Global pandemics are an extinction level threat.

In the United States, the definition of what is an emergency, or a disaster, is very fluid. The Federal Emergency Management Agency (FEMA) has an incident typing system, where five is the lowest in terms of resources required and one is the highest, as shown in Figure 1: Emergency management has as one of its standards to “ratchet up and down” the resource support needed, based on the incident type. For example, when a Type 4 incident is still scaling up – meaning not yet under control or expanding – additional resources should be requested as if the incident could grow to a Type 3. While COVID-19 may have started in the United States as a small outbreak in Washington state on January 20, 2020, it expanded exponentially across the entire country in a matter of weeks. And at the same time, the virus spread worldwide.[4] The size and scope moved the incident typing off the scale[5]; there were no additional resources available anywhere, nor was there proper planning in place for this level of incident. There are hazards and threats for which emergency management cannot plan, organize, equip, train, and exercise because they are too complex. One way to quantify them is to describe them as Type 0 – Extinction Level Events.[6]

Diseases cause extinction all the time in animals. Humans are just as susceptible.

McCallum 12 Professor Hamish McCallum is the Director of the Centre for Planetary Health and Food Security at Griffith. He is an infectious disease ecologist with a particular interest in modelling the dynamics, impact and transmission of parasites and pathogens in wildlife populations. This research has two broad applications: in conservation biology and in managing zoonotic pathogens. He was the senior member of a team awarded the Eureka prize for Environmental Research in 2011 for work on Tasmanian Devil Facial Tumour and was awarded the Vice Chancellor’s award for research leadership in 2019. (Hamish, October 19, “Disease and the dynamics of extinction”, National Library of Medicine,)

Almost all cases of disease-threatening extinction are a result of a host encountering a pathogen to which it has had no previous exposure in evolutionary time. We clearly need to be particularly concerned with ‘pathogen pollution’ [77,78], in which pathogens are introduced into naive populations or communities. Populations in habitats that have been relatively isolated until recently are therefore likely to be particularly at risk from disease. Habitat fragmentation and ecological transformation are also likely to bring susceptible host populations in contact with novel pathogens [79]. Although DFTD is a counter example, most pathogens threatening to cause extinction have one or more disease-tolerant reservoir hosts that can maintain a high force of infection even as the threatened host declines towards extinction. Low genetic diversity in host populations not only may make them initially more susceptible to disease, but also reduces their potential to respond evolutionarily.

(b) Identifying incipient disease threats

High prevalence of a parasite in a declining population or among morbid individuals is not in itself evidence that the pathogen is responsible for decline. In fact, parasites that cause little mortality are more likely to occur at high prevalence than those that are highly pathogenic [9]. Other criteria that indicate a pathogen may be responsible for population decline are needed. A possible approach is to apply the Bradford Hill criteria [80], which have also been suggested as being useful in understanding the emergence of infectious wildlife diseases [81]. Bradford Hill developed nine criteria to suggest that environmental factors have a causal relationship to human disease. He discussed these primarily in relation to smoking as a cause of lung cancer, a non-infectious disease causation, but they can be applied in other contexts. The criteria (table 2) are essentially a series of circumstantial associations, which suggest a causal relationship if several are satisfied.

The criteria may even be of use in suggesting the influence of infectious disease in population decline, without having identified a putative disease agent. Laurance et al. [48,82] postulated that an infectious disease was responsible for precipitous declines in several frog species in tropical and subtropical Australia. At that time, no agent was suggested. Of their lines of evidence (many of which were refuted at the time [49,50]), there was limited experimental evidence that some mortality-causing agent could be transferred in water (criterion 8), evidence that declines were widespread in similar habitats (criterion 2), evidence that moribund individuals could be collected from affected populations (criterion 6) and a suggestion of a wavelike progression of extinctions from south to north (perhaps evidence supporting criterion 4). Their final line of evidence, ‘absence of plausible alternatives’, could perhaps be aligned with criterion 7.

5. MANAGING EXTINCTION-THREATENING DISEASES

The options available to manage extinction-threatening diseases in wild populations are limited [83,84]. They can broadly be considered as: (i) isolating uninfected populations from disease; (ii) culling either infected animals only or all animals to reduce the force of infection; (iii) reducing transmission by habitat modification to reduce foci of infection; (iv) genetic management to spread resistance alleles within the host population; and (v) vaccination. Treating individuals will rarely be practical. A fallback option to preserve some individuals of species threatened with extinction is to bring them into captivity, with the possibility of reintroduction following extinction in the wild, if the disease problem has been solved.