Foot and Mouth Disease

FMD FMDV

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

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

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
Lameness
- Also produces cutaneous erosions in interdigital cleft, at coronet and bulbs of heals
- These feet lesions often take a long time to heal as secondary bacterial infections may ensue and produce true deep ulcerative dermatitis
Teats on animals that are suckling may also develop vesicles
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

Pigs and Sheep:
- Lesions less obvious, but vesicles around nose, mouth, and coronary band
- Pigs have vesicles on snout, which are quickly traumatised to leave an eroded lesion
- 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

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
1967 + 2001 major outbreaks in UK
Still widespread in many parts of world especially S. America, far East
Foot and Mouth disease is NOT a highly fatal disease: approximately 5% mortality (usually young animals); older animals recover

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
Serology for virus antibody can determine past infection
- ELISAs used to detect subclinical carrier sheep
- Cannot be done on vaccinated animals
RT-PCR has been suggested for on-farm diagnosis, but has flaws:
- RNA is readily degraded by tissue enzymes
- RNA must be purified before converting to DNA for PCR
- False positives can occur easily by contamination with previously amplified DNA
May see animals that have discoloration of tongue due to having had FMD
- In these cases take scraping of retropharyngeal region, put scrapings in transport medium and send for testing

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

ANY sign of lesions in a susceptible animal is NOTIFIABLE to the Divisional Veterinary Officer and local police
Once diagnosis is confirmed, all animals on the premises must be slaughtered and incinerated
Further disinfection of the premises
Movement is controlled within a 10-mile radius
Follow-up serology must be performed to ensure no spread has taken place
Ring vaccination with relevant subtype to create a barrier of immune animals (although this was not done in the 2001 outbreak)

Disease cannot be prevented by slaughter due to large numbers of carrier stock
Annual Inactivated whole virus vaccination using local subtypes
- Inactivated by azuridines, using alhydrogel adjuvant for cows, and oil for pigs
- Attenuated virus reverts to virulence
- Subunit vaccines ineffective
- Expensive
- 2 initial injections at 4 months (if dams vaccinated), followed by boosters every 6-12 months
- Induces virus-neutralizing antibodies
Vaccination DOES NOT render meat harmful to consumers, but does affect when it can be exported

Initially - hyperaemia of mucosa (e.g. catarrhal inflammation) then within 12 hours produces fluid filled vesicles on dorsum of tongue, may be other places
Small vesicle coalesce to produce big ones -i.e. Bullae
Very quickly rupture; epithelium appears dirty grey in colour because of necrosis - sloughed skin, very good for diagnosis
Leave painful, hyperaemic epithelium
Looks like "ulcer "with ragged edge but not a true ulcer as stratum germinativum retained and will rapidly heal completely in about 2 weeks unless becomes secondarily infected

Degeneration of prickle cells
Cells "balloon" as fill with fluid and then die to produce vesicle containing straw coloured or clear fluid

Caused by Apthovirus
Main presentation are vesicles
May also involve skeletal and heart muscle
Grossly:
- Yellow streaks and grey foci
Histologically:
- Segmental myofibre necrosis
- Infiltration of lymphocytes and neutrophils