Seroprevalence of zoonotic diseases (leptospirosis and Japanese encephalitis) in swine in ten provinces of Vietnam: a Bayesian approach to estimate prevalence

Background Leptospirosis is an important zoonotic disease with a global distribution, affecting a wide range of mammalian animals and humans. Japanese encephalitis (JE) virus is the major vector-borne zoonotic disease in the Asia-Pacific region. The main objective of this study was to evaluate the seroprevalence of serovar-specific Leptospira and JE in swine from 10 provinces in Vietnam. Methods Samples were initially collected for swine influenza surveillance from March to April 2017 at large-scale farms (with at least 50 sows and/or 250 fattening pigs) with pigs that tested positive for influenza in the previous surveillance period (2015-16). Findings A total of 2,000 sera samples were analyzed from 10 provinces. Overall, the seroprevalence of leptospirosis was 21.05% (95% CI: 19.28-22.90) using a cut-off titer of ≥ 1:100. The apparent prevalence of JE was 73.45% (95% CI: 71.46-75.37) while the true prevalence was slightly higher (74.46%, 95% credible interval: 73.73-86.41). We found a relatively high presence of leptospirosis and JE in pigs kept on large farms. Prevalence was comparable with other studies suggesting opportunistic testing of samples collected for other surveillance purposes can be a valuable tool to better understand and prevent the potential transmission of these zoonotic diseases from pigs to people in Vietnam. Conclusion Our study provides evidence to veterinarians and animal health professionals for evidence-based practice such as diagnosis, vaccination and zoonotic control. Further investigation into the possible role of different domestic animals, wildlife species or environmental factors is needed to identify the potential risk factors and transmission routes in Vietnam.


47
Leptospirosis is an important zoonotic disease with a global distribution, affecting a wide range 48 of mammalian animals and humans. Japanese encephalitis (JE) virus is the major vector-borne 49 zoonotic disease in the Asia-Pacific region [1][2][3][4][5]. Leptospira spp. are maintained in a wide range 50 of reservoir hosts, such as cattle, pigs, dogs, rats, raccoons, skunks and opossums [6][7][8][9][10]. 51 Leptospirosis is caused by gram-negative bacteria, and more than 260 pathogenic serovars have 52 been identified [11]. Pigs are considered to be reservoir hosts for serovars Bratislava,Pomona 53 and Tarassovi Mitis, and also become infected with Icterohaemorrhagiae from rats and Hardjo 54 from cattle [12][13][14]. JE virus is a flavivirus and the main cause of viral encephalitis in Asian 55 countries, resulting in 13,600-20,400 deaths per year [15,16]. It is estimated that more than 3 56 billion people live in endemic areas [17]. JE virus circulates between Culex sp. mosquitoes and 57 domestic/wild birds or pigs [18]. JE in pigs can cause abortion, stillbirth and infertility which 58 may result in significant economic losses to producers [19,20]. Pigs are considered to be 59 amplifying hosts and an important risk factor for transmission of JE to humans [21,22]. Bird 60 migration may play an important role in the spread of JEV [15]. 61 In Vietnam, leptospirosis is a notifiable disease in humans, but few cases (less than 20 cases per 62 year) have been reported to the Ministry of Health. Despite the low numbers of reported cases, 63 leptospirosis is considered to be endemic and serovar Bratislava is commonly observed [23][24][25][26]. 64 A study in the Mekong delta reported a seroprevalence of 18. 8%  Giang, Quang Ninh, Quang Ngai, Binh Duong, Dong Nai, Dong Thap and Soc Trang) (Fig. 1).

85
The sampling was targeted at large-scale farms (with at least 50 sows and/or 250 fattening pigs) 86 with pigs that tested positive for influenza in the previous surveillance period . In each 87 province, 10 blood samples from piglets (4-8 weeks old) and fattening pigs (9-12 weeks old) and 88 five blood samples from sows were collected from four-large scale farms, but the sex of the pigs 89 was not recorded. Pigs with symptoms of influenza were prioritized for sampling.  (table 1). For JE, we estimated the true prevalence (TP) from apparent prevalence (AP) 104 by adjusting for the diagnostic test sensitivity and specificity using a Bayesian framework [32].

126
( the transmission cycle to humans as well as the impact of the national immunization program.

201
We found less discrepancy between them as sensitivity and specificity are relatively high (> 202 95%) as well as a higher proportion of positive sample. A Bayesian approach provides an 203 opportunity to combine prior information with investigated data and estimate values for both 204 prevalence and diagnostic characteristics of tests [57]. If a diagnostic test with less than 100% 205 sensitivity and specificity is used to estimate the prevalence of a disease, our results would be 206 biased. Therefore, a Bayesian approach was used to estimate the TP from AP. In general,

207
Bayesian models are sensitive to the selection of the priors, so we employed a Jeffreys prior to 208 minimize the influence of prior to the posterior distribution as no prior information was available 209 for JE prevalence in our study area. Our study was the first attempt to estimate the TP for JE in 210 Vietnam, which showed how a Bayesian approach can be used to better estimate the prevalence 211 of animal diseases.
This study shows the added value of opportunistic use of samples collected for one purpose in 213 providing valuable information on other diseases. An advantage is the large sample size and 214 wide cover. A disadvantage is potential bias linked with sample collection from large farms with 215 previous influenza detection and preference given to apparently sick pigs.

217
We found a relatively high presence of leptospirosis and JE in pigs kept on large farms.