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ASF and the role of insects in its transmission

Dr Jafar Pazani, Technical Manager, MedIrAlis Pty Ltd

African Swine Fever (ASF) has never been diagnosed in Australia. There is no treatment or vaccine for curing or vaccinating against ASF. Although ASF is not a human health threat, it is a devastating disease for pigs and the pork production industry. ASF is a highly fatal viral disease of domestic pigs manifesting as a haemorrhagic fever. The mortality rate caused by ASF can be 100%. ASF is so serious disease that it is listed in the OIE’s Terrestrial Animal Health Code. OIE Member countries are committed to reporting outbreaks of ASF. AFS is in the national list of “Emergency Animal Diseases” of Australia, defined exotic to Australia and must be reported immediately if detected even by suspicion.

How does ASF spread?

African Swine Fever (ASF) has proven able to spread rapidly across borders and over long distances. Therefore, understanding the ways ASF is transmitted and the factors facilitating its spread is so critical to consider and adapt effective options for prevention and control of it. ASF virus can be transmitted through direct contact of susceptible pigs with infected ones; or by indirect contact through fomite and mechanical vectors including: feed, bedding, equipment, clothing, footwear and vehicles. Transmission of ASF by indirect contact can occur via pests, including: insects contaminated with blood, faeces, urine or saliva from infected pigs. It also can be transmitted by consumption of the pork meat from infected pigs and by the bites of infected ticks from argasid family or soft ticks (Ornithodoros spp.).

Border protection and Biosecurity management are the only ways to protect Australian pig industry against African Swine Fever. This is also explained by The World Organisation for Animal Health (OIE): “Prevention in countries free of the disease depends on stringent import policies, ensuring that neither infected live pigs nor pork products are introduced into areas free of ASF.”

According to many studies, it is now clearly known that people play an extremely important role in spreading ASF virus. In fact, people are the most responsible carriers moving pigs and/or their products over long distances. They also act as transmitters of the virus by moving from contaminated areas to uncontaminated without proper and sometimes no hygienic precautions.

ASF virus will survive in a protein environment for long periods. Therefore, pork meat from pigs with ASF slaughtered in infective stages is a good source of virus. The virus is also very resistant to high heat. Exposure to a temperature of 60C or higher for at least 20 minutes is required to inactivate ASF virus, 70C for 30 min is reported more efficient. Due to these facts not only fresh and frozen pork but also smoked, salted and dried pork may contain infective quantities of ASF virus. ASF virus is also able to survive in bone marrow, despite putrefaction or high temperature. One high risk area for commencing an ASF outbreak is swill feeding in backyard, hobby and small farms with food scraps containing contaminated pork meat from infected animals.

The Role of Insects

The other factor in ASF transmission is the roles of arthropods and insects. ASF virus is a member of arboviruses which stands for “Arthropod Born Viruses”. It is the only DNA virus in arbovirus category. It is well known that in sub-Saharan Africa where ASF is first recognised and reported, warthogs (Phacochoerus africanus) are natural hosts of the ASF virus without showing any sign of diseases; bush pigs (Potamochoerus larvatus and Potamochoerus porcus) are also considered to be the reservoir for the ASF virus in Africa. In this endemic condition there is a sylvatic cycle of the virus between the warthogs and soft ticks of Ornithodoros spp. as the biological vector, specifically soft ticks of the Ornithodoros moubata complex, that live in their burrows. Transstadial, transovarial and sexual transmission have been demonstrated in these ticks which is the main way to maintain the infection in their population. However, it is also known that warthogs are unable to transmit the virus directly to domestic pigs and the disease is transferred to pigs through soft tick-pig cycle and the soft ticks serve as a reservoir for persistence of the virus in the environment. Despite these findings, the Ornithodoros spp. had some roles in the epidemy of ASF in Central and Eastern Europe where direct or indirect contact with infected animals was the most efficient way of virus transmission. During the outbreaks in Spain and Portugal it is demonstrated that Ornithodoros erraticus acted as a biological vector. Some additional species of Ornithodoros have been infected in the laboratory.

It is now known that the natural arthropod host for ASF virus is Ornithodoros porcinus porcinus (Walton) ticks although there has been some confusion in earlier reports since the vector tick often has been referred to as either O. moubata porcinus or simply O. moubata coplex. These ticks and many other species in Ornithodoros genus can be infected by ASF virus.

It is well indicated that such ticks after feeding on viraemic swine can maintain ASF virus infection for several months or even years; therefore, they can be as biological reservoirs involved in maintaining the long-term source of transmission of ASF virus. In a study done in Madagascar, ASF virus was isolated from ticks found on a farm where no pigs had been kept for at least 4 years. In other study done in Portugal, ticks that had harboured the virus for more than 5 years were determined as the cause of an outbreak. In fact, this could seem more serious knowing that eradication of ticks from old piggeries is almost impossible due to the tick’s long life and ability to survive for a long time without feeding.

Stable flies, Stomoxys spp. are the only other arthropods that have been shown to be capable to maintain ASF virus for a considerable period of up to 48 hours and transmitting it to pigs by biting. Stable flies have been able to transmit ASF virus after 24 hours of feeding on the infected pigs in experimental condition. Sofie Olesen et al, National Veterinary Institute in Technical University of Denmark, in their study for finding an explanation for unexpected transmission of ASF virus into the pig facilities with high biosecurity standard, indicated that pigs ingesting stable flies of Stomoxys calcitrans fed on ASFV-spiked blood became infected with the virus.

Other blood sucking insects including mosquitoes and bitting flies may transmit ASF virus mechanically. Blood-feeding flies from family Tabanidae which are strong flyers, have been considered to be studied as a suspicious transmitting vector. The risk posed by insects is much more serious in free ranging farms especially organic types with restricted choice of products for insect control.

Prevention/Damage Control

In any case of unfortunate outbreak in Australia, the second important protection would be detecting any suspicious infections, and confining and eliminating the source of risk in the point of start. This will require a fast diagnosing process be established. In such cases, high standard of biosecurity will be an efficient way to keep an infection from starting in the first place. Well established biosecurity will help stop the possible spread of ASF virus to wild swine population, and therefore stop it from entering domestic pig farms. Most of the reported outbreaks are caused by contact, between improperly confined pigs and wild swine.  Entering ASF virus into any possibly present and susceptible tick population will exacerbate the situation, and initiate an unexpected endemic situation. Regulations for pig farming in the control area for surrounding the pig farming facilities with a pig-proof barrier, usually double fencing; are reported as very efficient biosecurity measure to keep farms free from ASF.

The other level of good biosecurity which can be achieved efficiently is in operational level. Controlling and restricting any unnecessary entrance of people, vehicles and fomites into farms, controlling staff, educating the staff about the risk of their contact with pork meat from unapproved sources, food prepared from such pork meat, not bringing any food prepared from pork meat into farm would be items of good biosecurity practice.

The other serious and significant area in biosecurity management is pest control. In the case of ASF the importance and roles of insects and arthropods are discussed above. It is indicated that arthropods with flying ability including bitting flies and mosquitoes can reservoir the virus and transmit it either by bitting or mechanically. The ability of insects to fly and reproduce in large number makes it harder to control them especially in swine farms that they could have easy access to faeces, in case of flies, and static water ponds in case of mosquitoes. Insects can also mechanically carry the virus from any contaminated food scraps disposed improperly.

In conclusion a successful eradication programs would involve rapid diagnosis, slaughter and disposal of all animals on infected premises, thorough cleaning and disinfection, disinsectization, movement controls and surveillance.

Nor Mite as part of your Biosecurity Plan

Nor-Mite a non-chemical, organic and cost-effective way to control insect infestation in pig farms

 Nor-Mite in our Nor-feed range can be considered as a very potent insect repellent to be included in pest management in pig farms biosecurity plans.  Usage of Nor-Mite has a very low labour and application cost. Nor-Mite can be used via water in liquid form or via feed in powder form. Nor-Mite liquid is highly soluble in water and can be used either by adding it to header tanks, or injecting it into pipes with a dosing machine.  Nor-Mite is an herbal product, which is also approved to be used in organic farming by “Bureau Veritas” who are well recognised in Australia too. By using Nor-Mite the herbal metabolites in it (polyphenols and triterpenes) get into the pig faeces and urine; and hence all the animals and the farm will have the repellent effects. The powder form also contributes to repelling dust mites and weevils from feed.

Nor-Mite has proven its high potency in contributing efficiently to repelling several types of insects including: some fly species, mosquitoes and several acarida and litter beetles.