describe in brief etiology, pathogenesis, diagnosis and control of fmd in veterinary medicine
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foot and mouth disease FMD etiology pathogenesis diagnosis control veterinary
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Foot-and-Mouth Disease (FMD) in Veterinary Medicine
1. Etiology
Causative agent: FMD virus (FMDV), an Aphthovirus belonging to the family Picornaviridae.
- Structure: Non-enveloped, icosahedral, single-stranded positive-sense RNA virus. Buoyant density in cesium chloride is ~1.43 g/mL.
- Acid lability: The virus is unstable below pH 6.8 and above pH 9 - this is important for disinfection.
- Serotypes: There are 7 distinct serotypes - O, A, C, Asia 1, SAT 1, SAT 2, and SAT 3 (SAT = Southern African Territories). Serotype C may now be extinct in the wild. Over 50 subtypes exist, and there is further antigenic diversity between strains within serotypes. This diversity is critical because immunity to one serotype does not confer protection against others.
- Host range: Cloven-hoofed animals of the order Artiodactyla - cattle, pigs, sheep, goats. Wild artiodactyls (African buffalo, bison, giraffe, deer, antelope) and even Asian elephants are susceptible. The virus has been recovered from at least 70 mammalian species.
- Zoonotic potential: Rare; humans may be infected via contact or ingestion, experiencing fever, salivation, and oropharyngeal/foot vesiculation.
2. Pathogenesis
Route of entry and primary replication:
- Infection most commonly occurs via the respiratory tract (inhalation of aerosols from infected animals), but also through ingestion, skin abrasions, and mucosal contact.
- The virus initially replicates in the epithelial cells of the pharynx (particularly the dorsal soft palate in cattle) - this is the primary site of replication.
Spread and viremia:
- From the pharynx, the virus spreads via lymphatics to regional lymph nodes and enters the bloodstream (primary viremia).
- Viremia distributes virus to epithelial tissues throughout the body - particularly the tongue, dental pad, lips, nares, feet (coronary band, interdigital space), and teats.
Vesicle formation:
- Virus replication in stratified squamous epithelium causes intracellular and intercellular edema, leading to formation of vesicles (aphthae). These initially appear as blanched areas, then rupture to form painful erosions.
- Rupturing vesicles release enormous quantities of virus, making this the period of highest contagion.
Persistence (carrier state):
- After recovery, cattle (and some other ruminants) can become carriers, harboring virus in the pharynx for up to 3.5 years (cattle), even in the presence of circulating neutralizing antibodies. Pigs do not become long-term carriers.
- Immunity after infection is short-lived and serotype-specific.
Mortality: Usually low (~2-5% in adults) but can reach 70% in young animals (myocarditis - "tiger heart" lesion in calves). Morbidity is high. Economic losses from reduced milk/meat production are severe.
3. Diagnosis
Clinical Diagnosis
Suspect FMD when there is:
- Sudden onset fever (40-41°C), followed within 24 hours by excessive salivation, lameness, and anorexia.
- Vesicles on the tongue, dental pad, lips, nares, interdigital cleft, coronary band, and teats.
- Ruptured vesicles leaving raw, painful erosions.
- Rapid spread through the herd.
Differential diagnoses to rule out: Vesicular stomatitis, swine vesicular disease, vesicular exanthema (in pigs), bluetongue, bovine viral diarrhea, rinderpest.
Laboratory Diagnosis
Laboratory confirmation is mandatory before official notification.
| Test | Purpose | Notes |
|---|---|---|
| Real-time RT-PCR | Detect FMDV RNA (primary test) | Two assays targeting different genome regions used together; very sensitive; works even on poorly stored samples |
| Antigen ELISA | Detect viral antigen + serotype | Used on fresh epithelial homogenate from lesions |
| Virus isolation | Confirmatory | Cell culture (BHK-21 cells); slower |
| Nucleotide sequencing | Serotyping and phylogenetic analysis | Identifies strain, source tracing |
| Serology (antibody ELISA, VNT) | Detect past infection or vaccination response | NSP (non-structural protein) ELISA distinguishes vaccinated from naturally infected animals |
Sample collection: Epithelial flap from unruptured/recently ruptured vesicles is the gold standard. Oropharyngeal fluid (probang sample), blood, and nasal swabs are also used.
4. Prevention and Control
Control strategy depends on whether the country is FMD-free or endemic.
In FMD-Free Countries (e.g., USA, Australia, UK)
- Stamping-out (slaughter) policy: All infected and in-contact susceptible animals are culled and carcasses destroyed - this is the traditional approach and most rapid method to eradicate an incursion.
- Movement controls and quarantine: Strict quarantine zones established around the infected farm. Area not declared safe until susceptible animals remain symptom-free for 30 days after last case.
- Import bans: Prohibition on importation of potentially infective materials (fresh meat, animal products) from FMD-endemic countries.
- Emergency vaccination (ring vaccination): May be used in addition to stamping-out to create a firebreak around infected zones - "vaccinate to die" (animals are vaccinated then slaughtered) or "vaccinate to live" (retained but require long-term surveillance).
In Endemic Countries
- Systematic vaccination programs using inactivated (killed) virus vaccines, grown in tissue culture (BHK-21 cells) and adjuvanted. Vaccines must match the circulating serotype(s) - multivalent vaccines covering several serotypes are used.
- Limitations: Vaccines do not provide long-lasting immunity; boosters needed every 4-6 months. Conventional vaccines do not prevent the carrier state.
- New generation vaccines: Recombinant DNA/subunit vaccines (virus-like particles, adenovirus vectors) are under development to improve durability and allow DIVA (differentiation of infected from vaccinated animals) strategies.
- Biosecurity: Isolation and quarantine of sick animals, restricted movement, thorough disinfection of premises (sodium hydroxide, sodium carbonate, citric acid are effective; glutaraldehyde, formaldehyde), and proper disposal of carcasses.
WOAH (OIE) Framework
FMD is classified as a notifiable disease by the World Organisation for Animal Health (WOAH). The Global Framework for the Progressive Control of Transboundary Animal Diseases (GF-TADS) coordinates the international effort toward FMD-free status in endemic regions using a progressive zonal approach.
Sources:
- Jawetz, Melnick & Adelberg's Medical Microbiology, 28th ed., p. 549 - Picornaviridae/Aphthovirus section
- MSD Veterinary Manual - FMD in Animals (updated Feb 2026)
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