Weather and Worms
Knowing how the weather impacts parasites can be a useful tool in any farmer’s box. The risk of infection is often seasonal as parasites are weather dependant, spending most of their life-cycle in the environment outside of the animal host.
Roundworms
Winter survival of eggs and larvae on the pastures influences infection rates at turnout in the spring. Typically, we see peak worm burdens in July and August in summer rainfall areas. Whilst very dry periods can kill off larvae on the ground, worms can survive by migrating down into the soil or within dung pats. When there is a burst of rain after this period, there can be an explosion of worms – often seen with lungworm.
Liver and rumen fluke
The mud snail is the core of the fluke life cycle, being used as an intermediate host and thrives in wet weather in spring and summer. Once the fluke are shed from the snail they can infect grazing cattle. This usually happens from autumn to winter but animals can be infected early in the spring season by liver fluke that overwinter in the snails or on the grass. Harsh, cold weather or droughts kill off the mud snails therefore reducing the liver fluke.
Knowing how to react post weather events, for example:
- A dry spell followed by heavy rainfall - be aware to watch out for clinical signs in your calves such as coughing (lungworm).
- After a cold winter – possible higher worm infections earlier in season – start faecal sampling earlier to monitor.
- If it’s been a wet summer, factor in your “flukey” fields, try to avoid them or if this is not possible, try to fence off the wettest areas.
- Monitor animals closely when worm burden peaks are expected.
There is no one size fits all in the case of parasite control, rainfall differs around the country, soil type can be different just a field away, etc. Each farm has their own specific circumstances, meaning an individual approach is needed. Having tools such as understanding weather effects on parasites can make the job a lot easier.
Additional information
What other tools are available?
In some countries, parasite forecasts are calculated and released. These give an estimate of the parasite risk based on weather conditions that year.
Examples of recent parasite forecasts:
Nematodirus forecast Ireland: https://www.gov.ie/en/department-of-agriculture-food-and-the-marine/press-releases/nematodirus-forecast-2025/
Nematodirus forecast UK: https://www.scops.org.uk/forecasts/historic-data/2025-data/
Liver fluke forecast Ireland: https://www.gov.ie/en/department-of-agriculture-food-and-the-marine/press-releases/liver-fluke-forecast-november-2025/
Sustainable Parasite Control in Grazing Ruminants
Ongoing | 2024-2027
- Main funding source
- Horizon Europe (EU Research and Innovation Programme)
- Geographical location
- Belgium, France, Spain, Poland, Italy, Netherlands, Greece, Ireland, United Kingdom