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RAIN, FLOODING AND JAPANESE ENCEPHALITIS

Using a granular IGR to tackle mosquito populations can give significant performance benefits, especially when pest pressure is very high.

With the continued rains and flooding, mosquito numbers have exploded. With increasing mosquito numbers comes the increased risk of mosquito-borne diseases. In particular, experts are concerned about the rise in cases of Japanese encephalitis. So what is Japanese encephalitis? What actions should homeowners and businesses take to protect themselves? And what mosquito treatments are most effective?

Japanese encephalitis is caused by the Japanese encephalitis (JE) virus and is spread through mosquito bites. JE virus belongs to a group of mosquito-transmitted viruses that can cause inflammation of the brain (encephalitis). The main hosts of the virus are pigs and wading birds. Although it can get passed on to humans through mosquitoes, humans are considered a dead-end host, as the virus does not develop into high enough concentrations to re-infect mosquitoes.

In Australia, Japanese encephalitis is normally confined to the tropical north – Cape York, the Torres Strait and the Tiwi Islands. However, with the continued rains and flooding across much of Australia, the resulting wetlands have drawn migratory birds from the tropics across much of inland Australia. Along with the resulting explosion in mosquito populations, the JE virus has been transferred to pigs. JE virus has now been detected in more than 70 piggeries across four states – QLD, NSW, VIC and SA.

With the increasing virus load in the pig and wading bird populations, Japanese encephalitis has become a risk in areas of Australia where JE virus was previously unknown.

Although there may be large numbers of infected mosquitoes in hotspots, representing a significant risk of receiving a bite from an infected mosquito, the ‘good’ news is that less than 1% of people infected with the JE virus develop severe symptoms. As of October 19, 2022 there have been 42 cases of Japanese encephalitis across the NT, NSW, QLD, VIC and SA, with seven deaths.

Most people are asymptomatic or only develop mild symptoms, such as a headache or fever. However, the unfortunate few that go on to develop encephalitis will also become tired and develop nausea and diarrhoea. As the inflammation develops, confusion, seizures and abnormal movements may occur, sometimes resulting in permanent brain damage and death. Symptoms develop within 5-10 days after a mosquito bite and the presence of the virus can be detected by a blood or spinal fluid test. There is no specific treatment for Japanese encephalitis and severe symptomatic cases need to be hospitalised. However, a vaccine for the JE virus exists and it may be an option for those in hotspots or working on farms with infected pigs.

In Australia the main disease vectors are Culex mosquitoes, in particular Culex annulirostris and Culex quinquefasciatus. Both carry a range of other viruses, such as Murray Valley encephalitis, Barmah Forest virus and Ross River virus. Culex annulirostris is typically found in inland riverine areas and Culex quinquefasciatus, which breeds in stagnant water bodies, is often the concern around piggeries.

 

Any transient puddle that lasts for up to 2 weeks could be a potential mosquito breeding site

 

People in areas of concern should wear long-sleeved shirts and trousers, and use personal insect repellent when outside to reduce the chances of getting bitten. Pest managers can recommend a range of actions to help reduce mosquito populations around homes, businesses and farms.

As with managing most mosquito problems, taking actions to eliminate as many breeding sites as possible is a great first step. However, with the continued rains this is a big challenge! As such, the treatment of water bodies with insect growth regulators to break the mosquito breeding site is a great option.

Sumilarv Granular insecticide from Sumitomo with its pre-dosed sachets is easily applied to small water bodies around homes and farms. However, for larger water bodies Sumitomo also has a range of larvicides for broadcast treatment – Vectobac (Bti larvicide), Vectoprime (Bti + s-methoprene and Vectolex (Bs larvicide). Vectolex is ideal for high organic matter water bodies, so could be a good option for farms.

 

Sumilarv Granular

 

Mosquito larvicides can be very effective in reducing mosquito populations. The pyriproxfen in Sumilarv is active at extremely low levels, with a LC50 as low as 0.012 parts per billion on Aedes aegypti (Sichuincha et al., 2005). Coupled with the phenomenon of auto dissemination – where females mosquitoes visiting treated-water bodies transfer pyriproxyfen to untreatedwater bodies – a Sumilarv treatment can effectively treat additional water bodies beyond the initial treatment area.

A word of warning with respect to insecticide treatments in piggeries: application of chemicals needs to be carefully considered due to the potential impact of pigs absorbing traces of chemicals resulting in measurable insecticide residues in pork, which can have a significant impact on trade. There are no residue issues with any of the Sumitomo larvicide products when used in accordance with the labels.

 

Reference: Sichuincha et al. (2005). Potential use of pyriproxyfen for control of Aedes aegypti (Diptera: Culicidae) in Iquitos, Peru. J. Med. Entomol. 42 (4): 620-630.