Know your enemy: Understanding the lungworm lifecycle

Wednesday, May 25, 2022
 
Parasitic bronchopneumonia is a prevalent disease in dairy and beef cattle in Ireland caused by the lungworm Dictyocaulus viviparus.  Economic losses from lungworm are conservatively estimated to be €167 per dairy cow. This strongyle has a direct lifecycle with seasonal onset of clinical signs, so it is important to understand the lifecycle to optimise management throughout the grazing season.

LUNGWORM LIFECYCLE

During optimal conditions, L3 larvae migrate from the faecal path to the nearby grass where they are ingested by the cow.  These larvae move through the digestive tract and then penetrate the intestinal wall.  Migration through the peritoneum to mesenteric lymph nodes occurs, where they will moult into L4.  L4 larvae continue their migration through blood and lymph where they penetrate capillaries and enter alveoli.  From ingestion to arrival in the alveolus can occur in 8 days.  Within the lungs, the L4 continue to grow and moult into L5 adults reaching up to 8 cm in length.  As they develop, they will migrate further into the bronchi where the females will lay eggs.  The eggs are coughed up with phlegm and swallowed, where they pass through the digestive tract hatching into L1 which are shed in the faeces.

L1 will moult into L2 and L3 within the faecal path, maintaining the protective cuticle to enable survival over the winter period.  Under optimal conditions of >15-20°C and damp, L3 will emerge and migrate in the environment in as little as 5-7 days after faecal L1 shedding.  L3 can then attach to Philobolus (fungal) sporangium growing on faeces to become aerosolised and widely spread in the environment. This unique dissemination poses a challenge and makes low-risk grazing unachievable. 

LONGWORM LIFECYCLE PHASE, PATHOGENESIS AND CLINICAL SIGNS

Understanding this lifecycle gives us an understanding of clinical signs, diagnostic challenges, and larval burdens observed in Ireland (Fig. 1).  Cows will show clinical signs when the L4 reach the lungs, as early as 8 days after infection.  However, patency with a positive faecal test is not achieved for a minimum of 26 days.  Additionally, cows will cough following patency (after anthelmintic treatment or natural immunity) due to dead worm debris, inflammation, and phlegm. 

Fig. 1: Dictyocaulus viviparus lifecycle phase, pathogenesis and clinical signs

In the environment, a sudden change to optimal temperature of 20°C accompanied by rain can cause an en masse dispersal of larvae in the pasture (and subsequently the lungs in 8-15 days).  This poses a significant challenge to the cow and can result in severe clinical disease.

Lungworms’ ability to cause significant prepatent disease, widely disseminate on pasture, and rapidly emerge under optimal conditions poses management challenges that often necessitate anthelmintic treatment in adult cows. 

References available upon request.

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