AVIS - Foot and Mouth Disease

The disease

Clinical disease and diagnosis

Ageing of lesions

In any epidemiological investigation of an outbreak of FMD, it is essential to estimate for how long the infection has been present on the infected premises. A thorough clinical examination is made of all the animals on the holding and an estimate is made of the age of the lesions on each animal. The animal with the oldest lesions is usually the index case. The date when infection is likely to have first entered the farm can be estimated by adding one incubation period (2 to 14 days) to the age of the oldest lesion. Ageing of lesions is also important for estimation of airborne virus production when examining the possibility of windborne spread.

Days of clinical disease
Day 1	Blanching of epithelium followed by formation of fluid-filled vesicle.
Day 2	Freshly ruptured vesicles characterised by raw, bright-red exposed dermis, a clear edge to the lesion and no deposition of fibrin.
Day 3	Lesions start to lose their sharp demarcation and bright-red colour. Deposition of fibrin starts to occur.
Day 4	Considerable fibrin deposition has occurred and regrowth of epithelium is evident at the periphery of the lesion.
Day 7	Extensive scar tissue formation and healing has occurred. Some fibrin deposition is usually still present.