Roundworm: lungworm life cycle
The lungworm (Dictyocaulus viviparus) of cattle, follow a direct life cycle that also involves a migratory phase to the lungs.
Figure 1. Lungworm life cycle. Image created by Madison Mayfield
First-stage (L1) larvae, not eggs, are passed in the dung.
- Second (L2) stage larvae develop into infective third (L3) stage larvae.
- L1 and L2 feed on bacteria.
- L3 (infective larvae) do not feed.
- L3 develop within about 4-5 days under ideal conditions.
- L3 are very sluggish and most do not migrate from the dung pat by themselves.
- Instead they access clean grass by congregating on the spore structure of the fungus Pilobolus, which grows on cattle faeces. When the fungal sporangium explodes, larvae are propelled up to three metres from the dung pat.
Pasture stage: spreading of infection
- Lungworm larvae on pasture prefer cool moist conditions.
- The survival time on pasture is shorter than that of most other roundworms.
Final host stage in cattle: new infection and re-infection
Infective larvae are eaten along with the pasture. Uneaten larvae die.
Migratory phase to the lungs.
- Ingested L3 exsheath (shed their outer membrane) in the small intestine.
- They penetrate the wall of the intestine and
- move with the lymph and blood systems to the lungs.
Adult worms in the lungs.
- Pre-adults develop into adult worms and commence egg laying.
- A few eggs hatch before leaving the lungs.
- Most eggs are coughed up and swallowed.
- Eggs hatch into first stage larvae during passage through the gut of the host.