CLASSICAL SWINE FEVER (hog cholera)
Aetiology Epidemiology Diagnosis Prevention and control References
AETIOLOGY
Classification of the causative agent
Virus Family Flaviviridae, genus Pestivirus
Resistance to physical and chemical action
Temperature: Partially resistant to moderate heat (56°C) pH: Inactivated by pH <3.0 or pH >11.0 Chemicals: Susceptible to ether, chloroform, ß-propiolactone 0.4% Disinfectants: Inactivated by cresol, sodium hydroxide (2%), formalin (1%), sodium carbonate (4% anhydrous or 10% crystalline, with 0.1% detergent), ionic and non-ionic detergents, strong iodophors (1%) in phosphoric acid Survival: Survives well in cold conditions and can survive some forms of meat processing (curing and smoking) EPIDEMIOLOGY
Hosts
Pigs and wild boar are the only natural reservoir of classical swine fever virus
Transmission
- Direct contact between animals (secretions, excretions, semen, blood)
- Spread by farm visitors, veterinarians, pig traders
- Indirect contact through premises, implements, vehicles, clothes, instruments and needles
- Insufficiently cooked waste food fed to pigs
- Transplacental infection
Sources of virus
- Blood and all the tissues, secretions and excretions of sick and dead animals
- Congenitally infected piglets are persistently viraemic and may shed the virus for months
- Infection routes are: ingestion, contact with the conjunctiva, the mucous membranes, skin abrasions, insemination, percutaneous blood transfer
Occurrence
The disease occurs in much of Asia, Central and South America, and parts of Europe and Africa. Many countries are free of the disease
For detailed information on occurrence, see recent issues of World Animal Health and the OIE Bulletin
DIAGNOSIS
Incubation period is 2-14 days
Clinical diagnosis
Acute form
- Fever (41°C), anorexia, lethargy
- Multifocal hyperaemia and haemorrhagic lesions of the skin, conjunctivitis
- Cyanosis of the skin especially of extremities (ears, limbs, tail, snout)
- Transient constipation followed by diarrhoea
- Vomiting (occasional)
- Dyspnoea, coughing
- Ataxia, paresis and convulsion
- Pigs huddle together
- Death occurs 5-15 days after onset of illness
- Mortality in young pigs can approach 100%
Chronic form
- Dullness, capricious appetite, pyrexia, diarrhoea for up to 1 month
- Apparent recovery with eventual relapse and death
Congenital form
- Congenital tremor, weakness
- Runting, poor growth over a period of weeks or months leading to death
- Clinically normal but persistently viraemic pigs, with no antibody response
Mild form (sows)
- Transient pyrexia and inappetence
- Fetal death, resorption, mummification, stillbirth
- Birth of live, congenitally affected piglets
- Abortion (rare)
Lesions
Acute form
- Leucopaenia and thrombocytopaenia
- Widespread petechiae and ecchymoses, especially in the skin, lymph nodes, larynx, bladder, kidney, ileocaecal junction
- Multifocal infarction of the margin of the spleen is characteristic but not always present
- Enlarged haemorrhagic lymph nodes are common
- Encephalomyelitis with perivascular cuffing
Chronic form
- Button ulcers in the caecum and large intestine
- Generalised depletion of lymphoid tissue
- Haemorrhagic and inflammatory lesions are often absent
Congenital form
- Central dysmyelinogenesis, cerebellar hypoplasia, microencephaly, pulmonary hypoplasia, hydrops and other malformations
Differential diagnosis
- African swine fever (indistinguishable clinico-pathologically. It is essential to send samples for laboratory examination)
- Infection with bovine viral diarrhoea virus
- Salmonellosis
- Erysipelas
- Acute pasteurellosis
- Other viral encephalomyelitis
- Streptococcosis
- Leptospirosis
- Coumarin poisoning
Laboratory diagnosis
Procedures
For details, refer to the OIE Manual
Identification of the agent
- Direct immunofluorescence test on cryostat sections of organs from affected pigs
- Virus isolation in cell culture, with virus detection by immunofluorescence or immunoperoxidase. Confirmatory identification with monoclonal antibodies
Serological tests
- Neutralisation peroxidase-linked assay
- Fluorescent antibody virus neutralisation
- ELISA
(prescribed tests in the Manual) Samples
Identification of the agent
- Tonsil
- Lymph nodes (pharyngeal, mesenteric)
- Spleen
- Kidney
- Distal ileum
- Blood in EDTA (live cases)
kept under refrigeration and shipped to laboratory as quickly as possible Serological tests
- Serum samples from suspect recovered animals, from sows with suspected congenitally infected litters, or from pigs under surveillance
PREVENTION AND CONTROL
No treatment is possible. Affected pigs must be slaughtered and the carcases buried or incinerated
Sanitary prophylaxis
- Effective communication between veterinary authorities, veterinary practitioners and pig farmers
- Effective disease reporting system
- Strict import policy for live pigs, and fresh and cured pig meat
- Quarantine of pigs before admission into herd
- Efficient sterilisation (or prohibition) of waste food fed to pigs
- Efficient control of rendering plants
- Structured serological surveillance targeted to breeding sows and boars
- Effective pig identification and recording system
Medical prophylaxis
Vaccination with modified live virus strains is effective in preventing losses in countries where classical swine fever is enzootic, but is unlikely, on its own, to eliminate infection entirely. In countries which are free of disease, or where eradication is in progress, vaccination is normally prohibited
Response to outbreaks
- Slaughter of all pigs on affected farms
- Disposal of carcasses, bedding, etc.
- Thorough disinfection
- Designation of infected zone, with control of pig movements
- Detailed epidemiological investigation, with tracing of possible sources (up-stream) and possible spread (down-stream) of infection
- Surveillance of infected zone, and surrounding area
REFERENCES AND OTHER INFORMATION
- Reference experts and laboratories
- Classified as an OIE List A disease (A130)
- Chapter 2.1.13. in the Manual of Diagnostic Tests and Vaccines for Terrestrial Animals.
- Terrestrial Animal Health Code
- Other references - see the Index
- World Animal Health .
- Current Animal Health Status (Disease Information)
---------------------------------------------
Contact : scientific.dept@oie.int