Issue 102: Limiting the infection toll on a warming planet

Written by Nobel Laureate Professor Peter Doherty

The first thing most of us think about when it comes to protection against any infection is vaccination. As we’ve discussed (#97-99), there are good vaccines against Japanese encephalitis virus (JEV) and yellow fever virus (YFV), but we’ve never developed one for Murray Valley encephalitis virus (MVE) or for Ross River virus (RRV). Though RRV is a reasonably prominent cause of infection and (often lingering) clinical impairment in Australia, it does not kill people and the financial case for expensive vaccine testing and later deployment has not been considered strong enough. Early studies suggest that an effective, inactivated whole virus RRV vaccine would work fine, and it’s likely an appropriate mRNA product that could be made fast if needed. As things stand, with a dangerous virus like JEV that causes periodic outbreaks in, say, the Murray Darling region, the use of currently available, approved and long-acting JEV vaccines could be appropriate for those living in the region. Developing vaccines to protect against malaria is problematic but, again, the use of mRNA technology may provide new possibilities.

Mosquito control is a different option and, in regions of Australia where mossies are a problem, there are well developed strategies for such environmental action. Spraying, including aerial spraying, is an obvious way to go, and there are possibilities for using natural products that kill off the larval stages of mosquitos. Delivered as a spray, Bacillus thuringiensis will be familiar to many, and there are alternatives, including Saccharopolyspora spinosa. These formulations can also be applied as ‘dunks’ in some readily dissolving solid form that can be dropped into ponds. When it comes to spraying for the elimination of adult mosquitos, the main products used are organophosphates. Very widely used in agriculture, these inhibit acetylcholinesterase and are essentially nerve agents, so there are environmental concerns.

Back in the 1960s when some very famous scientists like Australia’s Sir McFarlane Burnet were speculating that ‘the age of infectious disease is over’, they were no doubt thinking to some extent of the effectiveness of dichloro-diphenyl-trichloroethane (DDT) spraying for controlling insect vector populations. Unfortunately, DDT ingested by birds can cause them to form weak eggshells, which led to dramatic reductions in numbers for some iconic species, including the American bald eagle.  DDT deployment for environmental control has been banned, though this cheap and effective chemical used for spraying inside houses in tropical villages, and to impregnate mosquito bed nets. As the product is still around though, there are concerns re illegal use. Some forms of eggshell speckling are evidently an indicator that DDT is out ‘there’.

The problem with sprayed insecticides is that they remove a food source for insectivorous birds and bats. Another innovative strategy, originated by Scott O’Neill at Monash University, is to disable (or diminish) the capacity of mosquitos to replicate arboviruses by persistently infecting them with variants of the nonlethal (for mosquitos) Wolbachia bacterium. The microorganism is first introduced into mosquito eggs in the laboratory, before the adults that mature in insectaries are released into the wild. The bacteria then transmit stably from mosquito generation to generation, progressively taking over from uninfected insects. Large field studies in Southeast Asia, Northern Australia and elsewhere have shown that this can dramatically reduce dengue virus transmission. However, the approach is not as simple as it sounds as the various arboviruses (Zika, JEV, YFV, RRV) carried by different mosquito species may be optimally inhibited by genetic variants of Wolbachia. The same may be true for malaria. There is still a lot of research to be done in this area.

We’re all familiar with spray-on or roll-on mosquito repellents, and anyone who is likely to be in the tropics during mosquito season is advised to carry them. Long sleeves and long trousers are also protective, though that is obviously less attractive when it is very hot. Haemorrhagic dengue is uncommon in young adults who have grown up in endemic areas. But it can be an issue for those who first encounter these viruses – for example Peace Corps volunteers - in their twenties.

A further concern with respect to global warming and infection relates to increased risk during extreme heat waves. We already know that heat-related urban deaths may be greater in number than those associated with the actual fire zones in the Australian bushfires. Some of that may also reflect smoke pollution, but it is the case that any of the common respiratory infections can be more dangerous under such conditions.

That’s one of many reasons why we should be legislating and enforcing building codes mandating that new residences and public buildings, especially hospitals, are designed and constructed to protect their occupants. Climate change is not just about heat: it’s about increasing the energy of earth systems (big winds, storm surges) in ways that have massive implications for environmentally vulnerable areas (coastlines) and built environments. In fact, as the planet continues to warm, almost any important area of human activity, from infectious disease prevention, to food production and infrastructure design, needs to be evaluated in the context of extreme weather events that could have deleterious consequences for complex life forms like us.