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Control of Internal Parasites in Calves

30 June 2017
Type Media Article

By Anthony O'Connor, Teagasc Advisor, Galway/Clare

A good dosing regime is critical to effective management of worm infestation in calves. 

Stomach worms and lungworms (hoose) are the two main parasites of calves at grass. Both are favoured by warm humid conditions. In the absence of a control programme there is a steady build up of stomach worm larvae on grazing pasture through the months of May, June and July. Lungworm or Hoose outbreaks tend to occur a bit later in the summer.

Stomach Worms:

Stomach worms in calves causes scouring, lack of thrive and in severe cases anaemia.
The reduction in growth rate could be as high as 50% which is a serious loss in performance.  


July is the month when dosing will begin for many spring-born calves. Dairy-born calves will be much more susceptible to stomach worms than suckler calves because there is a dilution effect from the cow and they are less reliant on grass in their diet at this stage in their life. Lungworm will have to be dealt with once calves begin coughing.

Faecal Sampling: 

Taking dung samples to assess whether to dose or not will tell you what the worm burden is like in the calves while pinpointing which parasite is causing problems, whether it is Stomach Worms, Lungworms or Liver Fluke.  In consultation with your Vet, pooling samples from 10-15 calves and having samples analysed at a Veterinary Laboratory will be worth doing. In summary, faecal sampling is a more targeted approach to dosing.

Veterinary Treatment:  

By now there could be significant levels of infection in calves, so treatment could be required especially on the higher stocking rates.  Spring-born suckled calves should receive their first worm dose from late July onwards.  There are a wide range of anthelmintic drugs available to control worms in summer.

Benzimidazoles (white drenches) and Levamisoles (yellow drenches) have no persistency and will only kill lung and stomach worms present on the day of the dose.  Avermectins, on the other hand, have persistency and these ranges from three to six weeks, depending on the type of product used, or Chloromectins (combination products for the control of worms and Liver Fluke) can be used.

Consult your Vet on the veterinary product best suited to use on your herd. Pay particular attention to manufacturer's instructions and withdrawal periods for stock.   Whatever veterinary product used, it should be effective against both adult worms and larvae. If there is a Veterinary plan or protocol existing on your farm, then follow recommendations carefully. 

Lungworm or Hoose Infection: 

Lungworm infection is less predictable than stomach worms. The symptoms are a husky cough especially when calves are being moved. It also causes a reduction in growth rate and makes calves more susceptible to respiratory diseases due to lung damage. Thus, it is important to treat calves for lungworms well in advance of weaning. Lungworms can be controlled by dosing in July and again in the weeks before sale. The drugs that control stomach worms also cover lungworms. 

Drug Resistance: 

Research at Teagasc, Grange has shown over the last few years that there is drug resistance building amongst worm populations. To prevent the build-up of drug resistance, calves should get the full recommended dose rate according to animal weight as per manufacturer's instructions. Another suggestion is that the drug group type be changed each year. If you plan to use a Levamisole now, switch to an Avermectin or a white dose for the next dose but keep alternating. Your Vet will advise you on the differences between the various drug groups.