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 housefly) 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 viral 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 larval gut and antagonism by gut microflora maintained a generally low level of pathogens. More than one-fifth of houseflies and almost two-fifths of green bottle blowflies (Lucilia spp) were sterile on emergence.

The main risk of disease spread is from adult houseflies contaminated with bacteria, protozoa and virus particles. Houseflies could theoretically transmit pathogens on their feet but opportunities for attachment to such small surface areas are considered tiny and microbes are susceptible to desiccation.

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 housefly 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 intermittently 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 Escherichia 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 a dedicated disease of poultry but AI is now under the spotlight because the H5N1 highly pathogenic strain is a zoonosis transmitted to humans from animals. 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 of 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 months.

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 farms.

Following reports in 1985 of house flies 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 Lancaster County, 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 these three groups by the blood-sucking activity of mosquitoes, mainly Culex spp.

WNV 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 WNV 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 1937, the virus now threatens many parts of the world and spread right across North America from New York State 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 farms, along with other strict biosecurity measures.