Coughing cows from mid summer onwards - is lungworm responsible?

Having dairy and suckler cows milking on grass should be one of the most profitable periods of the year for most Irish farmers. However, over the last five years, many dairy farms have noticed outbreaks of coughing among adult animals from July onwards.

The severity and extent of the problem varies between farms, but it can persist for weeks, be sufficiently marked to cause milk drop across the group and sometimes lead to small numbers of clinically ill animals.

The exact cause for this condition is unresolved, although the involvement of lungworm is currently strongly suspected. So, could lungworm really be making a resurgence in mature cows and why now?

Life cycle

Lungworm is unusual in that, off the animal, the parasite is infectious primarily as larvae and not as an egg. Lungworm larvae actually move up blades of grass to maximise the chance they are eaten by cattle.

Once eaten, they migrate through the gut wall to the animal’s lungs, which takes about two weeks. In the lungs, they establish in the windpipe and larger airways and begin laying eggs – usually by a further one to two weeks.

The eggs are then coughed up and swallowed by the animal. They hatch in the animal’s stomach and live larvae are passed out in the pat and the whole cycle begins again.

Lungworms operate quite differently to stomach worms – relatively few overwinter on grass, with carrier animals being the principal source between grazing seasons, and some larvae becoming inhibited inside the animal as temperatures drop in autumn.

Animals usually develop immunity as they are exposed to micro-organisms and parasites. Parasite control is essentially a careful balancing act between worm numbers and animal immunity.

Again, lungworm is unusual in that there are two independent phases of immunity – a short-lived migration immunity, which prevents larvae moving from the gut to the lungs, and a much longer-lasting maturation immunity, which prevents adult lungworms maturing to egg laying in the lungs.

This is important because each year the migration immunity can fade during housing, so that even adult cows are susceptible come the new grazing season.

Cause of coughing

As lungworms tend to overwinter poorly, there’s no low-level exposure instantly available on early grass to build up that migration immunity again. In fact, if worming occurs at turn-out, it can miss having any significant effect against lungworm.

However, lungworm larvae numbers accelerate very quickly through spring, so that weeks later, huge waves of consumed larvae migrate from gut to lung unhindered due to lack of migration immunity. It is this migration which is thought to lead to the outbreaks of coughing in cows at grass.

Most of these lungworms never get to the egg-laying stage, as the maturation immunity is still in place, which makes the condition hard to diagnose – but with clinical disease still very apparent.

Too much worming

Too much worming as a calf, weanling and heifer can short-cut natural exposure, so that the animal arrives into adulthood over-protected with no real natural immunity.

Essentially, such cattle could be meeting lungworm larvae, without anthelmintic assistance, for the first time in their first or second lactation.

Inevitably, this type of immunity imbalance could further exacerbate the situation described above, so over- zealous worming regimes in early life may also be part of the problem.

Dose properly

As ever, both under-dosing and over-dosing of wormers needs to be avoided. Weighing scales are the ideal option but remain scarce on Irish farms.

It’s easy enough to self-test your ability to estimate cattle weights at the mart or factory. Many have been surprised!

Remember, too, that a dairy cow’s udder can weigh 50kg or more at peak lactation.

And whatever the product – drenches, injectables and especially pour-on types – it is important to follow the instructions provided.

Typically, too much soiling with mud or manure may affect the efficiency of pour-ons. Time is money – all of the effort in dosing, plus the cost of the wormer, is wasted when products are used incorrectly.

There are very few recorded incidences of Dictyocaulus viviparous (aka lungworm) showing actual wormer resistance. The worm in its larva stages remains relatively easy to kill using almost all modern wormers when they are used correctly, but more research is needed in this area.

Managing grass

Modern pasture management could also be involved. Grazing to a very low sward height would ensure that almost all the available lungworm larvae are hoovered up.

Grazing on very quick paddock rotation cycles (about three weeks) means that larvae are not being given time to die off on pasture, so that the maximum infective numbers are being eaten.

It takes only one week for lungworm larvae in the dungpat to mature to their infective stage, which is much quicker than larvae for stomach worms.

Wet summers would ensure that larvae survive longer on grass and heavy rain washes them outwards across the pasture.

Pushing out the paddock rotation cycle is an input-free approach of dealing with this, but may require some re-thinking in overall grazing management.

Finally, there is some evidence that highly productive cows may be more predisposed towards getting this condition. Irish researchers are now starting to explore this area.

A robust solution against lungworm on Irish farms must deal with the condition in all age groups of cattle.

Traditionally, we have relied on the build-up of immunity to largely deal with worms in adult cattle, concentrating most management effort on younger animals, but lungworm appears to be side-stepping that approach.

The problem needs to be worked through in a systematic fashion and guidelines developed.

However, it is likely that the disease can be closed out with a little extra thought and almost no extra spending, so that Irish farmers and their cows can get back to concentrating on maximising kg MS/hectare – without all that coughing.