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.