African Swine Fever- An Emerging Infectious Disease

Introduction      

Pig farming is one of the most fast growing industries in the world where pigs are raised and bred as livestock. Moreover, livestock sector is critical for rural livelihood as well as for economic development of the country. India is one of the largest contributors of the livestock wealth in the world. Total Pig Population in the country is 9.06 Million during 2019 and has decreased by 12.0% over previous Livestock Census (2012). About 1.7% of the total livestock is contributed by pigs. The main threat to this growing industry is the occurrence of disease which ultimately decreases the population by increasing the mortality. Emerging infectious diseases are infections that have recently occurred within a population or those whose incidence or geographic range is rapidly increasing or threatens to increase in the near future. African swine fever is one of them. It is an important viral disease of pig which is responsible for major economic loses in the pig industry worldwide. African swine fever is an OIE listed transboundary disease that infects domestic swine and other members of the family Suidae, including warthogs (Potamochoerus aethiopicus), bushpigs (P. porcus), and wild boar (Sus scrofa ferus), which was originally reported in Africa and parts of Europe, South America and the Caribbean. It was first detected in 1921 in Africa and later on it has been reported in different countries across Africa, Asia and Europe. Recent outbreak has been reported from Arunachal Pradesh in January 2020 the first time ever reported time in India. Further the disease has caused mass mortality of about 13,382 pigs in Assam and has also spread to the adjoining states Meghalaya. African swine fever is caused by tick born DNA viruses known as Asfavirus which are sensitive to lipid solvents and has the ability to spread over long distance.

Transmission

This disease can be transmitted through not only by the contact of infected pigs to the susceptible ones, but also due to consumption of the meat from infected pigs (Wilkinson, 1984), even by the bites of infected ticks (Ornithodoros spp.) and by contact with material or objects or fomites like (bedding, feed, equipment, clothes and footwear, vehicles) which are contaminated by virus-containing biological material such as blood, faeces, urine or saliva from infected pigs. Although warthogs are considered to be the natural hosts for this virus but it has been demonstrated that they are unable to transmit the virus directly to domestic pigs. The African swine fever virus is maintained in biological vectors (soft ticks) after feeding on the viremic pig and thus it can transmit the disease among the pig population (Murphy). The mortality rate is varying between 0 and 100% depending on the virus, the host, the dose and the route of exposure to the virus (Costard et al 2013)

Pathogenesis

The incubation period is 4 to 19 days of post exposure (Arias and Sánchez-Vizcaíno 2002). Natural route of infection is the alimentary tract, but other routes like respiratory tract, skin injuries, tick bites are also involved.  On entry of the viruses they first replicate in the mononuclear phagocytic cells predominantly on the monocytes and the macrophages also can infect endothelial cells and hepatocytes. Then viruses spread via blood and lymph causing primary viremia occurs in the 8 hours of post infection in case of new born piglets and later on secondary viemia occurs within 15-24 hours post infection where they further spread to almost all the tissue from the primary sites. After 30 hours of post infection it can be spread to the target organs like spleen, bone marrow, liver, lung, kidney causing massive haemorrhages after destruction of macrophages. Virus can be first detected in the tonsils, mandibular lymph nodes, and in circulating leukocytes in the blood one day post infection (Heuschele, 1967). Organ destruction is mainly due to the release of active substances including cytokines, complement factors and arachidonic acid metabolites (Penrith et al 2004). Thrombocytopenia occurs in the acute and sub-acute infection (Blome et al 2013) 

Clinical Signs

Clinical symptoms are categorized based on the time period and the severity of the disease. In per acute form pigs die within 1-4 days without showing any symptom due to the infection with highly virulent strain of the virus.

In acute phase of the disease the animals die within 3-8 days post infection showing different clinical signs like high body temperature (40.5–42°C), anorexia, depression, cyanotic chest and reddening of the skin of ears as well as the abdominal skin, respiratory involvement like nasal discharge and dyspnoea, vomition, nervous involvement like incoordination and convulsion. In domestic swine, the mortality rate often approaches 100%.

Sub-acute form occurs due to infection with moderately virulent strain of the virus and death generally occurs 18-45 days of post infection. Symptoms are milder than the acute form with lower fever, enlarged spleen etc. However, mortality rate is lower like 30–70% but varies widely.

Chronic form of the disease occurs due to low virulent strain of the virus causing recurrent fever even abortion in the sows as well as emaciation, this form can ultimately result in the carrier animals. The disease is often confused with the clinical symptoms shown in classical swine fever which is quite similar but in African swine fever the symptoms are much more pronounced.

Gross lesions

ASF specific lesions are observed specially in acute phase where petechial haemorrhages are seen on different internal organs like spleen, lymph node, kidney, heart, bladder accompanied by splenomegaly also haemorrhages are pronounced on the duramater in brain as well as in pleura. Kidney haemorrhage is more visible during subacute infection rather in acute form. Necrotic skin is observed in chronic form. (Wozniakowski et al 2016).

Diagnosis

Diagnosis is based on clinical symptoms and laboratory tests. Laboratory diagnosis involves identification of the agent by isolation in cell cultures showing haemadsorption and antigen detection. Antigen detection can be done by fluorescent antibody test (FAT) Polymerase Chain Reaction techniques can be used to detect the viral genome and particularly useful when samples may be unsuitable for virus isolation or antigen detection  in case of conditions like putrefaction.

Serological tests like Enzyme-linked immunosorbent assay (ELISA) which is the prescribed test for international trade, Indirect fluorescent antibody (IFA) test and Immunoblotting test can be used for diagnosis. Serum should be collected within 8–21 days after infection in convalescent animals.

However differential diagnosis is very important to distinguish African Swine Fever from Classical swine fever (CSF also known as hog cholera), Porcine reproductive and respiratory syndrome (PRRS), Erysipelas, Salmonellosis, Aujeszky’s disease (pseudorabies), Pasteurellosis and other septicaemic conditions.

Prevention and Control

“Prevention is better than cure” plays an important role for containment of the disease. As no vaccine and no direct treatment can be implemented except symptomatic treatment, the disease requires a set of basic preventive measures to be followed in order to minimize risk of ASFV spread. Measures can be taken at the institutional or at the individual level, e.g. the farmer, the middleman, the butcher, etc. Being a trans boundary disease risk analysis with stringent policies is a necessary step to be taken while doing export or import of any pork and pork products, hides, semen etc. to any free countries. However, African swine fever is non-zoonotic i.e. it poses no potential threat to human. Strict biosecurity measures should be implemented including hygiene and cleanliness. Swill feeding should be discouraged and feeding pigs with fresh fodder harvested at risk areas for ASFV exposure should be avoided. Access to pig’s sty should be restricted only to the care takers and pig owners and they should avoid visiting other farms. Workers should properly wash hands before and after leaving the premises. Effective disinfection methods like occasional fumigation and using footbath with disinfectant like potassium permanganate (0.3g per litre of water) at the entrance of sty is very important. In case of any outbreak, the disease should be reported immediately and separation of diseased and healthy animals should be done effectively. Inspection and feeding of diseased animals should be done at the end of the round. Regular monitoring of health of the animal is mandatory.  Proper disposal of dead animals is one of the key points to avoid spread of infection. Tick control programme should be held as they play important role as vectors to spread the infection. It is important to have good knowledge and management of the wild boar population as well as a good coordination among the Veterinary Services, wildlife and forestry authorities for effective prevention and control of African swine fever (ASF).

References

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2. Blome, S., Gabriel, C. and Beer, M., 2013. Pathogenesis of African swine fever in domestic pigs and European wild boar. Virus research, 173(1), pp.122-130.
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