Special topics on swine diseases: What is Foot-and-Mouth Disease?

Pathogen:

Foot-and-mouth disease (FMD) is a disease caused by the foot-and-mouth disease virus. It commonly affects cloven-hoofed animals such as pigs, cattle, and sheep. The foot-and-mouth disease virus belongs to the family Picornaviridae, genus Aphthovirus, and includes seven main serotypes: O, A, C, Asia-1, SAT1 (South African type 1), SAT2 (South African type 2), and SAT3 (South African type 3), each of which has several subtypes. While animals infected with different serotypes of the virus show similar clinical symptoms, there is no cross-immunity between serotypes. For example, pigs that recover from FMD caused by the O-type virus will only have immunity to the O-type virus, and infection by other serotypes will still cause foot-and-mouth disease.

Epidemiology:

Animals susceptible to the foot-and-mouth disease virus primarily include cattle, pigs, sheep, and various wild animals, with about 33 species being affected. Naturally infected animals are usually limited to cloven-hoofed animals. The main sources of infection are diseased animals and carrier animals. The virus is highly contagious and is shed through vesicular fluid, excretions, secretions, and exhaled air, contaminating feed, air, equipment, and the environment. Meat, organs, blood, skin, and wastewater from slaughtered animals that have not been disinfected are major sources of spread. After pigs are infected with the FMD virus, the virus first accumulates and multiplies in the epithelial cells of the throat and lungs. The amount of virus exhaled by infected pigs into the air is approximately 20 times that of cattle, making airborne transmission the primary route of spread within pig farms.

The disease is typically transmitted through the respiratory and digestive tracts, but it can also be spread through wounds or even intact mucous membranes and skin. Semen and milk contain a large amount of virus and can also be sources of transmission. FMD can occur throughout the year, but natural conditions like temperature and light intensity directly affect the virus's survival, so the disease shows seasonal variation, with outbreaks more common in the colder winter and spring months, and less common during the hot summer and autumn months. Pigs raised in high-density environments are highly susceptible to infection, and once infected, outbreaks can spread rapidly.

Symptoms of Foot-and-Mouth Disease in Pigs:

Symptoms appear as vesicles and ulcers of various sizes on the skin of the coronet, heel, cleft, interdigital space, and snout, as well as on the mucous membranes of the mouth, palate, cheeks, and tongue. Vesicles may also appear on the teats and udder of sows. Infected pigs show signs of lethargy, fever, and loss of appetite. When the virus affects the hooves, hoof temperature rises, and lameness becomes severe, often leading to deformed or detached hooves. Infected pigs may lie down and be unable to stand. The vesicles are filled with clear or slightly cloudy serous fluid and rupture quickly, exposing a clean-edged, dark red, ulcerated surface. If no secondary bacterial infections occur, the affected areas heal with scabs in 1-2 weeks. If the hooves are severely affected, healing may take over 3 weeks. The mortality rate of adult pigs infected with FMD is generally less than 3%.

When piglets are infected, vesicular symptoms are less noticeable, and the main symptoms are gastroenteritis and myocarditis, with a mortality rate of over 80%. Pregnant sows may experience abortion. After the virus enters the body of a susceptible animal, it first multiplies in the epithelial cells at the site of entry, causing serous exudation that forms primary vesicles (first-stage vesicles), which are usually hard to detect. 1-3 days later, the virus enters the bloodstream, causing fever and systemic symptoms. The virus then reaches its preferred sites such as the oral mucosa, hooves, and udder skin, and forms secondary vesicles (second-stage vesicles). After the vesicles rupture, the body temperature drops to normal, the virus in the bloodstream gradually decreases and disappears, and the infected pigs enter the recovery phase. The virus can invade the myocardium of piglets, leading to degeneration or necrosis of heart muscle tissue, resulting in grayish-white or pale yellow streaks or spots, commonly known as "tiger heart."

Prevention and Control:

1.Vaccination and Immunization:

The goal is to protect susceptible animals, enhance their immune levels, and reduce the severity and spread of FMD outbreaks. When the injection density of inactivated oil-adjuvant vaccines reaches over 80%, it can effectively control FMD outbreaks. Vaccination programs can include annual planned immunization, surrounding-ring immunization, and unilateral zone immunization around the outbreak area. Vaccine selection, dosage, and frequency should be based on the outbreak situation. Routine vaccination should ensure 2-3 doses per year, with each dose containing at least 3 PD50 (the dose required to protect 50% of the population). Emergency vaccination should increase the dose to 6 PD50 per animal and may involve additional booster doses.

2.Emergency Measures During FMD Outbreaks in Pigs:

•(1) Immediately cull infected pigs and their cohorts by electrocution or injection with a suitable drug. Disinfect the environment with 2-3% caustic soda. Vaccinate healthy pigs in the same area with 5 ml of the routine vaccine or 3 ml of a high-efficiency vaccine. After 15 days, administer a booster shot. Spray disinfectant (Roxycide disinfectant powder) at a 1:100 concentration 1-2 times daily for thorough disinfection of the pigsty and surrounding environment. Restrict pig movements, closely monitor the epidemic situation, and report to the local epidemic control center to prevent further spread.

•(2) If the outbreak is severe and emergency vaccination is ineffective, immediately block the entire pig farm, report to the epidemic control department, and organize personnel to monitor surrounding areas for susceptible animals. Interrupt transportation and set up disinfection checkpoints. The farm should remain locked down for 2-3 months.

3.Treatment Methods:

•(1) Exogenous foot-and-mouth disease antibodies (Superimmune Protein-V) can be administered by intramuscular injection, at a dose of 0.1 ml per kg body weight (double the initial dose). This can effectively control myocarditis and reduce mortality. It can also be used for emergency immunization. (If there are mixed infections, antibiotics sensitive to the condition should be used in combination.)

•(2) For topical treatment, use 0.1% potassium permanganate solution to rinse the affected area, followed by the application of iodine glycerin or gentian violet solution at 1%-2%.