by Ma. Albert Camille J. Bayoneta, Janelle Louise G. Deducin, and Desiree Mindanao

Beside the Malinta Barangay Hall in Los Baños lies the then-African swine fever overwrought piggery. 

“Naubos lahat. Walang natira. Patay lahat,” [Everything was wiped off. Nothing was left. Everything died] mourned Alfredo Gayahan, 72, over his losses due to African swine fever, a deadly and contagious viral disease of pigs that struck the Philippines in 2019. Gayahan lost 36 heads of pigs due to ASF amounting to P150,000 in losses.

As he tries to bounce back from the crisis, he is hopeful for a vaccine against ASF, “secure ang kabuhayan ko kung merong bakuna,” [my livelihood is secured if there’s a vaccine] Gayahan said. 

Challenges in ASF vaccine development

Decades after its first case in Kenya and after many first outbreaks across Europe and Asia, the world now awaits the go signal of the Food and Drug Administration for the availability of a vaccine in commercial markets against ASF. A UPLB veterinarian explains the challenges and causes of delay in ASF vaccine development. 

A study on the biology and vaccine approaches for ASF virus published in the journal Advances in Virus Research noted that the virus can interfere with how cells communicate. This interference results in immunomodulation or the adjustment of the pig’s immune system which makes the development of a vaccine against ASF virus challenging. 

Dr. Fletcher P. Del Valle, researcher and Assistant Professor III from the Department of Veterinary Clinical Science of University of the Philippines Los Baños, explained how immunomodulation occurs, “[The ASF virus] affects white blood cells, particularly what we call the phagocytic cells […] and it so happens that when your phagocytic cells are infected by the ASF virus, the ASF virus is able to create proteins that could control the behavior of your non-phagocytic cells. Now [white blood] cells normally have this process that when they are infected by a pathogen—viruses, bacteria, they would try to kill themselves. But the ASF virus is able to halt this process of white blood cells.”

“[T]hen, the ASF-infected phagocytic cells will create proteins to instruct the non-infected non-phagocytic cells to kill itself. So now you have an animal whose white blood cells or protective antibodies are not functioning,” Del Valle added. 

ASF is able to interfere with how cells communicate with each other resulting in the incapacity of the pig to regulate its immune system to fight against the virus. 

Furthermore, the findings of the study published in Veterinary Microbiology on the challenges for ASF vaccine development concluded that there is still much to be known about the viral proteins making up the ASF virus strains. 

Del Valle further explained the reason why it is challenging, “you need a large number of pathogens [of the virus] so that you can find out what proteins are involved. […] The challenge with ASF is because it is selective of cells, it only attacks the neutrophils, eosinophils, and macrophages […] thus it is difficult to [culture the virus] in other cell types so you’d need to kill a pig then you’d [have to] take the white blood cells [and] only then can you reproduce the virus in large amounts to create a vaccine.” 

Recently however, researchers are able to develop an alternative way to reproduce ASF that led to the ongoing development of the ASF vaccine candidate ASF-G-ΔI177L which proved to be effective under controlled clinical conditions but below satisfactory in field conditions or in actual pig farms/backyard farms. 

On June 7, GMA Integrated News State of the Nation reported the effectiveness of the ASF vaccine candidate in controlled clinical conditions/trials (emphasis intended). According to Del Valle in an interview on May 19, “they [the researchers] would have to do trials all over again in field conditions.” 

Del Valle also recognizes the rigorous process that vaccines have to go through before being available for public use. Thus, the availability of a vaccine in the marketplace may take a while. 

However, while many pig raisers and pork meat sellers are in a festive mood about the possibility of a vaccine, Marivic Balen, 46, a pork meat seller at Los Baños public market, feels otherwise. “Hindi ba nakakatakot ‘yon? Pwedeng makain ng tao [yung bakuna] […] kasi absorbed na siya sa katawan ng baboy–nakakatakot kapag nakain ng tao,” [Isn’t that scary? People might ingest [the vaccine] […] because the pig absorbs the vaccine–it’s scary if ingested by humans] said Balen perturbed about the possibility of transfer of vaccine from pork meat to human because of ingestion. 

Fasting finisher pigs (pigs that are sellable) is standard procedure for slaughterhouses or before any slaughter of pigs to maintain the quality of the meat and to avoid the immunity of humans to certain vitamins and medicine due to the consumption of vitamins and medicine-induced pork meat. 

Thus, Balen’s hesitation with the vaccine against ASF must be taken with a grain of salt due to the, understandably, recency of the vaccine candidate and due to the fact that it is still under trial. Needless to say, this aspect of the vaccine remains to be insufficiently researched. 

Potentials of ASF zoonotic spill-over

Lucky for us humans, even without a vaccine, ASF does not affect us. 

However, Del Valle expresses the possibility of the ASF virus affecting human health, “It’s a dark possibility. Pathogens and humans, we are in an arms race, so it’s always a competition—the pathogen wants to be pathogenic to us, we want to be resistant to it…and historically, they have won.” 

The transmission of pathogens or diseases from animal to human is referred to as the “zoonotic spill-over” in the scientific community. A study published in Nature estimated that around 60% of human-infecting diseases came from animal species. Examples of zoonotic spill-over are the Ebola virus and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus that causes COVID-19. 

To date, the world remains without a field–effective vaccine against ASF and thus the continued transmission of the ASF virus. With the threat of a possible zoonotic spill-over, institutions, and veterinarians call for strict compliance and effective implementation of biosecurity measures to prevent further transmission of the virus. 

Indicators of ASF viral infection 

According to FAO’s ASF detection and diagnosis manual, when ASF virus is examined under the microscope, the virus looks like a loose french braid and a rubik’s cube with 20 faces — it is a large, double-stranded DNA with particles that are icosahedral-shaped with a diameter of roughly 200 nanometer or 0.0002 millimeter. 

Although the size of ASF virus is big relative to other viruses, we cannot see the virus via the naked eye. Nevertheless, this virus causes various physical manifestations that are deadly to pigs. 

The field manual for ASF of the FAO noted that the initial symptom of ASF virus infection is a high and persistent fever  usually reaching 40-42 °C. This hot shower-like temperature leaves the animals weak and disoriented. 

As the ASF attacks the pig’s digestive system, the loss of appetite follows, causing drastic weight loss and frailty. For severe cases, ASF manifests in pigs through diarrhea and vomiting that could lead to dehydration that further worsen their condition. 

Since the virus could target the respiratory system and negatively affect the pig’s lungs and respiratory tracts, they may also experience respiratory difficulties such as coughing, sneezing, rapid and labored breathing, and nasal discharge.

As the infection progresses, the pigs could exhibit internal and external bleeding which is the most distinctive and dangerous effect of ASF. The affected internal organs such as the liver, lymph nodes, and spleen would exhibit bleeding and swelling. On the other hand, the external manifestation of the disease includes skin discoloration, blood in feces, bloody nasal discharge, and bloodshot eyes. 

Pig infected with ASF; red, jelly-like and foamy nasal discharge. Photo credits: PIADC & The Center for Food Security & Public Health.

The FAO’S ASF detection and diagnosis manual for veterinarians noted that the severity of ASF’s impact varies, with some pigs succumbing almost immediately, while others endure prolonged suffering. However, the most devastating aspect of ASF is its near-certain fatality, with mortality rates ranging from 80% to 100%. Infected pigs, once a source of sustenance and livelihood, succumb to the disease within 7 to 20 days. 

“This virus is very damaging aside from the immune response, it can alter circulation, liver process, kidney function that’s why it’s very dangerous to pigs,” Del Valle stated, emphasizing the harmful impact of the virus on the pigs.

ASF Transmission and Biosecurity Measures 

“Ang driver [ng pagkalat ng sakit] talaga ay human movement,” [the driver [of the transmission] of the disease] is really human movement, said Del Valle regarding the primary cause of the spread of ASF virus.

Del Valle underscores the critical role of human behaviors and practices play in the unintentional dissemination of ASF. He emphasizes that illegal movements of pigs, motivated by economic factors or the circumvention of regulatory measures, significantly contribute to the introduction of ASF into new areas.

These illicit movements involve the transportation of infected animals from affected regions to unaffected ones, thereby perpetuating the spread of the virus and igniting fresh outbreaks. 

The lack of proper quarantine measures and monitoring further compounds the risk associated with illegal pig movements.

Unknowingly, as people visit infected piggeries or come into contact with ASF-contaminated environments, they are likely to carry the virus on their clothing, footwear, equipment, or vehicles. These objects are what scientists refer to as fomites. 

The contamination happens when people unsuspectingly interact with infected animals or environments and transport them to other locations.

According to Del Valle, the ASF virus can survive for almost nearly three months in objects and tools contaminated with ASF-infected pigs’ blood. More alarming is that it can endure in ASF-infected frozen meat for up to 1,000 days or almost 3 years. 

Backyard farms as most vulnerable to ASF outbreak

According to data from the Philippine Statistics Authority, the Philippines’ Swine Industry landscape consists of 71.1% backyard swine farms. 

These small-scale swine farms are the most susceptible to ASF outbreaks as their farms are structurally more accessible to outside contaminants. Since they constitute the majority of the Philippines’ Swine Industry, Del Valle and fellow animal scientists emphasize their crucial role in adhering to preventive measures to protect their herds and the broader swine industry from ASF. 

Backyard hog raisers may not have access to sophisticated spraying systems or extensive resources, but Del Valle emphasizes that simple, yet effective practices can make a significant difference in safeguarding their herds.

“It’s a very hardy pathogen [ASF virus], but the surprising thing is, it’s easy to destroy with soap and disinfectants,” Del Valle explains. “The simple act of taking a bath, changing your slippers, changing your clothes before you come into contact with your animals, that would be a huge step,” he added. 

Del Valle’s insights align with the protocols posted by FAO which also highlight the importance of proactive biosecurity measures. By diligently implementing the following strategies, backyard hog raisers can fortify their farms against ASF transmission:

• Perimeter Control, like establishing physical barriers, such as fences, to control access to the farm and prevent unauthorized entry. This helps to limit contact between pigs and potential sources of infection.

• Restricted Access, specifically implementing strict protocols for farm entry, including visitor registration, disinfection of vehicles and equipment, and appropriate protective clothing or footwear. This reduces the risk of introducing the ASF virus to the farm.

• Quarantine and Segregation, which entails effective quarantine and segregation measures for incoming animals, ensuring they are kept separate from the resident herd for a designated period. This helps detect any potential ASF infection and prevents the spread of the virus.

• Biosecurity Signage, putting up clear and visible signs at the farm entrance and other strategic locations to communicate biosecurity protocols and remind personnel and visitors of their responsibilities in preventing disease transmission.

• Personnel Hygiene, enforcing strict hygiene practices for farm workers, including handwashing, use of disinfectants, and dedicated work clothing and footwear. This minimizes the risk of human-mediated transmission of ASF.

• Vehicle and Equipment Hygiene, regular cleaning and disinfection of vehicles, equipment, and tools used on the farm, especially those that come into direct contact with pigs or their environment. This reduces the potential for fomite transmission.

• Pest Control, implementing effective measures to control and prevent the entry of pests, such as rodents and wild boars, which can act as carriers of the ASF virus.

Additionally, although Del Valle and fellow animal scientists strongly discourage the consumption of ASF-infected pork meat, he specifies how the virus is sensitive to heat and can be exterminated by cooking for not less than 30 minutes at a temperature of 60°C. 

“It’s considered already unfit for human consumption,” Del Valle declares, “If we consume it, do we know what’s going to happen? You would not be affected. But if the meat is undercooked, then the virus is in you, but it’s not replicating. It’s just there.” 

This ties back to the discussion in the previous section regarding the potential of an ASF zoonotic spill-over. Even when the risk is currently considered just a “possibility,” this should still not be dealt with complacency.

While waiting for the vaccine, backyard hog raisers can significantly reduce the risk of ASF spread, protect their livelihoods, and contribute to the overall health and stability of the swine industry. Key to this is reporting any signs of ASF infection in pig farms to the proper authorities. 

Role of slaughterhouses in the prevention of ASF virus transmission

Slaughterhouses also have a responsibility to enforce strict biosecurity measures to mitigate the risk of an ASF outbreak and prevent zoonotic spill-overs. This includes proper screening and inspection of pigs before slaughter, ensuring that only healthy animals are processed. 

In the Los Baños Municipal Slaughterhouse, for example, the following documentary requirements are to be presented before pigs are allowed inside the premises for slaughter:

• Veterinary Health Certificate

• Shipping Permit

• Certificate of Ownership (Large Animal)

According to Ricky M. Flores, the Los Baños Municipal Slaughterhouse Supervisor, these requirements are in place in order to ensure proper traceability, food safety, and compliance with government regulations. 

“Kami [Los Baños Municipal Slaughter House] ay regulatory, ” Flores affirmed. 

“Sa batas, RA 9296 [The Meat Inspection Code of the Philippines], nirerequire nila na ang pambentang hayop, dapat sa municipal slaughterhouse or sa AA [National Meat Inspection Service Accredited] Accredited Slaughterhouse kinakatay,” [The law RA 9296 requires that sellable animals should be slaughtered in either the municipal slaughterhouse or in AA Accredited slaughterhouse] he added.

When suspected ASF-infected pigs are brought to the Slaughterhouse, they are immediately isolated and brought to the condemnation pit where animals declared to be unfit for human consumption are destroyed by incineration. 

The absence of a vaccine and lack of properly implemented biosecurity measures led us to grapple with the unprecedented socio-economic crisis caused by the ASF virus.  As the world awaits the vaccine, the key against further aggravation is to, unironically, not wait. As Del Valle among the many veterinarians and institutions has urged: prevention is key. 

For reports on actual and suspected ASF virus: 

Department of Agriculture: 

Phone: +63 (2) 8928-8741 

            +63 (2) 8273-2474

Address:  Elliptical Road, Diliman, Quezon City, 1100

Email:  [email protected]

Social Media:  https://www.facebook.com/dacentralphilippines

DA-Bureau of Animal Industry: 

Phone: 8528 2240

Address: BAI Compound, Visayas Ave., Diliman, Quezon City, Phil.

Email: [email protected]

Social Media: https://www.facebook.com/bai.gov.ph