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Special topics on swine diseases: What is Classical Swine Fever

2024-11-28

Classical Swine Fever (CSF):

Classical Swine Fever (CSF) is caused by the Classical Swine Fever Virus (CSFV), which has only one serotype. The virulence of CSFV strains can be classified into highly virulent, moderately virulent, and low virulent strains. Highly virulent strains can cause acute CSF with high mortality, while moderately virulent strains lead to subacute or chronic infections. Low virulent strains cause variable outcomes depending on the age, immunity, and nutritional status of the infected pigs.

This disease only occurs in pigs. Pigs of all breeds, sexes, and ages are susceptible. Infected pigs are the primary source of transmission. Infected pigs can excrete large amounts of virus through oral, nasal, tear, fecal, and urinary secretions. Dead pigs also contain large amounts of virus in their blood, meat, and organs. Healthy pigs can become infected through direct contact with sick pigs or by contact with contaminated feed, water, pigsties, or transportation vehicles. The infection route is primarily through the digestive tract, but it can also occur through the respiratory tract, reproductive tract mucosa, or skin wounds. The virus can also be transmitted transplacentally, infecting the fetus, which may result in mummified fetuses, stillbirths, weak piglets, or congenital tremor in piglets. Surviving pigs may become persistent carriers of CSF. Outbreaks are more common in winter and spring.

Symptoms: The incubation period is 2–21 days, with an average of around 7 days.

1.Peracute form: This is most commonly seen in the early stages of the disease. The onset is sudden, with rapid progression of symptoms, including persistent high fever, cyanosis, and petechial hemorrhages on the skin and mucous membranes, generalized convulsions, limb twitching, and recumbency. Death typically occurs within 1–8 days.

2.Acute form: This is the most common form. Initially, the body temperature rises to 41–42°C and remains elevated. Pigs become depressed, sluggish, and exhibit signs of anorexia, excessive thirst, and difficulty in movement. The eyes may show conjunctivitis with mucopurulent discharge, and the mucous membranes become pale, often accompanied by hemorrhages in areas such as the ears, lips, limbs, abdomen, and genitalia. Initial constipation may give way to diarrhea, and the urine may become cloudy and foul-smelling. In young piglets, acute CSF presents with neurological symptoms, such as seizures, opisthotonus, grinding of teeth, and spasms, often leading to death. Most pigs with acute CSF die 10–20 days after infection.

3.Subacute form: Symptoms are similar to the acute form but less severe, with occasional improvement. The disease lasts 3–4 weeks.

4.Chronic form: In the early stages, pigs show reduced appetite, depression, and fever. This is followed by slight improvement, but the pig remains emaciated, anemic, and weak. The pig may have a slow, uncoordinated gait and alternates between constipation and diarrhea. Some pigs may develop purple spots or necrotic scabs on the tips of the ears, tail, or limbs. The disease lasts for more than a month, and some pigs may survive for over 100 days. Non-lethal cases may become stunted and develop into "stiff pigs."

5.Mild form of CSF: Sows infected with mild or low-virulence strains of CSFV often show no clinical signs but can transmit the virus transplacentally to their fetuses. This congenital infection can result in abortion, mummified fetuses, deformed fetuses, stillbirths, congenital tremor piglets, or weak piglets. Piglets born seemingly healthy may later exhibit symptoms like reduced appetite, depression, conjunctivitis, dermatitis, diarrhea, or ataxia. This condition is referred to as delayed onset CSF. Some congenitally infected piglets may develop normally but become carriers of the virus within the herd.

Prevention and Control:

1.Treatment: There is no effective treatment for CSF. For valuable breeding pigs, CSF serum may be used early in infection to save the animal. However, this is not economically feasible for all pigs.

2.Preventive measures: Preventing the introduction of infected or carrier pigs is crucial. A comprehensive vaccination program should be followed, with vaccines administered according to the immunization schedule. In general, both boars and sows should receive one dose of the CSF vaccine in the spring and fall. In some cases, double doses may be used. For piglets, two immunization schedules are recommended:

•Normally, the first vaccine dose should be administered at 20–30 days of age, considering the interference of maternal antibodies. A 3–4 times the standard dose is preferred. The second dose should be given at 60–70 days of age.

•In farms that have experienced an outbreak of CSF, piglets should receive a double dose of the CSF vaccine before they begin suckling and should be allowed to nurse 2 hours after vaccination. A booster dose should be administered at 60–70 days of age.

3.Control measures during an outbreak:

(1) Quarantine: Lockdown the outbreak area, halting the movement of pigs and pig-related products. Quarantine should last until 3 weeks after the last infected pig has been handled or died and thorough disinfection has been completed.

(2) Surveillance and culling: Measure the temperature and perform clinical checks on all pigs. Culled pigs should be slaughtered immediately, and their blood, organs, and contaminated waste should be buried deep on-site. If the outbreak is detected early, it is essential to promptly handle infected pigs and contaminated materials.

(3) Emergency vaccination: Immediately vaccinate presumed healthy pigs in the affected and threatened zones with an increased dose of the CSF vaccine (2–4 times the normal dose).

(4) Restrict access: Prohibit outsiders from entering the farm, and ensure farm personnel do not move between farms to prevent disease spread.

(5) Culling of carrier sows: Sows that carry the virus but do not show symptoms should be culled, as they can transmit the virus to their piglets via the placenta, resulting in stillbirths, weak piglets, or continued viral shedding. Such piglets may develop immunity tolerance and become sources of infection within the herd.