Climate change and dairy management practices mean the threat of lungworm to UK herds is increasing. We look at the potential impact of the disease and the steps producers can take to prevent outbreaks.
By Rebecca Dawson
Lungworm is an unpredictable and potentially deadly parasite. It has a widespread distribution across the UK, but producers may not be aware of their herd’s lungworm status since its presence does not always lead to clinical disease.
Cattle become infected with lungworm (Dictyocaulus viviparus) by ingesting larvae from contaminated pasture. The worm’s development outside the host is heavily influenced by weather conditions, which in turn determine patterns of disease. Warm, wet weather favours development and survival of larvae, while hot and dry weather causes desiccation and death of larvae, which have migrated onto the grass in a matter of days, reducing the risk of infection.
“That said, larvae may remain protected within the moist core of crusted dung pats and sudden heavy rain can lead to their release onto the pasture,” says Boehringer Ingelheim Animal Health’s vet Sioned Timothy. “This can result in a high challenge and potential outbreaks of parasitic bronchitis or husk.”
Changing patterns of lungworm disease may mean the threat lungworm poses to UK dairy herds is increasing. During the past 40 years there has been an increase in the number of reported cases, particularly in Scotland. “Outbreaks are now seen in spring and autumn, outside the traditional window of between July and August, particularly in Wales and the south west regions. To compound this, husk has moved from being a young stock disease to a condition that also frequently affects adult dairy cattle.
“The reasons for this shift are poorly understood. Factors including climate change and management practices may have played a role. But producers cannot afford not ignore it and must address the specific risk of lungworm as part of a herd parasite control plan,” stresses Ms Timothy.
After ingestion, the larvae break out of the small intestine and migrate through the lymphatic system to the lungs. They mature as they travel up through the airways, developing to egg-laying adults within three or four weeks of infection.
Low-level exposure to the parasite can induce immunity without disease, but a heavy challenge can result in severe respiratory issues. Clinical signs, such as coughing and increased respiratory rate, can occur as soon as the larvae reach the lungs – as early as 10 days post-infection. Severe cases can lead to secondary infection, permanent lung damage, and even death.
“As well as causing clinical disease, lungworm can significantly impact dairy cow performance. A sudden milk drop can precede other clinical signs,” says Ms Timothy. “The overall economic impact of an outbreak has been estimated at approximately £140 per adult cow in the herd, with milk production losses amounting to 4kg per cow per day.”
She adds that whether or not lungworm causes clinical disease rest on achieving a fine balance between challenge and immunity. Exposed animals develop two levels of immunity.
“One is a first line of defence, which prevents immature larvae from migrating to the lungs. This develops rapidly after exposure (or vaccination), but is relatively short-lived, persisting for approximately six months.
“A second level of defence, which eliminates larvae and adults from the lungs, is slower to develop but can persist for years after infection.”
Although the reasons behind the increased risk of disease in adult cows are poorly understood, it is likely to stem from reduced or destabilised immunity. “Many factors can contribute to this, including changes in heifer management that limit exposure to lungworm before they enter the milking herd. Rotational grazing practices can lead to heavy pasture contamination and the introduction of naïve animals to an infected herd, or infected animals to a naïve herd, can rapidly result in disease.
“Re-infection syndrome, which occurs when cattle are exposed to a lungworm challenge after their first-line immunity to infection has waned, can be an important component of disease in adult cattle. In these animals the long-lasting second level of immunity prevents invading larvae from developing into adults, but can result in severe lung inflammation,” she explains.
Adopting an integrated approach to lungworm control is essential, in both dairy young stock and adult cows, on farms where lungworm is endemic. The objective should be to reduce reliance on treatments by assessing and managing the risk and promoting the development of effective immunity.
“Vaccination can be a useful first step in cattle of all ages. But it can also pose practical challenges as both doses must be administered ahead of turnout, and ongoing protection relies on low-level exposure to boost immunity during the grazing season.
“Strategic gutworm treatments, administered during the grazing season, may also control lungworm. But there is no guarantee the risk period for these different types of parasite will coincide, and disease may still occur at other times of the year.”
She stresses that a vigilance-based approach can overcome this risk. “Often the earliest signs of lungworm in dairy cows at grass is a deep and dry cough, heard when they are brought in for milking.
“Picking up on this early, and seeking advice from a vet or animal health adviser, can help ensure early and targeted treatment of the whole group. Testing of faecal samples for the presence of larvae can confirm the diagnosis, although they may not be detectable early in the course of disease or in cases of reinfection syndrome.”
When it comes to treatment, products from all the three major wormer groups, are effective against adult and immature stages of lungworm. But macrocyclic lactones have persistent effects, which can protect cattle from further infection for a period after treatment. “Practical considerations may also influence product choice,” says Ms Timothy. “A topical treatment, for example, may be preferable to an oral drench in animals that have respiratory compromise. And remember only products containing eprinomectin have a zero milk withhold, allowing treatment at the optimal time, without concerns about lost milk.”