Changing UK weather patterns and youngstock rearing approaches, as well as an over-reliance on persistent anthelmintics, mean lungworm or ‘husk’ is a serious threat to dairy cattle health once again.
Historically, lungworm infestation – and the parasitic bronchitis it causes – has been most commonly associated with first grazing season youngstock, but a greater proportion of cases are now being seen in herd replacements in their second grazing year and even in adult cows.
According to Colin Mason, veterinary centre manager at the SAC Disease Surveillance Centre (DSC), based in Dumfries, a disease which was once adequately controlled through vaccination many years ago is once again causing many milk producers significant financial losses.
He says: “Parasitic bronchitis caused by lungworm infection is a significant disease threat on many grazing dairy units and, unfortunately, every autumn we see highly costly outbreaks in adult cattle.
“This disease incidence is probably down to a combination of issues, such as reduced vaccine use, over-reliance on long acting wormers and due to young heifers being housed more because farmers are chasing higher growth rates.
“Cattle are simply not having the opportunity to develop an adequate immunity to lungworm.”
He adds that climate change could also be having an effect.
“We know warmer, wetter summers will increase the larval challenge on contaminated pastures, whereas milder, drier autumns allow farmers to keep cattle out for longer. The main problem is this is becoming a very unpredictable disease.”
Parasitic bronchitis is caused through the ingestion of the lungworm parasite Dictyocaulus viviparous, which is widespread on UK pastures, particularly in the wetter western half of the country.
Once the parasitic larvae are in the gut they migrate through its wall and move to the lungs.
Mild, wet weather over the grazing season can create a steady increase in lungworm populations on the pasture. Heavy rainfall aids the dispersion of lungworm larvae from cowpats, further increasing the chances of infection in naïve animals.
The results are inevitably serious to stock which have little or no immunity and disease outbreaks are most commonly seen in the late summer and autumn.
Mr Mason says lungworm outbreaks are becoming so unpredictable that spring vaccination against this devastating disease is now a ‘no-brainer’.
He says: “The best way of preventing parasitic bronchitis in cattle in their first grazing season is immunisation with lungworm vaccine.
“The vaccine is made from lungworm larvae exposed to radiation which are incapable of causing disease. It should be administered orally to calves from eight weeks old prior to turnout.
“Two doses are given four weeks apart and, to allow high levels of immunity to develop, vaccinated calves should be kept inside or off pasture for at least two weeks after the second dose.”
MSD Animal Health, manufacturer of the lungworm vaccine, says it induces a very good immune response in cattle.
Crucially, though, it does not prevent all worms from pasture infections from completing their life cycle - which allows the development of natural immunity. This often fails to occur when there is an over-reliance on long acting anthelmintics.
However, Control of Worms Sustainably (COWS) guidelines state although vaccination against lungworm is effective in preventing clinical disease, it does not completely prevent establishment of small numbers of lungworms. This means pastures may remain contaminated, albeit at a low level.
“For this reason, it is important that where lungworm is endemic on a farm, all calves are vaccinated whether they go out to pasture in the spring or later in the year – and a calf vaccination programme must be continued annually,” says Mr Mason.
COWS guidelines do say control of parasitic bronchitis in calves can be achieved by strategic early season treatments or by the giving of rumen boluses, in much the same way as for the control of parasitic gastroenteritis (gut worm).
“However, a potential downside is through rigorous control in the first grazing season, exposure to lungworm larvae may be so limited cattle remain susceptible to infection. In such situations, it is advisable to consider vaccination prior to the second year at grass.”
Dairy farmers seeing coughing and respiratory disease in cattle at grass should call their vet and arrange to send samples to the lab to confirm the diagnosis of lungworm.