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Foot and Mouth Disease (FMD)

Foot and Mouth Disease (FMD) also known as Aphthous fever, Fast Moving Disease and Endoepidemic disease.

Foot and mouth disease is a highly contagious viral disease of cloven hoofed animals, including cattle, buffalo, pig, sheep and goat and wild animals.

Foot and Mouth Disease is clinically characterized by high rise of temperature, lameness, profuse salivation, vesicles on the mouth, muzzle, teats and foot.

Etiology

It is caused by Aphthovirus of the family Picornaviridae.
It is a single stranded RNA virus, the capsid encodes 4 structural polypeptides, namely VP1, VP2, VP3 and VP4.
There are seven major immunologically distinct serotypes O, A, C, Asia1, SAT1, SAT2 and SAT3 are recognized.
The serotype SAT1, SAT2 and SAT3 are restricted to African continent.
The serotype O, A have immunologically and serologically distinct subtypes contain more than 60 strains. There is no cross protection between serotype.
The Serotype O is the most common serotype worldwide. Other serotypes also cause serious outbreaks; however, serotype C is uncommon and has not been reported since 2004.
In India serotypes, O, A, C and Asia 1 are known to occur. Among which A₂₂, A₅ & A ₁₀ subtypes are more commonly reported.
In india 80% of outbreak is caused by serotype O.
The virus is inactivated at pH range 6.0 – 9.0. The 4% sodium carbonate,1- 2%sodium hydroxide or formalin, 0.2% citric acid or acetic acid is an effective disinfectant.

Epidemiology

Distribution: World wide
Morbidity rate: 100%
Mortality rate: 1-2%, sometimes may reach 100% in malignant form

Host Affected

Cattle, buffalo, pig, sheep and goat, deer, antelope, elephant and giraffe, among which cattle are highly susceptible.
Young calves are highly susceptible than adults.
Domestic pigs are amplifier host and is important in propagating the virus.
Sheep and goat act as maintenance host.

Source of infection

Foot and mouth disease virus (FMDV can be found in all secretions and excretions from acutely infected animals, including expired air, saliva, milk, urine, faeces, semen and aborted foetus in sheep.
The amount of virus shed by each route can be influenced by the host species and viral strain Pigs produce large amounts of aerosolized virus, and the presence of large herds of infected swine may increase the risk of airborne spread.
The carrier status occurs in esophageal-pharyngeal fluid of recovered animals 3.5 yrs in cattle, 9 months in sheep and 4 months in goat but pigs do not be a carrier.

Transmission

The disease is mainly transmitted through inhalation of aerosol, under favourable condition the virus can travel in air for long distance even upto 250 km.
Ingestion of contaminated feed and water.
Pigs acquire infection through ingestion of untreated swills or direct contact with an infected animals.
Through skin abrasions.
Sexual transmission occur in SAT type viruses.
Mechanical transmission by fomites or vectors.
People can act as mechanical vectors for FMDV, by carrying the virus on clothing or skin. The virus might also be carried for short time in the nasal passages.

Pathogenesis

After inhalation or ingestion of the virus, the Primary replication takes place in the mucosal and lymphatic tissue of the nasopharynx/pharynx/tonsil. Viremia follows primary multiplication with further replication in the lymph nodes, mammary glands and other organs as well as the epithelial cells of the mouth, muzzle, teats, interdigital space and coronary band. In these areas of stratified squamous epithelium, vesicle formation results from swelling and rupture of keratinocytes in the stratum spinosum.

Clinical Manifestation

Incubation period is variable 1-21 days, usually 3-6 days (In pigs, the incubation period is as short as 1-3 days).

Cattle

Sudden onset of drop in milk yield.
High fever (40-41⁰C) accompanied by loss of appetite, depression, appearance of painful stomatitis.
Profuse salivation, the saliva hanging in long, ropy strings, a characteristic smacking of the lips, and the animal chews carefully.

Vesicles and bullae (1-2 cm in diameter) appear on the buccal mucosa, dental pad, tongue and udder.
The vesicles are thin walled contain a straw coloured fluid. Vesicles rupture within 24 hours, leaving a raw painful surface, which heals in about 1 week.
Vesicles appear on the feet, particularly in the clefts and on the coronet leads to lameness.
The lesions on the tongue often heal within a few days but those on the feet and nasal cavities are contaminated with bacteria, maggots interfere with wound healing.
Vesicles may also develop on the teats, which results in severe mastitis.

Ruptured Vesicles in Foot and Mouth Disease in Cattle

The virus does not cross the placenta but the abortion mostly during second trimester of pregnancy mainly due to fever.
Calves less 6 months old die suddenly due to acute myocardial failure.
The malignant form of infection in adults causes acute myocardial failure.
The common sequel of FMD is dyspnoea, panting (heat intolerance), over growth of hair and poor semen quality in bulls.

Sheep and Goats

The pronounced clinical sign is sudden onset of acute lameness.
The vesicles develop in the inter digital space of the feet, which rupture in about 2-3 days.
Oral lesions are rare or they may develop only on the tongue and upper palate but there is no salivation.

Pigs

Fever 107⁰ F.
Large vesicles and bullae develops on the snout and feet.
Lameness is the first sign, affected animal is lethargic, huddle among others.
Vesicles then develop on the coronary band and heel, and in the inter digital space.
The foot lesions are very painful; the pigs crawl rather than walk. The horns of the digits are sometimes sloughed.
Vesicles in the mouth are very less prominent.
Young pigs up to 14 weeks of age may die suddenly from heart failure; piglets less than 8 weeks of age are particularly susceptible.

Necropsy findings

Acute myocarditis occur in calves and malignant form of infection in adults.
Heart- Epicardial haemorrhage with or without pale area, ventricular walls appear streaked with patch of yellow tissue interspersed with normal myocardium giving the typical tiger heart appearance.

Sample Collection

Fresh vesicular fluids, lesions of tongue, feet, udder or lips of about 1 gm, pieces of cardiac muscle, mammary gland and pancreas should be collected in 50% phosphate buffered glycerol saline for virus isolation.
The outside of the container should be disinfected with 4% sodium carbonate or 0.2% citric acid.
Paired sera sample.

Diagnosis

Based on clinical signs.
Viral isolation-Inoculation of virus or tissue homogenate in monolayers of bovine thyroid or pig kidney cells, Lamb kidney cells or BHK-21 cell line. The cytopatheic effect (CPE) develops within 24-48 hours at 37°C.
The viral antigens are usually identified with ELISA and RT-PCR.
The persistent or subclinical infection can be detected in oesophageal/pharyngeal fluid by RT-PCR.
Virus infection associated antigen test (Enzyme Linked Immuno Electrotransfer Blot ) is used to detect non structural protein (NSP) which are expressed during viral replication.
Complement fixation test (CFT) is the most commonly used to identify the type of virus.
The virus neutralization test and ELISAs are used to detect serotype specific antibodies in serum.
RT-LAMP- Reverse transcription loop mediated isothermal amplification.
Lateral flow Immunochromatographic test strip test.

Differential Diagnosis

Vesicular stomatitis
Vesicular exanthema of swine
Blue tongue
Swine vesicular disease
Bovine viral diarrhoea
Rinderpest
Malignant catarrhal fever
Pox infection of mammary gland
Foot rot in sheep

Treatment

Cleaning of oral lesions and foot lesions with mild antiseptic solution.
Antibiotics to prevent secondary bacterial infection.
In Kenya, Ethano-veterinary practices are followed at the time of outbreak. Oral lesions are cleaned with Soda ash solution (97%) to remove necrotic tissue after that honey and finger millet flour was applied on the lesions to improve healing for 3 days.

Prevention

Inactivated Oil adjuvented trivalent or polyvalent inactivated vaccines are currently used.
Calves from unvaccinated dam should be vaccinated at 4^th month of age and revaccination should be done at 8^th months of age.
Calves from vaccinated dam should be vaccinated at 6^th month of age and revaccination should be done at 10^th months of age.
Adults should be vaccinated once in 6 months regularly.
Sheep and pigs should be vaccinated at 6 months of age.

Control

Ring vaccination- vaccination of all susceptible population around 5-10 km radius from infected premises to prevent the further spread of infection.
Restriction of animal movement.
Quarantine of newly purchased animals.
Stamping out policy, slaughter of infected and in contact animals.
Proper disposal of dead animals.
Disinfection of premises with 4% Sodium carbonate or 1-2% sodium hydroxide or formalin solution.
Animals in high risk area, the ruminants are vaccinated three times a year. With a medium risk, animals are vaccinated twice a year.
Avoid feeding of uncooked swill/garbage/meat scraps to the pigs.
Quarantine of all animals 16-24 km radius from outbreak area.