20 Apr 2022
Issue 101: Arboviruses and other infectious disease concerns for a warming planet
Written by Nobel Laureate Professor Peter Doherty
Having discussed (#97-98) Japanese encephalitis virus (JEV) and the mosquito-borne arboviruses, this is a good time to say a little about the likely increase in infectious disease threat level that will come with the progression of anthropogenic climate change. With respect to viruses, the pathogens of most obvious concern are those that multiply in mosquito and tick ‘vector’ species (after feeding on an infected animal or person) then transmit the infection when they take a ‘blood meal’ from us. The defining factor related to climate change here is obviously the environmental temperature sensitivity of the vector species.
But it’s not just viruses that we worry about with climate change. A major fear is, for example, that mosquito-vectored protozoan parasites like malaria will move into what were formerly temperate regions. Specifically in Kenya, malaria has long been a problem in the coastal city of Mombasa while Nairobi, at 1795 metres, is at much lower risk. That profile extends to the Kenya Highlands, though the malaria threat increases as people descend into the valleys. Malaria is a major problem in Papua New Guinea and it has occasionally been an issue for us in the islands of Torres Strait. We are likely protected by the very low human population density in our far north a situation that, as the planet warms, would not be expected to become more problematic.
Other climate concerns that relate to extreme weather events, (already increasing in both severity and frequency) include impacts on the availability of clean, safe drinking water. The flooding of sewage treatment plants and sewage spill-over into flood waters obviously raises the risk from the water-borne bacterial diarrheas and septicaemias, like cholera and typhoid. On the other hand, drought is also a problem for poor communities as local water sources used by both animals and humans shrink and are more readily contaminated with organisms like Escherichia coli that cause gastrointestinal problems
Has the recent invasion of JEV that we’ve been detecting as far south as the Murray/Darling been a consequence of climate change? We don’t know, and likely will never know. What does look certain is that, as pigs are a major maintaining host for JEV, it is here to stay. We can vaccinate domestic pigs so that they don’t become infected and circulate the virus in their bloodstream to infect feeding mosquitos, but that’s impossible for feral pigs.
Will other ‘exotic’ insect- borne viruses – like Zika virus that has been active in the Pacific Island states - become established here? As the climate warms, the relevant mosquitos may be around for longer and spread further south. Australia has a whole spectrum of possible vector species for different arboviruses, along with the possibility that much of our wildlife (native or imported) could act as potential maintaining hosts. Additionally, studies in South America are indicating that various bat species can, as with the coronaviruses, also be infected with arboviruses, including dengue virus. Dengue has long been an occasional invader into our north, with the virus likely arriving in the blood of an infected person who is then bitten by a female Aedes aegypti mosquito.
An obviously important parameter with viruses like Dengue (also called ‘break bone fever’) where we are the major maintaining species is human numbers in afflicted areas. In the main we’ve only seen ‘classical’ dengue here. That’s an unpleasant condition with severe headaches, fever, vomiting, rashes and pain in muscles, bones and joints, but people generally start to feel better after two to seven days.
What we haven’t experienced is the more serious, and potentially lethal haemorrhagic dengue that has been associated with devastating outbreaks in South-East Asia. Why is that so? The reason is that there are four distinct strains of dengue virus. Infection with one provides little protection against the others, but a prior exposure to one can lead to the production of cross -reactive, ‘enhancing’ antibodies (#20) that, instead of neutralising infection with a different variant (#21), helps the new virus to enter, and damage cells that it would not normally access. When this involves the blood vessel walls, this can lead to bleeding, tissue swelling and a form of shock. Early on, haemorrhagic dengue killed a lot of children in countries like Thailand, but that toll decreased when it was realised that most could be saved with inexpensive rehydration and fluid therapies.
Why don’t we see haemorrhagic dengue in Australia? Dengue incidence has been so low that people (especially children) are not normally infected sequentially with different strains. Otherwise, while the enhancing antibody issue (#20) has been a concern with vaccine development (as it was early on for COVID vaccines #47), we are starting to see the approval of dengue vaccines for the 9-16 year old age group. In general, when we talk about health issues in relation to climate change my perception is that, in the absence of major social instability, we should be able to handle any increased infectious disease threat. Next week, we’ll look a little more at related issues and some strategies we can use to protect ourselves.