Ross River virus hits Victoria: increased risk for southern Australia*

"Say hello to my little friend..."

 

 

The combined effects of heat and extreme flooding have seen the mosquito population explode in Victoria. So much so that a surge in cases of Ross River virus (RRv) is hitting those flood affected parts of Victoria:

 

VICTORIANS are being warned to brace themselves for outbreaks of diseases such as Ross River fever and Barmah Forest virus as mosquito populations explode across the state.

Health Minister David Davis said Victoria’s higher rainfall meant there would be more risk from mosquitoes this summer.

”The department is very aware of these risks and is in close communications with local authorities,” he said.

There has been a significant rise in the number of reported cases of Ross River fever and Barmah Forest virus as unseasonably wet and warm conditions in the first two weeks of January provided perfect breeding conditions for mosquitoes.

In the first 12 days of January there were 23 cases of Ross River virus confirmed by blood test in Victoria, and a further 21 cases of Barmah Forest virus – more than four times the number of reported cases for the same period last year, when there were six cases of Ross River fever and four cases of Barmah Forest virus.

In 2008, there were just four reported cases of both viruses in the first 12 days of January.

”I think the data speaks for itself,” said Health Department spokesman Graeme Walker.

”We were prepared for this, and issued a health warning just before Christmas.”

The “peak” period for infections is due over the next few weeks, as mosquitoe larvae mature and hatch.

What is happening in Victoria is also true for South Australia:

 

Rising flows in the Murray have led to a surge in mosquito-borne infections in South Australia.

Cases of Ross River virus and Barmah Forest virus are now beyond 350, compared with little more than a dozen cases at the same time a year ago in the Riverland.

Dr Grant Baker, from the Riverland Division of General Practice, said it was a cause for great concern.

“They need water and obviously up here in the Riverland we’ve gots lots of water at the moment,” he said.

He said mosquitoes did not travel far, but the virus could be carried further by birds and animals such as marsupials.

Dr Baker said Ross River symptoms included fever, rashes and aching joints.

What is the Ross River virus?

The New South Wales government provides an overview of the disease here:

 

What is the Ross River fever?

• Ross River virus is one of a group of viruses called arboviruses (or arthropod-borne viruses), which are spread mainly by blood-sucking insects.
• Ross River virus is a germ that infects people, particularly in rural areas, sometimes causing a flu-like illness with joint pains, rash and fever.
• Ross River virus is not fatal.

What are the symptoms?

• Many people who are infected with the virus will never develop symptoms.
• Some people will have flu-like symptoms that include fever, chills, headache and aches and pains in the muscles and joints.
• Some joints can become swollen, and joint stiffness may be particularly noticeable in the morning.
• Sometimes a rash occurs on the body, arms or legs. The rash usually disappears after seven to 10 days.
• A general feeling of being unwell, tired or weak may also occur at times during the illness. This may affect work performance.

See also the Victorian Department of Health for further information.

Even though it is not a fatal disease, by all accounts it sounds very uncomfortable. There is no real “cure”: those contracting RRV have to “ride it out”.

RRv in Australia: past and present

Each year there are about 5000 cases reported, with outbreaks more generally associated with the tropics – but cases are reported all over Australia.

As the above map makes clear, incidences occur frequently up North. RRv was first documented in 1928, but the virus itself was only isolated in 1959:

 

The first documented outbreak of RRV occurred in 1928 in Narranderra and Hay in New South Wales with subsequent outbreaks described during World War II among troops in the Northern Territory and Queensland. The virus was isolated in 1959 from an Aedes vigilax mosquito along the Ross River near Townsville in Queensland, although it was not until 1985 that it was isolated from an Australian patient with polyarthritis. Outbreaks have since occurred in all Australian states, including Tasmania, and have occurred in metropolitan areas of Sydney (New South Wales), Perth (Western Australia) and Brisbane (Queensland). Most notifications are from Queensland, with high case rates also reported from Northern Territory and the Kimberley region in Western Australia.

However, as the climate changes, RRv may be on the move.

The connection between RRv and climate change

The rate of transmission of RRv is dependent on a number of factors, as this 2008 paper makes clear. However climate change is having an impact. As the authors note, increased humidity create the perfect conditions for increased populations of mosquitoes:

 

Relative humidity influences longevity, mating, dispersal, feeding behaviour, and oviposition of mosquitoes. At high humidity, mosquitoes generally survive for longer and disperse further; they have a greater chance of feeding on an infecting animal and surviving to transmit a virus to humans or other animals. Relative humidity also directly affects evaporation rates from vector breeding sites. Clearly, humidity is another factor contributing to outbreaks of RRv disease, particularly in normally arid regions.

And that climate change is playing a part:

As global warming continues, it is important to assess the potential public health consequences of such change, including its impact on the transmission of infectious diseases. The results of this study indicate that climate variability/change may influence the transmission cycles of RRv, and its impact appears to differ between coastline and inland regions. These findings may have implications in the development of public policy for mitigation and adoption of climate change.

An early warning sign of a changing climate

One of the anticipated impacts of climate change is the steady march south (and north) of tropical diseases. Indeed, their movement is regarded as an “early warning” sign. The Union of Concerned Scientists have noted:

Climate change affects the occurrence and spread of disease by impacting the population size and range of hosts and pathogens, the length of the transmission season, and the timing and intensity of outbreaks (McMichael, 1996; McMichael et al., 1996; Epstein et al., 1998; Epstein, 1999). In general, warmer temperatures and greater moisture will favor extensions of the geographical range and season for vector organisms such as insects, rodents, and snails. This in turn leads to an expansion of the zone of potential transmission for many vector-borne diseases, among them malaria, dengue fever, yellow fever, and some forms of viral encephalitis. Extreme weather events such as heavy rainfall or droughts often trigger disease outbreaks, especially in poorer regions where treatment and prevention measures may be inadequate.

Mosquitoes in particular are highly sensitive to temperature. The mosquitoes that can carry malaria (Anopheline spp.) generally do not develop or breed below about 16° C, and the variety that transmits dengue fever (Aedes aegypti) is limited by winter temperatures below 10° C. Mosquito survival also drops at their upper temperature threshold, about 40° C. With sufficient moisture, warmer temperatures will generally cause an increase in mosquito abundance, biting rates, and activity level, and will accelerate the incubation of the parasites and viruses within them.

Warmer global temperatures will allow an expansion of the geographic range within which both the mosquito and parasite could survive with sufficient abundance for sustained transmission. Model predictions indicate that a 3° C global temperature rise by 2100 could increase the number of annual malaria cases by 50-80 million (not considering factors such as local control measures or health services) (Martens et al., 1995). The largest changes will occur in areas adjacent to current risk areas, at both higher altitudes and latitudes. In these regions, a temperature increase can convert areas that are malaria-free into areas that experience seasonal epidemics. In many cases, the affected populations will have little or no immunity, so that epidemics could be characterized by high levels of sickness and death.

This is inline with model projections:

 

Recent disease outbreaks are consistent with model projections that warmer, wetter conditions will lead to greater transmission potential at higher altitudes and elevations. Mosquito-borne diseases are now reported at higher elevations than in the past at sites in Asia, Central Africa, and Latin America (Epstein et al., 1998).

Already, climate change has (most probably) brought malaria and other diseases back to Italy:

 

Sandwiched between temperate Europe and African heat, Italy is on the front line of climate change and is witnessing a rise in tropical diseases such as malaria and tick-borne encephalitis, a new report says.

Italy was declared free of malaria in 1970, but it is making a comeback, said the Italian environmental organisation Legambiente. Tick-borne encephalitis, a virus which attacks the nerve system, is also on the way back. While only 18 cases had been reported before 1993, 100 have been since, mostly around Venice.

“Illnesses are arriving from Africa, while tropical animals and plants are attacking our biodiversity, droughts and flooding are on the rise, and semi-desert areas are appearing,” said Legambiente’s director general, Francesco Ferrante.

A third ailment, visceral leishmaniasis, carried by sandflies and potentially fatal, is expanding rapidly, the report added. Cases in Italy have risen to 150 a year from 50 before 2000, with the southern region of Campania a hotspot.

Of six sustained droughts in Italy in the last 60 years four have occurred since 1990. The average temperature has increased by 0.4C in the north in 20 years and by 0.7C in the south. Ten million hectares “are at risk of desertification”.

Twenty percent of the fish now swimming in the Mediterranean, including barracuda, are types that have migrated from the Red Sea as water temperatures rise.

Italy’s combination of sea coast, mountains, deep valleys and plains gives rise to a rich variety of food products but climate change could tip the balance, Mr Ferrante said. “We are at the southern edge of the globe’s temperate area and that is why Italy is being particularly hit by the collapse of the climatic equilibrium.”

But of course, this is all a co-incidence.

* Title edited for clarity, hat tip to reader John Byatt