African Swine Fever: A Set Back to Growing Pig Industry and Socioecomic Marginal Farmers of India
Kritika Dhial¹* and Himani Sharma²
- D. Scholar, Vety. Microbiology, RAJUVAS, Bikaner, Rajasthan
- D. Scholar, Vety. Public Health and Epidemiology, CVASC, Pantnagar, Uttrakhand
*Corroponding author: kritikadhial@gmail.com
Abstract
African swine fever is a highly contagious, fatal, and transboundary viral disease of domestic and wild pigs. This disease is caused by the African swine fever virus. It causes 100% morbidity and mortality. African swine fever causes heavy economical loss to the pig industry of the affected country. Any region of world having pig population is prone to African swine fever. African swine fever virus is highly stable to environmental conditions that attribute to the spread of this disease to a long distance from its origin. There is no treatment and vaccine available for African swine fever, only strict preventive and control measures can be used to save the pigs from African swine fever. In this article, different aspects of African swine fever are discussed in detail including a recent outbreak of African swine fever in India.
Keywords: African swine fever, ASFV, mononuclear phagocyte system, Ornithodoros moubata
Introduction
Responsible for huge losses in swine population and drastic economic consequences, African swine fever has emerged as a major crisis for pork industry in recent years. African swine fever is a highly contagious, transboundary, deadly, and economically important disease of pigs. ASF is a notifyable disease, caused by a double-stranded DNA virus, African swine fever virus (ASFV) and characterized by high morbidity and mortality rate in domestic and wild pigs (Costard et al., 2009). The spreading of virus from endemic area to virus-free area is facilitated by both legal and illegal movements of live animals, as well as the importation of animal products, byproducts, and animal feed (Brown and Bevins, 2018). As per the 20th livestock census, a decline of 12.03% was observed in the pig population over the previous livestock census due to different factors like exotic and re-emerging pig diseases, lack of pig farming at the commercial level, lack of marketing availability for pork and pork products, lack of Government policies, etc.. Recently, ASF has been reported in the Northeastern region of India during early 2020. In this article, different aspects of African swine fever are discussed in detail including a recent outbreak of African swine fever in India.
Epidemiology
ASF was described for the first time from Kenya in 1921. In 1957, it spreaded to Pourtgal and other European countries including France, Italy, Malta, Belgium, and the Netherlands. In 2007, major outbreaks occurred in Georgia, Armenia, Azerbaijan, and the European part of Russia, Ukraine, and Belarus. So far 50 countries around the globe are endemic for ASF. In Asia, China was the first county to report the ASF outbreak in August 2018 followed by Mongolia (January 2019), Vietnam (Februray 2019), Cambodia (March 2019), North Korea (May 2019), HonKong (May 2019) and Mayanmar (September 2019). Recently in early 2020, India has reported for the first time the outbreak of ASF in the Northeastern region (Assam and Arunachal Pradesh) adjoining to China. The representative specimens from dead pigs were tested in the National Institute of High-Security Animal Diseases (NIHSAD), Bhopal, India and were confirmed for the presence of ASFV. The ASFV persists in nature under different cycles that include the sylvatic cycle, the tick-pig cycle, and the domestic (pig-pig) cycle. Recently, a wild boar cycle has also been described (Olesen et al., 2020). Warthogs are considered as a reservoir of ASFV and participate in the sylvatic cycle with ticks of the Ornithodoros moubata complex (Jori et al., 2013). ASFV persists in the lymph nodes. ASFV spread from warthogs to domestic pigs either via direct contact or indirect contact (fomites, vector, and feeding contaminated pork products to the domestic pigs). Mechanical transmission via Stomoxys calcitrans has been reported within 48 hr of a blood meal on an infected pig (Olesenet al., 2020).
African swine fever virus
African swine fever virus(ASFV) belongs to the family Asfaviridae. ASFV contains a single molecule of dsDNA as a genome of length 170-190 kbp. It has icosahedral symmetry and the capsid consists of 1892 or 2172 capsomeres. The capsid is surrounded by a lipoprotein membrane. Twenty-three genotypes have been described based on the partial sequences of the p72 gene (Brownand Bevins, 2018). But, only genotypes I and II have been found outside of the Africa continent. The mononuclear phagocytic cells are the primary target cells of ASFV. As ASFV is a very stable virus and can sustain in raw or processed pork products including chilled, fried, smoked, offals, blood, skin, etc. ranging from 11 days to 1000 days (FAO, 2013).
Pathogenesis
There are two scenarios; in the first scenario ticks are involved in disease production and second scenario ticks are not involved in disease production. In the first scenario, ASFV gains entry into the host through tick biting. In the second scenario, ASFV gains entry into the body through the tonsils or dorsal pharyngeal mucosa to the mandibular or retropharyngeal lymph nodes.From there, the virus spread to other sites and produce viremia. ASFV is detectable from all the tissues of the body. But, ASFV is found at higher titer in the mononuclear phagocyte system (spleen and lymphnodes).The infection of dendritic cells is observed which may interfere with humoral immune responses. ASFV activates the secretory and phagocytic activity of macrophages. These activated macrophages secrete proinflammatory cytokines such as IL-1, IL-6, and TNF-α. This induces fever and vascular changes like vasodilatation and increased permeability. These pro-inflammatory cytokines also cause interstitial edema and fibrin microthrombi formation in septal capillaries (Bloome et al., 2013).
Clinical disease
The incubation period of ASF varies from 4-19 days. Clinical disease occurs in four forms; peracute, acute, subacute, and chronic (Yooet al., 2020). In peracute form affected animals die without any visible clinical symptoms with mortality rate upto 100% (Gallardo et al., 2017). An acute form of the disease caused by highly virulent strains and are typically characterized by a high fever, anorexia, lethargy, weakness, recumbency, diarrhea and/or constipation, abdominal pain, hemorrhagic signs, respiratory distress, nasal and conjunctival discharge, and abortions in pregnant females. Death often occurs within 7–10 days after the onset of clinical signs. Moderately virulent strains result in subacute infection (often with high mortality in young animals and much lower mortality in older animals), where the clinical signs often include abortion, fever, and transient hemorrhaging with death or recovery occurring within 3–4 weeks (Sargsyanet al., 2018). Chronic infections are characterized by intermittent or low fever, loss of appetite, depression, and in some instances result in a fatal infection (Sargsyanet al., 2018). Infected domestic pigs may start shedding viruses before the appearance of African swine fever. The recovered pigs may shed infectious virus for 1 month after the disappearance of clinical signs (FAO, 2013).
Diagnosis
Following techniques can be used for the diagnosis of ASF-
- Pig leukocyte cells, bone marrow cultures, porcine alveolar macrophages, and porcine blood monocytes can all be used in ASFV culture. ASFV can be isolated from the live pig (Nasal swab, blood, bioptic tissue from lymph nodes) or postmortem tissue (spleen, kidney, tonsils, and lymph nodes) of a dead animal (OIE, 2019).
- Conventional PCR, real-time PCR, direct fluorescent antibody test, and antigen ELISA tests can be used for detection of ASFV antigens (FAO, 2013).
- ELISA test, an indirect fluorescent antibody (IFA) test, by indirect immunoperoxidase test (IPT) can be used for the detection of ASF antibody (Laddomada et al., 2019).
At field level due to lack of infrastructure, equipment, reagents, and/ or unnecessary delay in reaching the sample to the laboratory may delay the diagnosis process. These limitations can be overcome by pen-side tests. Pen-side tests helps in initial screening of disease at spot tests before laboratory test reports come. There are two commercially available pen-side tests by INGENASA for ASF; one pen-side test is for detection of antibodies against using lateral flow device (LFD) and other pen-side for detection of ASFV antigen using lateral flow device (LFD) using blood as sample (Sastre et al., 2016; Cappai et al., 2017).
Postmortem Findings
In postmortem (PM) findings, there can be bluish-purplish discoloration of the skin of extremities, chest, and abdomen, erythema (peracute ASF), hyperaemic splenomegaly, petechial hemorrhage on the kidney (mainly in the cortex) and gallbladder, severe alveolar edema, hemorrhage in epicardium and endocardium (acute ASF). Abortion, fibrinous pericarditis, pleuritis and pneumonia. Hemorrhages and ulcers can be appreciated on the lining of the stomach, gastrointestinal tract (GIT) may be congested and the contents of GIT may contain blood (chronic ASF), yet there are no pathognomic lesions for ASF (Sánchez-Vizcaíno et al., 2015).
Differential diagnosis
ASF should be differentiated from other similar looking diseases that include- Classical swine fever (CSF), Porcine reproductive and respiratory syndrome (PRRS), Porcine dermatitis and nephropathy syndrome (PDNS), Erysipelas, Aujeszky’s disease, Salmonellosis, Septicaemias, Poisoning.
Prevention and control
There is no specific treatment and vaccine available against ASF. The following preventive and control measures can be adopted to control the ASF-
- Slaughter and proper carcass disposal of infected as well as suspected pigs.
- ASF is a vector-borne disease, so the elimination of its vector is an important step in controlling the disease.
- Restrict movements of live pigs, their products from an infected area to non-infected area.
- Strict biosecurity measures should be adopted in commercial as well as backyard farms.
- Fomites (clothes, utensils, vehicles, shoes, etc.) are also an important source of ASF. So, there should proper arrangements for the disinfection of fomites.
- Fences must be installed to block intruders such as roaming wild boar.
- Avoid Swill feeding.
- There should be strict regulations and controls on food waste from international flights, and ships.
Conclusion
African swine fever (ASF) is a one of the most serious threats to the pig industry worldwide , leads to a highly contagious and deadly disease affecting domestic and wild pigs. Recently in early 2020, the outbreak of ASF has been reported from Northeast states of India and can adversely affect animal husbandary if proper measures are not taken on time. Since till date, there is no specific treatment or liscensed protective vaccine available against the disease, the Government of India must take proper precautionary and preventive measures to control the ASF and protect the poor and marginal pig farmers of India.
Reference
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.
Brown VR and Bevins SN, 2018. A review of classical swine fever virus and routes of introduction into the United States and the potential for virus establishment. Front Vet Sci, 5: 31
Cappai, S., Loi, F., Coccollone, A., Cocco, M., Falconi, C., Dettori, G., Feliziani, F., Sanna, M.L., Oggiano, A. and Rolesu, S. (2017). Evaluation of a commercial Field test to detect African swine fever.J. Wildl. Dis., 53(3): 602-606.
Costard S, Wieland B, De Glanville W, Jori F, Rowlands R, et al., 2009. African swine fever: how can global spread be prevented? Philos Trans R SocLond B BiolSci, 364(1530): 2683-2696
FAO, 2013.Food Outlook Biannual Report on Global Food Markets. ISSN: 0251-1959. http://www.fao.org/3/a-I5703E.
Fenollar F and Mediannikov O, 2018.Emerging infectious diseases in Africa in the 21st century. New Microbes New Infect, 26: 10-18
Gallardo C, Soler A, Nieto R, Cano C, Pelayo V, et al., 2017. Experimental infection of domestic pigs with African swine fever virus Lithuania 2014 genotype II field isolate.TransboundEmerg Dis, 64(1): 300-304
Jori F, Vial L, Penrith ML, Pérez-Sánchez R, Etter E, et al., 2013. Review of the sylvatic cycle of African swine fever in sub-Saharan Africa and the Indian ocean. Virus Res, 173(1): 212-227
Laddomada A, Rolesu S, Loi F, Cappai S, Oggiano A, et al., 2019. Surveillance and control of African swine fever in free-ranging pigs in Sardinia. TransboundEmerg Dis, 66(3): 1114-1119
O.I.E. (2019).African swine fever. In Manual of Diagnostic Tests and Vaccines for Terrestrial Animals 2019, OIE. World Health Organization for Animal Health: Paris, France, pp. 355-398.
Olesen AS, Belsham GJ, Bruun Rasmussen T, Lohse L, Bødker R, et al., 2020. Potential routes for indirect transmission of African swine fever virus into domestic pig herds.TransboundEmerg Dis, 00: 1-13
Sánchez-Vizcaíno, J.M., Mur, L., Gomez-Villamandos, J.C. and Carrasco, L., 2015.An update on the epidemiology and pathology of African swine fever.Journal of comparative pathology, 152(1), pp.9-21.
Sargsyan MA, Voskanyan HE, Karalova EM, Hakobyan LH and Karalyan ZA, 2018. Third wave of African swine fever infection in Armenia: Virus demonstrates the reduction of pathogenicity. Vet World, 11(1): 5-9
Sastre, P., Pérez, T., Costa, S., Yang, X., Räber, A., Blome, S., Goller, K.V., Gallardo, C., Tapia, I., García, J., Sanz, A. and Rueda, P. (2016).Development of a duplex lateral flow assay for simultaneous detection of antibodies against African and classical swine fever viruses.J. Vet. Diagn.Invest.,28(5): 543-549.
Yoo D, Kim H, Lee JY and Yoo HS, 2020. African swine fever: Etiology, epidemiological status in Korea, and perspective on control. J Vet Sci, 21(2): e38.
https://www.pashudhanpraharee.com/classical-swine-fever-and-african-swine-fever-a-comparative-study/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7704300/
