The confirmed outbreak of foot-and-mouth disease in Co Louth is a grim reminder of our vulnerability to exotic disease. Although the island of Ireland might be expected to escape the major problems faced by countries with land frontiers, where movements of domestic animals and wildlife are difficult to control, when free movement of animals is permitted, free movement of pathogens follows.
The EU has had a major influence on agriculture in Ireland and in many instances the outcome has been beneficial. However, we are no longer masters of our own destiny and the problems encountered with animal diseases highlight the interdependence of member states and the necessity for effective control measures relating to exotic infectious diseases.
Foot-and-mouth disease is caused by a virus which belongs to the genus Aphthovirus of the family Picornaviridae.
Seven immunologically distinct serotypes are recognised and infection with one serotype does not confer immunity against another.
In addition, a large number of strains are recognised within each serotype. Thus, control through vaccination presents many problems for countries where the disease is endemic.
Foot-and-mouth disease is endemic in parts of Asia, the Middle East, Africa and South America. Before the recent confirmed outbreak in Co Louth, Ireland had been free of foot-and-mouth since 1941.
The virus responsible for the current outbreak in the UK has been identified as a pan-Asiatic strain of type O. This strain is considered to be particularly virulent.
The virus is stable in the environment at pH values close to neutrality and it can survive for many weeks on the walls of buildings, on clothing and footwear and in food.
In the carcasses of infected animals stored at 4 degrees, this virus has been recovered from the bone marrow after six months. In the muscle of dead animals, virus survival is short due to acid production post mortem.
In the environment, survival of virus is influenced by ambient temperature, humidity, sunlight and the type and amount of organic matter present.
The virus of foot-and-mouth disease which is shed in milk is exceptionally resistant to heat treatment. It can survive pasteurisation at 72 degrees for 15 seconds (the usual pasteurisation process) and 93 degrees for 15 seconds.
Heating at 100 degrees for more than 20 minutes may be required to inactivate the virus.
Cattle, sheep, goats, pigs and several wildlife species are susceptible to foot-and-mouth disease. Although the virus can be transmitted by many methods, most susceptible animals acquire infection by inhalation or ingestion. Virus shedding begins during the incubation period, about 24 hours before the appearance of clinical signs. Transmission can occur by direct contact, by aerosols, in transport vehicles, on footwear or clothing and through animal products such as meat, offal, swill and milk.
Infected groups of animals, particularly pigs, may shed large quantities of virus in exhaled air. Virus in aerosols may spread 10 km or more over land under favourable conditions of high humidity and moderate wind-speed. Because there is usually less turbulence over water, a spread of up to 200 km can occur.
Spread to cattle and sheep may occur through contaminated feed, hay, straw or transport vehicles.
Footwear and clothing are important methods of spread, hence the importance of foot-baths and protective clothing. Infected animals are the most important source of virus and, accordingly, control of animal movement is one of the most important measures when dealing with an outbreak of disease.
The incubation period ranges from two to 14 days but many animals show signs within a week of infection. Following inhalation, the virus spreads throughout the body and infects many tissues. The disease in cattle is characterised by fever, depression, loss of appetite and a drop in milk production and, later, by vesicles in particular locations.
Vesicles (blister-like lesions) occur on the tongue, in the mouth, on the lips, feet, teats and skin. Affected cattle drool saliva, often accompanied by smacking of the lips. Lameness is evident and pregnant cows may abort. Vesicles rupture quickly leaving raw ulcers which heal gradually over two to three weeks. Bacterial infections may delay healing.
In pigs, marked lameness is the main presenting sign. Foot lesions are severe in pigs and hooves may slough. In sheep, goats and wild ruminants, the disease is usually milder and careful examination may be required to detect lesions. Rapidly spreading lameness accompanied by fever may be evident in groups of sheep.
Although human infection can occur in those exposed to the virus, the disease is usually mild.
Although the disease may be suspected in animals with clinical signs suggestive of foot-and-mouth disease, laboratory confirmation is required. The Institute of Animal Health, Pirbright Laboratory, Surrey, is designated the World Reference Laboratory for foot-and-mouth disease.
Since 1991, the European Union has adopted a non-vaccination policy for the control of foot-and-mouth disease and member states implement a slaughter policy when the disease is confirmed in domestic animals.
If foot-and-mouth disease is confirmed on a premises, the Department of Agriculture implements a well-defined plan. The movement of people and animals into the infected place is prohibited, infected and in-contact animals are slaughtered and a vigorous disinfection programme is implemented. Foot-baths are placed at all exits and a bed of straw or foam rubber soaked in an approved disinfectant is placed at gateways for disinfection of vehicle wheels. Buildings, transport vehicles and equipment are thoroughly cleaned and disinfected. Hay, straw or other contaminated material are either burned or buried and the premises is usually left vacant for several months.
Disinfection of footwear and vehicle wheels feature in preventing the disease spreading at ferryports and airports or where people assemble for work, religious gatherings or social events. For this measure to be effective, it requires the use of effective disinfectant at the correct concentration applied over the entire surface being treated.
At the entrance to farms, foot-baths should be replaced when they are visibly discoloured or diluted by heavy rainfall.
Those entering a farm should wear clean waterproof footwear which should be immersed to a depth of 15 cm for at least one minute.
Control of foot-and-mouth disease requires the co-operation of the public in general and the agricultural community in particular. Rigorous control of farm-animal movement is the first and most important control measure.
Controls on the importation of live animals and food of animal origin require careful re-examination in the light of recent experience. Notification of suspicion of infection with foot-and-mouth disease virus in susceptible animals to the Department of Agriculture is particularly important during the weeks ahead.
It is evident that reliable identification of cattle and sheep using ear tagging, or electronically by chip implantation, would allow more rapid and effective identification of illegal animal movement.
Maintaining Ireland free of exotic disease is in the national interest and each citizen has a role to play.
By strictly observing the control measures currently being applied it may be possible to limit the spread of this exotic disease. Once foot-and-mouth disease arrives in the State, the time, effort and expense required to eradicate it affect not only the farming community but ultimately all sectors of society.
Prof P.J. Quinn is professor of veterinary microbiology and parasitology at the faculty of veterinary medicine at UCD.