Author: Prof Gavin R Thomson
Licensed under aCreative Commons Attribution license.
Introduction
No non-zoonotic disease (i.e. diseases that do not affect the health of people) causes as much disruption to animal production and international trade in commodities and products derived from animals as FMD. This situation results from historical approaches and policies that have evolved over many decades. Consequently countries in sub-Saharan Africa and other parts of the developing world have difficulty in exporting animals and animal products to high-value markets. Paradoxically, especially in extensive livestock production systems, FMD usually causes relatively mild disease in the wide range of domestic and free-living cloven-hoofed animals that are susceptible to it.
Salient features of FMD
Foot and mouth disease is a contagious viral disease of cloven-hoofed animals. It is a systemic infection that results in transient fever and loss of appetite with accompanying weight-loss, drop in milk production and the development of vesicles ‒ which soon rupture leaving painful erosions in the mouth, skin-hoof junction of the feet and teats ‒ that may cause salivation and lameness. Most animals recover uneventfully from the infection with 10-21 days.
In some circumstances FMD viruses are able to spread extraordinarily efficiently, including over long distances on air currents, and infect large numbers of animals over a wide area in short time periods. For this reason it is often referred to as the most contagious disease of mammals. However, in other circumstances, for example those that prevail in sub-Saharan Africa (arid climate with low stocking densities), the infection often spreads slowly and inefficiently. So the mechanisms and rate of spread of FMD are highly variable.
Tongue lesions commence as blanched foci
which develop into vesicles containing serous fluid
Ring lesions on the claw of a bovine that recovered from FMD showing the separation between the new outgrowing tissue and the horn of the hoof that were present distal to the coronet prior to lesion development
Viruses that cause FMD are classified within the genus Aphthovirus and comprise a constantly evolving quasi-species that is divisible into 7 serotypes named A, O, C, Asia 1, SAT1, SAT2 and SAT3. The SAT serotypes and the 4 others form two distinct sub-groups. The currently available evidence indicates that the SAT serotypes evolved in sub-Saharan Africa in association with African buffalo while the other 4 evolved in Eurasia, probably in association with livestock.
In eastern and southern Africa, unlike the rest of the world, wildlife populations (buffalo primarily) maintain SAT serotypes of FMD virus, i.e. provide a constant reservoir of infection. For that reason the epidemiology of FMD is more complicated in these regions and results in conflict between livestock development and wildlife conservation policies.
Where does FMDoccur?
SAT serotypes are still confined to Africa while the other serotypes occur widely in Africa, Asia, South America, parts of central/eastern Europe and the Middle East. Eradication of FMD has been achieved in North America, most of Europe, parts of the Pacific Rim and Oceania. In sub-Saharan Africa, SAT1, SAT2, A and O serotypes are widely prevalent; infections caused by C are currently rare, as is the case for SAT3 except in buffalo populations. Asia 1 infection has never been recorded in Africa.
What triggers an outbreak of FMD?
Because FMD is contagious the infection is usually precipitated by close contact between susceptible and acutely infected animals. Another common method of spread occurs where scraps of meat from infected animals are fed to pigs through swill-feeding. Pigs excrete large quantities of virus and especially on large piggeries may become a potent source of infection for other susceptible animals.
Some ruminants may retain the infection in the oro-pharynx for weeks to months following clinical recovery. However, so far only African buffalo have been shown to be capable of so-called carrier transmission.
Prevention and control
Because FMD is a contagious disease, methods of control are based on prevention of contact between infected and susceptible animal populations and exposure of susceptible animals to fomites and other sources of indirect exposure. Vaccination can also be an effective way of preventing infection from spreading to susceptible populations but the diversity of the viral population (i.e. intra-typic variation) and the fact that the currently used inactivated vaccines induce only short-term protection, complicates achievement of the high levels of herd immunity required to manage FMD effectively in domestic livestock.
In southern Africa extensive fencing systems have traditionally been used to prevent contact between infected and FMD-free animal populations. These fencing systems sometimes divide ecosystems with unfortunate ecological impacts.
A ‘Progressive Control Pathway for FMD Control’ (sic: so-called PCP-FMD) has been accepted by the OIE (World Organisation for Animal Health) and FAO (Food & Agriculture Organisation of the UN) as the recommended way forward for the eventual global eradication of FMD. However, the proposed pathway creates a significant problem for southern and eastern Africa because it takes no account of the wildlife problem and clashes in approach to rural development between livestock interests and those of effective wildlife (bio-diversity) conservation.
Find out more
The CPD module on FMD provides a detailed account of the disease and the viruses that cause it with emphasis on the way the disease commonly behaves in southern Africa, i.e. its epidemiology. Detail is therefore provided on the role of wildlife in the maintenance and spread of the disease. Other aspects covered by the module are the pathogenesis of FMD, its diagnosis and differential diagnosis, including clinical signs and pathology, and methods for controlling the disease. The module concludes with a section on the effect of FMD on trade in animals and animal products, i.e. the area in which this disease has its greatest impact.