Vets from our Waimate clinic were recently called to a farm where 4 calves had died, one calf was recumbent, and about 40 others showed signs of coughing and respiratory distress. The history was consistent with lungworm, and this was subsequently confirmed on postmortem (see photo below) where large numbers of adult worms were present in the large airways of the calf. We rarely see clinical lungworm cases, but this case saw a perfect storm; wet weather, long drenching intervals, and calves on the same paddocks for multiple years.
The lungworm lifecycle, much like gastrointestinal worms, revolves around the ingestion of L3 larvae from pasture. Grazing paddocks which have previously held calves (as in this case) will obviously increase the risk of infection. However L3's from lungworm are relatively inactive compared to their gastrointestinal cousins, and are incapable of traveling more than 5cm from the cow pats they are carried in. Instead they rely primarily on a mushroom (P klenii) which grows in the cow pats and bursts to disperse the larvae as far as 3m. The recent wet warm weather has been perfect mushroom weather, and has therefore increased the risk of lungworm.
A routine drenching programme normally limits lungworm as they are highly susceptible to anthelmintics. In particular pour-on abamectin compounds (such as Eclipse) have a persistence efficacy of over 14 days. In this case, in an effort to reduce costs the farmer used an oral abamectin/levamisole drench 8 weeks apart. We traditionally recommend 4 weeks between oral drenches and up to 6 weeks for pour-on products at this time of year.
This case highlights the need to be aware of drenching intervals in young stock, particularly with the warm, wet weather we were experiencing. As in all diseases prevention is much better than cure. In this case the farmer has lost 4 calves, has another 40 severely compromised, and the whole mob will have reduced growth rates - severely outweighing the cost of an extra drench. Talk to your Prime Vet about a Young Stock Health Plan if you don't already have one in place.