Journal vet: lungworm – the unpredictable parasite

The headline might be a little misleading as a farmer only yesterday rightly pointed out that we are getting into lungworm weather. So yes, there are certain times of year where we can expect problems with the parasite, but unfortunately there are many other factors that make lungworm unpredictable.

Lungworm, or dictyocaulus viviparous is a parasitic worm that affects the airways of both cattle and sheep. We need to look at how this parasite operates to get a step ahead when we draw up a control plan for the weeks and months ahead.

The lifecycle

So where do lungworm come from to reinfect stock each year at pasture? There are two sources: carrier cows and larvae that survive over winter until spring grazing. The parasite apparently can burrow into soil or be carried by the wind. This year we had a mild short winter which, combined with a wetter summer, is likely to lead to more lungworm than last year. Hot and dry weather is not favourable for lungworm, which is why we had so few issues during the drought last year.

The adult parasite lays eggs in the lungs which develop into larvae or young lungworm. These are coughed out and swallowed. They then migrate from the dung with the help of moisture and a fungus. They perch on grass waiting to be consumed. When inside the animal, these larvae burst through the gut and make their way to lung tissue and airways; this takes one to three weeks. In the lungs they develop into adults and begin the cycle again.

This is an important point for diagnosis. In young stock, coughing and a negative faecal sample doesn’t mean it’s not lungworm. They could have larvae (immature lungworm) in the lungs causing coughing.

Immunity

Cattle and sheep can develop an immunity to lungworm in their first grazing season after low exposure over time.

This life cycle from gut into lungs also means there are two places where the animal’s immune system can react to the invading parasite – first at the gut and second in the lungs.

Lungworm is complicated because the immunity at the gut only lasts six months to a year while immunity developed at the lungs lasts for years. This means even though animals can destroy the parasite in the lungs it can still get there in older animals. This is what we refer to as re-infection syndrome.

So when we see older animals getting lungworm in their second grazing and beyond, we have to ask two questions: has the immunity worn off or has no immunity developed through low exposure or over-worming in their first grazing?

With more and more lungworm issues in older cattle, I now consider the oral lungworm vaccine a very viable options for farms to control the parasite, particularly where adult dairy cows are affected.

The young worms affect the soft tissue deep in the lung and the adults are in the airways causing bronchitis. This causes a lot of physical irritation which results in coughing, particularly when walking or running, which causes more air intake and more irritation.

While mild infections cause coughing, they also cause damage to the airways leading to less performance and dramatically increasing the risk of pneumonia from bacteria and viruses.

Not every cough means your cattle have lungworm, but a distinctive deep lung cough after exercising or movement is pretty characteristic. Where we see secondary discharges from the noses, we can expect some viral or bacterial components as well.

In severe outbreaks I have seen lungworm cause death in animals after heavy pasture larval levels developing under the right conditions rapidly. These animals will have open mouths and be struggling to take in air.

In adult dairy cows I have seen milk drops before clinical signs of coughing actually develop.

How can we confirm a diagnosis?

The gold standard test is now a lung wash which can pick up adults and even immature worms.

In fairness, this isn’t routinely used as most will feel comfortable with coughing animals at pasture and clinical signs to make that diagnosis. While faecal counts pick up most worms, they can be misleading as lungworm larvae in the lungs cause coughing and do not produce eggs.

With adult cows, reinfection syndrome may also mean coughing with no larvae detectable in faecal samples. Any animal that dies at pasture needs to be opened up and never miss an opportunity to check the lungs for lungworm.

All of the three classes of wormers will kill lungworm, though some resistance has been reported in some studies. The challenge for dairy farmers is the availability of products to use on adult lactating cows. It is often limited to eprimectin because it has zero milk withdrawal and is also licensed for that use.

One of the challenges for treating heavy infestations is a rapid kill will lead to lots of dead lungworm, causing a post-patent bronchitis. This is often seen in its milder forms when animals with lungworm get worse after being dosed. Very sick animals respond well to steroids and covering antibiotics.

There are no grazing strategies to reduce lungworm, but beware that overstocking on grazing platforms makes all parasites more of an issue.

Where herds have had serious issues with lungworm, vaccination must now be considered. This is an oral vaccine given to calves over eight weeks of age and requires two doses before turnout. This makes it difficult with regards to turnout dates but some farms may now have no option, with boosters given one month before turnout as yearlings also.

Most animals will recover from lungworm well, but some will have chronic damage which will lead to coughing even after lungworm have gone. Farmers now need to be vigilant of symptoms and treat early to avoid damage and pneumonia potentially developing.

Remember, not all coughs are lungworm and, if not improving, always investigate with your vet.