Vet Watch: Getting your worming strategy right

Turnout time is near and for calves the first grazing season can be challenging with regards to parasite infestations.

Even though we have had one of the coldest winters for some time, this is unlikely to have reduced pasture contamination with worm larvae.

Ostertagia larvae (the main cattle gut worm) tends to survive better in colder winters. Therefore, it’s essential a good parasite control programme is in place – one that allows immunity to develop without causing production loss.

The increased frequency of lungworm cases and production losses in dairy cows due to gut worms implies youngstock don’t always get a chance to develop a proper immunity before entering the milking herd. It has been suggested our modern wormers might be too efficient at giving short term protection, which doesn’t allow for immunity to build up. Immunity against Ostertagia can take more than two years to fully develop and this should be an important consideration in any parasite control programme.

Gutworm

Every year calves pick up low levels of over-wintering larvae in the spring, which develop into adult worms and lay eggs. The rate at which these eggs develop into infectious larvae depends mainly on the temperature – the higher the temperature the quicker they mature.

Due to the increasing average temperature most of the larvae become infectious more or less at the same time, which is at the end of June. In dry summers this peak might be slightly delayed. However, it does give us the opportunity to maximise our protection in the summer when it is needed the most, allowing immunity to build up in the early and late season.

There are various products and protocols giving good protection and your vet can advise on the most appropriate choice for your farm. Some farms may only require a strategic dose late June, whereas others will have to rely on a full three month’s cover. One of the main considerations will be to not overprotect by using a product offering longer protection than the remaining grazing season.

Additionally calves can be moved to “safer” pastures in July (aftermath or pastures not grazed by young stock before) if heavy pasture contamination is suspected.

Lungworm

Control programmes against lungworm using wormers are often disappointing, due to the unpredictable nature of the pasture contamination with lungworm larvae. The build up of infection can be quick and is dependent on weather conditions, making it impossible to predict when pasture infection reaches dangerous levels. This is further complicated by the fact that only small amounts of lungworm can cause clinical symptoms. Any control programme therefore, should give protection from the moment calves are turned out.

The best way of protecting animals against clinical disease is by vaccination. This will only give the animal a limited amount of immunity, which needs further challenging by exposure to natural lungworm larvae during the grazing season to fully develop.

Control programme

• Vaccinate first grazing calves before turn out
• Delay turn out until mid May if possible to reduce the over-wintering larvae population
• Avoid grazing calves with older youngstock as they can carry over-wintering larvae, which can increase pasture contamination dramatically
• Use wormers during the summer months July to September, depending on turn out date, weather conditions, farm history and pasture management
• House calves by the end of October and worm after housing to reduce the amount of over-wintering larvae. This could be combined with a liver fluke treatment, as recent wet summers have also seen an increase in fluke