Why has a farm veterinary surgeon got any interest in the control and management of a human viral pandemic? What help can the veterinary profession be in sticking their noses in what is a human infection, running out of control and with a confused and ineffective management strategy? Should we stand back and watch, with the regular desperate sighs of ‘what did you expect?’ and ‘I could have told you that’?
Infectious disease management is the bread and butter of the livestock veterinary surgeon — it is our day job. We have developed systems and management procedures to prevent, control and eradicate infectious diseases that may impact on the health, welfare and productivity of our precious livestock populations. We understand that the management of any infectious disease requires the measurement and control of the four basic pillars of herd health: biosecurity, biocontainment, surveillance and resilience.
Our first approach to any disease management strategy is to define the objective. We use an aspiration hierarchy that reflects the wishes of the farmer (the owner of disease), the practicality of achieving the objective, and the economic benefits. The economic element not only involves financial objectives, but social requirements too. Total eradication is often not achievable, however desirable, but limiting the impact through proper control and protection of those animals that are likely to be adversely affected is a perfectly reasonable disease objective. For example, most dairy farms would find it impractical and uneconomic to eradicate staphylococcal mastitis from their herds, yet most can readily achieve low levels through control of spread and infection risks, managing resilience through vaccination, and preventing spread by early detection and treatment.
Similarly, bovine respiratory disease (BRD) is a recurrent disease that can decimate a population unless it is properly managed. It has close similarities to the management of COVID-19.
The management of any infectious or transmissible disease requires robust management of the four pillars of herd health.
Biosecurity: the management of the risks of disease entering a population
The difficulties in controlling the coronavirus pandemic is a reflection of the unmanageable biosecurity risks of large numbers of people moving between populations and dispersing within those populations with no control. Now that the infection is endemic within those populations, there is little hope that restoring biosecurity controls will have any major effect on the current UK prevalence. Those countries that claim virtual eradication have now found themselves facing the difficulty of effective biosecurity, without shutting their borders and restricting movements.
With common veterinary infectious diseases, we look to protect some specific sub-populations by robust biosecurity management, so protecting specific, valuable and sometimes very vulnerable groups within the overall populations.
Nearly half of all European COVID-19 deaths (as reported by WHO) have been in care homes or similar institutions for the elderly, which, along with other institutions that contain highly susceptible populations, need robust protection. Many of our livestock farms, including dairy farms, have farm-specific biosecurity plans tailored to their own particular risks, and delivered by specially trained veterinary surgeons engaging with well educated farmers. Surely, every care home and other high risk institution should have its own, specific, biosecurity plan to protect its occupants, whatever the cost and required resources.
Biocontainment: the management of the risks of disease spreading within a population
The key disease controls for endemically infected populations are based on preventing new infections by systems of biocontainment. Biocontainment control systems require the assessment of risk pertinent to the particular pathogen, to identify and control transmissions and new infections. Risks which enable one-to-many transmissions are of greatest concern as they disproportionately inflate R0: these tend to involve inoculation points where many susceptible animals can become infected by one shedder — a super-shedder event. We commonly see this with paratuberculosis infections in multiple calving yards, or calves being reared on automated multiple feeding systems. Of less concern are the one-to-one transmission risks that have minimal effect on spreading the disease. The latter might be more easily managed, but the effective management of the former are critical to prevent rapid and extensive disease spread.
COVID-19 is a respiratory disease; it appears that transmissions are mostly by the inhalation of significant doses of virus that can cause disease. It is likely that high dose infections lead to more serious clinical impacts. Droplet transmissions, such as those seen with BRD, are most effective in cold damp environments, where there is a high density of susceptible animals. This is simply a matter of physics and maths rather than anything particularly biological. Water droplets do not evaporate in cold humid atmospheres, and so the viral packets are protected. One-to-many transmissions are very likely in high density populations, especially when the environment has low air movements and poor quality air. It should be becoming clear through research and investigation of new infections as to what are the particular risks of major coronavirus transmission. These can then be assessed and managed, with the biggest risks managed most robustly. In the cattle world, we have software algorithms to aid the assessment and management of risks, and it is astounding that such systems do not yet exist for the management of COVID-19.
Surveillance: the detection, monitoring and measurement of disease
Scanning, active and passive surveillance systems all have their place in the management of infectious disease. Surveillance systems that focus on the detection and isolation of shedders, including tracking and tracing, to prevent spread is a system that works well with diseases that are relatively slow moving and where shedding can be quickly detected prior to any risk of spread. Tests have to be sensitive and fast. The science of diagnostics has been one of the more successful outcomes of the current pandemic, with new tests meeting the needs of surveillance systems.
In farm practice, antibody tests are commonly used to detect exposure within populations to give an overview of disease status: these tests are less time dependent than organismbased tests, as they reflect historic infections. They are not a particularly good reflection of resilience, as immunity is not simply determined by the presence of measurable antibody. Innate immunity, cellular immune responses, and genetic resilience also play a part in preventing individuals from becoming infected and infectious.
Using tests as part of the control plan for managing disease outbreaks can be difficult, and requires rapid tests of good specificity and sensitivity to work: one of the failures of the current bovine tuberculosis (TB) problem is that the tests are insufficient to deliver the objectives. In many situations, early disease detection is better achieved by the careful and robust observation for clinical signs indicative of early disease, often seen as pyrexia in the first instance. Systems such as FeverTags and Whisper automated auscultation systems are used to detect early BRD in susceptible cattle populations, but in many instances a simple digital thermometer and good stockmanship is all that is needed to detect early infection, isolate shedders and implement early treatment for the best prognosis. Perhaps every care home and susceptible high risk individual should be issued with a box of digital thermometers rather than a crate of pharyngeal swabs. Early detection and treatment is key to good clinical outcomes when it comes to animal respiratory diseases.
Resilience: the management of resistance to disease
The ultimate steady state of any infectious disease is when the population achieves a level of resilience such that new infections become rare. The disease may never die out, but it becomes clinically insignificant. This is very difficult to achieve in dynamic populations such as our livestock herds where there is continual renewal. It is generally agreed that the end of COVID-19 is when population resilience (or immunity) is achieved, either through exposure or vaccination, or both. This resilience will come in the form of acquired immunity through exposure (humoral and/or cellular), innate immunity, and genetic resilience. Simply measuring humoral immunity might give an indication of population exposure, but it will not tell the whole story.
Establishing population immunity without vaccination can be achieved without undue losses by managing exposure while attempting to minimise the impact of disease. Exposing animals (or people) that are likely to have minimal clinical impacts to low doses of infection creates an immune population and so naturally suppresses the virus. It is critical in this management process to protect the vulnerable — those most likely to have adverse clinical outcomes.
It seems that the main COVID-19 control objective, laudable and generally agreed by everyone, is to minimise hospital admissions and prevent deaths, in the same way that our approach to any major disease outbreak is to minimise losses with minimal cost. With the high numbers of COVID-19 victims that have already been hospitalised, and unfortunately died, it is becoming clearer whom this highly susceptible and vulnerable population comprises, and they can be readily identified and protected. Some will inevitably become infected, but early treatment systems (rather than waiting for them to become severe clinical cases) may well improve their prognosis and avoid the need for hospitalisation. No farmer would wait for severe clinical disease before instigating treatment and we even implement metaphylaxis protocols for outbreaks of BRD on our cattle farms to ensure very early treatment of high risk contacts.
The control and management of infectious disease has become an established part of farm animal veterinary medicine, not least because our modern farming systems have inadvertently increased the risks and impacts of disease. National, regional and individual disease control plans have become the norm. Some have been more successful than others, and the example of bovine TB can hardly be held up as a good lesson in disease management, but the management of disease outbreaks such as classical swine fever, Aujeszki's disease and foot and mouth disease have been great successes. More lately, voluntary schemes for the control of bovine viral diarrhoea (BVD) and Johne's disease have been great examples of industry collaboration, education and engagement.
Many of us that have been involved in such schemes, look on at the management of the current COVID pandemic with frustration and despair. We appreciate the scale, the difficulties with ethics, economics, and limited resources. We see a strategy that is ultimately undeliverable, inadequate and ineffective. Our leaders are kicking the can down the road in the hope that an effective vaccine will save the day through creating population resilience, while dismissing the opportunity to achieve the strategy through managed exposure and protection of the vulnerable. We now see the potential for unintended and deleterious consequences from the introduction of targeted compensation schemes. We sympathise with those who have to offer scientific solutions that have to be backed by evidence, and the reluctance to offer advice based on judgements and informed decisions that are part of our own work ethic. We hope that any criticism is taken constructively, and we offer our help.