Poultry population dynamics and mortality risks in smallholder farms of the Mekong river delta region

Abstract

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.

Keywords: Avian influenza; Infectious diseases of poultry; Livestock demography; Population biology; Poultry production; Smallholder farms; Southeast Asia; Veterinary epidemiology; Vietnam.

Fig. 1

Histograms of the sizes of poultry flocks in the study farms. Top: all species aggregated; bottom: plotted by species. Solid lines represent best-fit distributions (mixture of two gamma components (ducks) or three gamma components (chickens and Muscovy ducks))

Fig. 2

Distribution of age-at-departure and age-specific removal and mortality rates in the populations of chickens, ducks and Muscovy ducks of the study farms during the 20-month study period. Top: distribution of ages at departure (death or removal). Middle: rate of removal (sale, gift, or home slaughter) as a function of age. Bottom: disease-related death rate as a function of age. Shaded regions in the middle and bottom graphs show the 95% range using imputed values; solid lines show the median. The time series of median, 2.5, and 97.5% quantiles were smoothed using local regression (span factor: 0.5)

Fig. 3

Flow diagram representation of the life history of chickens, ducks, and Muscovy ducks present in the study farms during the 20-month study period. The diagrams show the average duration of the production period in the broiler and layer-breeder classes and the proportion of poultry used for different purposes at the end of their production period

Fig. 4

Frequency of association between disease-induced mortality and reported symptoms or reported suspected causes, based on all symptoms or causes listed for each reported death event in the populations of chickens, ducks, and Muscovy ducks of the study farms during the 20-month study period. E.g., for 75% of Muscovy duck disease-induced deaths, lethargy was reported. LET: lethargy, weariness; DIG: digestive symptom (diarrhea, flatulence or abnormal color of feces); RES: symptoms related to the lower respiratory tract (dyspnea or amplified respiratory sounds); SUD: sudden death (birds died before any symptoms could be noticed); SC: swollen crop; PW: paralyzed wing; SE: anorexia; CYA: cyanosis; RN: symptoms related to the upper respiratory tract (runny nose); FP: fowl pox; CD: coccidiosis; DRY: dry legs; RH: retraction of the head; PL: paralyzed leg; SULF: intoxication with sulfate; CNS: nervous-system symptoms; SN: shrinking neck

Fig. 5

Fraction (proportion of animal-months) of poultry in a particular housing type (top) and undergoing particular prevention practices against infectious diseases (bottom) in the populations of chickens, ducks, and Muscovy ducks of the study farms over the 20-month study period

Fig. 6

Time series of disease-induced mortality rate, population size, and absence of disinfection over the study period in the populations of chickens, ducks, and Muscovy ducks of the study farms over the 20-month study periods (from June 2015 to January 2017). Top: Dashed lines are median estimates of death rates (from imputation) while solid lines are smoothed rates obtained through local regression (span factor: 0.5)