AUSTRALIA AND INVASION

Brendan Duffy, guest blogging

Australia split off from Pangea maybe 120 million years ago, before us Eutherian ‘placental’ mammals had evolved from our pouched relatives in what was to become Asia. All across Pangea the inferior pouched model was supplanted by the latest triumph in evolution with its long internal gestation, but not in Australia. Australia (and Antarctica) travelled the southern ocean in isolation, carrying a cargo of ‘ancient’ flora and fauna into the present. Antarctic marsupials became extinct.

Waves of exotic placental invaders have washed up on our shores, particularly ones that could get here under their own steam, such as the flying and swimming ones; bats, seals, and dugongs, although rodents made it, too. It took the walking ones quite a while. The first wave of human invaders, as Tim Flannery tells us in The Future Eaters, hunted the marsupial megafauna to extinction, then were forced to develop a more sustainable resource management strategy, or harmony. Chris Gregory tells it a slightly different way in his excellent short story ‘Mock Chicken’.

The second wave of human invaders occurred 220 years ago. Since then Australia has suffered more species loss than the rest of the world combined.

In 1859 twenty four European rabbits were released near Geelong, and by 1910 they had spread across most of the continent, occupying all but the alpine and tropical areas. Current Australian rabbit population is estimated at 200-300 million, descended from the genetic bottleneck of those 24 founders. Rabbits cause approx $600 million damage to agriculture per annum, severe environmental degradation, desertification, and compete with native wildlife for resource and habitat.

Typical local control methods have included destroying warrens through ripping, ploughing, blasting, and fumigating; poison baiting; shooting and hunting with dogs and ferrets; rabbit proof fencing; and, unfortunately, releasing predators such as cats and foxes. Against such a huge rabbit population these largely manual methods have a small impact, so, on a national scale, Australians did what they were good at: biological ‘control’ – introducing MORE exotic animals to combat the ones now reaching plague proportions. Much like the old lady who swallowed a fly, things can escalate.

Myxomatosis, a mosquito borne myxoma virus, was released in 1950 and killed an estimated 95-99% of all rabbits within 18 months, but with no nationwide follow up to manually finish the job, or at least keep numbers low, the rabbit population soon bounced back with increased immunity. Furthermore, the host-pathogen relationship changed in the wild, and new strains emerged which were less virulent. Still, myxomatosis remains an active biological control agent whose efficacy appears to have reached an equilibrium, keeping rabbit populations at half that of the 1940s.

Efforts to increase the efficacy of the virus included the introduction of two new Myxomatosis carrying flea species to Australia, the European rabbit flea in 1968 and the Spanish rabbit flea in 1990. Both saw the myxomatosis transmission rate increase. Australia’s exotic menagerie continues to expand.

The rabbit calicivirus disease, RCD, was first recorded in China in 1984. Modes of transmission include the blowfly, which will transmit the virus from an infected carcass, unlike myxomatosis which requires the mosquito vector to sample live blood. Also unlike Myxoma, RCD is an RNA virus and mutates 10⁶ times faster. In 1995 Australian researchers were testing this virus as a possible biological control agent under quarantine conditions on Wardang Island, 5 km off the South Australian coast. It jumped to the mainland, presumably on flies, and rapidly spread across south eastern Australia, with a 95% kill in peak areas. Stories abounded of people driving around the country selling infected rabbit carcasses to farmers in order to spread the virus more quickly and alleviate pressure caused by the rabbit plague. It is estimated that more than 30 million rabbits died in southern Australia during October–November 1995.

However, the new black in biocontrol is Immunocontraception. It is a method whereby a sexually transmitted viral disease from the target species is genetically modified. Basically, genes encoding the target species’ fertility factors, such as sperm and/or egg proteins, are spliced into the viral genome downstream from the viral coat proteins, so that when the virus replicates, the progeny display their normal viral antigens PLUS the host animal’s fertility factors. Infected animals make antibodies against the virus AND their own fertility factors. As a consequence they become sterile while their health and quality of life remain unaffected, unlike myxo and RCD. Immunocontraception is lauded as humane (although some have suggested using lactation controlling genes so that young will starve).

Immunocontraceptive methodologies can be applied to all of Australia’s pest species. It targets their most successful attribute – high fertility. Alpha males and females spread the disease fast, and their symptomless behaviour allows them to compete as normal, stopping uninfecteds from breeding. Immunocontraceptive biocontrol has not yet been trialed in Australia, but success has been achieved with the house mouse in lab conditions.

So are biological agents safe, effective, and controllable? Where did RCD come from? Why are there genetically related viruses with different hosts? Is it because viruses can mutate and change hosts? How did Brisbane horse trainer Vic Rail die? A mystery virus, eventually identified as the Hendra virus, closely related to the emerging Nippah virus that plagues Malaysian pig farms, occasionally kills the farmers, and can be carried by migratory flying foxes (all eutherian mammals). Most mammals have the same genes and many those genes have highly conserved coding sequences. So, viruses emerge, mutate, and jump species. What will happen if Immunocontraceptive viruses are released?

Australia has seen the introduction of many pest species (as Robert VanderMeer would know). Biocontrol of pest species should be dealt with carefully. Successes like the control of Patterson’s Curse and Prickly Pear are not always attained. One thing is sure, if rabbit numbers are to be reduced, the numbers of introduced rabbit predators such as the red fox should likewise be reduced, less the native wildlife fall prey to hungry mouths.

Brendan Duffy has a PhD in the evolution of mammalian sex chromosomes, and was involved with screening marsupial Y chromosome libraries for candidate immunocontraceptive genes to target the Australian Possum, an animal introduced to New Zealand to provide a fur trade. NZ’s lack of predators has lead to a plague of possums running rampant in New Zealand. They are a bioreservoir of bovine tuberculosis, and threaten the NZ ecology and dairy industry. Many Australian pest animals are currently being considered for Immunocontraceptive management strategies. Brendan has a story about Australian Immunocontraception appearing in ASIM 21.