African Swine Fever

Dr Terija Rajkumari / Dr Thiyam Ramesh Singh *

African swine fever is a highly contagious viral disease of the swine population that affects all ages and breeds of pigs and has a high fatality rate in both domestic and wild boars.

It has been listed into category of notifiable disease by (World Organisation for Animal Health (OIE). ASF has a severe negative impacts on regional and national trade in pork products as well as posed significant challenges to countries that raised pigs. However, African Swine Fever does not infect humans or other livestock species (Non Zoonotic Disease).

African swine fever (ASF) was first discovered in Kenya in 1921 and initially occurred in sub-Saharan African countries.ASFV genotype I was first introduced to Europe from western Africa in 1960. In 2007, ASFV type II was introduced from eastern Africa and spread widely across Europe, and in 2018, the virus was introduced to China via Russia .

In Shenyang, Liaoning Province, China, on August 3, 2018, the World Organization for Animal Health (OIE) reported the first case of ASFV infection and the strain falls under genotype II.The first report of ASF outbreak in INDIA is reported from Assam and Arunachal Pradesh wherein, a total of 3701 pigs died from 11 outbreaks(Morbidity-38.45%and Mortality- 33.89%).

ETIOLOGY:

ASF is caused by a large double-stranded DNA virus, African Swine Fever Virus (ASFV)of the genus Asfivirus belonging to the family Asfarviridae.

STABILITY OF THE VIRUS:

o African swine fever virus is thermolabile and sensitive to lipid solvents.The virus is very resistant to a wide range of pH (several hours at pH 4 or pH 13), and survives for months and even years in refrigerated meat.
o ASFV can remain infectious for 15 weeks in frozen meat, for 6 months in cured ham, and for 399 days in hams.
o In liquid fertilizer, stability was observed for more than 100 days.
o In liquid blood, the virus can survive for 18 months at room temperature, for 6 years at 4 æ%C, and longer when frozen.
o ASFV can survive for 18 days in salami, 60 days in pork belly, and 83 days in tenderloin.
Another study showed that ASFV in pig manure was inactivated after 4 h at 40 degree C. Urine ASFV remains infectious for 4 days at 37 degree C, and ASFV in faeces remains infectious for 3 days at 37 degree C.

SUSCEPTIBILITY OF VIRUS:

ASFV can be inactivated at 60 degree C for 30 min. Sodium hypochlorite is one of the compounds recommended for the inactivation of ASFV.

Others chemical compounds effective in inactivation of ASFV are
1. 1% formaldehyde
2. Sodium hypochlorite (0.03% to 0.0075%)
3. 2% caustic soda solution (the strongest virucidal agent)
4. 1% sodium or calcium hydroxide (effective at virus inactivation in slurry at 4æ%C)
5. Phenols—lysol, and creolin.

CLINICAL SIGNS:

ASF can be classified asacute,subacute, or chronic.

1. ACUTE ASF -

o which is caused by highly virulent strain pigs usually die within 4- 20 days post infection, with high mortality rate of 95-100%.

Symptoms includes-
o High fever with temperature of 40-42°C,
o Lethargy,
o Anorexia,
o In coordination of gait.
o Cyanotic extremities (ears, limbs)
o Oculonasal discharge, respiratory distress
o Vomiting, blood stained diarrhoea,
o Increased pulse and respiratory rate,
o Prostration.

2. Subacute ASF: similar clinical signs as those observed in acute ASF.Hemorrhages and oedema in the subacute form of the disease can be more intense than the acute form.

3. Chronic ASF: multifocal necrosis in the skin and arthritis, weight loss, emaciation, respiratorydistress, and abortion.

Infected adult warthogs do not develop clinical disease , constitute wild reservoirs of the virus.

POSTMORTEM LESIONS:

o Enlarged, friable and dark-red spleen.
o Enlarged haemorrhagic gastro-hepatic and renal lymph nodes.
o Petechial to echymotic haemorrhages in the kidney.
o Oedema of gall bladder and interlobular oedema of lungs.
o Petechial haemorrhages in the urinary bladder.
o Visceral peritoneum and haemorrhages in lymph nodes of abdominal cavity and myocardium.

MODE OF SPREAD/ TRANSMISSION

o Direct contact with diseased animals
o Farm worker, farm machinery, fomites- Unrestricted movement of men and materials between farms, vicinity of farm to forest areas wherein wild pigs come in contact with farm pigs.
o Improper disposal of carcass, slaughtering of pigs by farm side, sale of sick pigs, unknown source of replacement stock, feeding of swill.
o Soft Ticks (Ornithodorosmoubata, Ornithodorouseraticus) .

DIAGNOSIS:

ASFV infection is very difficult to differentiate from CSFV infection either by clinical or post-mortem examination. Hence, it is essential to confirm the ASFV infection through laboratory diagnosis only.

LABORATORY DIAGNOSIS

1. Isolation in porcine macrophages culture,haemadsorption of pig RBC.
2. Ag detection by FAT, AGPT, ELISA
3. Ab detection by CFT, AGPT and ELISA
4. PCR

TREATMENT:

As of today, there is no vaccine and treatment for the disease.

PREVENTION AND CONTROL:

o At present, there are no vaccines available com­mercially. SO, testing of infected and in contact pigs, culling all positive animals are the only way of pre­venting the spread of infection.

Some of the Biosafety and biosecurity at farm level to be practiced.

1. Disinfect the pig shed and premises by ASFV approved disinfectants like 8:1000 NaOH (30 min), 1-2% sodium hypochlorite (30 min), 3:1000 formalin (30 min), 3% ortho-phenylphenol (30 min), 2-3% iodine compounds, quaternary ammonium compounds.

2. Separate workers for handling of diseased and healthy animals.
3. Pigs should be purchased only from authorized source.
4. The newly purchased pigs should not be mixed immediately with other pigs.
5. The newly purchased pig should be kept in quarantine for 30-45 days and monitor their health daily for any observable sign of illness.
6. People/labors handling the infected pigs should take all biosafety precautions such as wearing of protective equipment such as aprons, spectacles, gloves, and gumboots and should not visit the other sheds.

7. Gumboots should be washed with 2% sodium hydroxide immediately after use with disinfectant.
8. Rapid culling of all infected and in contact pigs and proper disposal of cadavers, litter, and waste food is essential.
9. All these things should be buried deeply (6 feet depth) in the vicinity covered with lime and salt, not to be transported to distant places to avoid spillage.
10. Complete ban on the movement of live pigs, slaughtered pig and meat products inside and outside of the infected areas.
11. The dead animals should not be thrown to the nearby water bodies like rivers, lake, ponds, etc.
12. Thorough cleaning (with water) of farm/infected area and disinfection (Disinfection may be carried out with 2% sodium or calcium hypochlorite/sodium hydroxide or a detergent, if tick population is high, one can use acaricide depending on the need.

13. Farm utensils should be cleaned with detergents and washed properly.
14. Any suspected cases of ASF should be immediately reported to Veterinary doctors and to the District Veterinary Officer.
15. All the pigs within 1km radius (As per the National Action Plan for control, containment and eradication of ASF, DADF, GOI) must be slaughtered by humane methods, whether or not they currently show signs of the disease.

16. After slaughter, disposal and decontamination procedures must be completed and premises left destocked for a period of minimum 40 days.
17. After 40 days of the destocking period, if farmers want to reintroduce the pigs than only 10% of the normal stocking rate on the previously infected farms.
18. These reintroduced pigs must be observed closely for 6 weeks to ensure they are free from ASF, and then reintroduce the full population of pigs.

As there is no effective vaccine or treatment for ASF, strict biosecurity measures, proper sanitation, and hygiene practices are needed to prevent the spread of pigs or pigs products from infected areas.

About the authors

Dr. Terija Rajkumari, MVSc Scholar, Department of Veterinary Microbiology.
Dr. Thiyam Ramesh Singh, MVSc Scholar, Department of Veterinary Public Health and Epidemiology.
College of Veterinary Sciences and Animal husbandry,
Selesih, Aizawl,
Mizoram,
CAU, IMPHAL