European rabbits (Oryctolagus cuniculus) have been exposed to rabbit haemorrhagic disease virus (RHDV) and myxoma virus (MYXV) in their native and invasive ranges for decades. Yet, the long‐term effects of these viruses on rabbit population dynamics remain poorly understood.
In this context, we analysed 17 years of detailed capture–mark–recapture data (2000–2016) from Turretfield, South Australia, using a probabilistic state‐space hierarchical modelling framework to estimate rabbit survival and epidemiological dynamics.
While RHDV infection and disease‐induced death were most prominent during annual epidemics in winter and spring, we found evidence for continuous infection of susceptible individuals with RHDV throughout the year. RHDV‐susceptible rabbits had, on average, 25% lower monthly survival rates compared to immune individuals, while the average monthly force of infection in winter and spring was ~38%. These combined to result in an average infection‐induced mortality rate of 69% in winter and spring.
Individuals susceptible to MYXV and immune to RHDV had similar survival probabilities to those having survived infections from both viruses, whereas individuals susceptible to both RHDV and MYXV had higher survival probabilities than those susceptible to RHDV and immune to MYXV. This suggests that MYXV may reduce the future survival rates of individuals that endure initial MYXV infection.
There was no evidence for long‐term changes in disease‐induced mortality and infection rates for either RHDV or MYXV.
We conclude that continuous, year‐round virus perpetuation (and perhaps heterogeneity in modes of transmission and infectious doses during and after epidemics) acts to reduce the efficiency of RHDV and MYXV as biocontrol agents of rabbits in their invasive range. However, if virulence can be maintained as relatively constant through time, RHDV and MYXV will likely continue realizing strong benefits as biocontrol agents.
Rabbit capture-mark-recapture and serology (disease) state data
Rabbit capture-mark-recapture and serology (disease) state data of 2,200 selected rabbit individuals with unequivocal serology data for all capture events live-captured at Turretfield Agricultural Research Centre (34°33′S, 138°50′E, South Australia) between January 2000 and August 2016 at 8-12 week intervals. All live captures were uniquely marked with serially numbered ear tags, weighed to the nearest 10 g and sexed. Blood was collected from an ear vein for serological tests of rabbit haemorrhagic disease virus (RHDV) and myxoma virus (MYXV) antibodies. Additionally, the study area and warrens were regularly surveyed for dead rabbits at intervals ≤ four weeks, increasing to weekly searches during spring (Sept-Nov) when epizootics were most likely to occur, and at 1-7 day intervals following any evidence of disease-related mortality.
Capture events are assigned to capture session (n = 83) The date of the capture session was calculated as the median date of all captures made during a capture session and given as the number of days elapsed after the first date of capture (i.e. 19/01/2000 corresponds to day count ‘1’). Capture sessions were also assigned to one of the following (southern hemispheric) seasons according to local climate: 1 = Autumn: Mar – May, 2 = Winter: Jun – Aug, 3 = Spring: Sept-Nov, 4 = Summer: Dec-Feb.
The full data set with further details is hold by Biosecurity SA, Adelaide, Australia.
Data are arranged in 7 spreadsheets with the content described below.
1) ‘y’
Table of capture events/ individual encounter histories y, denoting the capture of individuals in the different capture session as “1” if an individual has been captured and “0” otherwise. Individuals are identified in the column “indNo”, the columns entitled “trip_1” to “trip_83” refer to the different capture session.
2) ‘z’
Table of the (partially known) state variable z, denoting of whether an individual is known to be 1= alive, 0 = dea or NA = in an unknown state. Individuals are identified in the column “indNo”, the columns entitled “trip_1” to “trip_83” refer to the different capture session.
3) ‘d_RHDV'
Table of RHDV disease state, classifying rabbits as 1 = seronegative (“susceptible”) 2 = seropositive kittens with maternal antibodies (“protected young”) and 3 = seropositive due to previous infection (“immune”). Individuals are identified in the column “indNo”, the columns entitled “trip_1” to “trip_83” refer to the different capture session.
4) ‘d_MYXV'
Table of MYXV disease state, classifying rabbits as 1 = seronegative with no detectable antibodies and therefore susceptible to infection/disease (“susceptible”) or 2 = seropositive with antibodies against disease. Individuals are identified in the column “indNo”, the columns entitled “trip_1” to “trip_83” refer to the different capture session.
5) ‘individuals’
Table of individual attributes used in the capture-mark-recapture analysis, namely "sex": individual gender; "birthmin": minimum day count of possible birth date; "birthmax": maximum day count of possible birth date; "trip_LowerLimit": earliest capture session for which individual data are modelled according to unknown states variables z; "trip_UpperLimit": latest capture session for which individual data are modelled according to unknown state variables z.
6) ‘capture session’
Table of capture session classifications, with "trip_No": identification number of capture session “daycount”: day count of capture session; “season” season classification (see above); “year”: year of capture session (‘1’ corresponds to the year 2000); “deltaT”: scaling factor to account for unequal time intervals between capture sessions.
7) ‘individuals’
Table of body mass measures of rabbit individuals during various individual capture events with “indNo”: individual number; weight” body mass measure (in gram); “daycount”” day count of individual capture events.
Data_Rabbit.Dz_CMR.analysis_Dryad.xlsx