Lifecycle of the lungworm

The lifecycle of the lungworm is about 4 weeks long i.e. from the ingestion of larvae to the excretion of infective larvae by the affected animal. In the worst case, within four weeks of ingesting lungworm larvae, the cow or calf can be shedding literally millions of fresh larvae onto the pasture. A faecal output of 1,850,000 larvae per day has been recorded!

Infective larvae (L3) eaten from pasture.	Migrating, at 1 week, as L4 out of the intestinal wall via the bloodstream to the lungs.	Becoming L5 (immature adults) within a few days, moving up to upper respiratory tract. L5 can over-winter in carrier hosts. Immunity established/active at this stage	Become mature adults: slender white worms up to 8cm long, living in the lungs.
	The infection now becomes patent. Eggs containing L1 are coughed up and swallowed by the same cow, hatching to L2 on the way down and out in faeces.	After 1 week on pasture, become infective L3. Scattered by fungus/rain for over a metre. Thought to over-winter on pasture in some circumstances.

Clinical disease

Clinical disease has an incubation period of around three weeks from ingestion, and can therefore occur a few days before larvae appear in the faeces. By the time clinical disease is present, the health of the cow or calf is already severely compromised, whatever treatment is then chosen. Because of the extensive damage that can be caused by worms in the lung tissues, this is a classic case of prevention being better than cure.

However, it is important to note that in contrast with other parasitic diseases, immunity to lungworm can develop relatively fast, achieving a steady state in which no clinical signs are observed (it is, after all, a poor parasite that kills its host). But without vaccination, such immunity can only develop if the cow or calf has received just enough natural challenge from the pasture in the past - neither too much, nor too little.

Unfortunately, the level of natural challenge depends on far too many factors to be regulated simply by farm management or worming routines.