Epidemiology of lungworm infection in cattle

What influences the level of challenge?

 

Farm history

It has been suggested that a farm’s history of lungworm outbreaks is a reliable indicator of the likely degree of future challenge – and it is certainly true that larvae can survive a long time on pasture.

However, following extensive analysis of countrywide data it now seems more plausible that the mere presence of cattle predisposes that pasture to lungworm, even if only at very low levels. The parasite and its host have a long-established relationship, and it is only a sudden change in circumstances or immunity status which allows lungworm growth to accelerate to pathological levels.

Where there are cattle, there is likely to be lungworm.

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Humidity and rainfall

Rain can disperse larvae in contaminated faeces, while warm, moist conditions keep infective larvae alive and encourage fungal growth. Larvae often make use of the fungal spore of Pilobolus to disperse themselves on pasture. Generally, conditions which favour the growth of pasture also favour development of the infective larval stage L3, which is why outbreaks peak in the late summer and early autumn.

A dry season followed by a damp one has always encouraged outbreaks, as this creates a natural immunity gap. This entirely natural situation is one we are now mimicking through our long acting worming policies.

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Geographical location

Thanks to varying climates and cattle densities, some counties are more prone to infection than others.

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Replacement stock

The introduction of immunologically-naïve replacement stock can severely destabilise a herd’s balance of immunity. In such circumstances a much lower level of existing challenge is sufficient to cause an outbreak.

The introduction of ‘carrier’ animals among replacements is thought to be less of a problem, although it has been recorded.

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Off farm or reserved pasture for replacement stock

A growing trend is to graze dairy replacements on separate pasture, away from the main herd. Frequently this stock is subject to continuous worming, perhaps in both the first and second seasons. When the replacement group finally enters the main herd, they are completely naïve and an outbreak is highly likely.

This situation contrasts strongly with beef suckler herds, where calves are exposed to continuous low level challenge from their immunologically-competent adult companions.

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