The half-packed suitcase sits open on the bed, its owner reluctant to bring their Asian holiday to a close. The last of the packing can wait until after lunch and a final trip to the markets before heading to the airport.
A black and white banded mosquito that is sharing the accommodation has already had ‘lunch’. Sated with a meal of blood possibly infected with a nasty virus, she settles in a quiet, dark corner of the suitcase, ready to hitch a ride to a far off place called Australia.
By the time she’s arrived in her new home, it will be time for another meal. This time, as well as drawing blood, she will also pass on the virus, infecting her unwitting travel companion, a family member, a house-mate, and/or a neighbour.
It’s actually a well-recognised phenomenon called ‘luggage transmission’ and it occurs more often that you might think.
In fact, experts like Sydney-based entomologist, Dr Cameron Webb believe luggage transmission is the most likely explanation for at least some of the ‘locally acquired’ cases of mosquito-borne disease.
Tiny ‘stowaways’ most likely culprits
“We’re seeing cases of dengue where the patient has no history of travel and there are no known local populations of mosquitoes that can spread the virus,” he said
“As unlikely as it sounds, an infected stowaway mosquito is the most likely culprit.”
‘Locally acquired’ cases usually begin when travellers are infected overseas and come home to be bitten by Aedes aegypti mosquitoes breeding in or around their homes. In turn, the newly infected mosquito eventually infects the traveller’s family members or neighbours.
In Australia, most of the 1824 dengue virus infections and the 129 caused by Chikungunya last year were ‘imported’ by travellers, mainly from Asia. In North Queensland, where Ae. aegypti populations are firmly established, imported cases lead to local outbreaks – involving many hundreds of cases in some years.
However, ‘luggage transmission’ sees an already-infected mosquito simply emerge from luggage to pass on the virus to the first person it encounters.
Once an Ae. aegypti or Ae. albopictus female becomes infected with dengue, Chikungunya or one of the other pathogens she might be carrying, she will continue to pass on the infection several times throughout her entire life cycle.
Luggage transmission is similar to ‘airport transmission’, where infected mozzies emerge from the cargo bays of planes and infect airport workers or residents living in nearby streets.
Many cases of luggage transmission
Here are just 3 cases suspected of being luggage-transmitted.
CASE 1: A man travelled from a West African country to Paris last year. He unpacked his bag for the first time since leaving home in a friend’s room at a hostel. Soon after the visitor left Paris his friend, who had no recent history of overseas travel, was diagnosed with Plasmodium falciparum malaria. Read more.
CASE 2: A 51-year-old German woman contracts dengue fever last August while picking grapes at a vineyard outside the Japanese city of Fuefuki during a 2-week holiday spent exclusively in Japan. The woman’s infection was the first case of dengue in Japan since the 1940s, nor have any other cases been reported there since her case. Read more.
CASE 3: A man from the Western Australian coastal town of Point Samson contracted dengue last October, despite not having left the country in several decades. It was the state's first locally acquired case in more than 70 years. As Ae. aegypti aren’t found in the area, authorities concluded he was bitten by a mosquito which either arrived in luggage, on a boat from Asia, or even a caravan from far North Queensland, where outbreaks occur each summer. Read more.
In the French case, luggage transmission is the standout likely cause: Indeed, it is the only feasible explanation as France lacks populations of mosquitoes which transmit malaria, and the man had not left the country for some years. As for the second and third cases, Dr Webb believes luggage transmission is the most likely explanation.
Aedes species prefer our blood
“The reason luggage transmission happens, especially with dengue and Chikungunya, is because the two vector mosquitoes are found almost exclusively in and around the places humans live,” he said.
“They need a blood meal to lay their eggs and they actually prefer human blood. This intimate relationship with humans is the reason for the large-scale dengue and Chikungunya epidemics we’re seeing in Asia and the Pacific – in fact, throughout the tropical world.
“They simply take refuge where it’s dark and humid. An open suitcase provides exactly the conditions they prefer.
“In the case of a one-off dengue infection in Japan, all the mosquito has to do is survive a short journey from Asia. She arrives inside the luggage of another traveller who most likely flew into Japan from an Asia destination and when their bag is opened, perhaps in the accommodation provided for the grape pickers, she bites the unlucky German traveller.
“The only alternative is for an infected person to arrive and get bitten by one or more of the local mosquitoes. And, that’s assuming that there are suitable vector mosquitoes in that region of Japan.
“Even given a vector species is present, the mosquitoes would have to take that blood meal, and survive in a new environment for a week before they become infective for dengue and can pass on the virus to humans.
“Of course, that’s possible, but I think that’s a far less likely scenario than having a mosquito survive a short trip from Asia in a traveller’s bag.”
Baggage class Tiger could settle in south, too
Dr Webb fears luggage transmission could very well be one of the ways the Asian Tiger mosquito will ultimately reach Australia – for good.
“I think mosquitoes hitch a ride with travellers in bags and vehicles pretty frequently,” he said.
“I can’t help but think how lucky we’ve been that we haven’t already had the Asian Tiger mosquito become established in various parts of Australia.
“If the only times we find out about them are when people are actually infected with the virus, there’s probably many other times when a mosquito isn’t infected and they just fly out the window, maybe to bite dogs and cats. It’s miraculous that there haven’t been little spot fires of them springing up – especially across northern parts of the country.”
Dr Webb believes the Ae. albopictus will ultimately arrive here through a ‘back door’. In fact, the temperate climate-tolerant species could find conditions in Australia’s southern cities to its liking, where as Ae. aegypti prefers the tropics.
While a concerted quarantine program was underway in the Torres Strait in 2012 to prevent the species from establishing itself on the northern tip of the Australian mainland, an ambush of Tigers very nearly slipped into Melbourne in a shipment of Lucky Bamboo pot plants imported from Asia.
“It was incredibly lucky that someone spotted these mosquitoes during the secondary quarantine for the plants in a glasshouse very close to a market garden and they were destroyed,” Dr Webb said.
“The garden would have provided any number of places for the mosquitoes to lay eggs.
“I just hope we can keep them at bay for a while yet.”
- by Laurie Sullivan