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Avian influenza outbreaks have been occurring on smallholder poultry farms in Asia for two decades. Farmer responses to these outbreaks can slow down or accelerate virus transmission. We used a longitudinal survey of 53 small-scale chicken farms in southern Vietnam to investigate the impact of outbreaks with disease-induced mortality on harvest rate, vaccination, and disinfection behaviors. We found that in small broiler flocks (≤16 birds/flock) the estimated probability of harvest was 56% higher when an outbreak occurred, and 214% higher if an outbreak with sudden deaths occurred in the same month. Vaccination and disinfection were strongly and positively correlated with the number of birds. Small-scale farmers – the overwhelming majority of poultry producers in low-income countries – tend to rely on rapid sale of birds to mitigate losses from diseases. As depopulated birds are sent to markets or trading networks, this reactive behavior has the potential to enhance onward transmission. The past few decades have seen the circulation of avian influenza viruses increase in domesticated poultry, regularly creating outbreaks associated with heavy economic loss. In addition, these viruses can sometimes ‘jump’ into humans, potentially allowing new diseases – including pandemics – to emerge. The Mekong river delta, in southern Vietnam, is one of the regions with the highest circulation of avian influenza. There, a large number of farmers practice poultry farming on a small scale, with limited investments in disease prevention such as vaccination or disinfection. Yet, it was unclear how the emergence of an outbreak could change the behavior of farmers. To learn more, Delabouglise et al. monitored 53 poultry farms, with fewer than 1000 chickens per farm, monthly for over a year and a half. In particular, they tracked when outbreaks occurred on each farm, and how farmers reacted. Overall, poultry farms with more than 17 chickens were more likely to vaccinate their animals and use disinfection practices than smaller farms. However, disease outbreaks did not affect vaccination or disinfection practices. When an outbreak occurred, farmers with fewer than 17 chickens tended to sell their animals earlier. For instance, they were 214% more likely to send their animals to market if an outbreak with sudden deaths occurred that month. Even if they do not make as much money selling immature individuals, this strategy may allow them to mitigate economical loss: they can sell animals that may die soon, saving on feeding costs and potentially avoiding further contamination. However, as animals were often sold alive in markets or to itinerant sellers, this practice increases the risk of spreading diseases further along the trade circuits. These data could be most useful to regional animal health authorities, which have detailed knowledge of local farming systems and personal connections in the communities where they work. This can allow them to effect change. They could work with small poultry farmers to encourage them to adopt efficient disease management strategies. Ultimately, this could help control the spread of avian influenza viruses, and potentially help to avoid future pandemics.

Background Poultry farming is widely practiced by rural households in Vietnam and the vast majority of domestic birds are kept on small household farms. However, smallholder poultry production is constrained by several issues such as infectious diseases, including avian influenza viruses whose circulation remains a threat to public health. This observational study describes the demographic structure and dynamics of small-scale poultry farms of the Mekong river delta region. Method Fifty three farms were monitored over a 20-month period, with farm sizes, species, age, arrival/departure of poultry, and farm management practices recorded monthly. Results Median flock population sizes were 16 for chickens (IQR: 10–40), 32 for ducks (IQR: 18–101) and 11 for Muscovy ducks (IQR: 7–18); farm size distributions for the three species were heavily right-skewed. Muscovy ducks were kept for long periods and outdoors, while chickens and ducks were farmed indoors or in pens. Ducks had a markedly higher removal rate (broilers: 0.14/week; layer/breeders: 0.05/week) than chickens and Muscovy ducks (broilers: 0.07/week; layer/breeders: 0.01–0.02/week) and a higher degree of specialization resulting in a substantially shorter life span. The rate of mortality due to disease did not differ much among species, with birds being less likely to die from disease at older ages, but frequency of disease symptoms differed by species. Time series of disease-associated mortality were correlated with population size for Muscovy ducks (Kendall’s coefficient τ = 0.49, p -value < 0.01) and with frequency of outdoor grazing for ducks (τ = 0.33, p-value = 0.05). Conclusion The study highlights some challenges to disease control in small-scale multispecies poultry farms. The rate of interspecific contact and overlap between flocks of different ages is high, making small-scale farms a suitable environment for pathogens circulation. Muscovy ducks are farmed outdoors with little investment in biosecurity and few inter-farm movements. Ducks and chickens are more at-risk of introduction of pathogens through movements of birds from one farm to another. Ducks are farmed in large flocks with high turnover and, as a result, are more vulnerable to disease spread and require a higher vaccination coverage to maintain herd immunity.

Avian influenza outbreaks have been occurring on smallholder poultry farms in Asia for two decades. Farmer responses to these outbreaks can slow down or accelerate virus transmission. We used a longitudinal survey of 53 small-scale chicken farms in southern Vietnam to investigate the impact of outbreaks with disease-induced mortality on harvest rate, vaccination, and disinfection behaviors. We found that in small broiler flocks (≤16 birds/flock) the estimated probability of harvest was 56% higher when an outbreak occurred, and 214% higher if an outbreak with sudden deaths occurred in the same month. Vaccination and disinfection were strongly positively correlated with flock size and farm size, respectively. Small-scale farmers – the overwhelming majority of poultry producers in low-income countries – tend to rely on rapid sale of birds to mitigate losses from diseases. As depopulated birds are sent to markets or trading networks, this reactive behavior has the potential to enhance onward transmission. A cohort study of fifty three small-scale poultry farms in southern Vietnam reveals that when outbreaks occur with symptoms similar to highly pathogenic avian influenza, farmers respond by sending their chickens to market early, potentially exacerbating the effects of the outbreak.

This study describes the demographic structure and dynamics of small scale poultry farms of the Mekong river delta region, one of the world's highest-risk regions for avian influenza outbreaks. Fifty farms were monitored over a 20-month period, with farm sizes, species, age, arrival/departure of poultry, and farm management practices recorded monthly. The history of poultry flocks in the sampled farms was recovered using a flock-matching algorithm. Median flock population sizes were 16 for chickens (IQR: 10 - 40), 32 for ducks (IQR: 18 - 101) and 11 for Muscovy ducks (IQR: 7 - 18); farm size distributions for the three species were heavily right-skewed. There was substantial flock overlap on almost all farms, with only one farm practicing an all-in-all-out management system. The rate of interspecific contacts was high, with two out of three farms housing at least two bird species. Among poultry species, demographic dynamics varied. Muscovy ducks were kept for long periods, in small numbers and outdoors, while chickens and ducks were farmed in larger numbers, indoors or in pens, with more rapid flock turnover. Most chicks were sold young to be fattened on other farms, and broiler and layer ducks had a short production period and higher degree of specialization. The rate of mortality due to disease did not differ much among species, with birds being less likely to die from disease at older ages, but frequency of disease symptoms differed by species. Time series of disease-associated mortality and population size were correlated for Muscovy ducks (Kendall's coefficient = 0.49, p value < 0.01).

This study describes the demographic structure and dynamics of small scale poultry farms of the Mekong river delta region, one of the world’s highest-risk regions for avian influenza outbreaks. Fifty farms were monitored over a 20-month period, with farm sizes, species, age, arrival/departure of poultry, and farm management practices recorded monthly. The history of poultry flocks in the sampled farms was recovered using a flock-matching algorithm. Median flock population sizes were 16 for chickens (IQR: 10 – 40), 32 for ducks (IQR: 18 – 101) and 11 for Muscovy ducks (IQR: 7 – 18); farm size distributions for the three species were heavily right-skewed. There was substantial flock overlap on almost all farms, with only one farm practicing an all-in-all-out management system. The rate of interspecific contacts was high, with two out of three farms housing at least two bird species. Among poultry species, demographic dynamics varied. Muscovy ducks were kept for long periods, in small numbers and outdoors, while chickens and ducks were farmed in larger numbers, indoors or in pens, with more rapid flock turnover. Most chicks were sold young to be fattened on other farms, and broiler and layer ducks had a short production period and higher degree of specialization. The rate of mortality due to disease did not differ much among species, with birds being less likely to die from disease at older ages, but frequency of disease symptoms differed by species. Time series of disease-associated mortality and population size were correlated for Muscovy ducks (Kendall’s coefficient τ = 0.49, p value < 0.01). The structure and dynamics of poultry populations kept in small scale farms in the Mekong river delta were accurately described.Poultry farms mix poultry of different species, ages, and production types, with multiple overlapping flocks present on a farm at all times. This promotes the persistence of pathogens.The three main farmed poultry species of poultry (chickens, ducks and Muscovy ducks) are managed differently and have different demographic characteristics. The structure and dynamics of poultry populations kept in small scale farms in the Mekong river delta were accurately described. Poultry farms mix poultry of different species, ages, and production types, with multiple overlapping flocks present on a farm at all times. This promotes the persistence of pathogens. The three main farmed poultry species of poultry (chickens, ducks and Muscovy ducks) are managed differently and have different demographic characteristics.