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How To Manage Worms On Your Farm

How To Manage Worms On Your Farm

Summer Worms

Worms at grass across the summer months;

  • Gut worms (Ostertagia)
  • Lung worms; Lung worms can occur anytime cattle are at grass, outbreaks trend to occur when a dry spell is followed wet weather. Lungworm is diagnosed when cattle start coughing up the worms- this is an emergency and needs immediate dosing.
  • Rumen fluke can also be an issue in young stock grazing areas that are prone to flooding.  Rumen fluke may not show up in an egg counts as it is the larva that are causing the issue.

In young stock, severe infection can reduce growth rates by up to 30%

Immunity at turnout:

1st year grazing = no immunity, dose when symptoms appear

2nd year grazing = some immunity, dose if symptoms appear, may require 1 dose

3rd year + grazing (Adult cows) = good immunity (unless immuno challenged or poor nutrition), dose if symptoms appear

Do I need to dose?

Calves should be left at least three weeks after turnout to build up natural immunity against gut worms, but ideally calves should only be treated for the first time when symptoms show. Take dung samples and monitor egg counts monthly.

What are the symptoms of stomach worms?

  • Loose dung
  • Poor thrive – A weight gain for calves of 0.7 kgs plus per day indicates a very low risk from parasites.
  • Eggs detected in dung samples.  A faecal egg count of greater than 200 eggs per gram may have an impact on performance and may indicate a need to treat for gut worms
  • Not all the group may be affected just some

What are the symptoms of lung worms?

Lungworm is diagnosed when cattle start coughing up the worms- this is an emergency and needs immediate dosing.

Reducing the risk:

  • Use a leader follower system
  • Don’t force the calves to graze the paddocks out too tight
  • Graze Clean pastures post bales or silage

Control of gut worms;

An effective control strategy should control the worm burden while allowing the calve to build up its natural immunity.

There are currently 3 classes of wormers licenced in Ireland;

  • Benzimidazole (commonly known as white wormers),

These kill the worm on the day but offer no residual effect.  White wormers come as oral doses and in a bolus.

Repidose is a white wormer that offers season long control from handling the calves once while also allowing the calves natural immunity to develop.

  • Levamisole (commonly known as yellow wormers)

These kill the worm on the day but offer no residual effect.  These are available as injectables, pour-ons and oral doses.


  • Macrocyclic lactones (commonly known as clear wormers e.g. ivermectin, moxidectin and eprinomectin).

These products will kill worms on the day of dosing and for the following 2 weeks. Therefore, you would need to dose every 5 weeks with these products (3-8-13 weeks).

These products are available as injectable and pour-ons.  Cydectin LA is a macrocyclic lactone that offers 18 weeks of continuous cover from time of dosing, but it will not allow the calves natural immunity to develop, until the end of the season.

Anthelmintic resistance

The genes responsible for anthelmintic resistance are believed to be present in all worm

populations but at very low levels. When animals are treated with an anthelmintic, this kills all the susceptible worms allowing only the resistant worms to survive.

The surviving resistant worms output eggs with the dung onto pasture which results in resistant worms making up a much greater proportion of the worm population in subsequent generations.


A 2017 Teagasc survey found:

  • Resistance to benzimidazole anthelmintics (white wormers) of 75% on dairy calf-to-beef farms
  • Resistance to macrocyclic lactone anthelmintics (clear wormers) of 100% on dairy calf-to-beef farms

How to avoid resistance

  • Read the manufacturer’s instructions
  • Avoid under dosing animals, weigh cattle if possible to get correct weight.
  • Check dosing equipment to ensure correct amount is applied
  • Avoid the continual use of wormers from the same class and only use a combination anthelmintic product when it is necessary to treat for both fluke and worms
  • Use good biosecurity protocol for all bought in animals
  • Reduce the amount of dosing and allow the animals build immunity.
  • Take dung samples and only dose on rising counts or when animals coughing.
  • Do not dose all the animals in the group target the low weight calves and those with dirty tails.
  • After dosing leave in old paddock and move to clean pasture in 3 days
  • Extend the period between doses by using bales or silage ground
  • Dung sample before and 2 weeks after doses and 1 week for the levamisoles to ensure a response to treatment

Gut worms in milking cows:

Bulk Milk testing for Ostertagia will identify if there is a problem with worms in the herd, However, recommendations for dosing should be done in conjunction with a dung sample.

If the Ostertagia result in milk are over 0.7, there will be value in treating the younger cows in the herd for worms. If the result is higher than 1.0, all cows

will benefit from treatment due to an increased worm burden from the grass.  If your only treat a section of your stock (i.e 1st calver) do not use a pour-on, as untreated animals

can lick the dose from the treated animal back, leading to partial dosing and worm resistance.  If you’re only treating the 1st calvers use an injectable.

Lung Worm in milking cows:

Diagnosis of lungworm in the herd is not as simple as for Ostertagia.  Stomach worms build up slowly leading to increasing egg counts and milk levels.

Lungworm explodes into a full-blown outbreak in a few days. Cows will also start coughing before they start shedding eggs in their dung.

For farms where there is a known risk of lungworm, treatment should be used once a few cows begin to cough. All cows in the herd should be treated for lungworm as the infection will have been picked up by all animals when grazing

Only Eprinomectin products have zero milk withhold in lactating animals.

NOTE: When treating cows for lungworm, even though the product will work quickly, animals may continue to cough for a period of time until all of the dead worms have been cleared from the lungs



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