Delta

Coronavirus is back as Covid-19 continues to emulate a fast running invasive mammal biosecurity incursion.

On August 17 2021 the New Zealand Government announced that an infection of the Delta variant of the SARS‑CoV‑2 virus had been positively identified in the community.  As a result, the whole country immediately moved from the least restrictive Level 1 Covid-19 alert level to the most restrictive Level 4 lockdown.

​This event occurred almost 18 months after the virus had been first detected in New Zealand in late February 2020.  New Zealand’s swift and decisive action to suppress coronavirus through the autumn of 2020 resulted in its successful eradication from the country.  Over the ensuing months positive cases of infection were continuously detected at the border and these were mostly successfully controlled by the Managed Isolation and Quarantine system designed to reinforce the country’s closed international border.

---

Between May 2020 and August 2021 small and localised incursions of the virus into the community occurred in August 2020 and February 2021.  These were quickly detected by surveillance testing and were eradicated with aggressive contract tracing efforts combined with regional rather than national lockdown restrictions.  Over this period, surveillance testing for the virus covered on average several thousand people per day, with a peak of around 25,000 people per day in the August 2020 breakout.

Daily coronavirus surveillance testing in New Zealand (yellow bars) with the 7-day moving average (purple line). Graph taken from Ministry of Health website.

However, during this time the virus was also changing as it collected mutations to become both more infectious and more virulent.  From the wild-type virus, the Alpha variant emerged in the UK in September 2020 followed by the Beta variant in South Africa in October 2020.  In January 2021 the Gamma variant was detected in Japan after it arose in Brazil and the Delta variant, first detected in India in late 2020 became the dominant strain globally from June 2021.  Many other coronavirus variants have been detected but did not become variants of concern.

​The genetic mutations that the Delta variant of coronavirus has accumulated on its spike protein has made it one of the most transmissible human respiratory viruses known.  These mutations have also enhanced its ability to avoid both natural antibodies and Covid-19 vaccines and have led to repeat waves of infection sweeping through communities where it is prevalent.  Each wave of infection demonstrates the classic hockey stick “J” shape of exponential population growth.

Coronavirus pandemic infections (thousands per week) in sub-Saharan Africa showing development of a third wave of infections as at June 2021. Graph taken from IMP website.

The data behind these graphs is likely to be robust simply because Covid-19 is such a debilitating disease that those who are infected with coronavirus, especially in areas where rates of vaccination are low, such as sub-Saharan Africa, need to seek medical care and they can be readily identified.

Comparable percentage coronavirus vaccination rates. Graph taken from IMP website.

So does the arrival of the Delta variant and the re-establishment of coronavirus in New Zealand have any further lessons for predator control and eradication?

Certainly coronavirus is a biosecurity issue, just as invasive predators are, as it is an unwanted organism that causes both injury and death in the population that it preys upon.  New Zealand’s initial interaction with coronavirus during the autumn of 2020 was a colonisation that was successfully eradicated.  At that time the key factors were:

• Preventing further immigration of the unwanted organism by throwing up perimeter defences and
• Restricting the rate of reproduction of the unwanted organism then wait for natural mortality to drive the desired eradication.

New colonisations of invasive predators are still actively occurring, as demonstrated by stoats successfully establishing themselves on the Orkney Islands, so the parallels between coronavirus and invasive predators are real and striking.

Since its eradication from New Zealand in May 2020, coronavirus has been knocking on the door to re-invasion and now Delta has got inside our perimeter defences.  Continuous surveillance for the invader both at the perimeter defences but also inside the perimeter is critical to monitor for re-invasion and re-establishment, and then when an incursion is detected to ramp up the surveillance from a focus on simple detection to a focus on measuring distribution and abundance.  To illustrate this, daily coronavirus testing has recently peaked around 50,000 people per day, more than twice the previous maximum.

It is also important to remember that the unwanted organism could be different when it does reappear.  The Delta variant’s increased transmissibility is the equivalent of a faster breeding rate in a newly invasive population of predators and its improved ability to evade antibodies is the equivalent of a new predator knowing how to avoid traps or having different bait preferences, both of which make existing predator control techniques less effective and eradication harder.  Changed behaviour of predators has been seen previously with the development of warfarin resistance in rodents as they developed genetic resistance to this previously lethal toxin.

Predator control like disease management is an ongoing ecological arms race between the hunter and the hunted.  It requires an adaptive response based on real world knowledge provided by robust surveillance and must be fought on a number of fronts simultaneously including:
​

• Immigration at the perimeter boundary
• ​Reproductive rate within the boundary
• Dispersal of propagules to uncolonised habitats
• ​Avoidance behaviour to evade control measures

If eradication can be achieved then surveillance may be able to be scaled back but it is important that detection continues to focus both at the perimeter and throughout the control area.  Missing an incursion at the border gives the invader extra time to establish a viable population that could make re-eradication at least more difficult, or maybe even impossible.

Time will tell if this invasive organism can again be eradicated from our well defended shores.