The Best Australian Science Writing 2013

by Jane McCredie

  [F]rom so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved. – Charles Darwin, On the Origin of Species (1859)

  Memory

  Biblical battles

  Rekindled

  Sentinel chickens

  Peter Doherty

  The idea of ‘sentinel chickens’ seemed pretty incongruous when I first heard the phrase as a young undergraduate. My reaction was no doubt conditioned by recollections of the scatty and fussy hens that scratched about in the dusty chicken run in my grandmother’s backyard. The notion of the humble chicken waiting like a trained soldier, alert and focused, for some unseen and approaching enemy just didn’t seem likely. Hens en garde!

  Like most students in that distant era, I knew everything and knew nothing. Nowadays, any reasonably sophisticated young person would go immediately to the internet and find that, way back to mythological times, guard duty has been part of the avian job description. Gods with the body of a man and the head of a bird, like the ibis, falcon, hawk or heron, watched over the ancient Egyptians. In the Western tradition, the cockerel, or rooster, symbolises vigilance and has been widely used as a French heraldic device. Adopted as the national symbol at the time of the 1789 revolution, the proud, colourful rooster of France (le Coq Gaulois) went beak to beak with the black eagle of Germany during World War I.

  When it comes to warning us of imminent danger, sentinel geese have long been associated with the human story. Geese go on the attack and make an enormous noise if they perceive an incursion into what they regard as their patch. The trick is to provide feed and nurture so that they make our patch their patch. According to the Roman historian Titus Livius – better known as Livy – sacred geese in the temple of the Goddess Juno alerted the exhausted defenders of ancient Rome to a nocturnal attack by marauding Gauls. In modern times, Scottish whiskey distilleries are sometimes guarded by gaggles of geese that raise a loud hue and cry if a thief tries to make off with what many consider the most spiritual of all aqua vitae. Whiskey may be part of the local religion, but I doubt whether the pragmatic Scots would regard the birds as sacred.

  Then there’s the story of the ravens that somehow guarantee the integrity of the Tower of London and, beyond that, the continuity of the crown of England. Legend has it that the monarchy will fall when the ravens leave the tower. Following the spirit of a decree by Charles II, there are always at least ten ravens available, six on duty and four active reservists. The resident ravens are cared for by a raven master drawn from the ranks of the Beefeaters, the medievally attired tower guards. One wing is clipped to ensure they can’t fly away.

  * * * * *

  I first heard the term sentinel chicken from an older cousin, Ralph Doherty, a medical scientist who was then building a substantial reputation in the study of the insect- or arthropod-transmitted viruses, known collectively as the arboviruses, also called togaviruses because they have an outer ‘envelope’ or ‘coat’. Among the major achievements of his research group at Brisbane’s Queensland Institute of Medical Research was the discovery that the mosquito-borne Ross River virus (RRV) is the cause of the human disease epidemic polyarthritis with rash, a painful and debilitating condition that can persist for several months. With more than 4000 cases every year, this non-fatal disease is all too familiar to those who live in the northern parts of Australia and has been rapidly spreading away from the tropics.

  Like all viruses, the arboviruses can only reproduce themselves within living cells. What makes the arboviruses special is that they replicate in the tissues of very different types of animals, though the individual viruses in this very large group do vary considerably in their overall host range. The ‘virus production factories’ include biting arthropods, particularly mosquitoes and ticks, which, as they take their blood feed, either become infected or (if already carrying the virus) transmit the infection to warm-blooded species, including human beings and a whole spectrum of furry and feathered vertebrates.

  And that’s why we have sentinel chickens. The progressive spread of many arboviruses is monitored by placing caged chickens around the countryside at sites where they are likely to be bitten by mosquitoes. The widely distributed birds are sampled regularly, a comparatively non-intrusive process that involves taking a small amount of blood from the prominent wing vein. The blood is allowed to clot, and the yellowish serum supernatant is either frozen or taken on ice to a specialist laboratory, where the samples are analysed for seroconversion. That is, the technician uses a well-established assay to detect newly acquired (since the previous test) circulating antibodies specific for the virus in question. (That doesn’t work for all mosquito-borne infections. Chickens aren’t very susceptible to RRV, for example, which seems to prefer mammalian hosts, and they’re of no value for tracking malaria, for which we humans are the most sensitive sentinels. Birds have their own distinct malaria parasites.)

  If, for example, the birds were seronegative when taken to their guard station, then seropositive for some arbovirus six months later, it’s obvious that they were exposed to an infected mosquito at some time over that period. The relatively few virus particles injected by the feeding mosquito will have travelled via the circulation to invade susceptible cells in one or other organ of the new chicken host. Successive cycles of virus replication then lead to the presence of a great deal more virus in blood (viraemia), a process that terminates somewhere over the next 7–12 days or so, when the developing immune response will lead to the production of specific, neutralising antibodies. Those antibodies will continue to be made for the life of the bird. Once antibody-positive for the infection of interest, the chicken veteran is both permanently immune and eligible for honourable retirement and replacement with a new recruit.

  * * * * *

  Virologists further sub-classify the arboviruses into alphaviruses and flaviviruses. The alphaviruses include RRV and Barmah Forest virus (in Australia), eastern equine encephalitis virus (in the USA) and the Chikungunya virus that has lately been spreading from the Indian Ocean region to South-East Asia and the Mediterranean. Human infection with Chikungunya, RRV or Barmah Forest virus can lead to the development of persistent polyarthritis with rash, while chickens, at least, remain asymptomatic.

  All the flaviviruses are broadly related to yellow fever virus, the terrible pathogen that kills humans by a combination of haemorrhagic disease and liver destruction. That’s where the ‘flavi’ (Latin for yellow) comes from, describing the severe jaundice that characterises the lethally compromised patient. The main vector is the mosquito Aedes aegypti, which is present in tropical North Queensland, though there have been no cases of yellow fever in Australia. A vaccine was developed in the 1930s by the South African medical scientist Max Theiler, an achievement recognised by his 1951 Nobel Prize. There are, however, 70 known flaviviruses, with 30 of these being found in southern Asia and the Australasian region. Some are ‘orphan’ viruses that are not associated with any known disease.

  Way back in the 1960s, cousin Ralph’s involvement with sentinel chickens reflected the broad interests of his research group in arbovirus epidemiology, the study of how this diversity of infections spreads and is maintained in nature. Some arboviruses, particularly the tick-borne ones, can ‘overwinter’ by vertical transmission through the successive stages of an insect life cycle, but even when this does occur, it’s unlikely to be the main mechanism that keeps the virus going in nature. Though infectious disease epidemiologists search for the vertebrate ‘maintaining hosts’ that continue the mosquito–animal transmission cycle, the identity of the key species can be incredibly hard to nail down. Antibodies (the footprints of prior infection) to RRV have, for example, been found in marsupial and placental mammals and, less often, in birds, but that doesn’t prove that the levels of virus in blood were sufficient to cause widespread infection of the mosquito vectors. This two-way insect–vertebrate interchange probably continues throughout the year in the warmer parts of a continental
landmass, particularly in forested areas where there is no effective mosquito control. Migrating birds are, of course, likely culprits for any north or south spread away from the tropics with the onset of spring and summer.

  * * * * *

  Over the years, one of the medically important functions of Australia’s valiant sentinel chickens has been to serve as ‘birds of record’ for measuring the southern spread of Murray Valley encephalitis virus (MVE), a flavivirus that’s also called Australian encephalitis virus. This infection becomes a problem when the combination of warm weather and an abnormally wet season leads to a massive increase in mosquito numbers. If MVE is somewhere in the neighbourhood, perhaps at high enough levels in the blood of susceptible birds, then mosquitoes become infected and sporadic cases of encephalitis are seen in humans, particularly those living along the banks of major water courses like the Murray River. Though MVE has also been found in Papua New Guinea and Indonesia, the main threat to our north is the closely related, but much more dangerous, Japanese encephalitis virus (JEV), which causes severe disease in a relatively high proportion of infected people. Pigs, rather than birds, are known to be a major maintaining host for JEV, and one way of protecting humans is to decrease the ‘multiplier’ factor by vaccinating pigs. There are also effective human vaccines for JEV. According to public health doctors, JEV is not a cause of locally acquired disease in the USA, perhaps because of the lack of the main vector, Culex tritaeniorhynchus. This mosquito is also absent from Australia, but an alternative vector, Culex gelidus, has been identified in the tropical north, where there have been two fatal JEV cases.

  Staffed by successive generations of avian ‘volunteers’, at least some of those sentinel chicken outposts that were located around the country to inform us about the spread of MVE in the ’60s and ’70s still house birds on active duty as part of a continuing Australian surveillance network. Sentinels in the cooler south seroconvert to MVE from time to time, though most evidence of infection is found in tropical northern Australia where occasional human outbreaks continue to occur. The Australian chickens also pick up evidence for the circulation of the closely related (to MVE) Kunjin virus, an occasional cause of human encephalitis, and Barmah Forest virus. Kunjin recently (2011) caused a number of deaths in Australian horses.

  The use of sentinels depends, of course, on knowing the identity of the virus that’s being looked for. Otherwise, it isn’t possible to set up a specific antibody test to determine if any individual – whether poultry or person – has indeed been infected. Though human outbreaks of what was then called Australian X disease had been recognised as early as 1917, it wasn’t till 1951 that Eric French, then working at the Walter and Eliza Hall Institute in Melbourne, reported the isolation and initial characterisation of the MVE virus.

  Apart from the information from sentinel chickens and human cases, what else is known about MVE? The mosquito vector, Culex annulirostris, has been identified, but there are only indirect antibody results that implicate several species of cormorants and the Nankeen night heron as possible maintaining hosts. The Nankeen night heron is common in the wetter regions of southern and northern Australia and is generally regarded as a non-threatened species. It does depend heavily, though, on access to fresh water, and there was some cause for concern during the recent long drought, now broken by the return of an unprecedented La Niña climate system, bringing severe flooding and massive cyclonic activity. That, of course, is also likely to increase the incidence of mosquito-borne infections.

  Sometimes it’s a relatively straightforward matter to establish that a particular species of bird is susceptible to a given arbovirus infection and is capable of circulating the virus, thereby functioning as a maintaining host. For example, eastern equine encephalitis virus, an alphavirus that circulates in the USA and causes disease in both horses and humans, also kills significant numbers of ibises, starlings and emus. Both the birds that eventually die and the survivors can have very high levels of virus in their blood.

  In general, though, it’s been easier to identify the insect vectors that transmit these infections than to establish which particular wild birds or mammals support their overwintering. One reason for this is that arboviruses generally persist longer in mosquitoes, as they lack the type of adaptive, or highly specific, immune system that is characteristic of birds, mammals and the other bony vertebrates. Even when vertebrates suffer a severe infection, the virus is usually eliminated from the blood of survivors within 8–12 days. The other reason is that trapping and handling wild birds takes a lot of effort, while it’s relatively easy to catch large numbers of mosquitoes using light traps that emit carbon dioxide and other chemical attractants (like octenol), simulating the presence of warm-blooded animals. A more primitive technique is to allow them to bite, say, a tethered horse or your own arm, then capture them using some sort of suction device that may be as simple as a skilfully used drinking straw.

  Once trapped, the mosquitoes are classified by a medical entomologist, then those of the same type are pooled, frozen and later ground up in saline for injection into some detection system (such as tissue culture or suckling mouse brain), which will then grow any virus the mosquitoes were carrying. The freshly isolated viruses can then be identified by sequencing to determine their characteristic genetic code, using essentially the same technique that forensic experts employ to identify DNA from a rapist or murderer.

  The capacity to produce highly specific antibodies following natural infection in the field or forest is, of course, the basis of the sentinel chicken’s role. We feed and nurture these doughty guardians because birds have both a thymus that produces the immune T lymphocytes (including the killer T cells, which I’ve worked on for almost four decades) and the B lymphocytes or plasma cells that produce the specific antibodies we detect in blood.

  The avian and mammalian immune systems have evolved somewhat differently over the aeons, but they do the same job of controlling infection. Furthermore, this shared capacity for generating long-term immune memory is the reason why, in the past century or so, we have seen the development of numerous protective vaccines for both domestic birds and chickens.

  Arboviruses aren’t, however, on the chicken vaccine list, as they don’t affect commercial producers. How vaccines are used is always determined by practical considerations, and the fact that a product is used in one vertebrate but not another doesn’t reflect some sort of discriminatory ‘speciesism’. For obvious reasons, it’s pretty much impossible to vaccinate wild birds against anything. In the USA, valuable horses are vaccinated against the Venezuelan equine encephalitis alphavirus, while humans are not. People who live in the more prosperous countries are protected by the environmental control of mosquitoes that’s practised in most of the larger, warmer cities, by a more indoor lifestyle and by the judicious application of mosquito repellent when venturing into the countryside. We’ve never made a vaccine against MVE because the incidence is too low, but such a vaccine could be developed if, for example, the warming associated with anthropogenic climate change led to MVE becoming a more substantial threat to large numbers of humans.

  A more likely danger for Australians is that infections like JEV and malaria will simply migrate south as ambient temperatures rise, birds modify their migration patterns, and mosquitoes extend their host range. That is already happening in parts of Africa, as infected mosquito populations move inexorably into the cooler and higher regions of the continent, which were formerly malaria-free. In Europe, Chikungunya virus has now penetrated as far north as Ravenna. As land, air and water temperatures increase, the shift of viruses that depend on a mosquito–vertebrate (bird or mammal) lifecycle into what were temperate regions will inevitably continue.

  Vigilance

  The secret life of birds

  Climate change

  The science of shark fishing

  Ian Gibbins

  There’s not much you can do with a hook through your jaw.

  Apart from anything
else, you cannot escape

  that pervading taste of metal, that disorienting,

  somehow worrying, sensation of stainless steel,

  mixed, almost certainly, and against all hope,

  with your very own pulsating haemoglobin.

  It’s difficult to describe your disbelief and indecision.

  The pressure is unrelenting, even in those moments

  when you convince yourself to relax, to let yourself

  drift forwards a body length or two, slip backwards

  a metre or two, while refracted ripple-skies continue

  to be drawn just that much closer to your touch.

  In the end, weariness utterly overwhelms you.

  Surrounded by more oxygen than you ever have required,

  you find yourself aching for one more breath of the sea.

  You wish, perhaps, that evolution had provided you

  the wherewithal not only to bite, to maul and harangue,

  but simply, decisively, to get up and run away.

  Anthropomorphism

  Caught!

  Resigned

  On flatulence

  Nicholas Haslam