Difference between revisions of "Foot and Mouth Disease"

Revision as of 19:27, 22 October 2008

VIRUSES PICORNAVIRUSES

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
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
- 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

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

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

@@ Line 77: / Line 77: @@
 *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
 ====Control====