Reduced efficacy of anthelmintic drugs for cattle on pasture?

Background and Objective

Anthelmintic resistance (AR) plays an important role in organic farming, as we are supposed to minimize drug use. If young livestock grows ill, caused by worm infections (they are almost unavoidable in pasture systems), the drugs used have to act safely to avoid suffering and permanent damage to the animals. While the problem of AR is well known among small ruminant farmers and is already involved in many sheep and goat farmer’s management concepts, the spread in European cattle is still unclear.
The aim of our project was to check on-farm the efficacy of two active ingredients (albendazole and ivermectin) in collaboration with University of Veterinary Medicine Hannover and following an EU-wide project (PARASOL).

Approach

In the course of two consecutive grazing seasons every year 10 dairy farms were visited with more than 10 first season grazing calves and examined several times. Among others individual faecal samples were examined for the number of excreted eggs (EPG) of gastrointestinal nematodes (GIN). Using Fecal Egg Count Reduction Tests which detect the differences in the excretion of eggs before and after a deworming treatment, the efficacy of the active ingredient can be described. In addition, depending on the active ingredient, adapted laboratory - methods for determination of AR are performed. Further, the body weight of young cattle was estimated via heart girth measurement. To find out a correlation to EPG could possibly be used to select individual animals for targeted deworming.

Data and Methods

Some of the participating farms answered for the experiment according to an advertisement in a farmers’s journal, the rest were contacted by phone. The trials started when the herds of young cattle had developed sufficient egg excretion in bulk faecal samples. The limit was 100 eggs of GIN per gram of fresh faeces (EPG). Depending on the highest individual EPG - values 10 calves were selected for Faecal Egg Count Reduction Test. The entire herd of young cattle was dewormed, dosed according to individual body weight. In the year 2007 ten farms, including five with organic husbandry, are evaluated using albendazole. The active ingredient Ivermectin was evaluated in the years 2006 and 2007 on 8 and 6 farms, respectively. The albendazole-treated animals were examined again 7 and 14 days after drug input. The ivermectin-treated animals were 14 days, 21 days and 28 or 35 days re-sampled after deworming. This corresponds to the protocol of the World Association for the Advancement of Veterinary Parasitology.
On-station in Trenthorst, all calves were checked simultaneously by heart girth tape (ANImeter®) and body weight by livestock scales. Further, body condition score by Edmonson was detected.
In laboratory, the Larval Migration Inhibition Assay was performed for ivermectin-treated animals and the Egg Hatch Test for albendazole-treated animals. Genus differentiation by PCR from larval cultures and experimental infection of two calves with larvae gained from fecal samples of an AR-suspicious farm are also made.

Results

The results of this field study carried out in 2006 and 2007, provided for the first time evidence of reduced efficacy of ivermectin in deworming first season grazing calves in Germany. The same was observed within the European PARASOL project in Belgium and Sweden.

The active ingredient Ivermectin had revealed reduced efficacy at 4 of 8 farms in the year 2006 and 2 out of 6 farms in the year 2007. The reduction in EPG 14 days after treatment did not meet international criteria for full efficacy.
The drug albendazole revealed at all of the ten other farms no indications for limited efficacy.

The values for body constitution showed that regression from heart girth value on body weight is allowed (R² = 0.965) and between BCS and body weight (R² = 0.30) it is uncertain. Body weight and egg excretion showed no significant correlation.
The in vitro - Methods for determination of AR have been tested and verified. Both methods are suitable in principle. Due to the field presence of mixed infections, the cut-off limits have been identified poorly in this trial and require further research. The PCR resulted in most samples a mixed infection of Cooperia spp. and Ostertagia spp. The infection experiment confirmed the suspicion of AR, but a resistant strain could not be isolated.

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• Dissertation Nina Kleinschmidt