Cattle Parasite Management

Worm management for cattle in Spring

Spring is when you need to be watching for internal parasites in cattle, especially in those less than 2 years of age. If weaners and yearling animals have not already been treated, then now is the time to drench for worms and adult fluke.

If it is unclear if fluke is present, then a test can tell what, if any, treatments are needed (contact your District Veterinarian). Weaners and yearlings should receive a treatment in late spring to reduce the burdens of the small brown stomach worm (Ostertagia) which is the most serious of all the internal parasites in cattle.

This parasite is picked up in spring and either develops rapidly to an adult worm or can inhibit (hibernate) in the lining of the fourth stomach. Inhibition is especially important in stock moving through their second summer of grazing. A treatment program can return 4-6 times the drench cost in increased weight gain so is well worthwhile economically. Your District Veterinarian can help advise on what program to use for your area.

A good reference and regional specific guide can be downloaded on the MLA website at

Drenching for internal parasites

It is important when utilising chemical control for worms in cattle to observe the ACTIVE INGREDIENT instead of just the trade name.  The active ingredient is always listed on the label, usually under the trade name.

Drenches for cattle can be divided into three classes:

  • Macrocyclic Lactones (ML; names of active ingredients end with “-ectin”)
    • These are the most frequently used class of chemical due to ease of application (injectable or pour-on), wide range of effectiveness (they often cover ectoparasites such as flies and ticks as well as internal parasites), and extended efficacy.
    • However they are expensive, and there is potentially emerging resistance to this class of chemical across northern NSW.
  • Benzimidizole (BZ or “white” drenches; names of active ingredients end with “-zole”)
    • The BZ drenches are significantly cheaper than ML drenches, and often more effective against the economically significant worm species in our region.  However they are only available as oral drenches, requiring increased labour inputs for treatment.  There is some reported resistance within this class of drench.
  • Levamisole (Lev or “clear” drench)
    • Highly effective against cooperia, and some (off-label) effectiveness against stomach fluke.  Predominantly oral formulation, there is one pour-on combination version available.

Drenching programs should focus on young cattle up to around 18 months old, with treatment of adult cattle only on an as-needed basis when symptomatic.  Breakdowns in adult immunity tend to occur secondary to stressful events such as drought, nutritional deficiencies, calving, and occasionally in breeding bulls.  There is little evidence to support routine drenching of adult cattle, and it is only of economic benefit in symptomatic animals.

Ineffective drenching is a common occurrence, and it is important to differentiate this from true drench resistance in worm populations.  Ineffective drenching can occur when animals are treated with short-acting products and then immediately returned to heavily infested paddocks, or when they are dosed at sub-lethal levels

  • when weight is misjudged and animals are under-dosed
  • rain occurs soon after a pour-on product is applied
  • animals have dirty/dusty coat when pour-on products are applied

True resistance is when the worm population develops genetic resistance to the chemical active ingredient in the drench product, and will survive treatment despite appropriate dosing and application of product.  It is important to differentiate true resistance from rapid reinfestation. Risk factors which select for resistant survivors within a worm population include

  • Frequent drenching (especially without rotating drench type)
  • Long-acting formulations
  • Sub-lethal doses (under-dosing or the tail end of long-acting formulations)
  • Unnecessary drenching (treating animals who have a low worm burden, or drenching during periods unfavourable for worm survival)

Times when a drench is strongly recommended are at (and often again soon after) weaning, and a quarantine drench when new animals are brought onto the property.  It is important to ensure a quarantine drench is highly effective, and a drench with multiple actives from different classes is recommended for this.

It is also important to consider a drench’s effect on the dung beetle population of a property, as dung beetles are an important part of an integrated pest management strategy.

What drench to use?

The options can be bewildering. These can be given as a pour-on along the back midline (more convenient but more expensive), as injectables or by mouth (the latter is less convenient and requires a good crush).

Most pour-on products contain macrocyclic lactones (MLs) as the active ingredient (e.g., abamectin, doramectin, ivermectin, moxidectin, eprinomectin). Levamisole is also available in this form. While these are normally highly effective there is resistance developing in some parasites (see later for discussion). Levamisole has been added to some products to help overcomes this issue. The duration of the effect of the treatment varies according to product and parasite being targeted. Pour-on administration can lead to variable results due to the product being licked off by some animals (and not by others).

Injectables mostly contain similar macrocyclic lactones (MLs) to the pour-on products. They may have a flukicide added (e.g., triclabendazole, clorsulon, nitroxynil – these only need to be used if this parasite is present) or levamisole (to help manage resistance). The duration of effect varies greatly according to the active ingredient and targeted parasite.

Oral products have several different active ingredients and include benzimidazoles (so called white drenches), levamisole or macrocyclic lactones (ivermectin, abamectin) or a combination of all three. Some also have a flukicide added.

If it is unclear what drench to use (active ingredient(s), parasites to target or method of treatment) then consult with your District Veterinarian.

Drench resistance

Some genera and strains of internal parasites have developed resistance to the various active ingredients. This is not universal and can be quite sporadic. In some less pathogenic parasites this can be less relevant.

One technique used to assess if resistance is present is to check for egg counts in faeces taken 10-14 days after treatment (as is done routinely in sheep). This is appropriate for some parasites and in young cattle but not for all parasites.

If the response to treatment has not been as good as you expect then it is best to consult with your district veterinarian to help investigate further.

Worm testing and strategic drenching

Strategic drenching is where worm testing is utilised to ensure animals are only treated when worm burdens are likely to be pathogenic (reducing production).  Strategic drenching targets high risk (usually young) cattle over lower risk animals.

Worm testing is a method to “measure” the likely worm burden in animals.  A number of faecal samples are taken from individuals or randomly from the mob, and the number of worm eggs per gram (epg ) are calculated.  This can assist in not only selecting when is an appropriate time to drench animals (“strategic drenching”), but can also be utilised for a “drench test” to ensure a drench has been effective (a drench is classed as having been effective if it kills >95% of worms).

Integrated pest management

Integrated Pest Management focuses on increasing animals’ resistance to worms, reducing larval challenge within the environment (creating “safe” pastures), combined with strategic drenching as-needed.

Animal Resistance Factors

  • Genetic selection
    • Worm resistance is heritable, so if possible select animals who consistently have lower WEC  for breeding
  • Nutrition
    • Well-nourished animals are able to develop better immunity to worms
  • Stress
    • Stressed animals are less able to resist worm infestation, and more likely to succumb to disease at lower worm levels.  Stress can be caused by poor nutrition, increased metabolic demands (growth, pregnancy, calving) as well as management (high stocking density) and environmental factors (cold weather, prolonged wet weather, drought)
  • “Safe” pastures can increase interval between chemical treatments by reducing exposure to infective larvae
    • Long rests (although in this area larvae can survive up to nine months on pasture, which is often an impractically long time to have pastures out of grazing rotation)
    • Hay or silage making removes large numbers of eggs and larvae, as well as making conditions unfavourable for survival of remaining parasites
    • Cropping rotation or newly established pastures reduce parasite burdens due to soil disturbance as well as the long period ungrazed while the crop or pasture establishes.
    • Graze “long” pastures – most worm larvae are low down near the ground, so overgrazing increases risk of larval ingestion.
    • Rotation with other species such as sheep or horses as these animals do not share worm species with cattle.
    • Rotation of high risk young animals with adult low risk cattle for six months can also be effective
    • Avoid returning drenched animals to “dirty” paddocks
    • Avoid constantly running young animals on the same paddocks year after year (Calf nurseries)

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