Gastrointestinal parasites

Examples of gastrointestinal parasites in cattle:

  • Brown stomach worm
  • Trichostrongylus axei
  • Cooperia spp
  • Haemonchus placei

Conditions when likely to occur: High rainfall regions (>500mm annual rainfall), high risk above 600mm or irrigation.

High stocking rates, especially where high proportion of grazing pressure on property is cattle.

Cattle less than 15 months of age: after weaning especially over winter and early spring period when peak larval challenge occurs.

Rising 2–3 year-old cows: Type 2 ostertagiasis in summer and autumn, especially in association with calving or nutritional stress.

Older cows: Type 2 ostertagiasis less likely, but depends on property history.

Bulls and bull beef systems are higher risk given their lower immunity to gastrointestinal parasites (consider similar to cattle less than 15 months of age).

If no effective control program in place clinical disease is highly likely in high rainfall areas.

Intensification will exacerbate parasitism without adoption of proper prevention strategies, particularly in the case of bull beef production. History: Poor performance on pasture (growth rates of different classes of cattle is less than expected with known pasture availability and quality).

Clinical signs: Scouring (diarrhoea), weight loss, bottle jaw, anaemia (pale mucous membranes).

Management strategy to prevent disease:

Timing of strategic drenching to prevent gastrointestinal parasites will vary slightly between regions and local conditions. Timing of treatment is critical with younger cattle likely to require additional treatments.

Strategic drenching: (Refer to MLA’s Cattle Parasite Atlas for basic information on the more important internal and external cattle parasites and an integrated approach to internal/external parasite control in various regions).

  • Calves at weaning in all risk situations
  • May/June (>500–600 mm rainfall)

         –      17 months of age for summer drop calves

         –      14 months for autumn drop

         –      11 months for winter drop

         –      8 months for spring drop

  • July/August (>500–600 mm rainfall)

         –      10–13 months for winter–spring drop

  • Rising 2 and 3-year-old cows, all bulls in summer

Other drenches:

  • Pre-calving first calving heifers
  • Based on diagnostic Tool 6.2

Grazing management:

Graze weaners on pastures with low worm larvae contamination.

  • Low worm risk pastures include:

         –      new pasture or fodder crop

         –      pasture grazed by sheep for last 6 months

  • Moderate worm risk pastures include pasture grazed by mature dry cattle
  • High worm risk pastures include pasture grazed by young wormy cattle

Introducing new stock: Drench with a macrocyclic lactone (ML) on arrival.

Observe withholding periods and export slaughter intervals of anthelmintic treatments. These are available at

Macrocyclic lactones (ML) are generally the most commonly used drench type due to high efficacy and persistent activity. Benzimidazoles (BZ) are very useful and are cheaper, although they have to be given by mouth. In situations of high and ongoing larval challenge with stock less than 18 months-old, potent MLs are more desirable. 

Drench resistance is emerging based on trials across south eastern Australia with anthelmintic resistance developing to both BZ and ML drenches. Drench efficacy testing should be undertaken as part of normal management. To minimise the risk of developing drench resistance, ensure cattle are weighed and drenched to heaviest and drenching equipment is calibrated correctly. Drench frequency should be minimised and avoid over reliance on drenches. Plan grazing management to reduce weaned calf exposure to high risk pastures. In addition, total reliance on ML drenches should be avoided; rotate with other drench groups including BZ and combination drenches.

New cattle should ideally be drenched with combinations of drenches or at least ensure the drench used for introduced stock is fully effective by monitoring WECs 10 days after drenching.

Worm faecal egg counts (WEC): Some value in stock less than 12 months old. Be careful with interpretation as cattle with low WEC can sometimes still have high worm burdens, especially with Ostertagia which produces low numbers of worms compared with species such as Cooperia which generally produces higher worm numbers but are less pathogenic. Always consider the clinical condition of cattle.

Field trials to assess growth response from drenching can be very useful if uncertain of parasitological and economic benefit, particularly in regions where gastrointestinal parasites are not considered to be economically important.

A simple tool to evaluate whether drench resistance is present is to test WECs 10 days post drenching to determine if drenches are effective. A more comprehensive test is to set up a drench resistance trial in young cattle with WECs above 200 eggs per gram. Drench resistance can be measured by comparing WECs in undrenched with drenched cattle in groups of at least 15. The standard definition of drench resistance is regarded as when worm egg count reduction is above 95%, though ideally drench reduction should approach 100%.


See the MLA catte parasite atlas (a regional guide to cattle parasite control in Australia) for more information on the small brown stomach worm and other typical parasites affecting cattle, including advice on selecting a worm drench and safe pastures.