Avian
influenza has taken a great toll on the human population and on the poultry
industry over much of the world. One aspect that has occupied scientist is the
possible method of transmission, of which there appear to be many. Wild birds
have received much oh the attention until now but studies shows that flies can
be virus carriers too (Dr Terry Mabbett)
Poultry farms and houseflies need no introduction.
Universal infestation by Musca spp. and
close relatives is part and parcel of livestock production, especially in hot
climates. Musca domestica (common
houseply) is a dipterous insect of cosmopolitan distribution and versatile
within its environment, feeding and breeding on all kinds of organic matter
including food and animal feed, garbage, faeces, sewage and animal carcasses.
Houseflies constantly move between ‘dirty’ areas of
putrefaction teeming with pathogens and ‘clean’ areas including feed storage
and animal housing. As such they are a major source of disease and implicated
in the transmission of over 30 different diseases caused by bacterial,
protozoan and virus parasites.
Adult flies carry disease
Houseflies breeding in media brimming with bacteria,
including pathogens, might be expected to convey at least some from the larval
(maggot) stage to the adult fly emerging from the pupa case. But early research
showed the hostile chemical nature of the larva gut and antagonism by gut
microflora maintained a generally low level of pathogens. More than one-fifth
of houseflies and almost tow-fifth of green bottle blowflies (Lucilia spp) were sterile on emergence.
Hazards are focused on housefly feeding, sucking up
liquid from putrefying food and faeces supporting high concentrations of
pathogens. Transmission via vomit drops presents risks, but adult houseflies
vomit originates from sugary fluids stored in the crop and presents a
correspondingly lower risk than excretory deposits. Research which showed Salmonella typhimurium multiplying in
the mid-and hind-gut and passing out intermittenly over an interval of at least
one week lent huge weight to these arguments.
Houseflies are well established vectors of food
poisoning bacteria like Salmonella spp.
and Eschericia coli harboured by
birds. More recently, transmission of poultry virus diseases like Newcastle
disease and avian influenza (AI) by houseflies is considered, in addition to
spread by direct contact by contaminated faeces and bird secretions.
Houseflies carry virus disease particles
Recent research at North Carolina State University has
shed new light on the role of Musca
domestica in the potential transmission of Newcastle disease virus (NDV).
Adult flies carried an infectious dose in the gut for three hours after feeding
and researchers Drs Wes Dawson and James Guy considered this might be important
for spread of the virus when fly populations are high and in contact with
highly virulent velogenic NDV strains.
Newcastle disease virus is dedicated disease of
poultry but AI is now under the spotlight because the H5N1 highly pathogenic
strain is a zoonosis transmitted to human from animal. The march of H5N1 has
been far and fast leaving precious little time to study infection and spread in
detail. H5N1 virus spread by wild birds is a relatively recent focus since the
Qinghai strain H5N1 appeared in wild fowl in western China during may 2005 and
subsequently sped all the way to Western Europe and West Africa in just six
month.
There are several reported instances going back 20
years of houseflies carrying the AI virus and suspected as a mode of
transmission. In 2005, they were highlighted and summarised by scientists from
Novartis Animal Health as part of an article reinforcing the importance of good
fly control on poultry farm.
Following reports in 1985 of houseflies in poultry houses
contaminated with AI, a detailed study was presented at a conference in
Australia in the following year. The study was based on a serious 1983/4
outbreak of H5N2 in Lacaster Country, Pennsylvania, USA. Up to 90% of affected
flocks died and various modes of transmission, including direct contact between
birds, mechanical vectors and vector insects and especially houseflies, were
considered.
Fifteen different insect species-mainly flies and
beetles- were collected in 300+ species-specific samples, each containing 10-60
insects. More than one-third of the adult Musca
Domestica samples contained AI virus particles, as did one-third of samples
comprising less abundant fly species like Ophyra
(dump flies) and Coproica (dung
flies).
During the 2004 H5N1 outbreak, Asian scientists
identified the virus in blowflies caught near a poultry farm in Kyoto in
western Japan, which had experienced a disease outbreak in the preceding
months.
Parallels with West Nile virus
If the AI virus is present in Musca Domestica or other flies, it does not necessarily mean
transmission to poultry, let alone extra risk of human infection. However, the
possibility of spread by flies does open a whole new dimension on this virus
disease. A comparison can be drawn with West Nile virus (WNV), a flavivirus
causing disease in wild birds, horses and humans and transmitted amongst theese
three groups by the blood-sucking activity of mosquitoes, mainly Culex spp
West Nile virus is a zoonosis and an arbovirus, a
virus particle transmitted by an arthropod animal such as an insect, mite or
tick. This efficient means of West Nile virus transmission between humans, wild
birds and horses by airborne blood-sucking insects has facilitated a
breathtaking speed of spread. From a single case in the West Nile region of
Uganda in 1973, the virus now threatens many parts of the world and spread
right across North America from New York to California in just three years (1999
– 2002).
That the AI virus can be spread by winged insects as
well as wild birds underlines the need for efficient fly control on poultry
farm, along with other stric biosecurity measures.
Dr.
Terry Mabbet, Potters Bar
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