La seguridad alimentaria

This was a prospective cohort study carried out between 1 December 2008 and 3 April 2010 using a stratified, three-stage cluster sampling design. Four districts from the 129 districts in the MRD (Figure 4.1) were purposively selected for study and were treated as strata. Villages within districts comprised the first stage, poultry flocks within villages comprised the second stage and finally, individual birds within flocks comprised the third stage of sampling.

To be eligible for the study, villages were required to have at least three layer duck flocks and two of these flocks were required to be moved out of their home village for field run- ning throughout the study period. The second criterion was that each village had at least one broiler duck flock. Details of villages that met the eligibility criteria were provided by a cross-sectional survey conducted in the study districts in March 2008 (Minh, Morris, Schauer, Stevenson, Benschop, Nam & Jackson 2009). In total 27 of 60 villages met the eligibility criteria. Each eligible village was numbered from 1 to 27 and a sample of eight numbers corresponding to the village identifiers selected without replacement using the random number function within a spreadsheet.

Twenty birds from each of the enrolled flocks were sampled. This number was largely based on practical grounds (Bennett et al. 1991) because it was easy for field staff to

4.2 Materials and methods 39 collect and then manage 20 samples given the amount of time available for each flock. In addition, a sample size of 20 was considered appropriate based on the assumption that if virus was present within a flock, the prevalence of infection was likely to be greater than 80%.

The number of flocks to be sampled was calculated assuming that at the population level, the individual bird-level prevalence of infection was approximately 15% (Nguyen et al. 2005, Long 2008, Minh, Morris, Schauer, Stevenson, Benschop, Nam & Jackson 2009, Minh et al. 2010). A total of 157 flocks needed to be sampled to be 95% confident that the estimated individual bird-level prevalence was between 5% and 25% (i.e. an absolute error of 10%) when 20 birds were sampled per flock. For these calculations we assumed a design effect of 2 since no information on clustering of AI virus infection at the flock level was available. A design effect of 2 is similar to that determined previously for Newcastle disease (Otto & Kristensen 2004).

A sampling frame of all poultry flocks within the eight selected villages was compiled by staff of the Sub-Department of Animal Health in cooperation with staff from the District Veterinary Station and commune veterinarians. In total, 20 poultry flocks per village were selected at random using the procedure described above. The owners of the 160 (that is8×20) selected poultry flocks were then contacted by their respective commune veterinarians to seek their consent to participate in the study; the owners (n= 97) of 157 flocks agreed to take part.

At the first sampling round individual birds were selected using a systematic simple ran- dom sampling procedure. Selected birds were then identified using two leg bands and permanent aerosol dye. Ten of the 20 birds were kept without vaccination as sentinels; the other 10 together with all other (non-study) birds in the study flock were vaccinated under the requirements of the national vaccination campaign (MARD 2009).

In this paper we use the term ‘flock’ to refer to the group of birds present at an initial selec- tion visit and then repeatedly sampled throughout the study period. Using this definition, a flock is, in effect, a cohort of birds. Some households had many flocks throughout the study period (population turnover was rapid), whereas others had only a single flock that persisted for the entire period.

At the start of the study, a total of 76 flocks, including 19 broiler duck, 29 layer duck and 28 in-contact species (chickens, Muscovy ducks and geese) flocks were enrolled.

Any flock that dropped out of the study was replaced by a new flock that met the selection criteria described earlier. The first 19 broiler duck flocks were enrolled between December 2008 and March 2009 and an additional 19 broiler duck flocks were enrolled between December 2009 and April 2010. No broiler flocks were enrolled between March and November 2009 since this was considered to be the low-risk period for HPAI H5N1 in the MRD, and broiler duck production is seasonal. After enrolment broiler duck flocks were monitored for three sampling rounds. In contrast, layer duck and in-contact species flocks were enrolled at the start of the study and monitored over the entire study period. Owners of in-contact flocks were permitted to sell their birds after a minimum of three sampling rounds.

A total of 5,476 birds from 157 flocks were enrolled from 97 contracted households in the eight study villages. No broiler duck flocks needed to be replaced throughout the study period. In contrast, 59% (17 of 29) of layer duck flocks dropped out and were replaced by new flocks one time over the study period. Four (14%) layer duck flocks dropped out and replaced by new layer flocks twice over the study period. One of the main reasons for loss of layer duck flocks was that during the 17-month study period the price of eggs decreased sharply making it necessary for many flock owners to sell their birds. All (28 of 28) in-contact species flocks were replaced because their owners were permitted to sell them after every three sampling rounds. Eleven (38%) in-contact species flocks were replaced twice and two (7%) in-contact species flocks were replaced three times over the study period.

Contracted flock owners were asked to stop vaccinating their 10 sentinel birds after the date of flock enrolment. Replacement birds were enrolled when any of the study birds were removed for reasons such as death, loss, consumption or sale. Five additional, un- vaccinated birds were kept to allow replacement of lost sentinel birds.