Foot-and-mouth disease virus

Foot-and-mouth disease (FMD), caused by the foot-and-mouth disease virus (FMDV), is a globally widespread infectious disease that affects cloven-hoofed animals. Recognized as a "political and economic disease," FMD outbreaks not only pose severe threats to the health of affected animals but also inflict significant economic losses on livestock industries and international trade. While humans are mildly susceptible to FMD, the World Organisation for Animal Health (WOAH) has designated it as the top priority among Class A infectious diseases, mandating immediate reporting upon any confirmed case. To better understand the foot-and-mouth disease virus and effectively combat its spread, let’s explore essential knowledge about this virus together.

FMDV is a member of the family Picornaviridae and the genus Aphthovirus, belonging to the picornavirus group. The viral particles are non-enveloped, featuring an icosahedral symmetry with a spherical surface. Its genome consists of a single-stranded positive-sense RNA, approximately 8.3 kb in length, encoding a single open reading frame (ORF) of about 7 kb. Flanking the ORF are a 5' untranslated region (5’ UTR) and a 3’ untranslated region (3’ UTR), with a Poly(A) tail at the 3’ end. The virus's ORF comprises several key genes, including the L gene, the P1 structural protein gene, and the P2 and P3 non-structural protein genes. Notably, the P1 structural protein gene encodes four major capsid proteins—VP1, VP2, VP3, and VP4. Among these, VP1 is predominantly exposed on the surface of the viral particle and serves as the primary site for antigenic epitopes, directly determining the virus's antigenicity and serotype. It plays a crucial role in eliciting neutralizing antibodies in the host and conferring partial protective immunity. To date, seven distinct serotypes have been identified: O, A, C, SAT1, SAT2, SAT3, and Asia-Ⅰ. Importantly, there is no cross-protection among these serotypes or their subtypes. Each serotype is further subdivided into multiple topotypes: Serotype O includes 11 topotypes, Serotype A has 3 topotypes, Serotype C is divided into 3 topotypes, Asia-Ⅰ features just 1 topotype, while Serotype SAT1 encompasses 13 topotypes and Serotype SAT2 boasts 14 topotypes, and Serotype SAT3 comprises 5 topotypes [1].

Foot-and-mouth disease has previously occurred or become endemic in most countries and regions around the world, with major outbreaks concentrated in parts of Africa, the Middle East, Asia, and South America. Serotypes are unevenly distributed across the globe: Types O and A are the most widely prevalent, while SAT1–3 are primarily found in Africa, and Asia-Ⅰ is mainly endemic in the Middle East and South Asia. In China, Types O and A are the dominant strains currently circulating [2]. Specifically, the predominant Type O strains include CATHAY, Ind-2001e, and Mya-98, whereas Type A is represented by the Sea-97 strain. Meanwhile, Asia-Ⅰ type foot-and-mouth disease remains under control, with sporadic outbreaks of Type O reported—only one outbreak was documented in 2022. Overall, the foot-and-mouth disease situation in China remains relatively stable.

Foot-and-mouth disease primarily infects cloven-hoofed animals such as cattle, pigs, and sheep. Infected animals can serve as both carriers and transmitters of the pathogen. Transmission occurs through direct contact with infected animals, consumption of contaminated animal products, contact with contaminated equipment, vehicles, and tools, as well as via airborne routes. The disease can occur year-round, though it is more common during the spring and autumn seasons.

The FMDV replication cycle consists of nine distinct stages (Figure 1): attachment, internalization, uncoating, translation, processing, replication, capsid assembly, morphogenesis, and viral egress. The viral particle attaches its capsid to membrane receptors via the highly conserved RGD motif (Arg-Gly-Asp) on the VP1 surface—such as ανβ6 integrin, which is highly expressed in epithelial cells, or heparan sulfate (HS). Once attached, the viral particle is internalized through clathrin-mediated endocytosis. Inside the cell, the virus undergoes uncoating, where acidic conditions trigger the dissociation of the viral capsid, releasing the genomic RNA into the cytoplasm. Translation then initiates at the Internal Ribosome Entry Site (IRES), followed by processing into mature polypeptides and various intermediate products. These intermediates are subsequently cleaved by two viral proteases—Lpro and 3C—to yield fully functional viral proteins. Meanwhile, RNA replication begins with the synthesis of a complementary negative-strand RNA from the positive-sense RNA template, which is then used as a template to produce new positive-sense RNA strands. During this process, double-stranded RNA replication intermediates (RI) often form. Finally, the genomic RNA associates with the immature capsid, assembling into a nascent virion precursor. This precursor undergoes proteolytic cleavage of the VP0 precursor protein into VP2 and VP4, transforming the immature virion into a mature viral particle. The newly formed virus particles rapidly accumulate in the cytoplasm of infected cells, ultimately inducing cell lysis and facilitating the export of newly generated viruses from the host cell.

Figure 1: Foot-and-Mouth Disease Virus Replication Cycle [3]

Clinical symptoms after infection are generally similar across different animals, with typical signs including elevated body temperature in affected livestock, depression, excessive salivation, and the development of vesicles and erosions on the oral mucosa, hooves, and skin of the udder. Post-mortem examination reveals hemorrhagic inflammation of the gastrointestinal mucosa, diffuse hemorrhagic spots on the pericardium, and degenerative necrosis of cardiac muscle fibers. In severe cases of foot-and-mouth disease, the heart cross-section may show grayish-white or pale yellow speckles or striped patterns, characteristic of "tigerheart."

Foot-and-mouth disease is listed as a Class 1 animal infectious disease in the "Catalogue of Quarantinable Animal Diseases for Entry into the People's Republic of China," making it essential to conduct foot-and-mouth disease virus testing on animals involved in import and export trade to prevent both the introduction and spread of the virus. In addition, Beijing’s local standards mandate that foot-and-mouth disease be tested as a mandatory item for experimental pigs and miniature experimental pigs. Regarding the detection methods for foot-and-mouth disease, the export industry standard SN/T 1181-2010 and the national standard GB/T 18935-2018 outline specific protocols. These include: - Serotype identification using type-specific ELISA (where enzyme-linked plates are coated with antigens corresponding to different serotypes, enabling the capture and detection of antibodies against specific serotypes in serum samples); - Type-specific RT-PCR; - Multiplex RT-PCR for antigen-based diagnosis of foot-and-mouth disease virus; - qPCR techniques. For detecting antibodies against foot-and-mouth disease virus structural proteins (SP), the following methods are employed: - Virus Neutralization Test (VN); - Liquid Phase Blocking ELISA (LPBE); - Solid Phase Competitive ELISA (SPCE). To detect antibodies against non-structural proteins (NSP) of the virus, two approaches are used: - Indirect ELISA targeting 3ABC antibodies; - 3ABC Antibody Blocking ELISA. While both NSP and SP antibody tests are designated methods for import and export trade, the NSP test stands out for its ability to identify up to seven serotypes of the virus and distinguish between natural infections and vaccine-induced immunity, giving it a distinct advantage. However, in populations of animals that have undergone repeated immunizations—particularly when trace amounts of NSP may inadvertently remain in vaccines—it’s crucial to interpret results carefully, as this could lead to false-positive reactions.

Currently, foot-and-mouth disease remains widely distributed. To encourage global participation in controlling and eradicating the disease, the World Organisation for Animal Health (OIE) and the Food and Agriculture Organization of the United Nations (FAO) have jointly developed the "Global Foot-and-Mouth Disease Control Strategy." Since 2001, China has implemented a mandatory immunization strategy. The Ministry of Agriculture and Rural Affairs recently issued the "National Guidance on Mandatory Immunization against Animal Diseases (2022–2025)," emphasizing that the annual seroconversion rate for foot-and-mouth disease antibodies must consistently remain above 70%. The directive also mandates O-type and A-type foot-and-mouth disease vaccination for all cattle, sheep, camels, and deer nationwide, while ensuring that all pigs receive O-type foot-and-mouth disease immunization.

In conclusion, diagnosing the foot-and-mouth disease virus is a critical step in preventing and controlling its spread. We encourage animal health managers, veterinarians, and researchers to remain actively engaged in advancing diagnostic studies for FMDV, continuously refining and developing more accurate and sensitive testing methods. With our collective efforts, we are confident that we can better safeguard animal health, protect the livestock industry, and ultimately enhance human well-being.

References

[1] Gao Y, Sun SQ, Guo HC. Biological function of foot-and-mouth disease virus non-structural proteins and non-coding elements. Virol J, 2016, 13: 107.

[2] Ren HR, Li MT, Wang YM, Jin Z, Zhang J. The risk factor assessment of foot-and-mouth disease spread in mainland China. Journal of Theoretical Biology, 2021, 512: 110558.

[3] Sarry M, Vitour D, Zientara S, Bakkali Kassimi L, Blaise-Boisseau S. Foot-and-Mouth Disease Virus: Molecular Interactions with the IFN Response and the Importance of the Model. Viruses, 2022, 14(10).

[4] DB11∕T 1809-2020 Laboratory Animals — Microbiological Testing (Beijing Municipality)

[5] People's Republic of China National Entry-Exit Inspection and Quarantine Industry Standard SN/T 1181-2010 – Foot-and-Mouth Disease Quarantine Technical Specifications

[6] National Standard GB/T 18935-2018 — Foot-and-Mouth Disease Diagnostic Techniques