Biology
African Swine Fever virus is the only member of the genus Asfivirus in the family Asfarviridae. The root “Asfar” in this case is an acronym, which when combined with the suffix for “viruses” literally means “African swine fever and related viruses.” One common mistake is to compare African Swine Fever virus with Classical Swine Fever (CSF), although the symptoms of these two diseases are similar, the viruses are entirely unrelated.
African Swine Fever virus is a double-stranded DNA virus and replicates primarily in the lymphatic and vascular systems of infected pigs. The virus works by entering the bloodstream of pigs and infecting white blood cells. Once inside, the virus alters the DNA inside of the white blood cell, reprogramming the cell to start creating more of the virus. When the cell has reached the end of its usefulness, the virus destroys the cell and enters the bloodstream to infect more cells. Over time, the virus starts infiltrating cells in the spleen, lymph nodes, kidneys, gastrointestinal and respiratory tracts, resulting in widespread hemorrhages.
African Swine Fever virus is not a zoonotic disease, meaning that it does not transmit from animals to humans. African Swine Fever virus only affects the family Suidae, which includes domestic pigs, wild boars and warthogs. Because of this inability to affect humans, African Swine Fever virus is not a public health or food safety concern.
Transmission
In domestic pigs, the incubation period can range from five to 15 days. An incubation period is the time between infection and onset of symptoms. Even more worrisome is the fact that infected animals can start shedding the virus more than 48 hours before their symptoms start appearing, and recovered pigs can continue shedding the virus for up to a month after their symptoms stop. These factors are just a few of the reasons that this disease is so difficult to contain with traditional farm biosecurity measures.
While a pig is contagious, it can spread the virus via three primary pathways, described below in detail:
Direct contact: Direct contact with an infected animal, whether domestic or wild, can result in an infection. This occurs most often through swill feeding, where pigs are fed food scraps that contain meat or have come in contact with meat. Direct contact can also involve direct transmission of fluids, but this is less common.
Indirect contact: Indirect contact with contaminated food or surface is another method of transmission. Food or surfaces may be contaminated with blood, feces, urine, saliva or semen, which can all carry the virus for a long time. Contamination can then spread to swine through the consumption of contaminated food or through contact with contaminated surfaces. ASF virus may also spread through indirect contact if a worker touches contaminated material before handling live swine.
Vectoral transmission: ASF virus is the only known DNA arbovirus, meaning that it can be spread by arthropods, primarily ticks. The soft ticks from the genus Ornithodoros spread ASF virus by biting pigs and are also reservoir hosts for the ASF virus . This means that the virus can live within a tick host without negatively affecting its health, and can persist through the local tick population without needing to infect a primary host, in this case, a pig. Studies have also shown that pigs can become infected with ASF virus by ingesting blood-sucking stable flies, or Stomoxys calcitrans. Stable flies may carry the virus for up to three days after feeding on infected blood to transmit the virus to healthy swine.
Direct and indirect contact are the most common modes of transmission for the virus and are the focus of efforts for on-farm virus mitigation. Studies of 68 recent outbreaks in China found that 34 percent of the outbreaks were caused by swill feeding, 46 percent by contaminated equipment and handlers and 19 percent through transportation.
Symptoms
Domestic pigs can express a wide range of clinical symptoms — sometimes the symptoms are very obvious, while other times they can be challenging to detect, or easily mistaken with other conditions. Often, the severity of symptoms is based on the virulence of the virus and the type of pig affected. Generally speaking, the clinical signs of ASF virus come in two forms, being acute and chronic.
Acute forms of ASF virus are more severe and more deadly, with mortality rates as high as 100 percent, especially in areas where the disease is not endemic. The symptoms will generally lead to death within 6 to 20 days of onset, depending on the conditions of the case. Acute ASF virus is characterized by the following symptoms:
High fever
Decreased appetite and weight loss
Red, blotchy skin or skin lesions
Muscle weakness
Coughing and difficulty breathing
Bluish discoloration of the skin from oxygen loss
Spontaneous abortion in pregnant sows
Diarrhea and vomiting
Chronic forms of ASF virus, also called subacute forms, are generally caused by less virulent forms of the virus, therefore causing less severe symptoms. This form is more common in areas where ASF virus is endemic and is also seen in wild boar populations. Animals with chronic ASF virus tend to express symptoms for a longer time, and it can take days for the symptoms to become recognizable. Though the mortality rate for chronic ASF virus is lower, it still tends to range from 30 to 70 percent. The most common symptoms of chronic ASF virus include the following:
Intermittent fever
Decreased appetite and weight loss
Red, blotchy skin or skin lesions
Coughing and difficulty breathing
Arthritis
If a producer or veterinarian notices any of the above-listed symptoms, they should immediately report to state or federal animal health officials for a thorough investigation. Even if it hasn’t reached your country yet, it is essential to report possible incidences promptly to help maximize response and prevent spread of the virus.
Treatment
Currently, there is no ASF virus vaccine. Although previously infected pigs show immunity to subsequent infections, trial vaccines using a modified form of the virus have been unsuccessful at making pigs immune to the virus. While researchers continue working on a vaccine, ASF virus still poses a threat. Unfortunately, due to the high level of resilience of the virus, the only way to prevent ASF virus from spreading is to depopulate affected herds, disinfect all objects that came in contact with them and ensure strict biosecurity protocols are employed moving forward.
Interested how to prevent diseases through biosecurity, with ASF as a concrete example? Watch this presentation of Jeroen Dewulf, Professor in veterinary epidemiology at the University of Ghent.
