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Lungworm control in calves at grass
Be one step ahead when it comes to calf health during the grazing season. The first thing to do is reflect on last year and plan how to best prevent any likely issues that might flare up again.
Are there pneumonia or respiratory problems in your calves at grass?
Lungworms can be a common cause of pneumonia and bronchitis in youngstock during their first grazing season as, unlike older cows, they have not yet built up a full bank of immunity against these parasites.

Calves attain this infection through ingestion of larvae from grass, once ingested it migrates to the lungs of the calf where the larvae turn into an adult and produces eggs. The eggs hatch and larvae are coughed up, then swallowed and passed out in their faeces. The larvae continue to develop in the dung pat on the fields and the infective larvae are spread onto the grass which is eaten by the next calf that comes along.
Figure 1. Adult lungworms, Dictyocaulus viviparus. Image courtesy of A.R. Walker Wikimedia Commons
Clinical respiratory signs can be seen in calves as early as eight days after the initial infection. Immunity starts to develop from 10 days after a heavy infection.
The worm burden is generally low and manageable in grass at the beginning of the season (March – June), however come July onwards the worm burden increases, and this is when the worms can take over and more animals start showing clinical signs. However, it is important to bear in mind that the worm burden may be higher early on if pasture has been previously heavily grazed.
Tips:
- Keep stocking density low
- Graze youngstock ahead of older animals (as they are more vulnerable) so they aren’t grazing dirty (high worm burden) pasture from older carrier cows.
The weather plays a role in the dispersion of worms across the pasture. A high-risk period for worms is after a spell of warm dry weather followed by rain. Sudden heavy pasture larval challenge can occur because of the larvae sitting in moist cow pats and once the rain falls it disperses the larvae to a wider area leaving less clean areas for the calves to graze.
There are several choices of anthelmintics on the market and these come in the form of a pour on drench or injectable depending on the brand and product. They will all protect the animal for varying periods of time. It is important to liaise with your vet, follow administration instructions carefully and get the timings right to provide the calf with adequate protection.
If choosing to use a long-acting wormer/anthelmintic, one that will last 16 weeks or more it is good practice to adhere to the treatment guidelines which will most likely recommend administering this dose approx. three weeks after the calves are turned out to grass. This three week period allows the calves to be exposed to a low worm level enabling the calf to mount its own immune response to the worms and create their own immunity. The long-acting wormer can then help to mop up any further exposure the calves get as time goes on.
Alternatively, there are other short acting anthelmintics on the market which require more regular application. It is just a case of keeping good records as to when the next dose is required. It is always important to use anthelmintics according to the stated treatment regime and use responsibly bearing in mind the bigger resistance issues.
If there has been previous history of lungworm on the farm, vaccinating your stock against lungworm ahead of turnout can be a great tool to aid in the prevention of calves becoming ill due to high level of exposure. Faecal testing can establish the level of challenge to help develop a vaccination protocol. The calves should be fully vaccinated at least two weeks ahead of turnout to allow the calves time to build up an immune response to the vaccine prior to exposure. However, every farm is different, and an individual plan must be discussed with your local veterinarian.
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