A010 - FOOT AND MOUTH DISEASE

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A010 - FOOT AND MOUTH DISEASE

Nature of the disease
Foot-and-mouth disease (FMD) is a highly contagious disease of cloven-hoofed animals caused by a virus of the Picornaviridae family, genus Aphtovirus. Seven serotypes can be distinguished: A, O, C, SAT1, SAT2, SAT3 and Asia 1 and within each serotype infectivity, virulence and pathogenicity may vary.

The disease is characterised by the formation of vesicles (fluid-filled blisters) and erosions in the mouth, nose, teats and feet. Although not very lethal in adult animals, it causes serious production losses and is a major constraint in international trade.

Classification
OIE List A disease
Susceptible species
Cloven-hoofed animals (ungulates) are susceptible to FMD. Of the domestic species, cattle, buffaloes, pigs, sheep, goats and deer are susceptible. Infections have been (rarely) described in Rodents and in Humans.
Distribution
After a global recession of the disease in the late 90's, FMD resurged all over the world. 

In 2000 outbreaks occurred in Korea and Japan and in 2001 a major outbreak occurred in UK and subsequently spread in Western Europe. In October 2002 Western Europe countries and Japan had recovered the status of officially free of FMD without vaccination. 

The disease is still endemic in many parts of South America, Africa, Middle East and Central Asia. 

The Pacific nations are free of the disease.

Clinical signs (see pictures)
FMD should be suspected wherever vesicles are seen in cloven-hoofed animals. Vesicles begin as small white fluid filled areas that quickly grow to a blister about 3 cm in diameter. Two or more blisters may join to form a large one. The blisters usually burst leaving a raw surface. these heal over a few days. Other suggestive signs include:
  • Sudden lameness in a number of animals
  • Salivation
  • Fever
  • Loss of appetite
  • In dairy cattle, a sudden drop in milk yield

In cattle, the earliest signs are dullness, poor appetite, fever and reduction of the milk production.. Salivation and lameness may be seen after a few days. Vesicles appear inside the mouth on the tongue, cheeks and gums, lips and palate. Vesicle may form between the claws of the feet and along the coronary band. Vesicles often evolve to erosions. There may also appear on the teats and udder — mastitis and abortion are common.

The recovery generally occurs within 1 to 2 weeks, sometimes erosions are superinfected causing complications. Mortality in adults is usually very low. However, up to 50% of calves can dies due to cardiac involvement, secondary infections, exposure and malnutrition.

In pigs, the main sign is lameness. There is also fever and loss of appetite. Vesicles form along the top of the foot, on the heels and between the claws. The feet are sore and affected pigs prefer not to move. Vesicles may also be form on the snout, but tend to rupture quickly. Abortion is common. Mortality in piglets can be high.

In sheep and goats, the disease is usually milder with fewer lesions. Affected sheep are depressed and reluctant to stand. Some animals may get a sudden lameness. Vesicles form around the top of the foot and between the claws. Lesions may develop on the tongue and dental pad, but can be hard to detect. Significant mortality can occur in lambs and kids.

Post-mortem findings 
Vesicles, blisters and erosions can be found on the tongue, dental pad, gums, cheek, hard and soft palate, lips, nostrils, muzzle, coronary bands, teats, udder, snout of pigs, corium of dewclaws and interdigital spaces, rumen pillars, myocardium (particularly of young animals).

In young animals there is focal necrosis of cardiac muscle.

Differential diagnosis 
Foot-and-Mouth Disease cannot be clinically distinguished from other vesicular diseases:
  • Swine vesicular disease, 
  • Vesicular exanthema, 
  • Vesicular stomatitis

Other diseases to include in the differential diagnosis are:

  • Infectious bovine rhinotracheitis
  • Rinderpest
  • Bluetongue
  • Bovine papular stomatitis
  • Mucosal disease
  • Peste des petits ruminants
  • Mycotic stomatitis
  • Phototoxic dermatitis
  • Footrot
  • Chemical irritants and scalding
  • Traumatic lesions of mouth and feet
Specimens required for diagnosis 
Due to the difficulty to carry a clinical diagnosis, laboratory diagnosis is a matter of emergency.

If an history of vaccination exists, the prescribed diagnosis should be based on virus isolation. Diagnostic tests include ELISA, Complement Fixation and PCR. The tests can be performed on specimen of 1 g of tissue from unruptured or recently ruptured vesicles. Epithelial samples should be placed in a transport medium (antibiotics and phosphate buffer) which maintains a pH of 7.2-7.4 and kept cool. Also oesophageal-pharyngeal fluid collected by probang cup and frozen to below -40°C immediately after collection can be used.

If there is no history of vaccination, serological tests can be used. OIE prescribes the use of Virus Neutralisation and ELISA, both tests are serotype specific. The blood of suspicious animals should be collected in plain tubes and the sera separated.

The shipment of specimen of FMD are submitted to strict quarantine regulation.

Transmission   
FMD is one of the most contagious diseases of animals. Each blister can contain up to 1 million of infective particles, pigs produce the more particles, followed by cattle and sheep.  

Animals can become infected through inhalation, ingestion and through reproduction. The primary mechanism of spread is by direct contact, through inhalation of virus aerosols.

The virus can also survives for 2 weeks in dry fecal material and for 6 months in slurry, it resists for about 1 week in urine and in the soil depending on climatic condition.

Under the right conditions long distance spread (measured in kilometres) of FMD by wind-borne virus can occur. This general requires a potent sources of aerosol virus e.g. an infected piggery, high humidity, stable atmospheric conditions and exposed livestock downwind.

In the tropics the conditions are not very favorable for environmental transmission of the virus because of its faster inactivation.

Movement of infected animals is the most important method of spread between herds. Other sources of infection include contaminated vehicles, equipment, people and products. 

FMD virus can survive for long periods in meat if pH does not fall below 6.2. It can also survive in frozen lymph nodes, bone marrow and viscera. FMD virus will also survive well in salted and cured meats, and in non-pasteurised dairy products. Transmission to calves via infected milk was described. Contamination of pigs by ingestion of infective swills is also common (supposed origin of the 2001 outbreak in UK).

It has been shown experimentally that FMD can be transmitted through artificial insemination where the donor was infected.

Pigs, cattle and sheep seem to have different epidemiological role in the distribution of the disease. In a simplified manner, pigs produce the largest amount of viruses and amplify the disease. Cattle, which are very sensible because of their large respiratory volume, favor the dissemination of virus in the country through their frequent movement but are good indicator as the clinical signs are obvious. Sheep are a possible maintenance host as they can transmit the virus with little signs of the disease.

Risk of introduction   
The importation of animal, animal products and material or feed for animal husbandry should be assessed through risk analysis.

If strict quarantine procedures are followed, the highest risk of entry of FMD into Pacific Island countries is probably through contaminated meat or dairy products. Virus can survive for long periods in a range of fresh, partially cooked, cured and smoked meats, and in inadequately pasteurised dairy products. These could be brought in with passengers on aircraft and ships, through the mail or on fishing vessels or yachts.

The sterilization of swills and the destruction of leftover from aircraft and ships are safe measures that should be always practiced. 

During the latest outbreak of FMD, disinfections of passengers' shoes at the airport and at the wharf was implemented.

Control / vaccines  
OIE recommend to use inactivated virus vaccine containing an adjuvant. They are specific of each serotype but they can contain several valence (e.g. O, A, C strains). Nowadays it is recommended to produce a vaccine to respond to a specific outbreak, international laboratories and international bank of vaccines can produce million of doses within a week. The immunity lasts 6 months after two initial vaccinations, 1-month apart, depending on the antigenic relationship between vaccine and outbreak strains. 

The vaccinated animals have a positive serological reaction and which interferes with the diagnostic and the surveillance of the disease. Live vaccines with no serological positive reaction exist but are not recommended as they may transmit the disease. 

Vaccinated animals (with inactivated vaccines) can be infected with high infective doses, they do not have the clinical disease and no evidence of transmission of the virus from vaccinee have been found. 

The main reason to vaccinate in case of outbreak is to limit the destruction of stock and the consequent economic loss. 

Vaccination can be practiced with a simultaneous eradication policy. In that case 'ring-vaccination' is practiced to contain outbreaks

The eradication or stamping-out policy involves quarantine, movement restrictions and slaughter and disposal of all affected an in-contact livestock on affected premises followed by cleaning and disinfection. 

The main interest is to eradicate the disease faster and therefore to recover in a shorter period a favorable status for international trade (free from FMD without vaccination). It is more likely to be successful if the incidence of the disease is low and the capacity of emergency response of the country is high (preparedness level). Stamping-out policy can be very costly and country must insure that they have the resources to afford it. 

References
  • Foot-and-Mouth Disease, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 457-459
  • Foot-and-Mouth Disease, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 965-974
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, 440p
  • Office International des Epizooties, 2002
  • Website of the INTERNATIONAL VACCINE BANK FOR FOOT-AND-MOUTH DISEASE: http://www.iah.bbsrc.ac.uk/ivb/