Introduction
Foot-and-mouth disease virus (FMDV) is a contagious virus of
cloven-hoofed ungulates (Artiodactyla), of major trade-limiting
importance (Anonymous, 2020b). FMD is present in approximately
two-thirds of the world’s countries, in which it acts as a barrier to
trade. Parameters of acute FMD infection in naïve cattle are
well-defined, and continue to be refined by research (Arzt et al., 2010;
Stenfeldt et al., 2015). In contrast, the subclinical cycles of FMD in
the approximately 128 countries where the virus circulates are less-well
documented. Experimental work to assess viral shedding and immune
response in recently vaccinated, previously naive animals (Parthiban et
al., 2015) has partially filled gaps in knowledge of the virus in
animals with humoral immunity. In endemic regions, FMD lesions similar
to that reported in epidemics are documented, but the characteristic,
“fulminant” herd-wide disease may be observed or reported
inconsistently compared to what is documented in epidemic contexts (van
Andel et al., 2020b; Bertram et al., 2018). The driving factors in the
discrepancy between viral circulation and frequency of clinical FMD in
endemic regions have not been well-described. Almost fifty years ago,
Anderson et al. (1974) warned of Kenyan cattle where “the occurrence of
clinical outbreaks does not necessarily give a true assessment of the
amount of virus in the environment as subclinical or inapparent
infection could occur, particularly in partially immune cattle.” Much
uncertainty remains about how FMD manifests in endemically-infected
herds, and the various states by which subclinical infection exists
considering the distinct phases: neoteric and persistent infection
(Stenfeldt and Arzt, 2020).
Before considering endemic disease, it is useful to briefly review the
well-defined stages of epidemic disease in naïve cattle (Yadav et al.,
2019). The majority of naïve cattle exposed to FMDV will become infected
through exposure of the mucosa of the upper respiratory tract. In an
individual animal, there is a 2-14 day incubation period; a pre-clinical
viraemia typically within 48-72 hours of infection. Viraemia occurs
24-48 hours after initial detection of virus within nasopharyngeal
mucosa (Pacheco et al., 2016). Virus can also be secreted through
breath, milk, faeces, saliva, urine, and semen up to 4 days prior to
clinical illness. A 2-3 day period of clinical illness follows, and
within 24 hours there is rupture of epithelial vesicles, which slough
and erode, creating the classical fulminant lesions of FMD (Anonymous,
2020b). Resolution occurs in a characteristic pattern by local
reepithelialisation of erosions by remaining islands of basal epithelial
cells. Full disease resolution occurs by day 15 from the start of
clinical infection. Antibodies are a useful indicator of natural
disease, and may be partially cross-protective to future infection from
other serovars (Garland, 1974). Cattle with viral infection past 28 days
are defined as chronic carriers, and in these cattle, virus most
commonly persists in the dorsal nasopharyngeal mucosa (Stenfeldt et al.
2016; Stenfeldt and Arzt 2020). In live chronic carriers, the probang
technique provides the most useful sample of the area of viral
persistence, allowing for molecular detection and virus isolation
(Barend et al., 2016; Stenfeldt and Arzt 2020), however scraping this
area with a cuvette at slaughter also yields virus (Anderson et al.,
1974). Readers are directed to the recent review by Stenfeldt and Arzt
(2020) for a more detailed summary of the chronic carrier state.
Besides the chronic carrier state, other factors that influence
subclinical infection in cattle include: the pre-clinical (pro-dromal or
neoteric) state; previous infection and natural immunity to a particular
serotype; vaccination status; and potential breed or species resistance
associated with host-adaptation. Protective immunity is not mentioned as
a possible factor in subclinical infection in any document we could
find, except when antibodies were a product of vaccination (Kitching,
2002, Sutmoller P., Casas Olascoaga R. 2002). This omission is
interesting, since seroprevalence is one of the major differences
between FMDV-free cattle and cattle in endemically-infected populations.
In endemically-infected populations, the seroprevalence of the
population as a whole, by definition, is expected to be greater than
zero. In fact, reports suggest that seroprevalence in some cases is much
higher than zero: in non-vaccinated populations of large ruminants,
region-based seroprevalence is reported at between 19-71% in Africa
(Eldaghayes et al. 2017; Munsey et al. 2019; Wungak et al 2016), and
between 18-51% in Asia (de Carvalho Ferreira, 2017; Dukpa et al., 2011;
Blacksell et al. 2008).
Knowledge of in-country FMD epidemiology forms one of the major
requirements for countries participating in the Food and Agriculture
Organisation of the United Nations (FAO) and the World Organisation for
Animal Health (OIE)’s Progressive Control Programme for FMD (PCP-FMD),
which provides a benchmarking guide to countries wishing to progress
towards FMD freedom (Jamal & Belsham, 2013). Multiple factors,
including the naturally-acquired immunity of endemic herds, may
potentially influence the patterns of FMD observed in endemic regions.
The majority of research on FMD has been either experimental or has
occurred during outbreaks in previously-FMD-free regions. Reports of
differences in disease presentation and epidemiology in endemic regions
present striking contrasts to those from epidemic contexts, but the
reports themselves are limited in number. For example, while 90% of
FMDV-infected naïve cattle would be expected to develop disease upon
exposure, FMDV-exposed cattle in an unvaccinated herd in Cameroon had a
seroprevalence increase of 30% over a twelve month observation period,
despite the fact that only 6 of 100 animals were noted to have
FMD-associated lesions during this time (Bertram et al, 2018). In
addition, clinically-healthy cattle and buffalo from Pakistan produced
milk containing live FMDV up to 6 months following FMD outbreaks on the
same farms (Nawaz, 2019; Ahmed 2017). These examples suggest that cattle
and buffalo exposed early and frequently to FMDV might exist in a
different relationship with the virus than is commonly understood in an
epidemic context.
Control of FMD has been identified as a priority for the livestock
production sectors in Myanmar and Lao PDR, due to their perceived role
in regional transmission of FMD due to cattle movement (Blacksell et
al., 2019). Previous studies undertaken in these countries as part of
the PCP-FMD demonstrated widespread exposure of cattle and the
circulation of multiple strains of FMDV (Bo et al, 2019; Blacksell et
al., 2008; Khounsy et al. 2008; van Andel, 2020b) similar to that
reported in other endemic regions. Concerningly, analyses of outbreak
reporting in Myanmar failed to demonstrate that seroprevalence could be
accurately predicted either by government-level outbreak reports (van
Andel, 2020a), or by villager or headman observations (van Andel,
2020b), suggesting that more research is needed to better understand the
FMD epidemiology in Myanmar.
As part of understanding the circulation of FMDV in Southeast Asia, we
examined the subclinical presence of FMDV on the dorsal nasopharyngeal,
oral, and nasal mucosal surface of healthy Laos and Myanmar cattle and
buffalo at slaughter. Research was undertaken as part of the South East
Asia and China Foot‐and‐Mouth Disease Project (SEACFMD) supported by OIE
and the New Zealand Ministry for Foreign Affairs and Trade (MFAT) and
the New Zealand Ministry for Primary Industries (MPI). We obtained
post-mortem dry swabs and sera from 232 clinically-healthy adult large
ruminants, which were tested by real-time PCR and non-structural protein
(NSP) ELISA, respectively. The results of this study may help better
inform the epidemiology of FMDV in the Southeast Asian region.