Understanding Viral Threats

21st December 2020

Understanding Viral Threats

The arrival of Covid-19 has shown very dramatically the impact that a virus can have on the wealthiest, most sophisticated economies of the world.

Many billions of Yuen, Euros, Dollars and Pounds have been found to fight the pandemic, to search for vaccines and medicines in order to sustain life - even if, not life as we knew it.

As someone who has spent the last 40 years involved in animal agriculture, with particular emphasis on pig production, I feel battle-hardened when it comes to surviving the impact that an entirely new virus brings to a naive herd. Alarm, despair, and potential economic ruin!! - a gut-wrenching sensation, if ever there was one! PRRS, CVD, Parvo, PED, ASF have become commonplace in our vocabulary as we discuss vaccine programmes or potential viral threats that seem to be moving ever closer to our shores. ASFv has no known cure and no vaccine is available to protect our herds if infection arrives, while vaccines for PRRSv, PCVv and PARVOv have been in our armoury for some time.

But what is a virus?

The Concise Oxford Dictionary defines a virus as - 'A microscopic organism consisting mainly of nucleic acid in a protein coat, multiplying only in living cells and often causing disease,' while virologists have simply described a virus as a very small infectious particle that reproduces by "commandeering" a host cell and using its machinery to make more viruses.

In summary, a virus is an invisible strand of DNA (e.g. ASFv) or RNA (e.g. PRRSv or Covid-19) inside a protein shell. In addition, there may be an outer layer of fat surrounding the entire structure (such viruses are said to be Enveloped e.g. PRRSv). Perhaps surprisingly, informed opinion has stated that the 'enveloped viruses are usually more susceptible to temperature & pH changes or detergent attack' so are easier to kill.

One positive feature is that a virus cannot replicate outside a host cell - it does not have the capacity to do so. Consequently, the virus must have a mechanism to penetrate the host cell wall to gain access to replicate and spread within the host. This is good news in that it usually means that an individual virus is specific to a certain host species (e.g. PRRSv and pigs) as it only has the 'key' to enter the cells of the host species. It is only when the virus mutates, or changes in DNA or RNA coding, that this new variant may be able to penetrate the cells of more than one species (e.g. Covid-19 and mink/humans).

It is important to know that viruses have a Minimum Infectious Dose (MID: number of viral particles) necessary to establish an infection and a Minimum Lethal Dose (MLD: number of viral particles) necessary to result in a fatal infection for that particular virus. These numbers will vary dependent on the ability of the virus to gain access to the host cell, the susceptibility of the host (for example if the host immune system is impaired) and certain environmental conditions. Reports from the US, for example, indicate the extremely low dose level of PEDv that can elicit a deadly outbreak of this infection.

In the context of the current Covid-19 pandemic, most countries have implemented restriction recommendations that require us to wear surgical standard face masks when we are in close contact with others. This has been done to reduce the infectious dose we receive from others in the community. While mask-wearing has been controversial, Gandhi et al (2020) have reported that wearing a surgical standard face mask does help reduce the incidence of severe disease following Covid-19 viral infection to mild or even asymptomatic level expression of the disease. This, they suggested, is due to a significant reduction in the level of dose (also known as inoculum) that the wearer will receive, as most of the viral load in the aerosol will be intercepted by the material of the mask.

Covid-19 has shown society at large, what we in the pig industry already knew; that surviving a sustained viral challenge can be very expensive in many ways - economic and emotional, resulting from elevated levels of sickness and mortality on our farms. What the last 9-10 months have highlighted for us all in society, and what we should reflect on in the pig industry, is that prevention is better than a cure! On our farms, levels of biosecurity need to be high and effective. We should take some time to ask ourselves: Have we eliminated / can we significantly reduce all sources of infection - animal to animal, vectors (e.g. flies, vermin or birds) with our biosecurity measures? Do we and our staff understand why biosecurity protocols must be followed every day, every time? - with NO exceptions - and that even includes the Boss.

Looking forward to a world without Covid-19 and a pig industry free from the threat of ASFv in 2021. Back to Press Releases