The Best Australian Science Writing 2013 (26 page)

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

Charles Darwin's life was a vale of sorrows. His mother died when he was a boy, three of his own children perished at an early age and his great discovery of the theory of evolution by natural selection was met with hostility by prominent members of society and many of his fellow scientists. Through most of his adult life he also suffered mightily from disabling anxiety, depression and physical symptoms that included heart palpitations, eczema, vomiting and flatulence. In his mid-fifties he described the latter thus: ‘For 25 years extreme spasmodic daily & nightly flatulence … every passage of flatulence preceded by ringing of ears, treading on air & vision'.

Flatulence is universal. For some it is a minor annoyance or embarrassment, for others a source of amusement and for an unfortunate few it is a terrible burden. For Darwin, flatulence was the symptom of a deep malaise which combined fear, fatigue and black pessimism. According to his biographer, John Bowlby, it was the psychosomatic expression of an anxiety disorder linked historically to his mother's early death, exacerbated in the present by life stresses, and caused directly by a tendency to hyperventilate.

More often than not flatulence has kept more cheerful
company than Darwin's. It has been a staple of comedy since the ancient Greeks, reaching a peak in the ribald humour of the Middle Ages. Famous farts are documented in Chaucer's
Canterbury Tales
and in the
Arabian Nights
. Flatulence reaches heights of comic absurdity in the work of François Rabelais, who describes a fictitious volume in the library of St Victor on the art of farting decently in public and whose creation, the giant Pantagruel, unleashes one great fart whose ‘foul air created more than fifty-three thousand tiny men, dwarves and creatures of weird shape'. This spirit of high-spirited vulgarity, often directed by the young and powerless against authority figures and the well-bred, survives to this day in a thousand jokes, children's books and popular culture references.