Pathogenic Agent

Etiology

Picornavirus, genus aphthovirus, 7 types and several subtypes

FMD, highly contagious and one of the most feared diseases, is endemic to Asia, Africa, parts of Europe and South America.

Because the disease is easily transmitted and spreads rapidly, outbreaks cause enormous financial losses in terms of production and export revenue, and costs of eradication and vaccination programs.

Cloven hoofed livestock such as cattle, pigs, sheep and goats as well as several wild animals (deer, water buffaloes, antelopes, llamas, camels, giraffes, elephants, rats, hedgehogs) of all ages and sexes are susceptible to infection. Men in very rare cases may develop very mild forms of disease.

FMDV is resistant to cold temperatures and survives even freezing, but it is susceptible to pH less than 5, uv-light and heat and dryness.

Transmission is either via aerosol from animal to animal, or contaminated personnel, equipment or feed. Wind may spread the virus over long distances. Occasionally, it is imported to FMD-free countries by contaminated meat especially meat on the bone or garbage. Pigs are mainly infected by direct or indirect contact by ingesting infected food. Many outbreaks could be traced to consumption of uncooked waste (swill feeding) from ships or airplanes originating from FMD-infected countries. Even contaminated biologicals (MLV vaccines against hog cholera, or insufficiently inactivated FMD vaccines) and bad vaccination managements have been a source of infection in the past.
FMDV replicates in the pharyngeal area, gains access to the bloodstream and infects the epithelium of mouth, feet and teats as well as the heart muscle especially in young animals. Pigs excrete large amounts of virus by respiratory aerosols and are considered highly important in disease spread.

Morbidity approximates 100%, mortality rates vary with different strains. It ranges from 5% (adults) to 75% (young stock). While surviving ruminants remain virus carriers for up to two years, this status is not documented for swine.
Sheep normally show only mild signs of FMD, are sometimes not detected and play an important role in disease transmission.

Clinical Signs

Following an incubation period of 2 to 21 days (in most cases 3 to 5 days), infected animals develop fever, depression, nasal discharge and anorexia. In pigs, vesicles are most frequently localized around the snout, the coronary bands of the main and supernumerary digits, interdigital clefts and bulbs. Thus lameness is often the first sign noticed.

Vesicles begin as small blanched areas, which later raise above the surface and extend to blisters of 0,5 to 1 cm in diameter. They may also appear on the snout and lips, in the mouth and on teats of sows with litters. The vesicles contain a straw colored fluid. Two to 3 days after appearance the blisters rupture, leaving denuded areas, erosions and painful ulcers. Many pigs loose their horn shoes.

Frequently, the healing is delayed by secondary bacterial infection of the wounds. Deformation of the feet and detachment of claws are common sequels to the initial viral infection. Suckling piglets frequently develop fatal myocardial degeneration and die within a shortest time without showing any other clinical signs (=peracute disease).

Sporadic abortions are due to fever, because FMDV is not reported to infect the fetus.

FMD vesicle: cells of the str. spinosum are destroyed, cell debris and edema fluids fill the space

The following signs should be considered as possible FMD infections: highly contagious diseases among cattle and pigs, vesicular lesions especially on top of the snout, erosions and ulcers of the coronary bands and interdigital area, lameness, fever, abortions, sudden drop of milk yield and sudden death of calves and piglets.

Contact state authorities immediately.

Collect and submit samples of vesicular fluids, tissue samples of the blisters, heparinized and clotted blood in suitable containers to confirm the clinical findings. Tissue culture, ELISA, virus neutralization, complement fixation, or agar-gel precipitation and PCR are used for detection and typing of virus and antibodies.

At necropsy, vesicles in varying stages of development may be observed.

Histological examination of the vesicles reveals swollen cells (ballooning degeneration) of the stratum spinosum. The intercellular bridges brake down and edema fluids and cell debris infiltrated with leukocytes fill the spaces. The stratum basale usually remains intact (graphic).

Differential diagnosis

Vesicular stomatitis, swine vesicular disease (SVD), vesicular exanthema of swine, Anjeszky's disease

Control

Prevention

The OIE classifies FMD as a list A disease.

The International Animal Health Code of the OIE contains the criteria for a country or zone to be listed as FMD free. There are free countries with and without vaccination.

Free countries maintain their status by prohibiting or restricting the introduction of animals and animal products from countries in which FMD exists. Anyway these countries are always under a very high risk of infection. It is commonly practiced that such countries put contingency plans into place to be prepared for an accidental outbreak. Computer programs have been developed to simulate possible outbreaks and to evaluate the economic effects of eradication by compulsory slaughter, vaccination programs or different combinations respectively. The consequences of a wrong decision can be economically devastating.

FMD infected countries try to control the disease by setting up control and eradication programs.

• This will lead to status "allow incidence of outbreaks".
• Now an eradication program can be started.

Eradication programs are favored in countries with a low incidence of disease. The success depends on the thoroughness that is paid to the following points:

• immediate slaughter of all infected and suspicious cloven hoofed animals and those who had contact with them
• safe discarding of carcasses e.g. in rendering plants where the complete carcasses may be autoclaved at temperatures above 121°C
• restricted areas with a radius of 2 miles (3 km) where all animal movement is strictly prohibited and even movement of humans is restricted
• quarantine zones with a radius of 10 to 15 miles (16 to 24 km) of the outbreak
• setting restricted areas and quarantine zones geographical parameters, such as main roads, rivers, sea or lakes, meteorological parameters, such as winddirection, temperature, and bright sunshine as well as socio-economical behavior of people must be taken into consideration
• traffic through the quarantine zone must be reduced to a minimum
• cleaning and decontamination of the premises
• no possibly contaminated material may leave the premises nor the quarantine zone
• burning of all contaminated objects that cannot be thoroughly disinfected
• restocking of the premises after 30 days with test animals such as cattle, pigs or wild animals, which are monitored for clinical signs of FMD
• final permission for restocking after 30 days of careful monitoring
• prohibiting or restricting the introduction of animals and animal products from countries in which FMD exists

Vaccination

Regular vaccination programs in a form of a blanket vaccination are favored in countries with a high incidence of disease, where eradication does not seem possible in the near future. The enormous number of animals saved from stamping out and the prevention of losses due to clinical disease may outweigh the costs of vaccination programs. However, in very rare cases and depending on the quality of the vaccine, unexpected expenses may be caused by the occurrence of adverse reactions to vaccination in pigs and immunity breakdowns of individual animals.

There is no cross immunity among the different strains of FMD virus only among some subtypes.

Vaccination programs include

• regular basis immunization consisting of 2 shots 3 to 6 weeks apart of all cattle
• good farm and vaccination management
• monitoring of vaccination programs by state authorities
• administration of killed vaccines, approved by the OIE laboratory, WRL (World Reference Lab) Pirbright, UK and covering the strain of FMD causing the outbreak
• administration of oil-adjuvanted vaccine to swine for better efficiency
• typing of field virus in case of a new outbreak and administration of vaccine from locally isolated virus strains or covering the strain with a high probability

Treatment

Treatment is prohibited in several countries, because eradication programs demand the slaughter of affected animals. Treatment must be avoided even in countries where it is not prohibited, because treated animals shed high amounts of virus and are a very high risk for all remaining animals. If allowed, systemic antimicrobial drugs, mild disinfectants, immunomodulators and protective dressings are recommended to prevent secondary bacterial infection, alleviate the signs and strengthen the immune system.

Literature

• Blood D.C., Radostis O.M., Veterinary Medicine, 1989
• Fraser, C.M. et al., The Merck Veterinary Manual, 1991
• Leman, A.D. et al, Diseases of Swine, 1994
• Mayr, A., Medizinische Mikrobiologie, Infektions- und Seuchenlehre, 1984
• Mayr, A., Scheunemann, H., Infektionsschutz der Tiere, 1992
• Pensaert, M.B., Virus Infections of Porcines, 1989
• Plonait, H., Bickhardt, K., Lehrbuch der Schweinekrankheiten, 1997
• Smith B.P, Large Animal Internal Medicine, 1990