Feral Rabbit Population Boom: CSIRO Study Assesses Myxomatosis and Calicivirus Biocontrol Effectiveness

Australia is witnessing a dramatic resurgence in feral rabbit populations, with estimates placing their numbers at around 200 million nationwide. This boom, fueled by favorable breeding conditions from consecutive years of good rainfall and vegetation growth, has prompted renewed scrutiny of longstanding biocontrol measures. The Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia's premier government research body, has been at the forefront of assessing the ongoing effectiveness of two key viruses: myxomatosis and rabbit haemorrhagic disease virus (RHDV), particularly the RHDV1-K5 strain released in 2017. As rabbits adapt and develop resistance, experts warn of escalating environmental and economic threats unless new strategies are deployed swiftly. 63 62

🌱 The Resurgence: Why Rabbits Are Multiplying Again

Feral rabbits, originally introduced in 1859 by grazier Thomas Austin near Geelong, Victoria, have long been one of Australia's most destructive invasive species. A single pair can produce over 100 offspring in a good year, breeding every 28 days when green vegetation is abundant. Recent wet seasons have provided ideal conditions, triggering exponential growth. Reports from peri-urban, farming, and outback areas across New South Wales, Victoria, Queensland, and beyond indicate visible increases, with some regions seeing plagues reminiscent of the 1990s. 63

CSIRO monitoring, aided by citizen scientists submitting samples via the RabbitScan app, reveals that while viruses still circulate, their impact has waned. In cool, wet coastal zones, benign calicivirus strains confer cross-immunity, reducing lethal RHDV efficacy. Arid interiors fare better, but overall, populations are rebounding after a decade of relative suppression. 105 106

A Timeline of Invasion and Control Efforts

The rabbit saga spans over 160 years:

• 1859: 24 rabbits released by Thomas Austin; rapid spread at 100km/year.
• 1870s-1940s: Shooting, poisoning, and fencing fail to curb billions-strong plagues.
• 1950: CSIRO releases myxomatosis; kills 500 million in two years (99.8% mortality initially).
• 1995: RHDV accidentally escapes Wardang Island trial; intentional releases follow.
• 2017: RHDV1-K5 deployed, reducing populations by 60% for 8+ years.
• 2025-2026: Boom emerges amid resistance and good seasons.

This cycle of boom-bust underscores the need for ongoing innovation, as rabbits and viruses co-evolve. 63

Myxomatosis: The Pioneering Biocontrol

Discovered in Uruguay in the 1890s, myxomatosis spreads via mosquitoes and contact, causing blindness, tumors, and death in European rabbits. CSIRO's 1950 release was a game-changer, but by the 1980s, rabbit genetic resistance and milder viral strains reduced lethality. Today, it's less reliable, though still detected in samples. 63

Step-by-step process: Virus enters via bites, replicates in skin cells causing tumors, spreads systemically leading to pneumonia and death in 10-14 days for virulent strains.

Calicivirus (RHDV): Faster Killer, Emerging Resistance

RHDV causes rapid internal bleeding, killing in 3-5 days—better for welfare. The 1995 outbreak and 2017 K5 strain excelled in dry areas but struggled coastally due to benign RCV-A1 cross-protection. CSIRO data shows K5 failed to spread widely post-release, and immunity is building. Recent surveillance (Jan-Feb 2026) confirms calicivirus in 58% of samples, but populations persist. 106 84

Mechanisms of Resistance and Coevolution

Rabbits evolve genetic resistance (e.g., fewer tumors in myxo), while viruses attenuate for survival—needing live hosts. Benign caliciviruses vaccinate populations against lethal ones. CSIRO's organoid breakthrough (Journal of Virology, 2025) allows virus culturing without animals, speeding resistance studies. 51 96

• Genetic selection: Survivors pass resistance genes.
• Virus evolution: Milder strains spread better.
• Environmental: Wet areas favor benign strains.

Devastating Impacts: Economy and Environment

Rabbits cost $197-250 million annually in ag losses (grazing, erosion). They threaten 300+ natives via competition, predation boost (foxes/cats), weed spread. Low densities (1/ha) halt tree regeneration; warrens erode soil. Pre-biocontrol, wool losses $130M/year; post-RHDV gains $600M projected. 104 62

Case study: Malleefowl extinction risk heightened by rabbits destroying habitat.

Regional Case Studies: From Outback to Urban Fringe

In NSW Peel-Harvey, strong immunity noted (2026 testing). Victoria's northern trials echo 1950s success initially. Urban booms in Sydney outskirts challenge toxin use. SA and TAS show K5 detections, but numbers rise. 106

CSIRO's Cutting-Edge Research Pipeline

Organoids: 3D rabbit organ models grow viruses 40+ years after failure, per 2025 Journal of Virology. Enables passaging, titering without ethics issues. Gene drives: Fertility suppression (e.g., all-male offspring) in lab stages. Citizen science: 345 samples/year track strains. 51 105

Challenges: 8-10 years to field-ready; regulatory hurdles.

Stakeholder Perspectives: Farmers, Experts, Government

Heidi Kleinert (CISS): New virus every 10-15 years needed. Jack Gough (Invasive Species Council): Fund CSIRO now. Tanja Strive (CSIRO): Disaster without next virus. Govt: $1.5M extended, but states lead on-ground. 63 62

• Farmers: Push for warrens destruction, baiting integration.
• Conservationists: Prioritize natives.
• Pet owners: Vaccines available.

Future Outlook: Integrated Strategies and Innovations

Combine viruses with warren ripping ($10-20/warren), fencing, shooting. Gene drives promising but controversial. Urgent funding call: Pipeline dry since 2022. Actionable: Report sightings via CSIRO RabbitScan; support research jobs in biosecurity.

Photo by Hossein Anv on Unsplash

Conclusion: Time to Act Before the Plague Peaks

The feral rabbit boom signals complacency's cost. CSIRO's work offers hope, but sustained investment is key. Explore careers in invasive species research at higher-ed research jobs, rate professors on Rate My Professor, or seek advice via higher ed career advice. Protecting Australia's biodiversity demands vigilance.