Myiasis

Myiasis is the infestation of organs and tissues by the larval stages of dipteran flies. 

• Blowfly- Cutaneous. Important in Sheep.
• Oestrus Ovis- Nasal Bot in Sheep.
• Hypodermis- somatic in Cows. 

Calliphoridae 'blowfly' 

Commonly known as flystrike and also affects rabbits. 

Flies possess a brilliant green sheen. 

Main species:

• Lucilia Sericata responsible for 95% of all initial flystrike. L.Phormia can also initiate flystrike.
• Calliphora species are responsible for secondary flystrike infections and cannot initiate flystrike themselves. 

The flys are oviparous and lay eggs on wounds, infected or faecally soiled skin. The larvae pass through three larval instars whilst feeding on host tissues and when mature they wander away from the strike focus and drop to the ground to pupate. 

Life Cycle

1. Fly is attracted to odour on the sheep. Development of the larvae and eggs is development on the temperature and humidity.
2. 200 eggs per batch are laid on the sheep. They generally take 12 hours to hatch and larvae take 3-10d to mature. To feed larvae digest tissue via oral hooks and proteolytic enzymes. 
3. Once mature the larvae drop to the ground to pupate which takes 3-7 days before the emergence of the adult fly. 
4. Adult flies are generally  seen from April through to October so disease can occur in this time period. Larvae arrest in the environment and overwinter as pupae. 
5. The life cycle takes 3-6 weeks in total. Factors= temperature, humidity (rainfall) and host susceptibility. 

Rabbit

Pathogenesis

• Debilitation. 
• Loss of condition.
• Anorexia.
• Dull and isolated sheep.
• Wool NOT lost as seen in sheep scab but skin larvae can be seen in the wound.
• Eventual death as a result of toxaemia. 

Treatment & Control

• Separate sheep, clip lesion and treat. 
• Treatment= insecticide dips/pour ons or using an insect growth regular cyromazine. 
• Problems with treatment include the short time the larvae are on the host, repeated infections, rapid tissue damage and extensive breeding sites of the larvae. 
• Control-> tail dock, dag & crutch, prevent scour, good carcass disposal, wound treatment and 'mules operation.'