By Dr Eddy Bajrovic*
A Victorian man who holidayed in Bali recently has been hospitalised with the potentially fatal mosquito-borne virus, Japanese encephalitis (JE).
On the face of it, the 45-year-old wasn’t at any particular risk of this most Asian of diseases.
He had a typical short stay: a week in a villa in Canggu, which is just 15 minutes north from the holiday hotspot of Seminyak, in Bali’s southwest.
We’re told he didn’t spend any time in rice-growing areas, where most JE outbreaks occur. (Wading birds in rice fields and pigs serve as reservoirs, or ‘amplifying hosts’, in the virus’ transmission cycle. Mosquitoes transfer it between the two animal species before passing it on to humans, who are termed ‘dead-end hosts’ because the virus doesn’t pass from one human to another via mozzie bites.)
Indeed, the Victorian hardly left his villa, according to the state’s senior medical advisor, Dr Finn Romanes, whose case report was published on ProMED
JE cases are very rare
It seems he was just extraordinarily unlucky: His is the first ever case recorded in Victoria, while only 9 cases of JE having been reported in Australia since 2001 – most of them in Cape York Peninsula and the Torres Strait Islands. Worldwide, less than 70 have been recorded among travellers in the last 40 years.
In recent years a German woman who visited Bali for 2 weeks was infected with the virus in 2011, while in 2013 two 20-year-old men – one from Spain, the other from China – were infected during 3 week stays in different parts of Thailand. All 3 recovered fully after treatment in hospital.
None of the trio’s itineraries seemed to hold any particular risk of JE: no visits to villages with rice fields or pig farming – just typical tourist activities firmly on the proverbial ‘beaten track’.
The imperative of insect bite avoidance
So, should Australians planning to visit Bali or anywhere else in Asia where Japanese encephalitis occurs be concerned? The answer is yes and no.
No because for a traveller on a typical short holiday in Asia, the risk of getting JE is clearly so low that vaccination is very rarely recommended. The vaccine is more likely to be recommended for someone working in, staying in, or travelling extensively through, rice-growing rural areas for more than a month, especially in the wet season.
Yes because the Victorian case demonstrates that no-one travelling to a tropical destination – even well-travelled ones like Bali – can afford to be blasé about avoiding mosquito bites. Not anywhere. Not any time.
In fact, the Victorian was far more likely to have contracted either of the two better known mosquito-borne viruses, dengue or Chikungunya. While they rarely result in severe illness – especially among travellers – both are much more common than JE. As well as being found throughout Bali and the rest of Indonesia, the 2 viruses are widespread across vast areas of tropical and subtropical Asia, the Pacific, Africa, South and Central America, and the Caribbean.
The rise and rise of dengue and Chikungunya
There are an estimated 50 million cases and 22,000 deaths in 100 dengue-infected countries across the globe each year, according to the WHO.
Firm Chikungunya numbers are harder to find, but it has been associated with massive epidemics since the 1990s. In the past 14 months, there have been 1.2 million reported cases (and many more unreported) along with 181 deaths across the Caribbean and Latin America. In our region, there have been a conservative 60,000 cases and 14 deaths in French Polynesia, Samoa and many smaller island nations in the western Pacific in the past 6 months.
Last year, 1563 Australians returned from the tropics with dengue (although some of the 394 cases reported in Queensland were acquired locally in the state’s north), while Chikungunya was a very unpleasant holiday memento for another 103. The trend looks set to continue this year: we’ve had 265 cases of dengue and 22 of Chikungunya to date, according to national surveillance figures.
City slickers and country cousins
The reason for the high numbers is that the two types of Aedes mosquitoes that spread dengue and Chikungunya (and yellow fever in regions of Africa and South America) are urban dwellers.
They breed and feed in and around the places where they can most easily access their favourite food, human blood, which the females need to lay eggs.
Conversely, the species that spread JE (Culex) and malaria (Anopheles) are usually (but, clearly, not always) found in rural areas and bite mainly between dusk and dawn, so fewer travellers are infected. (I should add that there are many malaria-infected countries where cases occur in urban areas – even in major cities.)
Rare, but potentially severe outcome
An arbovirus, JE is closely related to other mosquito-borne viruses like West Nile, St Louis encephalitis, yellow fever, and dengue fever.
Most cases occur among local people living in rice-producing areas of China, the Indian subcontinent, and Southeast Asia. More recently JE has become established in Papua New Guinea and the Torres Strait Islands, with the occasional case in Northern Australia.
Most JE infections are mild: usually a slight fever and a headache. But, for around 1-in-100 victims, the infection is severe and statistics show that 20-30% of these patients will die while 30% - 50% will be left with a severe neurological disability.
Because so few returned travellers show signs of illness – unlike the luckless Victorian – JE’s incidence is almost certainly under reported here and elsewhere around the world.
Those at higher risk of JE
Inevitably, cases like the Victorian one fuel the vaccination question: Should short term travellers to Asian holiday resorts such as Bali be vaccinated?
The fringes of holiday resort areas and major towns and cities in Asia often do have rice paddy fields and pigs – including Seminyak. So, we shouldn’t be surprised that the JE virus is present in mosquitoes in these areas.
Typically, the vaccine is recommended for backpackers passing through regional areas for more than a month, for rural-based aid workers, and expats living or working in an endemic region for extended periods.
However, because travellers have been infected after shorter periods, the Australian Immunisation Handbook now recommends that doctors consider advising vaccination if:
– Travel is taking place during the wet season, and/or there is considerable outdoor activity, and/or the accommodation may not be mosquito-proof.
– The traveller is spending a year or more in Asia (except Singapore) – even if much of their time will be spent in urban areas, and
– The itinerary includes a month or more in Papua New Guinea, particularly during the wet season.
Pre-travel conversation has changed
There are two very effective vaccines against Japanese encephalitis for those at sufficiently high risk to warrant that level of protection or who simply chose to be vaccinated even if not at high risk. One of these vaccines (Imojev) affords protection for at least 5 years.
However, short-stay cases have changed the conversation doctors are having with patients during pre-travel medicals.
While the focus is still on a risk assessment and the full range of bite protection measures, the length of the trip is no longer the main consideration for vaccination. Increasingly, it is being discussed in terms of a long-term ‘investment’ in healthy travel, particularly for more the adventurous who regularly head off the beaten track in Asia, as well as those who expect to continue travelling there regularlyfor leisure or work and want the extra protection (and peace of mind) immunisation offers.
If that sound like you, discuss the merits of vaccination with your travel doctor at least 6 weeks before departure.
* Dr Bajrovic is Medical Director of Travelvax Australia.
Heading to Asia? Learn more about the risks of Japanese encephalitis and the potential travel health issues for your trip by calling the Travelvax Australia travel health advisory service on 1300 360 164 for the latest country-specific information.