FOOT-AND-MOUTH DISEASE

FOOT-AND-MOUTH DISEASE

is a highly infectious viral infection of
cattle, pigs, sheep, goats, buffalo, and artiodactyl wildlife species.
It is characterized by fever; vesicles in the mouth and on the muzzle,
teats, and feet; and death in young animals. Picture All species of
deer and antelope, elephant, and giraffe are susceptible to FMD, but
camels are resistant to natural infection, and the smaller camelids
such as alpacas and llamas, although susceptible, are probably of no
epidemiologic significance. Camels, mice, and guinea pigs can be
infected experimentally. FMD is endemic in the Middle East; in Africa,
excluding South Africa, Botswana, Namibia, and Zimbabwe and possibly
Tunisia and Morocco; and in South America, excluding Chile, southern
Argentina, Uruguay, Guyana, Surinam, and French Guiana. Europe is free
of FMD, although there have been occasional outbreaks that have quickly
been eliminated. Australasia, Japan, Indonesia, and Korea are free of
FMD, but FMD is endemic in all other Asian countries, including the
Philippines and Malaysia. Etiology: FMD is caused by an aphthovirus of
the family Picornaviridae. There are seven immunologically distinct
serotypes: A, O, C, Asia 1, and SAT (Southern African Territories) 1,
2, and 3. Within each serotype, there is a large number of strains that
exhibit a spectrum of antigenic characteristics; therefore, a number of
vaccine strains for each serotype, particularly O and A, are required
to cover the antigenic diversity. Strains are characterized by their
genomic relationships and their antigenic similarities with established
vaccine strains. (Previous classification into subtypes became
untenable as the number of subtypes rapidly increased.) The virus is
quickly inactivated outside the pH range of 6.0-9.0 and by desiccation
and temperatures >56،C, although residual virus may survive a
considerable time when associated with animal protein (for instance, a
proportion of FMD virus in infected milk will survive pasteurization at
72،C for 15 sec). The FMD virus is resistant to lipid solvents such as
ether and chloroform. Because of the sensitivity of the virus to acid and alkaline pH, sodium hydroxide, sodium carbonate, and
citric or acetic acid are effective disinfectants. Transmission,
Epidemiology, and Pathogenesis: Transmission of FMD is generally by
contact between susceptible and infected animals. Infected animals have
a large amount of aerosol virus in their exhaled air, which can infect
other animals via the respiratory or oral routes. All excretions and
secretions from the infected animal
contain virus, and virus may be
present in milk and semen for up to 4 days before clinical signs
appear. Aerosol FMD virus can spread a considerable distance as a
plume, depending on weather conditions, particularly when the relative
humidity is >60% and when the topography of the surface over which
it is dispersing does not cause turbulence. FMD has been transmitted to
calves via infected milk, and milk tankers carrying infected milk have
been implicated in the spread of disease between farms. Fodder can
become contaminated after contact with infected animals, and iatrogenic
spread of FMD has been reported, but mechanical spread by
nonsusceptible species of wildlife such as birds and dogs is unlikely.
A typical scenario for the introduction of FMD into a previously clear
area is for pigs to be fed imported food derived from an infected
animal (as meat, offal, or milk) and then virus spreads by aerosol from
the infected pigs to cattle, which are the species most likely to be
infected by the respiratory route because of their large respiratory
volume. FMD virus can survive in dry fecal material for 14 days in
summer, in slurry up to 6 mo in winter, in urine for 39 days, and on
the soil between 3 (summer) and 28 days (winter). Ruminants that have
recovered from infection and vaccinated ruminants that have contact
with live FMD virus can remain infected and carry the virus in the
pharyngeal region for up to 2 1/2 years in cattle, 9 mo in sheep, and
probably lifelong in the African buffalo. Experimentally, it has not
been possible to show transmission from a carrier bovid to an
in-contact susceptible, but there is evidence that under field
conditions these carrier animals initiate new outbreaks of disease. FMD
virus can be recovered from carrier animals by culturing a sample of
pharyngeal mucus and superficial cells (collected using a probang cup)
on susceptible tissue culture, such as primary bovine thyroid cells.
However, the technique is probably only 50% reliable in identifying a
carrier using a single sample because the quantity of virus found in
the pharynx varies on different occasions. The primary site of
infection and replication is usually the mucosa of the respiratory
tract, although the virus can enter through skin abrasions or the GI
tract. Replication then occurs in the local lymph node, and infection
subsequently spreads via the blood stream to predilection sites in the
epithelium of the mouth, muzzle, feet, and teats, and also to areas of
damaged skin (eg, the knees and hocks of lame pigs kept on concrete).
Vesicles develop at these sites and rupture, usually within 48 hr. The
viremia persists for ~5 days. Clinical Findings: The incubation period
for FMD is 2-14 days, depending on the infecting dose, susceptibility
of the host, and strain of virus. The clinical signs are more severe in
dairy cows and intensively reared pigs than in sheep and goats, and FMD
has frequently been ignored or misdiagnosed in small ruminants. After
the incubation period, a fever of up to 106،F (41،C) develops, the
animal is anorectic and salivates and stamps its feet as vesicles
develop on the tongue, dental pad, gums, lips, and on the coronary band
and interdigital cleft of the feet. Vesicles may also appear on the
teats and udder, particularly of lactating cows and sows, and on areas
of skin subject to pressure and trauma, such as the legs of pigs. Young
calves, lambs, kids, and
piglets may die before showing any vesicles
because of virus-induced damage to the developing cells of the
myocardium. Milk yield drops dramatically in milking animals, and all
animals show a loss in condition and growth rate that may persist after
recovery. Sheep and goats may develop only a few vesicles on the
coronary band and in the mouth. Vesicles in the mouth, even when
severe, usually heal within 7 days, although recovery of the tongue
papillae takes longer. Lesions on the mammary gland and feet frequently
develop secondary infections, resulting in mastitis, underrunning of
the sole, and chronic lameness. In pigs, the complete horn of the toe
may be lost. Cattle and deer may also lose one or both horns of the
foot. Diagnosis: In cattle and pigs, the clinical signs of FMD are
indistinguishable from those of vesicular stomatitis, QuickSearch and
in pigs from those of swine vesicular disease QuickSearch and vesicular
exanthema. QuickSearch Samples of vesicular epithelium or vesicular
fluid should be sent in phosphate-buffered saline (pH 7.4) to the
national laboratory responsible for the diagnosis of FMD, or otherwise
to the OIE/FAO World Reference Laboratory for FMD, Pirbright, UK, after
having previously made arrangements for submission of samples. Samples
are prepared as a 10% suspension and inoculated onto susceptible tissue
culture and directly typed by ELISA. Isolated FMD virus is
characterized by antigenic comparison with existing FMD vaccine
strains, and the nucleotide sequence of a segment of the 1D gene is
determined for comparison with other strains of the same serotype from
the same region to identify a possible origin of the outbreak. It is
essential that samples are kept as close as possible to pH 7.4 to
prevent destruction of the FMD virus and antigen, and that they are
securely packed in double leak-proof containers that comply with
national and, when appropriate, international regulations for the
shipment of pathologic and hazardous material. Treatment and Control:
The occurrence of FMD in countries previously free of the disease can
have a major effect on local and international trading arrangements.
Many countries free of FMD have a policy of slaughter of all affected
and in-contact susceptible animals and strict restrictions on movement
of animals and vehicles around infected premises. After slaughter, the
carcasses are either burned or buried on or close to the premises, and
the buildings are thoroughly washed and disinfected with mild acid or
alkali and by fumigation. Tracing is done to identify the source of the
outbreak and premises to which FMD virus could have already been
transmitted by infected animals or animal products, by contaminated
vehicles or people, or by aerosol. In areas or countries free of FMD in
which this is not possible, control is by movement restriction,
quarantine of affected premises, and vaccination around (and possibly
within) the affected premises. This has the disadvantage that many
carrier animals may remain after the outbreak, and quarantine may not
be sufficiently long to prevent their subsequent movement. In countries
in which FMD is endemic, protection, particularly of high-yielding
dairy cattle, is by a combination of vaccination and prevention of FMD
virus entering the dairy premises. This can be difficult if prevalence
of FMD in the unvaccinated population is high and climatic conditions
are suitable for