Lungworm, caused by the round worm Dictyocaulus viviparus, generally occurs in heifers less than 10 months of age. In particular, those heifers with no immunity that are exposed to high numbers of worm larvae when put onto pasture (such as a group of Spring-born calves weaned onto pasture grazed by Autumn calves).
Lungworm disease can present as coughing, loss of body condition, increased respiratory rate and, in worst the case, death. Lungworm can also cause disease in the milking herd, where they are exposed to large number of larvae, or where their immunity has waned. As well as the above signs, milk production will decrease.
How does lungworm infection occur?
The lungworm life cycle is a little complicated (see diagram below). Infection begins when cattle graze infective L3 larvae from the pasture. These larvae penetrate the gut, enter the lymphatic system and are carried to the lungs. During this time, they develop further, reaching adulthood in the large airways of the lungs. For the next month or so, adult worms lay eggs which are coughed up by the heifer or cow and swallowed. The eggs hatch in the intestines and L1 larvae are passed in the faeces. Adult worms are up to 8 cm in length and it takes 3 weeks (or sometimes less) from the initial infection to being able to lay eggs.
What happens on the pasture?
The L1 larvae passed out by infected animals develop within manure pats. The larvae can’t move very far from the pats on their own, however, they can attach themselves to fungi present in the manure. In warm conditions the fungi germinate: they burst and spread both their spores and the L3 larvae up to 3 metres away. Being a distance from the manure pat they’re more likely to be eaten.
Heavy rain/flooding can also spread the larvae in the top soil. When grass growth picks up (particularly in paddocks cut for silage) there can be a mass infection. With the right weather conditions, L3 larvae can remain infective for months in the manure pat or on vegetation and high levels of larvae can accumulate within 2–4 months.
Click here for an old-school YouTube video that has some great footage of the lungworm/fungi phenomenon.
Why do lungworm cause disease?
The presence of larvae and worms in the lungs fills the airways with mucus and exudate. There will be a slight cough early in the disease. However, if animals are badly affected the coughing becomes severe and breathing becomes difficult – they will stretch their heads and necks to try an improve airflow. Animals that survive the acute infection will develop immunity, though there may be enough lung damage to affect growth and production later in life. Other causes of pneumonia (e.g. bacterial, viral) can cause similar clinical signs so a correct diagnosis is important: either by looking for L1 larvae in the faeces or through post-mortems.
Treatment and Prevention
Once infection in identified in a group of animals, they should be drenched and moved to low worm burden paddocks. Ideally, choose a “mectin” drench which have a longer residual activity. In a milking herd, a drench with zero milk withhold needs to be used.
Clinically affected animals should be treated with antibiotics and anti-inflammatories. It is also important to try to reduce exposure of young cattle to large numbers of infective larvae. Young, susceptible animals should not be grazed on pastures contaminated by older animals especially during the spring and summer months.