Foot-and-Mouth disease (FMD) is a highly contagious viral disease affecting all ruminants, both domesticated and wild, and swine, in which it causes vesicles and ulcers in the mouth, on the udder and on the feet. While mortality is generally low, loss of production is severe. The disease is found in many areas of the world. South America, Africa, the middle East and Asia are endemic areas. Western Europe is normally free of the infection.

AETIOLOGY

Foot-and-mouth disease virus (FMDV) is an aphthovirus within the picornaviridae. Characteristics of the virus which are important in the epidemiology of the disease include the rapid growth cycle of the virus, the stability of the virus under a variety of conditions and the occurrence of serotypes.

Serotypes: There are 7 serotypes which are designated according to the geographical area where each was first recovered. Within each serotype there are several subtypes.

Disinfectants:

Once one or more animals in a herd have been infected the quantity of virus in the environment will be greatly amplified and then transmission by several different routes will be possible. Resulting incubation periods will generally shorten as disease spreads through the herd.

The incubation period depends on the species, dose, route and strain of virus. In within-farm spread the incubation period can vary from 2-10 days but generally it can be expected to be 2-4 days for cattle, sheep and goats; 1 or 2 days longer for pigs. In between-farm spread by airborne route the incubation period can range from 4-14 days, depending on the infecting dose.

The usual route of virus entry is via the mucosae and lymphoid tissues of the pharyngeal/tonsillar region. Less frequently initial infection can occur through cuts in the mouth or on the feet as a result of abrasions and contact with contaminated materials. e.g. food stuffs (swill, meat) and fomites.

During the acute phase of disease (vesicles less than 1 week old), all body secretions and excretions of infected animals will contain high amounts of virus.

Virus is also excreted from infected animals in aerosols of exhaled breath and in lymph from ruptured vesicles. The most important species in relation to airborne excretion is the pig. One infected pig can excrete around 4000 million infectious units per day.

Cattle, sheep and goats excrete much lower amounts of virus in breath; the highest recorded amount for these species is 10 infectious units per day. Therefore, as emitters of airborne virus, one infected pig is equivalent to around 3,000 infected cattle. For all species the excretion of airborne virus lasts for 4-5 days from the first development of vesicles.

Convalescent cattle, sheep and other ruminants, but not pigs, may carry the virus in their pharyngeal region for periods of many months. Vaccinated cattle can become carriers if exposed to infection without having developed clinical disease. There is some circumstantial evidence which suggests that transmission from carrier cattle may occur occasionally in the field but this has not been reproduced experimentally.

CLINICAL SIGNS

Cattle

Following contact exposure, lesions appear in 2-10 days. Some viral strains are associated with longer incubation periods than others. There is an early serous nasal discharge which often becomes muco-purulent. There may also be an ocular discharge.

Early lesions in the mouth generally appear as blanched, roughly circular areas on the tongue or dental pad. These become more prominent as fluid collects underneath until a vesicle is formed which stands out clearly from the normal surrounding epithelium. When viewed along its surface the tongue may appear corrugated. Vesicles on the tongue frequently reach several centimetres in diameter and are situated characteristically on the dorsum, but may be under the tip or on the sides of the tongue close to the molar teeth. Tongue lesions cause the animal much pain. In some cases stomatitis produces profuse salivation. Handling the tongue frequently results in large pieces of epithelium becoming detached, leaving deep, red ulcers with a ragged edge. Vesicles followed by ulcers also appear in other parts of the mouth particularly on the dental pad, hard palate, gums and inside the lips. The animal is anorexic.

In calves, vesicles and ulcers are less obvious and often difficult to find but deaths are not uncommon due to viral effects on heart muscle.

Healing of the ulcers on the tongue and other parts takes place surprisingly rapidly. Within 3-4 days the base of the ulcer becomes covered with a serofibrinous exudate and regeneration of the epithelium is complete within about 10 days unless secondary bacterial infection supervenes which leads to scarring.

Mortality in adult cattle very seldom exceeds 5% but may be much greater in calves. Cattle recovered from FMD are immune for about 2-5 years to clinical disease from that strain of the virus.

Pigs

Tongue lesions are much less dramatic than in cattle and heal much more rapidly. Vesicles occur as one or more raised white areas of about 0.5-1 cm diameter on the dorsum. They rupture readily to leave small ulcers. Frothy saliva may be present around the lips. Vesicles may also occur on the snout and on the lips and gums.

Foot lesions are generally prominent and lameness is marked. Vesicles and ulcers occur at the bulbs of the heels, around the coronary band and in the interdigital space. In piglets sloughing of the horns of the hooves and of the supernumary digits takes place readily leaving the sensitive laminae exposed.

There is depression, anorexia and loss of condition. Piglets frequently die suddenly.

Sheep and goats

Adult sheep and goats are not generally severely affected: indeed lesions in the mouth are often difficult to detect and those on the feet may be confused with foot-rot or abscesses. Vesicles and ulcers may develop on the tongue or particularly on the dental pad. They are usually small (1 cm). If foot lesions occur they usually do so at the coronary band or in the interdigital cleft and may cause marked lesions.

CARRIERS

Clinical recovery is the normal sequel to FMD but in ruminants virus is carried in the cells of the oesophagus and pharynx despite high levels of circulating antibody. Virus can also be recovered from oesophageal fluid samples of vaccinated ruminants following exposure to infection despite full clinical immunity.

Cattle may be carriers for up to 2 years; sheep for up to 9 months and goats for up to 2 months. Pigs are not known to be carriers.

PROPHYLAXIS

Inactivated FMD vaccines are widely used throughout the world and could only be used in this country in an emergency. 

Following vaccination, cattle are immune to challenge infection after about 14 days. Antibody may be detected on primary vaccination as early as 3-7 days. Satisfactory immunity can only be guaranteed against the homologous strain for 4-6 months after primary vaccination and on revaccination cattle may be protected for 9 months to 1 year. Repeated annual vaccination not only gives satisfactory high levels of protection against the homologous strain but increases the range of strains against which protection is afforded.

Pigs are not as readily immunised as cattle and high concentrations of virus and different adjuvants are necessary. Recently pigs have been satisfactorily protected for up to 9 months with vaccines incorporating oil adjuvants. 

CONTROL OF FMD

Two approaches to the control of FMD are currently applied. In the UK a policy of "stamping out" of the disease has been traditionally followed which relies on a well developed state veterinary organisation for the early recognition of disease followed by slaughter, disinfection and strict movement control. This policy has proved successful over the years and is less costly than compulsory annual prophylactic vaccination. The policy is combined with vigorous attempts to prevent the introduction of virus by the control of animal and animal product importation.

Between these two approaches many variations are practised notably strategic vaccination in the face of an outbreak where baffler or ring vaccination is applied (e.g. in some eastern European countries).

Control of FMD outbreaks in the United Kingdom

FMD is a notifiable disease so if the disease is suspected, the police or the Divisional Veterinary Officer (DVO) must be notified. Immediate restrictions are placed on the farm (Form A) to prevent movement of people, animals or things on or off the premises. The DVO inspects the farm. If positive or suspicious, Form C is served to prohibit movement of susceptible animals within 5 miles. Lesion material is sent to the World Reference Laboratory for FMD at Pirbright. If FMD is confirmed, movement restriction is extended to 10 miles around the infected place. An Infected Area Order is made and a Form D is served on premises which have been directly or indirectly in contact with the initial outbreak to restrict movement of animals for 21 days since exposure to infection.

Infected farm. All susceptible animals are valued, slaughtered and cremated or buried as soon as possible. There is immediate preliminary disinfection. Following disposal of animals, thorough disinfection takes place. Restocking is permitted 6 weeks alter slaughter or 4 weeks after disinfection, whichever is sooner, but depends on whether or not the disease has been brought under control in the area.

Infected areas. These areas are at least 10 miles around the infected farm. No animal may be moved, except to abattoir under licence. No markets may be held. Milk tankers may be fitted with filters and milk is heat treated before being fed to animals. No movement of slurry, litter or manure is permitted. There is no hunting, racing or shooting. The restrictions are lifted gradually; to 5 miles 14 days alter the last outbreak and then completely removed after a further 7 days.

Controlled areas. These are set up to allow tracing of animals and are usually of short duration. All animal movements require a licence. Markets are controlled. Disposal of manure and refuse from abattoirs is regulated. The hunting of deer is prohibited.