Foot-and-mouth disease overview
Cause
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.
Signs
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.
A variety of other signs also appear in cattle, pigs, and sheep and goats.
Incubation period
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
The incubation period is typically 2–14 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.
For the purposes of the World Organisation for Animal Health, the FMD incubation period is 14 days.
Transmission
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
- saliva
- milk
- semen
- faeces
- urine.
The main transmission method within herds or flocks is by direct contact or via respiratory particles and droplet.
Cattle (indicator species) are highly susceptible to aerosol infection and readily display clinical signs. Sheep (maintenance species) are equally susceptible to aerosol infection but are less infectious and may not show obvious clinical signs. Pigs (amplifying species) 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. Pigs frequently show obvious clinical signs.
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.
Virus persistence
FMD virus can remain infective in the environment for several weeks and possibly longer:
- in the presence of organic matter, such as soil, manure and dried animal secretions
- or
- on chemically inert materials, such as straw, hair and leather.
In carcasses that have undergone normal post-slaughter acidification, the virus is inactivated within 3 days. However, the virus can remain viable for months in chilled lymph nodes, bone marrow, viscera and residual blood clots.
The virus may be shed in milk from infected animals up to 4 days before the onset of clinical signs and for up to 3 weeks afterwards.
Experimentally, FMD can be transmitted by insemination with infected semen. FMD virus has been found in bull semen 4 days before, during and up to at least 37 days after the appearance of clinical signs. It has also been found in bovine semen stored at -50°C for 320 days. FMD virus has also been found in pig semen and is likely to occur in sheep and goat semen. The virus enters semen as a result of viraemia or lesions around the preputial orifice. Some ruminants may remain long-term FMD carriers, but their role in starting new infections in susceptible animals has not been demonstrated.