African Swine Fever: Understanding the Disease

African Swine Fever (ASF) is a highly contagious viral disease that poses a significant threat to the global pork industry. This article is based on information published by the World Organization for Animal Health (OIE) and aims to provide a comprehensive overview of ASF, including its etiology, epidemiology, diagnosis, and prevention and control measures.

Etiology of African Swine Fever

Classification of the Causative Agent

African Swine Fever is caused by the African swine fever virus (ASFV), a DNA virus belonging to the Asfarviridae family and the genus Asfivirus. Notably, ASFV is the sole member of its family. Viral genotypes have been identified through sequence analysis. The virulence of ASFV isolates can vary significantly, and standard nomenclature for isolates includes the city or country of isolation and the last two digits of the year of isolation (e.g., Lisbon ’60, DR ’78). It is important to note that ASFV is the only known DNA arbovirus.

Resistance to Physical and Chemical Action

ASFV exhibits remarkable resistance to various environmental conditions:

• Temperature: It is highly resistant to low temperatures but can be heat-inactivated at 56°C for 70 minutes or 60°C for 20 minutes.
• pH: ASFV is inactivated at pH levels below 3.9 or above 11.5 in serum-free medium. Serum can increase the virus’s resistance, extending its viability at pH 13.4 for up to 21 hours without serum and 7 days with serum.
• Chemicals/Disinfectants: While susceptible to ether and chloroform, ASFV can be inactivated by 8/1000 sodium hydroxide (30 minutes), hypochlorites (between 0.03% and 0.5% chlorine, 30 minutes), 3/1000 formalin (30 minutes), 3% ortho-phenylphenol (30 minutes), and iodine compounds. The effectiveness of disinfectants may vary depending on factors such as pH, storage time, and organic content.
• Survival: ASFV can remain viable for extended periods in blood, feces, tissues, and particularly in infected, uncooked, or undercooked pork products. It can also multiply in vectors, specifically Ornithodoros ticks.

Epidemiology of African Swine Fever

Hosts

ASFV has a range of hosts, including:

• All varieties of domestic and wild Sus scrofa (pigs).
• African wild suid species such as warthogs, bush pigs, and giant forest hogs, which are usually inapparently infected and act as reservoir hosts of ASFV.
• Ticks of the genus Ornithodoros are the only known natural arthropod hosts of the virus and act as reservoirs and biological vectors.

Transmission

ASFV transmission can occur through various routes:

• Direct transmission: This involves contact between sick and healthy animals.
• Indirect transmission: It includes scenarios such as feeding on garbage containing infected meat (ASFV can remain infectious for 3–6 months in uncooked pork products), biological vectors like soft ticks of the genus Ornithodoros, and fomites, which encompass premises, feeds, vehicles, implements, and clothing.
• Within tick vector: ASFV can be transmitted transstadially, transovarially, and sexually within the tick vector.

Sources of Virus

ASFV can be found in blood, tissues, secretions, and excretions of sick and dead animals. Additionally, animals that have recovered from acute or chronic infections may become persistently infected and act as virus carriers, especially in African wild swine and domestic pigs and wild boar in endemic areas. Soft ticks of the genus Ornithodoros also serve as a source of the virus.

Occurrence

ASF is present in wild or domestic pigs in regions of Asia, Europe, Africa, and the Americas. For the most up-to-date information on its global occurrence, refer to the OIE World Animal Health Information Database (WAHID) interface.

Diagnosis of African Swine Fever

Clinical Diagnosis

ASF can manifest in several clinical forms, including:

• Peracute: Characterized by sudden death with few signs.
• Acute: In domestic pigs, the mortality rate is usually close to 100%, with symptoms such as fever, leucopenia, thrombocytopenia, reddening of the skin (in white pigs), anorexia, listlessness, cyanosis, and vomiting.
• Subacute: Shows less intense signs, including slight fever, reduced appetite, and depression.
• Chronic: Presents various signs such as weight loss, irregular temperature peaks, respiratory signs, skin issues, arthritis, and a small number of survivors that may become virus carriers for life.

Lesions

ASF can result in various lesions, with pronounced haemorrhages, petechial haemorrhages, congestive splenomegaly, and oedematous areas being notable in acute forms. Chronic forms may exhibit focal caseous necrosis and mineralization of the lungs, among other lesions.

Differential Diagnosis

ASF can be difficult to differentiate from other swine diseases, such as classical swine fever (CSF or hog cholera). Laboratory examination is often essential for accurate diagnosis.

Laboratory Diagnosis

Laboratory diagnosis of ASF involves various methods:

• Isolation: ASFV can be isolated through cell culture inoculation, and most isolates produce haemadsorption.
• Haemadsorption Test (HAD): A positive HAD test result is definitive for ASF diagnosis, and negative HAD samples should be tested by PCR to rule out the presence of the virus.
• Antigen Detection by Fluorescent Antibody Test (FAT): A positive FAT result, combined with clinical signs and appropriate lesions, can provide a presumptive diagnosis of ASF.
• Detection of Virus Genome by PCR: PCR techniques are useful when samples may be unsuitable for virus isolation or antigen detection.
• Serological Tests: Enzyme-linked immunosorbent assays, indirect fluorescent antibody tests, immunoblotting tests, and immunoperoxidase staining can be used for serological diagnosis.

Prevention and Control

Preventing and controlling ASF requires a multifaceted approach:

• In free countries, ASF is a notifiable disease, and strict import policies for animals and animal products are enforced.
• During outbreaks, rapid slaughtering of infected pigs, proper disposal of cadavers and litter, thorough cleaning and disinfection, and the establishment of infected zones with movement controls are crucial measures.
• In infected countries, preventing contact between pigs, wild suids, and soft tick vectors, avoiding feeding untreated swill or kitchen scraps containing meat to pigs, and ensuring good biosecurity practices on farms and throughout the pork supply chain are essential.
• There is currently no specific treatment or vaccine for ASF.

Conclusion

African Swine Fever is a complex and highly contagious disease with far-reaching implications for the pork industry. Understanding its etiology, epidemiology, diagnosis, and prevention and control measures is vital for both affected regions and countries seeking to safeguard their swine populations. Continued research, surveillance, and international cooperation are essential in combating this significant threat to the global pig farming community.

A good approach that can help in the preventing the spread of the ASFV can be made by producing enough hogs for a local community and avoiding transport from one producing community to another hog producing community. You may read more from this ASF Mitigation Article.

See ASF Card from the World Organization for Animal Health.