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H5N1 HPAI in Different Species

3.6 Mammals and H5N1 HPAI

Cases of H5N1 HPAI have been recorded in cats (both domestic cats and captive wild felids) in areas where infection was present in poultry. Some onward transmission of virus has occurred among kittens in close contact experimentally, suggesting that virus could persist in a litter for at least a limited period (Rimmelzwaan et al, 2006). The role of cats in transmission of virus to humans or other species remains to be determined. However, as long as a cycle of infection does not become established in these animals, their role will remain secondary to that of infected poultry. The possibility of endemic infection in cats in Indonesia warrants further examination.

There is no evidence to suggest that H5N1 HPAI viruses are adapted to swine (Choi et al, 2005), although there have been isolated incidents in which pigs were found to be infected (Li et al, 2004b). At this stage, pigs should be monitored in areas where disease is occurring in poultry and ongoing serosurveillance of pigs at slaughter should be conducted to ensure that H5N1 viruses have not become established in this species.

As discussed in Chapter 2, H9N2 avian influenza viruses have been detected frequently in pigs (Peiris et al, 2001; Chen et al, 2006b) in China and theoretically pose a greater risk of spread to humans than H5N1 HPAI viruses given that they have already adapted to infect a mammalian host.

Cases of HPAI infection have been recorded in a number of other animal species, both naturally and experimentally, including laboratory mice and ferrets (Maines et al, 2005), Owston’s civet (Roberton et al, 2006), stone marten (WHO, 2006) and dogs (Thiry et al, 2007). As with cats, these all remain potential vectors of spread, but unless independent cycles of infection develop in any of these species the control of the disease in poultry will also limit their importance in spread of Asian-lineage H5N1 HPAI viruses.

3.6.1 Transmission of Asian-linage H5N1 avian influenza viruses to humans

The transmission of HPAI viruses from birds to humans is a serious public health concern and has been reviewed in depth elsewhere (see for example Peiris et al, 2007). Although Asian-lineage H5N1 avian influenza viruses pose a potentially serious threat of converting to a human pandemic strain, they are not yet easily transmissible to and between humans. They do not replicate efficiently in humans, although some subtypes of avian influenza can replicate within the human respiratory tract and cause disease. As of 14 December 2007, there had been 340 confirmed clinical cases of H5N1 influenza, of which 208 had been fatal (WHO, 2007). This probably underestimates the true number of cases but given the very large number of humans exposed to H5N1 HPAI viruses over the past 11 years, it is apparent that the attack rate for this disease is extremely low (i.e. many exposures but few cases of infection/disease). For example, studies in Nigeria suggest that despite considerable circulation of virus, no seroconversion in humans has been detected in high-risk occupational groups (Ortiz et al 2007), although there has been one confirmed fatal human case in that country.

Across Asia, most human cases have occurred in places where poultry are kept and where, usually, sick poultry are detected concurrently. Apart from a few instances where human-to-human transmission may have occurred (Kandun et al, 2006; Yang et al, 2007), most human cases have a probable link to infected poultry (Peiris et al, 2007).

Case control studies for human cases of HPAI have been performed in a number of locations, including Hong Kong SAR, Indonesia, Thailand and Viet Nam. The case control study in Thailand suggested that contact with inexplicably dead poultry was the strongest risk factor (Areechokchai et al, 2006). This differed from the situation in Hong Kong SAR in 1997 where a visit to a market selling live poultry was found to be a significant risk factor (Mounts et al, 1999), although it is now known that the live-poultry markets in Hong Kong were heavily infected at that time.

Much remains unknown about the factors that determine susceptibility to infection after exposure in humans. The possibility of a genetic determinant of susceptibility based on clusters in blood relatives has been suggested by some, although so far there is little evidence to support this (Pitzer et al, 2007). It is an area that warrants further exploration.

At present, the main role of humans in the spread of avian influenza is through activities related to the production and marketing of poultry. These are discussed in detail in Chapter 4.

If a human pandemic strain of influenza virus emerges (one capable of sustained human-to-human transmission) from an avian influenza virus, poultry will likely become largely irrelevant in the spread of the virus, which will occur instead through movement of infected people.

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