Foot-and-mouth disease overview
Foot-and-mouth disease (FMD) is caused by a picornavirus.
The virus has 7 distinctive serotypes. Immunity to one serotype provides little to no protection against other serotypes.
FMD is clinically characterised by vesicles and erosions:
- in the mouth and nostrils
- on teats
- on skin between the claws and at the coronary band.
FMD's incubation period is highly variable, and changes according to the:
- virus strain
- virus dose
- transmission route
- animal species involved
- conditions in which animals are kept.
Incubation is typically 2 to 14 days. For the purposes of the World Organisation for Animal Health, the FMD incubation period is 14 days.
Large numbers of animals in a group may be infected by the FMD virus simultaneously. However, they may display differing clinical signs depending on how long each individual animal has been infected.
FMD is one of the most contagious animal diseases known. Animals may be infectious before clinical signs develop.
Infected animals excrete virus in:
- fluid from ruptured vesicles
- exhaled air
The main transmission method within herds or flocks is by direct contact or via respiratory particles and droplets.
Cattle are highly susceptible to aerosol infection and readily display clinical signs (indicator species). Sheep are equally susceptible to aerosol infection, but are less infectious (maintenance species). Pigs are less susceptible to aerosol infection, but are potent amplifiers and excretors of the virus, especially in their breath. They serve as a significant source of virus to susceptible animals.
The spread of FMD between properties and areas is often due to the movement of:
- infected animals
- contaminated vehicles, equipment, people and animal products.
Under certain weather conditions, infected aerosols can spread the virus many kilometres by wind.
The FMD virus can remain infective in the environment for several weeks.
In carcasses that have undergone normal post-slaughter acidification, the virus is inactivated within 3 days. However, the virus can survive for months in chilled lymph nodes, bone marrow, viscera and blood clots.
In experimentally infected cattle, the FMD virus has been detected in milk for 23 days and in semen for 56 days.
Some ruminants may remain long-term FMD carriers, but their role in starting new infections in susceptible animals has not been demonstrated.