Dictyocaulus filaria – A potential cause of coughing sheep

What is Dicytocaulus filaria?

One or two sheep coughing in a flock – particularly a harsh dry cough – could be a sign of Dictyocaulus filaria infection. This is a “lungworm” with the adult worms that measure some 4 – 10cm in length – living in the bronchi and causing bronchitis.

Unlike other lungworms in sheep, this lungworm has a direct life cycle with larvae passed in droppings developing into infective larvae on pasture. Other lungworms have a so-called indirect life cycle as they need a snail or other invertebrate host in which the larval stage develops.

How do sheep become infected with Dictyocaulus filaria?

Infection typically perpetuates at a relatively low level which may be unnoticed within an adult flock, with the adult sheep passing small numbers of larvae onto pasture which will develop and infect young lambs and kids, which may show more marked signs than adult sheep.

Infection normally causes less lung damage or pathology than the other species of Dictyocaulus – Dictyocaulus viviparus in cattle, where a heavy infection in a naïve animal can be fatal. Nonetheless, infection in sheep can cause ill thrift and poor doing. 

How is Dictyocaulus filaria infection detected?

D. filaria infection can be confirmed by identification of the characteristic larvae in the faeces. They have a rounded protruberance on the front end of the head of the larva (Fig.1) and a rounded tail (Fig.2). They are isolated from faeces using a Baermann apparatus that consists of a funnel containing water with a tube on the bottom closed with a clip. A small sample of droppings are placed in a holder in the top of the funnel immersed in the water. Once in water, the larvae migrate out of the faeces and sink to the bottom of the tube. Some hours later the clip is undone and a sample of water collected into a tube. This water is then examined for larvae.

It is important to confirm the identity of the larvae as, for example, an old sample may contain larvae that have hatched from gut roundworms or even free living nematodes. Free living nematodes comprise a huge population of nematode worms almost all of which live throughout their lives in the environment. The only exception of which I’m aware is Strongyloides which curiously can be a free living, soil dwelling nematode in one generation and a parasitic worm in the next. You may have seen reference to this worm in cases of diarrhoea in puppies recently. We’ll look at Strongyloides in more detail in a future article. 

How can I test my animals for Dictyocaulus filaria?

If you would like to test your sheep or goats for lungworm or any other parasite, please go to our website (services) where you can find details of all the tests our laboratory can do for you.