In Nov. 2019, a stream near the border between North and South Korea turned red after being contaminated with the blood of 47,000 slaughtered pigs. Authorities in South Korea took this massive step to curb the spread of the African Swine Fever (ASF) virus, a deadly pathogen that spread like wildfire in China the previous year. This outbreak was first detected in Liaoning province in northeastern China in Aug. 2018 and quickly swept all mainland provinces in the country and its neighbors. In 2019, ASF outbreaks spread outwards to other Asian countries including Vietnam, Indonesia, North Korea and South Korea. China, the world’s largest producer and consumer of pork, was home to almost half of the world's pig population before the 2018 outbreak. However, it is estimated that about ~225 million pigs in China either died or were culled due to the outbreak, and almost 25% of the global pig population died of ASF from 2018-19.

ASF is an extremely contagious viral disease with high mortality rates (95-100%) in both domestic and wild pigs. First detected in East Africa in the early 1900s, the virus later spread to Europe in the late 1950s and has recently wreaked havoc in many Asian countries. The outbreaks have serious economic implications, particularly on farmers whose livelihood depends on the global swine industry and for consumers who are faced with the brunt of increasing pork prices.

Can Humans Contract African Swine Fever? 

Fortunately, the ASF virus cannot be transmitted from pigs to humans. Neither direct contact with infected pigs nor eating pork originating from infected pigs can transmit the ASF virus to humans. In fact, only domestic pigs and wild boars (warthogs and bushpigs) are susceptible to the virus, in addition to soft ticks belonging to Ornithodoros genus that may act as biological reservoirs and vectors. It is unclear why the virus is not transmitted to humans or other animals. ASF virus targets macrophages and monocytes in swine cells for entry and replication, although the identity of cellular receptors targeted by the virus for host cell entry are unknown. It is possible that the virus uses some specialized receptors on the host cell surface that are absent in non-susceptible species or certain steps in viral replication or maturation may not be supported in non-sensitive species. Low levels of ASF virus replication in human cell lines like Vero and HEK293T cells have been achieved by continuous passaging and adaptation. However, the adapted virus is less infectious in swine macrophages compared to the wild-type strain.

African Swine Fever and the COVID-19 pandemic 

The COVID-19 pandemic is unarguably one of the most serious challenges faced by humanity. To this day, ~500 million people have been infected with SARS-CoV-2 and ~6 million have died of the disease, not to mention the devastating consequences on the mental, social and economic well-being of countless others. ASF, which arose in prominence in Asia and Europe in 2018-19 had similar debilitating consequences on the pig population, with high mortality rates. It is rather uncanny that the 2 pandemics originated at almost the same time and place. Some researchers speculate that the shortage of pork and disruption in the meat trade following the ASF outbreak in China may have led to dietary changes in the local population. Meat from other farmed animals like chickens and cows was not sufficient to fill the overall demand. Hence, producers and consumers of food may have resorted to alternate sources of meat, including wildlife animals. This could have brought humans in closer contact with animals harboring zoonotic pathogens and result in a spillover event, although there is currently no direct evidence to support this. Nonetheless, humanity at large was unprepared for both pandemics, illustrating the importance of understanding the origins of pathogenic diseases and effective disease surveillance, management and mitigation.

What are the Symptoms of African Swine Fever Disease?

ASF can manifest a range of symptoms from acute to chronic, depending upon the virulence of the strain causing the infection and the immune status of the infected pig. In the case of acute disease, caused by highly virulent strains, pigs usually die within 4-20 days post-infection, with a high mortality rate of 95-100%. Symptoms include fever, followed by loss of appetite, depression, hemorrhages that cause blackening of the skin and coughing. Less virulent strains can cause a chronic disease, the symptoms of which include reduced growth, skin lesions, swelling and secondary infections. The mortality rate is typically lower in such cases (30-70%), and sometimes. Wild swine population of warthogs and bushpigs usually have an asymptomatic infection, constituting them as wild reservoirs of the virus. 

How is African Swine Fever Transmitted? 

ASF virus can transmit by direct or indirect contact between infected pigs. Pigs are highly social animals and stay happier and healthier with their kith and kin, making it difficult to contain the spread of ASF once it has started. Transmission occurs via contact with feces, bodily fluids or contaminated carcasses of infected pigs, as well as when pigs eat pork products that contain the ASF virus. The hardy virus can persist for approximately 5 months in boned meat stored at 4^o C and in salted dried hams. In an asymptomatic infection, the virus can persist for extended periods of time in tissue or blood.

ASF virus can also be transmitted via soft ticks that belong to the genus Ornithodoros. In the guts of Ornithodoros ticks, the virus does not cause any disease and can persist for long periods of time. However, feeding on neighboring pig populations will transmit the disease to new hosts. An ASF outbreak in Madagascar in the late 1990s is likely linked to the local establishment of a biological reservoir of O. porcinus ticks. When a farmer tried to introduce uninfected pigs to a particular farm in Madagascar a few years after the outbreak (and the farm didn’t house any pigs in the interim), he noticed that the pigs died of ASF no matter what precautions he took. Researchers later found that O. porcinus ticks infected with ASF virus were present on that farm at the time of the initial outbreak, had firmly established local colonies in the interim and were now transmitting the virus to the new pigs, years after the initial outbreak had subsided.

African Swine Fever Vaccines and Treatments 

To this day, there are no commercial vaccines or treatments available for the ASF virus, even though the virus was detected almost a century ago. The virus has an estimated incubation period of 4-19 days and pigs that survive mild infection can shed the virus for at least 70 days. The only way to contain the virus in case of an outbreak is to either quarantine the pig population, if possible, or slaughter them.

A vaccine against the ASF virus appears feasible, since pigs that recover from an infection are protected when challenged with a closely related strain. Developing a universal vaccine has been challenging, however, due to the limited cross-protection between different strains of the virus. Currently, 24 genotypes of ASF virus associated with distinct geographies in Africa have been identified based on the sequencing of the major capsid protein p72. Live attenuated vaccine strains can induce a long-term resistance to a homologous strain but not a heterologous strain. Such vaccines also need to be tested thoroughly due to safety concerns surrounding the potential for severe side effects like chronic viremia (persistence of the virus in the blood).

A promising and safer vaccination approach is to create strains that are deficient in virulence genes. In Jan. 2020, in collaboration with the U.S. Department of Agriculture (USDA), researchers developed an experimental vaccine that was 100% effective against the strain of the ASF virus that caused an outbreak in Georgia in 2007. Researchers deleted a gene called I177l that was responsible for the high virulence of the virus and likely interfered with the pig’s immune system. Animals inoculated with the modified strain were protected against the virulent strain when challenged 28 days later and did not show any virus shedding or viremia. The infected animals also demonstrated a strong virus-specific antibody response, suggesting the possibility of neutralizing antibodies in protection against the virus. Whether this modified strain also offers protection against the strain that caused the 2018 outbreak in China remains unstudied.

As this vaccine awaits commercialization, further efforts to identify candidate virulence genes and understand correlates of immune protection against the ASF virus are urgently needed to control the increasing number of outbreaks of ASF in different parts of the world.

Diagnostics and Nonpharmaceutical Interventions for African Swine Fever 

In the absence of vaccines and effective treatments, it is necessary to quickly detect and prevent the spread of the ASF virus in both domestic and wild swine populations. Early detection is key to curbing the rapid spread of the disease. Researchers are developing a rapid test that can detect the presence of the ASF virus within 30 minutes using a color-changing paper strip and saliva or blood samples. The development of such rapid, affordable and easy-to-use detection tests, alongside the discovery of promising vaccine candidates, can help alleviate the sufferings of the swine industry and prevent future outbreaks.

Strict biosecurity measures, proper sanitation and hygiene practices are also needed to prevent the spread of pigs or pig products from infected areas. One can easily carry the disease into a farm via boots infected from another location! Mitigating the spread of disease in domestic populations is one thing, but it is more difficult to detect and control ASF in wild boar populations. Prevention of wild boar hunting can help. So far, ASF has not been detected in the U.S., and the USDA is taking strict biosecurity measures to prevent the virus from entering the country.

Conclusion 

ASF is perhaps one of the most dangerous zoonotic diseases, with its high mortality rate, transmission and lack of effective treatments. The virus has serious consequences on health, economy and social well-being, bringing into light the concept of “One Health” which states that “animal health, human health and environmental health are intrinsically intertwined and interdependent.” Additionally, certain aspects of ASF demonstrate that preparedness, mitigation and containment strategies tend to be similar, for pandemic pathogens regardless of the primary species that is affected. For example, both SARS-CoV-2 and ASF have difficult-to-trace origins and epidemiology and are also connected to spillover from wildlife. We have been able to curb the transmission of COVID-19 to some extent with the help of safe and effective vaccines, masking and rapid diagnostic tests for timely isolation and contact tracing. Adoption of similar technologies, policies and surveillance strategies will be useful to prevent ASF outbreaks as well.

Beyond the scientific lens, we also need to look at these pandemic-prone diseases from perspectives of local, national and global health frameworks. This will involve bringing experts from different fields, including professionals in human, animal and environmental health and policy to prevent future outbreaks of ASF and other pathogens of concern.

---

Disease names often incorporate geography, referencing place of discovery or suspected origin, areas of high risk or major outbreak sites. While identifying a disease by location may seem harmless —maybe even helpful — the strategy can be problematic, particularly if those connections are not accurate.