagi and 900 animals with both developing and early L-4 stages of O. ostertagi. Similarly, 9/10 animals were parasitized with adult Cooperia spp. Fenbendazole and oxfendazole efficacy verses controls were >90% against adult Cooperia spp., while moxidectin caused an 88% parasite reduction post-treatment (P< 0.05). Ivermectin treatment resulted in no reduction in adult Cooperia spp. Based on geometric mean percent reduction versus saline controls, all four treatments were >90% efficacious against adults of O. ostertagi, while moxidectin and fenbendazole were equally effective against developing and inhibited early L-4 stages (P< 0.05). Ivermectin was not efficacious for developing or inhibited early L-4 stages of O. ostertagi. Oxfendazole failed to decrease O. ostertagi developing L-4 larvae by >90% but was efficacious for inhibited early L-4 larvae. Based on the results of this study, a source of multi-species anthelmintic resistance in cattle has been identified in the western United States (Edmonds et al., 2010).

The role of the drug efflux pump, known as P-glycoprotein, in the pharmacokinetic disposition (of the host) and resistance mechanisms (of the target parasites) of the macrocyclic lactone (ML) antiparasitic compounds has been demonstrated. To achieve a deeper comprehension on the relationship between their pharmacokinetic and pharmacodynamic behaviours, workers assessed the comparative effect of loperamide, a well-established P-glycoprotein modulator, on the ivermectin and moxidectin disposition kinetics and efficacy against resistant nematodes in cattle.

Fifty (50) Aberdeen Angus male calves were divided into five (5) experimental groups. Group A remained as an untreated control. Animals in the other experimental groups received ivermectin (Group B) and moxidectin (Group C) given alone or co-administered with loperamide (Groups D and E). Estimation of the anthelmintic efficacy for the different drug treatments was performed by the faecal egg count reduction test (FECRT). Nematode larvae were identified by pooled faecal cultures for each experimental group. Cooperia spp. and Ostertagia spp. were the largely predominant nematode larvae in pre-treatment cultures. A low nematodicidal efficacy (measured by the FECRT) was observed for both ivermectin (23%) and moxidectin (69%) in cattle, which agrees with a high degree of resistance to both molecules.

Cooperia spp. was the most abundant nematode species recovered after the different drug treatments. The egg output reduction values increased from 23% to 50% (ivermectin) and from 69% to 87% (moxidectin) following their co-administration with loperamide. Enhanced systemic concentrations and an altered disposition of both ML in cattle, which correlates with a tendency to increased anthelmintic efficacy, were observed in the presence of loperamide. Overall, the in vivo modulation of P-glycoprotein activity modified the kinetic behaviour and improved the efficacy of the ML against resistant nematodes in cattle. The work provides shows for the first time under field conditions, that modulation of P-glycoprotein may be a valid pharmacological approach to improve the activity and extend the lifespan of these antiparasitic molecules (Lifschitz et al., 2010).

Tests for anthelmintic resistance in cattle

Three different in vitro methods, the Larval Development Test (LDT), the Larval Migration Inhibition Test (LMIT) and the Micromotility Meter Test (MMT) have been adapted to detect anthelmintic resistance in cattle nematodes. Nematode eggs and third stage larvae of different Ostertagia ostertagi and Cooperia oncophora isolates were obtained from faecal cultures of experimentally infected calves. Additionally, adult C. oncophora were evaluated in the MMT for the detection of resistance to ivermectin (IVM).

For all three in vitro tests standard operating procedures (SOPs) were established and successfully used for the detection of responses of non-parasitic and parasitic stages to different anthelmintic substances and the description of dose-response curves. In the LDT ivermectin (IVM) and thiabendazole (TBZ) were tested, in the LMIT IVM and levamisole (LEV) and in the MMT only IVM was evaluated. Susceptible isolates of C. oncophora and O. ostertagi, an IVM-resistant isolate of C. oncophora and a TBZ-selected isolate of O. ostertagi were used in all (C. oncophora) or only some of these tests (O. ostertagi). For all isolates sigmoidal dose-response curves and EC50 values for the tested substances were obtained using a four-parameter logistic model. For the LDT, the previously reported problem in development of larvae was successfully overcome with mean development rates between 80% and 87% in negative controls.

Following optimization of incubation times, temperatures, mesh sizes (LMIT only), nutritive medium (LDT only) and group size (MMT only) all three test systems reliably detected significant differences in the response to IVM between the susceptible and IVM-resistant isolate of C. oncophora (p < 0.0001), resulting in an resistance ratio (RR) value of approximately 5 for IVM and 2.8 for LEV in C. oncophora. The LDT also detected differences in the response to TBZ between the susceptible and BZ-selected O. ostertagi isolates (p < 0.001) with an RR of 2 for TBZ. With the standardization of the described tests we report reproducible and reliable in vitro methods for the detection of resistance to IVM (LDT, LMIT and MMT) and TBZ (LDT) for cattle parasitic nematodes (Demeler et al., 2010).

February 2012