Skip to main content
SLU publication database (SLUpub)

Other publication2012

Limited efficacy of pour-on anthelmintics for cattle under Swedish field conditions

Areskog, Marlene; Ljungström, Bitte; Höglund, Johan

Abstract

It is commonly believed that deworming against gastrointestinal nematodes in cattle works well in Sweden. Globally, anthelmintic resistance is a well-known threat, due to years of excessive use of anthelmintics leading to genetic selection of resistant worms. However, other explanations to treatment failure may be present. A North European study published 2009 showed that the treatment effect, evaluated with a so called "faecal egg count reduction test" (FECRT) in herds on pasture, was lower than expected (Demeler et al 2009, Veterinary parasitology, 144. 74-80). As an attempt to further investigate the situation in Sweden, we investigated the effect of ivermectin (Ivomec pour-on®, Noromectin pour-on ®) and doramectin (Dectomax pour-on®) on the two most important parasitic gastrointestinal nematodes (Cooperia oncophora and Ostertagia ostertagi) in Swedish cattle herds.

The study was performed during the grazing season of 2010 and herds were recruited through advertisement in the farming press. Both dairy and beef cattle herds were included, and when the farmers requisitioned sample material they answered a questionnaire about their herd management. Four to six weeks after cattle turnout, 64 farmers sent in individual fecal samples collected from fresh dung pats from six first season grazers (FSG) for egg counts and larval cultures. In herds with a mean EPG (eggs per gram faeces) of ≥100, advice was given to apply anthelmintic treatment with either ivermectin or doramectin pour-on to all FSG in the tested grazing group. The animals were dewormed by the farmers in accordance with the manufacturer's dosage recommendations.

Faecal egg counts conducted 7 to 14 days post turnout (McMaster-method, sensitivity ≥ 20 EPG) revealed a mean of 119 EPG (min <20, max 1247) before treatment. Among the tested herds, 27 (42%) had an EPG higher than 100 EPG and the arithmetic mean was 248 EPG. Dairy herds (n=11) and beef herds (n=16) had a similar mean egg count of 283 EPG and 227 EPG respectively. In total, 26 herds were retested 7-14 days post treatment. The mean reduction was 87% (min 50%, max 100%) and 10 herds had a mean reduction of ≥ 95 %. Maximal reduction (100%), was seen in only eight herds. Beef herds had a mean reduction of 84%, which was slightly lower than the 92% reduction in the dairy herds. No significant difference (P=0.664) was seen between the groups treated with Ivomec pour-on® (n=8), Noromectin pour-on® (n=10) or Dectomax pour-on® (n=8). Larvae from post-treatment cultures were saved to determine the predominant surviving species using a species-specific ITS2 real time PCR. C. oncophora was the predominant species but five groups showed strong positive results for O. ostertagi, while the other 21 groups were weakly positive. Overall, this study showed that the efficacy of avermectin pour-on formulations was inadequate according to the FECRT standard (≥ 95% reduction). Only 42% of the herds had ≥100 EPG, which supports the concept of targeted selected treatments rather than blanket treatment without conducting diagnosis. This study also indicates that beef herds should consider parasite control in the beginning of the grazing period to a greater extent than is done in Sweden today. The dose limiting species C. oncophora was, in most cases, the survivor following anthelmintic treatment. However, it cannot be excluded that this may also reflect the development of resistance or altered pharmacodynamics towards avermectin compounds, especially on the few farms where the more pathogen O. ostertagi survived. Complementary laboratory investigations including pharmacokinetic studies of drug administration and uptake might further explain the poor efficacy under field conditions.

Published in

NJF Report
2012, Volume: 8, number: 7, pages: 49-50