This country has successfully been kept free of foot-and-mouth disease for almost 60 years. This is in no small way the result of vigilant controls which were put in place here during the various episodes in Britain, such as the major outbreak there in 1967 and the lesser one in 1981.
Weather can play a role in the spread of the disease and an understanding of this is important for the effectiveness of the control measures put in place.
Foot-and-mouth disease was probably first recorded in Italy, in the 16th century. It is a highly contagious disease of clovenhoofed animals - typically cattle, sheep, pigs, goats and deer. Movement of infected animals is the most important means of spreading the virus. Infection is also spread by contact with vehicles or people from infected areas, feeding animals infected meat or milk, or spreading infected slurry.
Other causes of virus dispersion are related to low-level winds during the period when infected animals have been emitting the virus. Airborne spread represents an essentially uncontrollable means of transmission, which is important only where the meteorological and epidemiological factors are favourable.
While the spread of the virus by the windstream is frequently restricted to short ranges - less than 10 km - under certain atmospheric conditions foot-and-mouth can be transmitted over long distances: spread is possible for a distance of 60 km over land and 250 km over sea.
The disease has been endemic in low latitudes, particularly in African, South American and Asian countries. The EU has been largely free from FMD since 1990 with exceptions of sporadic incursions. For Turkish Thrace and countries around the Black Sea the disease has been endemic. Some recent outbreaks include Italy in 1993, the Balkans in 1996, Greece in 1994 and 1996, and Bulgaria in 1991 and 1993. The outbreak in Albania in 1996 was attributed to the importation of infected buffalo meat from India.
The number of recorded outbreaks of FMD in Europe rose to an annual peak in the late 1960s. Following compulsory vaccination, there was a sharp decline and by 1998 the rate was 100 outbreaks annually. The more widespread stereotypes of the virus in Europe have been types O and A.
With the reduced level of incidences, the practice of vaccinating is being replaced in the EU by the so-called stamping-out policy, as already employed for the past 60 years in the UK and Ireland. With this system, infected animals or animals at risk are slaughtered and carcasses disposed of by burning and burial.
The slaughter method is costly and meteorological assistance to identify herds at risk is most helpful to the veterinary authorities. Knowledge of atmospheric dispersion becomes important when all other sources of infection (animal movement, milk, slurry, etc) have been controlled. Vaccination within the EU would be used to limit spread of the disease if "stamping out" proved inadequate.
During an animal emergency, time and resources need to be conserved. Studies have found that the main factors influencing the spread of the virus over distance were virus emission, virus survival, virus dispersion, virus deposition and susceptible livestock.
The use of atmospheric models, such as models adapted from those used to simulate pollution dispersion, can be helpful in estimating areas at risk.
Modelling aerosol transmission of the disease was pioneered in the 1970s/80s by the development of a computer programme by the UK veterinary and meteorological offices for the simulation of FMD virus dispersion. In the model, account is taken of veterinary, meteorological and topographical factors to indicate areas most at risk from secondary infection.
Virus emission: At peak time, emission from infected pigs amounts to 8.6 x 10s6] virus daily, while the corresponding figure from cattle and sheep is 5.2 x 10s3]. The incubation period is usually two to 14 days, occasionally longer, but it is more likely that two incubation periods have passed before being identified.
Virus survival: The emitted virus is encapsulated in a small respiratory moisture droplet exhaled by the infected animal, and this droplet must remain intact to sustain the virus in the atmosphere over periods of several days. Thus a relatively high atmospheric relative humidity is needed. The effects of temperature and sunlight are thought to be of secondary importance only.
Virus dispersion: For the probability of downwind infection of other animals to be high, the aerosol plume should remain concentrated. An aerosol containing the virus will be carried away from source and dispersed by a combination of two processes; one direct transport away by the wind and the other by diffusion or spreading sideways and vertically through the wind stream by turbulence.
The vertical and lateral spread of the virus plume depends on wind speed and direction, distance travelled and diffusion pattern of the aerosol - the latter being a function of both the horizontal and vertical currents of the low-level atmosphere as well as its vertical temperature structure (known as lapse rate).
Virus deposition: The most frequent cause of natural infection of FMD is by direct contact though inhalation rather than by ingestion and accounts for 95 per cent of outbreaks. Therefore virus viability and concentration in an aerosol are important factors.
Virus deposited on herbage by natural deposition or by precipitation effectively decreases the amount of airborne virus challenge to animals. Thus, for example, infected slurry which is disturbed through the act of spreading may release virus into the air. The role of precipitation is not entirely clear: whether it effectively removes virus out of harm's way, or whether other weather conditions which accompany rain reinforce the amount of virus available to animals.
Hills and valleys have considerable influence on the path of the virus plume. The effect of topography is to deflect and modify the characteristics of the virus plume. This effect is greatest in light winds and stable conditions. Thus channelling of wind can lead to animals in low-lying areas being at greatest risk. Stable atmospheric conditions frequently occur at night-time, so the effect of topography is greatest at night.
Susceptible livestock: As natural infection is more likely to occur through inhalation by animals than through ingestion, cattle having larger intake (larger lungs) are more likely to become infected in secondary outbreaks than sheep or pigs.
In the 1980s Met Eireann acquired a special aerosol type model especially to simulate the spread of foot-and-mouth. This model runs on hourly weather reports from the main meteorological stations throughout the country.
Starting from an identified source location of infection within the country, outputs from the model indicate areas and animals at risk from the virus, taking into account the volume of air exhaled or inhaled. Practice runs have been undertaken from time to time to be in readiness for an emergency such as is occurring at present.
Met Eireann is running the foot-and-mouth disease atmospheric model to be of assistance to Irish veterinary authorities, especially if or when needed. This model is reasonably accurate to a range of 10 km, where animals are most at risk from the virus emitted from a specified primary source.
Also, for long-range transport conditions, a separate model is employed to indicate in a more general way areas at risk. In time, it is envisaged the model would become a sub-model within a larger "Epidemiological Management control decision support system" known as EpiMan, requiring precise input on farms and livestock numbers and information on a whole range of diseases, such as foot-and-mouth, swine fever and tuberculosis, and other food-safety indicators.
Tom Keane was in charge of Agricultural Meteorology for over 20 years in Met Eireann, retiring in 1999. -