Controlling lungworm in dairy herds can be frustrating as outbreaks can be unpredictable. Lungworm (hoose) problems usually arise later in the grazing season from late summer to autumn but outbreaks have been seen earlier after warm, wet weather conditions, which favour worm development.
Close monitoring and treatment remain the most important factors in terms of lungworm control.
Clinical signs of lungworm
Initially, animals may show an increased rate or difficulty in breathing, although coughing is usually the first sign seen. These signs may be more noticeable when animals are being moved.
Open mouth breathing with the tongue protruding and the neck extended are signs of breathing difficulties. In dairy cows, drop in milk yield may be the first sign, although this can have many causes and is not specific to lungworm.
Lungworm can result in sudden deaths if animals are affected by a large number of worms. The coughing can develop into pneumonia and, due to lung damage, some animals may take a long time to recover even after wormer treatment. Additional treatments may be needed for these animals under the guidance of your veterinary practitioner.
Lungworm in young stock
Animals usually develop immunity to lungworm over time and first-season grazing calves or autumn-born calves are therefore at highest risk.
Lungworm are similar to gut worms in that there is a buildup of infective larvae on the grass over the grazing season, with their survival dependent on the weather, especially warm and wet conditions.
As calves increase their grass uptake, they will also consume more lungworm larvae but may not yet have developed a protective immunity and so can develop clinical disease. Pastures that were grazed by calves in the preceding year are at higher risk of contamination due to over-wintering of larvae.
Planned treatments can prevent buildup of larvae on the pastures and all of the main wormers are effective against lungworm. Ivermectins have a persistent activity against lungworm and will be protective against new infections for a number of weeks.
Calves with a lungworm infection may have increased susceptibility to other respiratory infections caused by viruses and bacteria, which can be problematic for calves if they are already under stress such as at transport, weaning and housing.
Dung samples are not always helpful to diagnose lungworm and a negative test does not rule out a lungworm infection because the immature larval stages of the worm can cause clinical signs.
High-risk animals should be closely monitored for signs of coughing at grass and dosed immediately.
Clinical signs from lungworm are becoming more common in adult animals. There are two main reasons why adults may be getting lungworm and these are related to immunity.
Firstly, dairy cows may not have been sufficiently exposed as calves to develop adequate immunity. This might happen where heifers are reared off-farm on land without lungworm.
When they return to grazing with the main herd, these animals might develop lungworm infection, similar to calves, along with noticeable losses in milk yield and body condition.
Alternatively, intensive wormer treatments given in their first grazing season may also prevent the exposure necessary to develop immunity. In following seasons, when treatments become less intense, the animals may now be at risk of infection. This is usually an issue in second-season grazers but can also extend to later grazing seasons.
Secondly, the naturally acquired immunity to lungworm is short-lived without follow-up exposure and as it wanes, cows may develop clinical signs.
High-pasture lungworm burdens especially can overwhelm the animal’s defences. Clinical signs can be severe in these animals – thought to be as a result of a strong reaction to adult parasites in the lungs.
Immunity to lungworm lasts for a few months and a long housing period combined with persistent wormers, for example, may prevent the natural boost needed to maintain immunity.
Vaccination should be considered where lungworm is a repeated problem – this can be given to calves before the grazing season, to returning stock or as booster vaccinations for older animals.
However, vaccinating spring-born calves can be a challenge as they need two vaccine doses before turnout.
Vaccinated animals need some exposure to maintain their immunity but can also be overcome by high infection rates. Anthelmintic treatments can interfere with vaccine effectiveness and should not be given at or before vaccination.
Animals returning to the farm or bought-in replacement heifers can be a source of lungworm infection and animals should undergo a quarantine treatment or vaccination as appropriate.
Discuss the best treatment and prevention protocols for the risk factors present on your farm with your veterinary practitioner.