Foot and Mouth Disease

VIRUSES PICORNAVIRUSES

Morphology

• Very small (25nm) RNA virus
• 12 capsomeres (1 per vertex)
• 5 subunits per capsomere
• 1 molecule of virus protein (VP) per subunit
• 4 virus proteins (VP1-VP4)
• VP1 is the attachment protein

Antigenicity

• FMDV was the first animal virus in which serotypes were isolated
• To date, there are (important in bold):
  • Oise (O)
  • Allemagne (A)
  • C (also German)
  • South African Territories (SAT) 1, 2, and 3
  • India (Asia-1)
• Each serotype has at lease three subtypes
• Serotype and subtype can be quickly identified by ELISA using guinea pig antisera
• All isolates are virulent

Hosts

• Cloven-hoofed animals, EXCLUDING the horse
  • Cattle
  • Sheep
  • Goats
  • Pigs
  • Deer
  • Elephants
  • Wild ruminants: buffalo, kudu, impala, etc

Pathogenesis

• Primary replication in the upper respiratory tract, tonsils, or upper alimentary tract
• Aerosol excretion during this incubation period
• Viremia
• Virus targets stratum spinosum of stratified squamous epithelia and mucus mebranes
• Secondary vesicles appear after incubation of 2-14 days
• Appearance of lesions by age:
  • 0-2 days: unruptured vesicles
  • 1-3 days: newly ruptured vesicles with adherent epithelia at margins
  • 3-7 days: ruptured vesicles, loss of epithelia, no marked fibrous margin
  • 7-10+ days: open lesions with marked fibrous margin
• In the young, without maternal antibody, virus will localize in the heart and cause death by myocarditis
• FMDV causes loss of condition and productivity but is NOT typically fatal

Pathogenesis by species

• Pigs and Sheep:
  • Lesions less obvious, but vesicles around nose, mouth, and coronary band
  • Lesion at coronary band means infection is usually less than a week old
  • Lesions grow down claw at a rate of 1mm per week
• Cattle
  • Lesions are seen inside mouth, around muzzle, in the interdigital cleft, around coronary band, and on teats
  • Excessive salivation, anorexia, secondary mastitis
  • PM: lesions in oesophagus and forestomachs

Epidemiology

• Highly contagious virus that is spread by aerosol, saliva, infected swill, direct contact, and fomites
• Pigs produce 3000 times more aerosol virus than cows
• Cows are much more susceptible to infection than pigs
  • Persistent infection of cattle can occur in unkeratinized lesions, but subclinical carriers do not usually transfer infection
  • Subclinical buffalo CAN transmit the disease

Diagnosis

• Clinical signs for provisional diagnosis
• Confirmed by ELISA for virus antigen
  • ELISAs are serotype-specific
• Should soon be replaced by immunochromatography-bedside ELISA to allow on-farm diagnosis
• Virus isolation can also be performed in kidney culture cells, and then serotyped by ELISA

Control

• Recovered animals show immunity ONLY to the subtype of first exposure, and even this is relatively short-lived
• Re-exposure to the original serotype after immunity as waned will still result in virus excretion, even without clinical symptoms
• Infection by a second serotype will result in clinical disease
• For these reasons, vaccination is not practiced in the UK
  • Further, vaccination would mean a loss of meat export markets