‘Climate change is already affecting vector-borne disease transmission and spread, and its impacts are likely to worsen. In the face of ongoing climate change, we must intensify efforts to prevent and control vector-borne diseases.’¹
JOACIM ROCKLöV & ROBERT DUBROW Dept of Public Health and Clinical Medicine, Umeå University
Tick and insect-borne diseases
Ectotherms do better in a warmer world. As average global temperatures exceed 1.2°C above pre-industrial levels, we can expect greater risks from arthropods, like ticks and mosquitos, in this changing climate.
Individual people usually have a preference for weather, including temperature, rainfall, and humidity. This is also the case for animals. Arthropods (e.g. insects and ticks) in particular will generally migrate to areas with their ideal climate.
As the planet continues to warm and we see more extreme weather events, the standard ‘climate’ of a region is altered. This effectively changes the suitable habitat for insects and ticks, either expanding or shrinking where they can live or just shifting its habitat to a different region entirely.
Often considered a nuisance, many arthropods actually pose a greater risk to people and to our animals as they can be vectors of disease.
With the changing climate we can expect that the distribution of some infectious diseases can increase, decrease or simply shift depending on the migration of these vectors. Irrespective, slight alterations in the abundance of the vector can cause large changes in disease.
Notable arthropods and their diseases
Tick bite paralysis- cats and dogs
Tick-bite paralysis in pets is a condition commonly caused by the eastern paralysis tick (Ixodes holocyclus). When dogs, cats, or even humans are bitten by this tick it can cause muscle paralysis, then respiratory dysfunction which, if not treated, can quickly become fatal.
This tick lives in hot areas with higher rainfall and it is predominantly found in the coastal regions of eastern Australia, from northern Queensland to Gippsland in Victoria. Prior to improvements in tick preventatives over the past decade, there had been an increase in case of tick paralysis and there are concerns that cases are now appearing in areas that have historically had few.
Babesiosis and theileriosis- horses, cattle, cats and dogs
Babesiosis and theileriosis are protozoal diseases transmitted by ticks to horses, ruminants, cats, dogs and humans. The protozoa cause haemolysis which can be fatal in peracute presentations. Chronic infections can cause persistent weight loss and anemia as well as predispose to laminitis, renal failure and infertility Most cases are found in tropical and subtropical Australia.
Ross River Virus - horses, humans
Ross River Virus (RRV) is the most common vector-borne disease of people in Australia. It is transmitted by mosquitoes and can cause clinic disease in both horses and humans. In horses it can cause significant muscle pain, joint pain, respiratory distress and it can involve neurological signs like ataxia and tremoring. While most horses survive, 10-15% can die directly from the disease or require euthanasia.
It is found across Australia, but more so in areas with greater rainfall and higher temperatures such as Queensland, Western Australia and the Northern Territory. However, RRV outbreaks have been noted to have increased recently with a greater incidence in areas where it had been absent previously.
Heartworm- cats and dogs
Dirofilaria immitis is a nematode transmitted by mosquitoes that can infect dogs and cats and cause heartworm disease. Infections can cause damage to blood vessels in the lungs which can lead to pulmonary hypertension, right-sided congestive heart failure and potentially a fatal pulmonary thromboembolism.
Given mosquitoes prefer hot, wet conditions, most cases are usually reported in Queensland and the Northern Territory but can occur in any state. Between 1970 and 1990, the prevalence of heartworm increased from 5% to 30% in New South Wales. While there haven’t yet been reports of changes in heartworm distribution in Australia, research in Europe and the US has already documented large increases in cases in areas where it is endemic and report it to be appearing in new locations.
The Culicoides midge is an important vector of bluetongue virus (BTV) which is usually a subclinical disease in cattle but can be severe in sheep. In sheep it is characterised by lameness, nasal discharge, significant swelling of the tongue and can reach mortality rates of 70 per cent in herds. Beyond the individual animal, outbreaks of BTV can have a significant impact on the livestock market- particularly the export trade.
BTV is currently endemic in areas of the Northern Territory, Queensland and Western Australia but is extending down into New South Wales as a result of different climatic conditions. This is attributed to the midge thriving in areas with hot and wet conditions.
Bovine ephemeral fever - cattle
Also known as ‘3-day-sickness’, bovine ephemeral fever (BEF) is thought to be transmitted by both midges and mosquitoes. It causes an acute onset of disease in cattle which is characterised by a high fever, nasal discharge, drooling and other non-specific findings. Although the mortality rate is low, outbreaks can cause considerable production losses.
Immediate milk drops can surpass 70% with full production often never regained, abortion, infertility, and paralysis have also been reported as a result of BEF. It is currently found in Western Australia, Queensland and New South Wales and is predicted to spread further south with the increases in temperature and rainfall.
Flystrike - sheep
While flies irritate most of us, they can cause dramatic disease in sheep. Flystrike (aka ovine cutaneous myiasis) is the result of flies (most commonly the Lucilia genus) feeding on the tissue of sheep. This usually happens in wet sheep in hot conditions. Flystrike is a major welfare concern and can cause death if left untreated. It also causes significant losses to farmers and it is estimated to cost the Australian sheep industry $280 million per annum.
There are two potential ways in which climate change can impact the incidence of flystrike:
1) increased temperature causing a longer fly season and a year-round flystrike risk period, or
(2) hot and dry conditions increasing the abundance of more pathogenic agents of myiasis
What can you do?
- Seek advice from your local veterinarian about how to best protect your animals from disease including: year-long prevention for heartworm and paralysis ticks, vaccination for BEF, using insect repellents and other management practices to reduce the impacts of mosquitos, midges, flies and ticks on domestic and farmed animals.
- Know the signs of important infectious diseases in your area and report any potential cases to your local veterinarian.
- Contact your local State or Federal MP to ask them to take greater action on climate change.
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- BLACK, P., MURRAY, J. & NUNN, M. 2008. Managing animal disease risk in Australia: the impact of climate change. Rev Sci Tech, 27, 563-580.
- MORCHÓN, R., MONTOYA-ALONSO, et al, 2022. What Has Happened to Heartworm Disease in Europe in the Last 10 Years? Pathogens, 11, 1042.
- NGUYEN, C., KOH, W. L., et al 2016. Mosquito-borne heartworm Dirofilaria immitis in dogs from Australia. Parasites & vectors, 9, 1-11.
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- Queensland Government 2023 Diseases Guide
- ROGERS, D.J. &ROBINSON, T.P. (2004) Tsetse Distribution. In: The Trypanosomiaises. (I. Maudlin, P. Holmes and M. Miles, eds.). Waingford: CABI Publishing. pp 624Rogers, D.J. and Robinson, T.P. (2004) Tsetse Distribution. In: The Trypanosomiaises. (I. Maudlin, P. Holmes and M. Miles, eds.). Waingford: CABI Publishing. pp 624
- SZÉLL, Z., BACSADI, Á., SZEREDI, et al T. 2020. Rapid spread and emergence of heartworm resulting from climate and climate-driven ecological changes in Hungary. Veterinary parasitology, 280, 109067.
- WALKER, P. J. & KLEMENT, E. 2015. Epidemiology and control of bovine ephemeral fever. Veterinary research, 46, 1-19.
- WALL, R., 2012. Ovine cutaneous myiasis: effects on production and control. Veterinary Parasitology, 189(1), pp.44-51.
- YU, W., DALE, P., TURNER, L. & TONG, S. 2014. Projecting the impact of climate change on the transmission of Ross River virus: methodological challenges and research needs. Epidemiology & Infection, 142, 2013-2023.