Discussion
This study provides the first long-term analysis of JEV seroprevalence
in a domestic pig population, and is in a region in India – eastern
Uttar Pradesh – in which a high incidence of cases in people occurs
annually (A. K. Singh et al., 2020). A key finding was that IgG
seroprevalence did not increase with age, indicating that IgG could be
transient, and also that JEV infection might not be an immunising
infection in pigs.
IgM is an indicator of recent JEV infection in pigs; IgM can be detected
4—7 days post-infection, peaking at approximately 2 weeks and
declining over the following 4 weeks (H. Dhanze, Kumar, et al., 2020).
If JEV infection was immunising, a reduction in IgM seroprevalence would
have been consistently observed with age, even if pigs were not relying
on a humoral response to JEV involving IgG. In a review of JEV
epidemiology in pigs, Ladreyt et al. (2019) reported that in
experimental studies, antibodies to JEV were detected for at least 28
days post-inoculation in two experimental studies, and a study of
piglets under field conditions found immunity for up to 3 years
(Geevarghese, Shaikh, Jacob, & Bhat, 1994). In this latter study, it
was possible that the pigs had been repeatedly infected with JEV and
that cross-reaction between antibodies to other flaviviruses (for
example, West Nile virus) had occurred. In an experimental case study,
IgG was found to decline after 40 days and remained at low levels at one
year in a single 100-day old pig experimentally injected with purified
recombinant NS1 protein as a JEV associated antigen, but this might not
be representative of the immune response following natural JEV infection
(H. Dhanze, Kumar, et al., 2020). In a field study of JE in Sri Lanka
(Peiris et al., 1993), point estimates of age-stratified prevalence of
neutralising antibodies decreased between 7-12 month and 13-24 month
pigs at one site (48% [90% CI 29-67] and 19% [90%CI 12-28],
respectively), and were similar at others (for example, 21% [90% CI
7-47] and 33% [90%CI 11-65], respectively). It is possible that
ages of pigs in the current study were misclassified, but we believe
this would be a small number because samples were taken by veterinarians
familiar with pig breeds in the region. It is also possible that further
stratification within districts would demonstrate increasing IgG
seroprevalence with age if JEV transmission has consistent localised
patterns within districts. Therefore, we recommend that longitudinal
studies of IgG levels in individuals in pig populations at continuous
risk of JEV infection should be undertaken to determine whether our
hypothesis is correct. There are public health implications where
intervention strategies have been based on models in which persistent
immunity in pigs was assumed; the force of infection on people from pigs
might be higher than estimated if susceptible pigs are not only
replenished through births and young pigs losing maternal immunity, but
also adult pigs losing acquired immunity.
The district-level spatio-temporal pattern of JEV IgG or IgM
seroprevalence did not follow a discernible pattern, which is consistent
with endemicity – waves did not reliably occur simultaneously in
districts across the region, and neither was a wave of spread apparent
from one district to the next. Seroprevalence of IgM was found
year-round, indicating a subtropical epidemiological pattern of JEV
transmission (Suresh et al., 2022). The seroprevalence throughout the
region was consistent with previous, shorter duration studies, with
large peaks observed, such as the annual seroprevalence of 60.8% in
Sant Kabir Nagar in 2016 and 69.5% in Gorakhpur in 2017 for IgG and
IgM, respectively, and some region-wide monthly IgG seroprevalence peaks
reaching 100% (H. Dhanze et al., 2014; Kolhe et al., 2015).
Decomposition of the monthly time series of seroprevalence for IgG and
IgM across all age groups demonstrated that most variance in JEV
seroprevalence was due to the unexplained ‘random’ portion of the
time-series, followed by the underlying trend. Although annual cyclical
waves with 3-4 peaks annually were apparent in the decomposition, the
variance of these waves was relatively small. These cyclical waves are
also consistent with the expected dynamics of an endemic infectious
disease, in which regular epidemic waves occur as the proportion of
susceptible pigs in a population increases through births – it does not
indicate necessarily that an infection is non-immunising although this
can contribute to the effect. Generally, seroprevalence peaks were
higher, or arose from a higher baseline, in the monsoon period,
indicating a seasonality which has been shown previously (Borah et al.,
2013; Kumari & Joshi, 2012; Pantawane et al., 2017; A. K. Singh et al.,
2020). In the monsoon season, temperature is warmer and rainfall is
higher, providing mosquitoes with more breeding habitat and promoting
shorter developmental cycles. The predominant vector of JEV, Culex
tritaeniorhynchus , has been shown to be most abundant in the monsoon
season in a previous study in Gorakhpur district (Kanojia, Shetty, &
Geevarghese, 2003).
Because most variance in the time series decomposition was due to the
random (unexplained) portion, autoregressive modelling of JEV IgG
seroprevalence in Gorakhpur district was not sufficiently nuanced to be
usefully predictive. Interestingly, the underlying trend of IgG
seroprevalence in Gorakhpur – higher overall seroprevalence in the
first half of the study period, similar to the region-wide trend – was
the inverse of the monthly mean relative humidity and total rainfall
which were both higher in the second half of the study period. This was
reflected in the best fitting model in which mean monthly relative
humidity was negatively correlated with seroprevalence. Previous studies
have stated that pigs could be used as sentinels for outbreaks of JEV in
people (Suresh et al., 2022). Given that seroconversion in pigs can be
highly correlated with cases in people 2-4 weeks later (study in Assam;
Borah et al. (2013), increased seroprevalence in pig populations could
theoretically be used as a predictor of human cases. Whilst the utility
of this might be useful for more immediate control options such as
fogging in areas detected with high seroprevalence in pigs, the utility
of this is low for predictive surveillance due to timeliness. Collection
of blood samples, analysis and reporting, as well as implementation of
longer-term control vaccination programs in people and the time required
to develop immunity would be greater than 2-4 weeks.
A strength of this study included the relatively large dataset over a
long time period, enabling comprehensive descriptive analysis of the
seroprevalence of pigs in this region. However, limitations included
sparse data in some districts, and the lack of longitudinal analysis of
pigs. In addition, reported cases in people from the same districts
during the study period would enable further evaluation of the value of
sero-surveillance in pigs as part of a public health surveillance
program.
Cases in people in Uttar Pradesh generally occur from June and peak in
September—October (Kumari & Joshi, 2012). This study demonstrates
that for sero-surveillance in pigs to be useful to predict the onset and
magnitude of human cases at district level so that public health
resources for interventions including vaccination programs, mosquito
control and bite prevention can be targeted to the highest risk
districts, more detailed data about known risk factors is needed at
local scales. It is possible that other risk factors such as local
availability of mosquito breeding habitat, ground surface temperature,
water bird abundance and other landscape factors might be useful in
predicting both pig and human cases. Pig sero-surveillance could have a
role in confirming predictions and the need for maintenance of
interventions in high risk periods for people.