Bird flu, mosquitoes and blowflies

A notice from ProMed yesterday alerted many of us to a new published report [subscription firewall] about H5N1 influenza detection in an arthropod species in the vicinity of an infected poultry farm. The arthropods were mosquitoes (Culex tritaeniorhynchus) in Thailand. Two years ago a similar report implicated blowflies (Calliphora nigribarbis and Aldrichina grahami) near some infected farms in Kyoto, Japan. Both papers suggested using arthropods near infected farms as surveillance tools. But both, especially the Japanese paper, raised the open question whether arthropods might play a part in transmission. I took at look at both papers.

The most recent paper (Thailand) examined mosquitoes trapped at a poultry farm in October 2005, a day before the farm reported an outbreak of H5N1. Females need a blood meal to ovulate so 148 engorged female mosquitoes were tested for H5N1 via RT-PCR. H5N1 identical to the Thai chicken strain were found in engorged females but not in females not gorged with blood. So the mosquitoes seemed to have picked up the virus after taking a blood meal, presumably from infected poultry on the farm. We don't know if the virus replicates in the mosquito or is just being carried mechanically. As one way to test whether mosquitoes can be infected, the authors inoculated a mosquito cell culture line, designated C6/36. These cells are not from the same species as found here (they are from Aedes albopictus) and the virus didn't come from the poultry but from a cloacal swab from a dead Asian open-billed stork found in 2004 (location not given). The virus had been previously propagated in dog kidney cells and it isn't clear if it was different in host range than the straight poultry-derived virus. But H5N1 did grow in this mosquito cell-line. What we know for sure is that detectable virus was in mosquito gut and able to persist in replicable form at least for hours. Mosquitoes have never been implicated in influenza transmission, either as mechanical "flying needles" or after additional replication and amplification. For them to be effective vectors they would have to deliver an infectious dose in a way that evades the usual host defenses.

The Japanese blow fly study is a bit more interesting. Four outbreaks of bird flu occurred in Japan in 2003 and 2004 and two farms near Kyoto were involved in the last two of these, in March 2004. Domestic flies (Musca domestica) are well known mechanical transporters of a wide variety of pathogens, but March is too cold in Kyoto for domestic flies. But at a team of medical entomologists in Japan wanted to know if other fly species might also be capable of transporting H5N1 virus. They set up traps within a 2.5 km radius of one of the infected farms and collected flies. One finding was that poultry farms attract flies. The closer to the farms the higher the fly density, but even more than 2 km away there were flies. Eight different species were trapped, none of them Musca domestica. More than 80% were the two blowfly species. These are pretty big critters, more than a centimeter long and they ingest the feces and secretions of infected birds. So it was not that much of a surprise to find essentially identical virus in the intestinal tracts of the blowflies as in the chickens and some infected crows from the same area. Since chickens and wildbirds also eat blowflies, the question as to the role of blowflies in spread of the virus on these poultry farms was reasonable. Reasonable but not demonstrated. So this is all we know at the moment. Blow flies are big and serve as food for birds but they are not biters and humans don't eat them. So they are very unlikely to be a vector for spread of H5N1 from birds to people. Still, the bird to bird question remains open.

Finding pathogens on arthropods isn't unusual. Decades ago I taught a course in environmental health and happened to have a microbiologist as a student. We decided to go to the kitchen of the hospital cafeteria and grab us a couple of cockroaches (the big "water bugs," Periplaneta americana). After throwing them in a blender and plating the stuff out on nutrient medium a lot of stuff grew out of these guys, including typical bacterial enteric pathogens like Salmonella spp. and E. coli). But it is rare in the medical literature to find any outbreaks traced to cockroaches. They've got the pathogens on them but they aren't a significant mode of transmission.

For people I would surmise it is the same. For spread between birds and poultry operations I think the verdict is out, although again I would guess this is a minor mode of spread compared to human and poultry movements between farms (with maybe some wild bird contribution).

There is still very little information about this, so we'll have to see what comes of this (if anything). Meanwhile I'd be much more worried about West Nile Virus, Eastern Equine Encephalitis, or in many areas, malaria from a mosquito bite. H5N1? Maybe, but doubtful.