Cattle drench resistance testing results and recommendations for cattle farmers

Page last updated: Monday, 11 March 2019 - 2:12pm

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Drench resistance in cattle worms has been found in tests in several countries in recent years, prompting an investigation into the situation in Western Australia. Testing was conducted on weaner cattle in their first year of grazing on 19 beef producing farms in the southern region of WA, during 2010-2011. The farms tested were spread across the agriculture regions and represent the variety of environments, cattle management and history of drench use.

beef cattle2.JPG

Drench groups tested

At the time of testing three chemical groups that were available for use in cattle in Australia were tested.

These are: macrocyclic lactones (ivermectin), white (benzimidazole -fenbendazole) and clear (levamisole) drench group. Comparisons of drench effectiveness were made to an untreated control group.

Resistance definition

A fully effective drench is one that reduces the worm egg count by 95% or more of parasites of a particular worm species and resistance is considered present if the reduction is less than this.

Resistance test results

Although there are a number of nematode parasites known to be present in WA cattle only the two species that make up the bulk of typical worm burdens are reported here. These two species are Cooperia oncophora (hair worm) and Ostertagia ostertagi (brown stomach worm).

Table 1 % reduction by species
	Cooperia oncophora			Ostertagia ostertagi
Test no	Ivermectin inj.	Fenbendazole	Levamisole	Ivermectin inj.	Fenbendazole	Levamisole
1	90	100	100	100	100	97
2	97	100	96	100	95	68
3	?	?	?	100	100	?
4	100	100	100	100	91	58
5	29	100	100	100	85	84
6	72	x	100	100	97	97
7	90	100	100	x	97	86
8	85	99	100	100	97	98
9	70	98	100	100	97	x
10	90	100	100	x	x	x
11	93	100	100	x	x	x
12	91	100	100	100	91	92
13	100	100	100	100	88	100
14	100	96	100	100	96	78
15	90	100	100	100	100	100
16	100	100	100	100	32	86
17	91	99	100	100	93	89
18	100	100	100	100	74	96
19	89	100	100	100	x	95
Farms with resitance	66% (12/18)	none	none	none	43% (7/16)	53% (8/15)

Note: 'x' denotes less than 10% species present in pre-test larval diff.
'?' denotes unsure, one animal (lev) with post drench count of 400, pre count of 12.5 epg.

Ivermectin resistance in Cooperia oncophora was detected in 66% of farms tested.
Ivermectin remained fully effective against Ostertagia ostertagi on all farms tested.
Fenbendazole (white drench) and levamisole (clear drench) remained fully effective against Cooperia oncophora on all farms tested.
Fenbendazole (white drench) and levamisole (clear drench) against Ostertagia ostertagi was less than fully effective on 43% and 53% of farms tested, respectively.

These results are generally in line with results of other testing carried out in beef producing countries around the world and in Victoria. However, it appears that WA is at a comparatively earlier point along the resistance timeline than many regions and provides early warning of the importance of applying best practice drenching regimes to maintain drench effectiveness.

Although these findings indicate the presence of resistance in the ML group in Cooperia oncophora on more than half of farms tested, it is unlikely that farmers using ML drenches are experiencing a major effect in terms of worm disease or production loss. This is because Cooperia is considered to have a relatively less severe effect than most other species. Importantly, the injectable MLs were fully effective against Ostertagia, which is the more important of the species.

There have been three single active drench groups available for use in cattle in Australia for the previous 30 years. In March 2012, a combination product was registered for use in cattle and this has added another drench group option. A recent survey indicated that an ML containing drench was being used 94% of the time. As drench choice is usually based on the ease of application, pour-ons are most commonly used as the popular choice. Combination products should be incorporated into the mix of possible chemical choices as they have an important role in drench resistance management.

Drench decision making should emphasise the use of drenches only when needed and farmers can refer to DPIRD drenching recommendations for beef cattle when making decisions. Drenching is recommended at set times of the year including in summer for weaners, heifers prior to calving and bulls before and possibly after joining. The majority of the breeding herd as adult cows will have a well developed immunity to parasites and will not benefit from regular drenching. Depending on seasonal conditions, stocking rates and individual animal genetic make-up, some adults within the herd may require an occasional drench treatment.