24 August 2001

Beware the lungworm challenge

By Hannah Velten

CATTLE with no previous history of exposure to lungworm are likely to develop clinical signs of husk when turned out on to infected pasture, as producers in south- west Scotland found to their cost.

After BSE culling, several dairy producers in Kirkcudbrightshire bought in two-year-old replacement calving heifers. Some animals developed husk when they were turned out to pasture resulting in a couple of deaths, expensive treatments and low milk yields, says local vet Graham Bell.

"Those animals infected by lungworm had never received a husk vaccine and as they came from relatively dry areas, such as Norwich, France and Holland, had not been exposed to infection so were naive."

The existing Kirkcudbrightshire herds had been vaccinated and, because of the higher worm burdens in this wetter area, had developed immunity against lungworm.

Mr Bell warns those producers with infected pastures, about to restock after foot-and-mouth culling, to check whether cattle have been vaccinated against husk, particularly those coming from dry areas of the country or the Continent.

"Any stock, whatever the age, that are likely to be husk-naive should be housed and vaccinated before spring turn-out."

But Intervets James Allcock warns that when there is a risk of high lungworm burden in pasture, even vaccinated animals from dry areas could need a booster vaccine, as they may not be fully immune because of lack of natural exposure.

Routine worming also does not guarantee cover against infection, unless animals have been challenged, adds Mr Bell. Faecal or blood samples can be used to determine level of immunity, says independent vet consultant Tony Andrews.

Mr Allcock believes that mixing naive youngstock with immune adult cattle can also cause previously husk-free herds to succumb to the disease.

"With a typical 20% replacement rate in herds, infected heifers will produce a huge worm burden on pasture, which could destabilise the natural balance between lungworm challenge and cow immunity."

But cattle producers should be wary of husk, even when stock is not bought in. This years mild, wet weather provides the best possible conditions for lungworm larvae to survive in pasture, says VLA Shrewsburys Graham David. Cases of husk in older cattle have been seen since May and it is anticipated that most infection will occur between August and September.

"Be vigilant, as husk is an unpredictable disease and can enter a previously clean herd via wind-blown larvae from neighbouring farms."

Overstocking on pasture as a result of movement restrictions could also increase risk, he adds.

Sales of husk vaccine have fallen significantly this year, says Mr Allcock, with many producers relying on long-acting wormers in the first grazing season. "Wormers act by reducing the exposure to infection, possibly providing no opportunity for challenge. When drugs become inactive, cattle could be at risk of developing husk in the autumn or in subsequent grazing seasons," he warns.

Any stock coughing at pasture or with breathing difficulties could be suffering from lungworm infection, says Dr Andrews. &#42

HUSK

&#8226 Vaccinate naive stock.

&#8226 Unpredictable disease.

&#8226 Most cases in autumn.

Hidden danger of lungworm… This years mild, damp weather is ideal for larvae survival, and they will challenge husk immunity.