African swine fever: how it spreads

China was first hit by an outbreak of African swine fever more than a year ago. The virus has now spread to most of the Asian countries and Europe has not escaped this dance either; The animal disease has spread rapidly, especially in Bulgaria and Romania. But in what ways can the virus actually spread, and are there solutions to prevent its spread?

1. Contact with infected pig or carcass
African swine fever spreads most easily through direct contact between infected pigs or boars, a carcass or via the blood of an infected animal. This virus can also be transmitted through direct contact between wild boars and domestic pigs, although this form of spread is almost non-existent in countries with few so-called 'backyard farms' (small-scale and unregistered pig farms).

In countries where there are many backyard farmers (such as China and Bulgaria), there is a good chance that wild boars will transmit the virus to domestic pigs. The pigs on these small farms often roam freely, which means there is a significant chance that they will come into (direct) contact with infected wild boars or be exposed to one of the other sources of infection.

The spread through direct contact between infected animals is a concern, especially on larger farms. Since the pigs only show signs of illness after 4 to 19 days, the entire stable can be infected with the virus in no time. After all, in the pigsty there is a very good chance that the animals will come into contact with each other. An example of this is the outbreak on one of the largest pig farms in Ukraine, which meant that 100.000 pigs had to be culled.

2. Spread through contaminated food
In addition to spreading through direct contact, there are also numerous indirect ways to spread African swine fever. The best known is the spread through food waste. Wageningen University writes that all outbreaks outside Africa probably started by feeding food waste (swill feeding). The virus can easily enter the food chain through the slaughter and processing of infected pigs; It is not surprising that this happens, especially in small pig farms. Due to the fact that people cannot become infected with the virus, it is often not noticed that food is contaminated.

Travelers in particular play a major role in this form of distribution. There is a chance that they (unknowingly) bring contaminated food from countries where African swine fever is prevalent. When the remains are then dumped along the highway or in nature, there is a chance that wild boars will eat them and become infected with African swine fever. This is one of the reasons that the Dutch Food and Consumer Product Safety Authority, but also many foreign authorities, frequently prevention campaigns to inform travelers of these risks.

In addition, there is theoretically a chance that African swine fever will be spread through livestock feed, although this chance is very small. Out research it appears that the virus can survive in soy meal. According to the researchers, a country infected with African swine fever would therefore be able to spread the virus through soy raw materials.

3. Spread via materials or water
The transport of pigs in uncleaned livestock trucks can also potentially be a source of infection with African swine fever. That is why, when transporting to high-risk countries, it is mandatory to have livestock trucks cleaned at an R&D location upon return. Tools, boots and/or stable bedding that are not regularly cleaned can also become contaminated with the virus. However, it is not clear how likely the animal disease is to spread via this route.

According to Guus Verhaag, water product specialist at Summit Water Systems, the animal disease can also be brought in by water from alternative sources (such as rainwater or surface water). Verhaag claims that many livestock farmers do not know that approximately 80% of viruses can spread through the drinking water system. Once African swine fever has entered the stable, there is often no stopping it. That is also the reason why prevention is considered more important than treatment.

4. Spread by flies
Researchers revealed last year that wild boars and domestic pigs can also become infected with African swine fever by eating infected flies. The research by the Danish researchers also shows that certain biting flies that have come into contact with the virus can also transmit the animal disease. This form of spread mainly explains outbreaks of African swine fever on farms with a high health status in, for example, the Baltic States; For example, air filters were also present at those companies.

The same research also shows that certain mosquito species can only transmit the animal disease over very short distances (within 1 pig farm), while other species have tested positive for long-distance spread (between different farms). It is not obvious that flies spread the disease over very large distances (between countries). In addition write scientists say it is unlikely that this form of infection is a common route of transmission.

5. Spread via tick bite
In parts of Africa, the virus cycle is maintained by the fact that many warthogs become infected with African swine fever through tick bites. These ticks feed on infected pigs, reproduce the virus and then pass it on to another warthog. It is striking that direct contact between warthogs plays a minor and/or no role in the infection, while in domestic pigs this is one of the largest forms of spread. The virus is regularly spread to domestic pigs via soft ticks.

On other continents, such as in Europe, this form of distribution plays almost no role. What is a danger, on the other hand, is the fact that the ticks can 'store' the virus for years. Wageningen University says there are examples of pig stables that have been empty for years after being culled due to African swine fever, but in which the animal disease was found again when repopulating. The researchers report that this is because infected ticks were left behind.

Looking for a solution
Pigs that initially survive the disease remain a carrier of the virus. Although they are less contagious at that time, they can still transmit the animal disease. Especially in wild boars, this regularly ensures that the virus does not die out so easily and keeps reappearing. That is why a global search seems to have started for effective preventive measures and a vaccine.

The United States Department of Agriculture (USDA) is already investing $1,7 million in the control of African swine fever and also Chinese researchers test different vaccines. However, no official vaccine has yet been launched on the market, although the number of illegal variants is increasing rapidly. The Chinese Ministry of Agriculture warns even ensure that these vaccines can cause the animal disease to spread further.

Another recent success is MS Schippers. This company reported in August that its 'Di-O-Clean' product has proven successful in combating viruses, including African swine fever. However, this product only focuses on contamination through water sources, which means it can only eliminate contamination source number 3. Scientists are also critical of other breakthroughs, such as in Vietnam. Since there is still little knowledge about the infection and immunity of the disease, they believe there is little chance that an effective vaccine will be developed in the short term. This makes the disease one of the biggest threats to the pig sector at the moment.