Although a parapoxvirus harmful to red squirrels is present in UK squirrel populations it has not been considered a major cause of red squirrel decline, and replacement by the introduced grey squirrel, mainly because diseased individuals are rarely observed. By developing a generic model we show that parapoxvirus is likely to have played a crucial role in the red squirrel decline even though the prevalence of infection is low. Conservationists are quite rightly concerned with the invasion of exotic organisms such as the grey squirrel. Our work emphasizes that they, along with other ecologists, should pay particular attention to pathogens, even when they occur at low prevalence.

/pdf/ecological-replacement-of-native-red-squirrels-by-invasive-2u1349f22e.pdf

Ecological replacement of native red squirrels by invasive greys driven by disease

Poxviruses and the evolution of host range and virulence.

The disease implications of novel pathogens need to be considered when investigating the ecological impact of species translocations on native fauna. Traditional explanations based on competition or predation may often not be the whole story. Evidence suggests that an emerging infectious disease, caused by a parapoxvirus, may be a significant component of the impact that the introduced grey squirrel has had on UK red squirrel populations. Here we validate the potential role of parapoxvirus by proving that the virus is highly pathogenic in the red squirrel while having no detectable effect on grey squirrel health.

/pdf/parapoxvirus-causes-a-deleterious-disease-in-red-squirrels-4ru2fm9qd1.pdf

Parapoxvirus causes a deleterious disease in red squirrels associated with UK population declines

Summary
Invasive organisms and emerging wildlife disease pose two of the greatest threats to global biodiversity and ecosystem functioning.
Typically, when parasites are considered in invasion biology, it is in the context of the enemy release hypothesis, wherein a non-indigenous species has greater probability of invasion success by virtue of leaving its natural enemies, including parasites, behind.
It is also possible that native parasites may prevent invasions, but it is clear that invasive organisms may bring infectious diseases with them that can infect native competitors (via spillover), or act as competent hosts for native diseases, increasing disease prevalence among native species (via spillback).
If the shared disease (either via spillover or spillback) has higher virulence in the native host (which is particularly likely with introduced parasites), there is the potential that the disease can act as a ‘biological weapon’ leading to a disease-mediated invasion (DMI).
Here, we review cases where disease may have been an important factor mediating a wide range of invasions in vertebrates, invertebrates and plants.
We then focus on the invasion of the grey squirrel into the UK as a case study of a DMI, and we discuss how mathematical models have helped us to understand the importance of this shared disease and its implications for the management of invasive species.
We conclude that (i) DMIs are a widespread phenomenon, that (ii) spillover is more common in animal invasions and spillback more common among plant invasions and that (iii) spillover DMIs are particularly important in explaining the replacement of native animals with phylogenetically similar non-indigenous species.

/pdf/invading-with-biological-weapons-the-importance-of-disease-1gfypueqee.pdf

Invading with biological weapons: the importance of disease-mediated invasions

Summary

1.  A stochastic individual-based model for simulating the dynamics of an infectious disease in sympatric red and grey squirrel populations is described. The model simulates the spread of parapoxvirus between squirrels in fragmented populations based on the dispersal of infected animals, the probability of encounters between individuals, exposure to the virus and subsequent mortality.



2.  The disease model was integrated with a spatially explicit population dynamics model that simulated red and grey squirrel populations in real landscapes, using habitat information held in a geographical information system. Latin hypercube sampling was used to create a range of realistic life-history and infection scenarios and the model was used to investigate the dynamics of red and grey squirrels in Norfolk between 1966 and 1980.



3.  The model predicted that parapoxvirus, like interspecific competition, could have led to the extinction of the red squirrel in Norfolk. The results suggest that the red squirrel–grey squirrel–parapoxvirus interaction represents a system of apparent competition mediated by an infectious agent, as seen in other interactions between resident and exotic species.



4.  The need for further epidemiological research on the virus is emphasized. We believe that the combined effects on disease transmission of habitat, behaviour and grey squirrels acting as reservoir hosts will lead to a patchy prevalence and sporadic incidence of parapoxvirus disease in red squirrels and a more rapid local replacement by grey squirrels.



5.  These results have implications for conservation management of the red squirrel in the UK. Schemes in which animals are translocated or given supplementary feeding may enhance disease spread by bringing infected animals into contact with others.

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2664.2000.00553.x

I.F. Keymer

Papers

Parapoxvirus infection of the red squirrel (Sciurus vulgaris).

Parapoxvirus infection of the red squirrel