Background and aim: In Indonesia, highly pathogenic avian influenza (HPAI) H5N1 outbreaks in poultry are still reported. The disease causes a decrease in egg production and an increase in mortality; this has an impact on the economic losses of farmers. Several studies have considered that ducks play a role in the HPAI endemicity in the country; however, little is known about whether or not the type of duck farming is associated with HPAI H5 virus infection, particularly within clade 2.3.2.1c, which has been predominantly found in poultry since 2014. A cross-sectional study was conducted to determine the HPAI seroprevalence for H5 subtype clade 2.3.2.1c in laying ducks that are kept intensively and nomadically and to determine the associated risk factors.

Materials and methods: Forty-nine duck farmers were randomly selected from ten sub-districts in Purbalingga District, Central Java, Indonesia; a cross-sectional study was implemented to collect field data. Based on an expected HPAI prevalence level of 10%, estimated accuracy of ± 5%, and 95% confidence interval (CI), the total sample size was calculated at 36 individuals. Samples must be multiplied by 7 to reduce bias; thus, 252 ducks were taken as samples in this study. Considering that the maintenance and duck handling were uniform and farmers complained that the effect of activity to take duck samples would reduce egg production, this study only took samples from 245 ducks (oropharyngeal swabs and serum). Those samples were taken from five birds on each farm. Hemagglutination inhibition tests examined the serum samples for HPAI H5 Clade 2.3.2.1c, and pool swab samples (five swabs in one viral media transport) were examined by real-time reverse transcription-polymerase chain reaction (qRT-PCR) test for influenza Type A and H5 subtype virus. Information regarding farm management was obtained using a questionnaire; face-to-face interviews were conducted with the duck farmers using native Javanese language.

Results: Serum and swabs from 245 ducks were collected in total. For individual birds, 54.69% (134/245) of serum samples were H5 seropositive. Seroprevalence among nomadic ducks was 59.28% (95% CI: 0.48-0.61), which was higher than among intensively farmed ducks (48.57%, 95% CI: 0.38-0.58). Farm-level seroprevalence was 50% (95% CI: 0.30-0.69) for nomadic ducks but only 28.57% (95% CI: 0.11-0.51) for intensively farmed ducks. The farm-level virus prevalence (proportion of flocks with at least one bird positive for influenza Type A) was 17.85% (95% CI: 0.07-0.35) for nomadic ducks and 4.76% (1/21) for intensively farmed ducks (95% CI: 0.008-0.23). All influenza Type A positive samples were negative for the H5 subtype, indicating that another HA subtype AI viruses might have been circulating in ducks in the study area. A relationship between duck farms that were H5 seropositive and their maintenance system was present; however, this relationship was not significant, the nomadic duck system detected 2 times higher H5-seropositive ducks than the intensive farming system (OR: 2.16, 95% CI: 0.33-14.31).

Conclusion: This study found that the seroprevalence of HPAI in the duck population level in Purbalingga was 54.69% and demonstrated that the nomadic duck farming system was more likely to acquire HPAI H5 infection than the intensive farming duck system. Other risk factors should be further investigated as the diversity of the farming system is partially related to HPAI H5 infection.