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The carrier state in foot and mouth disease--an immunological review
1Pirbright Laboratory, Institute for Animal Health, Pirbright, Surrey, UK.
Abstract:
The carrier state in foot and mouth disease (FMD) is characterized by the asymptomatic low-level excretion of foot and mouth disease virus (FMDV) from the oropharynx of ruminants for periods that are species and virus strain-dependent. Persistent infection with FMDV readily occurs following the failure of virus elimination at the acute stage of infection, a process thought to be mediated through the phagocytosis of antibody/virus immune complexes. Recent evidence supports the view that carrier cattle are important in the epidemiology of FMD in the field. The absence of histopathological change in persistently infected tissues and the reduced cytopathology of carrier virus isolates in tissue culture suggest that less lytic FMDV variants are generated or selected in the carrier animal. Altered virus replication, due to attenuation or interference, rather than antigenic variation may therefore allow evasion of the exaggerated FMDV-specific systemic and local humoral immune responses that occur in the carrier state. Although cell-mediated immune mechanisms for FMDV clearance have not been described, the eventual elimination of many persistent virus infections involves this arm of the immune system. Thus, further investigation of cellular elements of the immune response, more particularly the local interaction of mononuclear cell infiltrates with persistently infected cells, represents an area of research that has the potential to elucidate the carrier state problem.
Insights
Foot and Mouth Disease (FMD) carriers shed virus asymptomatically. Understanding this persistent FMD virus infection is key to controlling FMD transmission in cattle.
Area of Science:
- Veterinary Virology
- Immunology
- Epidemiology
Background:
- The carrier state in Foot and Mouth Disease (FMD) involves asymptomatic, low-level FMD virus (FMDV) shedding from ruminant oropharynx.
- Persistent FMDV infection arises from failed virus elimination post-acute infection, potentially involving immune complex phagocytosis.
Purpose of the Study:
- To investigate the mechanisms underlying the FMDV carrier state in ruminants.
- To explore the role of altered virus replication and immune evasion in persistent FMDV infections.
- To highlight the importance of cellular immune responses in understanding and potentially resolving FMDV carrier states.
Main Methods:
- Analysis of FMDV shedding patterns in ruminants.
- Investigation of virus-host interactions in persistently infected tissues.
- Assessment of viral cytopathology and replication characteristics of carrier isolates.
- Review of humoral and potential cell-mediated immune responses in FMDV carriers.
Main Results:
- Carrier cattle are epidemiologically significant in FMD field transmission.
- Persistently infected tissues show no histopathological changes, and carrier isolates exhibit reduced cytopathology.
- Altered FMDV replication (attenuation/interference) may facilitate immune evasion, rather than antigenic variation.
- Exaggerated FMDV-specific systemic and local humoral immune responses characterize the carrier state.
Conclusions:
- Less lytic FMDV variants are likely selected or generated in carrier animals.
- Altered viral replication, not antigenic drift, may be the primary mechanism for evading host immunity.
- Further research into cell-mediated immunity, particularly mononuclear cell interactions, is crucial for elucidating the FMDV carrier state.