Anthelmintic resistance in sheep in the semiarid region of Minas Gerais, Brazil

Anthelmintic efficacy was evaluated among sheep that had become naturally infected with gastrointestinal nematodes in 17 flocks located in the semiarid region of Minas Gerais, Brazil. Feces were collected individually from 1021 hairy sheep to determine the number of eggs per gram of feces (EPG) and for coprocultures to identify nematode genera the nematodes. Only the animals that presented EPG counts greater than or equal to 200 were included in the study (totaling 381 sheep). The animals were divided into three treatment groups: albendazole, ivermectin and levamisole. Fourteen days after the administration of anthelmintics, fecal samples were taken from all animals. In each flock, the pre-treatment and post-treatment arithmetic mean EPG were used to calculate the efficacy (FECR) for each of the treatment groups and the lower 95% confidence limit. Data were analyzed with the "eggCounts 2.3" package in RStudio, using a Bayesian model for paired design. The anthelmintics were classified as being efficacious (when the FECR was both equal to or above 95% and the lower 95% confidence limit was equal to or above 90%) or as encountering anthelmintic resistance (when the FECR was below 95% and the lower 95% confidence limit was below 90%) or inconclusive (when none of the other criteria were fulfilled). Albendazole and ivermectin were not effective in any of the flocks. Levamisole was effective against gastrointestinal nematodes in 25% of the flocks studied. Haemonchus, Trichostrongylus and Oesophagostomum genera were identified in this study in a semiarid region of Minas Gerais, Brazil. The genus Haemonchus was the most prevalent, followed by Trichostrongylus and Oesophagostomum. After anthelmintic treatment, the most prevalent genus was Haemonchus, followed by Trichostrongylus; the genus Oesophagostomum was not detected. The highest percentage of Haemonchus larvae was observed after treatment with ivermectin, followed by albendazole and levamisole. This study revealed the existence of gastrointestinal nematodes in sheep that present multiple resistance to all three main classes of anthelmintic drugs.

First Report of Resistance to Ivermectin in Parascaris univalens in Iceland

Horses in Iceland have been isolated for more than 1,000 yr but still harbor a similar range of gastrointestinal parasites as do horses across the world. The long isolation of the horses and their parasites presumably means that no resistance genes have been introduced into the Parascaris spp. population. It is therefore of particular interest to investigate the efficacy of ivermectin on Parascaris spp. infecting Icelandic foals. Potential treatment failure of ivermectin in Iceland will add substantial new information on how resistance can arise independently. This study aimed to determine the efficacy of subcutaneous injection of ivermectin for the treatment of Parascaris spp. infection in foals and to identify the Parascaris species present in the west and north of Iceland. A fecal egg count reduction (FECR) test (FECRT) was performed on 50 foals from 8 farms, including an untreated control group of 6 foals, from September to November 2019. The foals were between 3 and 5 mo of age at the start of the study and had not previously been treated with anthelmintic drugs. Each foal was treated subcutaneously with off-label use of Ivomec® injection 10 mg/ml or Noromectin® 1% at a dose of 0.2 mg/kg. The FECR for each farm was calculated in 2 ways, by the eggCounts package in R and by the Presidente formula (FECRT). Both calculation methods resulted in efficacy levels between 0% and 80.78%, indicating ivermectin resistance on all farms. We also confirmed, by karyotyping, that the species of equine ascarid present in the west and north of Iceland is Parascaris univalens. This study provides evidence for treatment failure of ivermectin against P. univalens infection in foals. Since Icelandic horses have been isolated on the island for more than 1,000 yr, this implies that resistance alleles have developed independently in the Icelandic Parascaris population. The actual clinical impact of ivermectin resistance is unknown but another drug of choice should be considered to treat Parascaris infection in foals in Iceland.

How to improve the standardization and the diagnostic performance of the fecal egg count reduction test?

Although various studies have provided novel insights into how to best design, analyze and interpret a fecal egg count reduction test (FECRT), it is still not straightforward to provide guidance that allows improving both the standardization and the analytical performance of the FECRT across a variety of both animal and nematode species. For example, it has been suggested to recommend a minimum number of eggs to be counted under the microscope (not eggs per gram of feces), but we lack the evidence to recommend any number of eggs that would allow a reliable assessment of drug efficacy. Other aspects that need further research are the methodology of calculating uncertainty intervals (UIs; confidence intervals in case of frequentist methods and credible intervals in case of Bayesian methods) and the criteria of classifying drug efficacy into 'normal', 'suspected' and 'reduced'. The aim of this study is to provide complementary insights into the current knowledge, and to ultimately provide guidance in the development of new standardized guidelines for the FECRT. First, data were generated using a simulation in which the 'true' drug efficacy (TDE) was evaluated by the FECRT under varying scenarios of sample size, analytic sensitivity of the diagnostic technique, and level of both intensity and aggregation of egg excretion. Second, the obtained data were analyzed with the aim (i) to verify which classification criteria allow for reliable detection of reduced drug efficacy, (ii) to identify the UI methodology that yields the most reliable assessment of drug efficacy (coverage of TDE) and detection of reduced drug efficacy, and (iii) to determine the required sample size and number of eggs counted under the microscope that optimizes the detection of reduced efficacy. Our results confirm that the currently recommended criteria for classifying drug efficacy are the most appropriate. Additionally, the UI methodologies we tested varied in coverage and ability to detect reduced drug efficacy, thus a combination of UI methodologies is recommended to assess the uncertainty across all scenarios of drug efficacy estimates. Finally, based on our model estimates we were able to determine the required number of eggs to count for each sample size, enabling investigators to optimize the probability of correctly classifying a theoretical TDE while minimizing both financial and technical resources.