Virus Detection and Identification

"In the majority of cases a definitive diagnosis of FMD is based on the presence of FMD virus or antigen. Tests identify whether or not virus is present and also the specific serotype. When tissue samples cannot be obtained, diagnosis may be based on the demonstration of specific antibodies in serum samples." (J64.11.w2).

Detection of virus (virus isolation or detection of virus antigen) is generally carried out on epithelium from intact vesicles (blisters), fluid from vesicles, or the tags of epithelium from the edges of ruptured  vesicles (erosions). Blood may also be tested for the presence of virus (viraemia) and other tissues/secretions are also used sometimes. Cells and mucus from the pharynx, sampled using a probang cup, may be used for the detection of virus in subclinically infected animals and carriers (J3.77.w3, J42.118.w1, W18.Apl01.sib1).

• Virus typing by complement fixation (CF) test, enzyme-linked immunosorbent assay (ELISA) or polymerase chain reaction (PCR) may be carried out directly on material expected to contain large amounts of virus, such as fluid or epithelium from intact or recently ruptured vesicles. Samples which are negative must then be tested by growth in cell culture followed by virus typing of the cell culture supernatant. Samples which are less likely to contain large amounts of virus are tested by growing the virus from the sample in cell culture, followed by virus typing. (B58, B209, B210.89.w89, B216, B219).
• Tests such as the complement fixation test or ELISA, performed on fresh material from vesicles (blisters) or vesicular fluids, allow a definitive diagnosis within a few hours.
• (J64.15.w3, B58, B209, B210.89.w89, B211).
• Direct serotyping using ELISA is able to produce a positive result within three hours from the time the test is set up. (J64.15.w3). This is dependent on the presence of sufficient antigen in the sample. If a test result is negative at this stage a cell culture is required which will either increase (amplify) antigen to to a level where it can be detected or confirm the negative result.
• Repeated sampling of oesophageal/pharyngeal fluid and detection of virus in this fluid is required for the detection of carrier animals as an animal negative on one occasion may be positive at a later date. The sensitivity of testing is probably 50%; this can be improved by repeated sampling.(J3.77.w3, J42.118.w1).
• Recently, work has been carried out developing kits which, subject to satisfactory validation, could be used for rapid pen-side testing for FMD virus (J71.144.w1, P22.2000App18.w1).

Detection of antibody is carried out on serum samples.

• Traditional tests detect antibody to structural proteins of the FMD virus and the serum must be tested for each of the seven different types of virus. These tests can be used to determine which types of virus are/have been present in an area.
• More recently, tests have been developed which detect non-structural proteins. These tests detect infection with any type of FMD virus.
• The oldest of the tests for non-structural antibodies detect antibodies to "virus infection associated antigen" (D3). However, animals which have been vaccinated repeatedly with inactivated vaccines may be positive with this test (for example due to contamination of some impure vaccines with this non-structural protein and because the virus particle contains some of this protein in its capsid (V.w23)).
• In the last several years, tests have been developed, mostly based on ELISAs to various non-protein antigens, which are sensitive, specific and suitable for screening large numbers of serum samples. These tests are able to distinguish between animals which have been infected with foot-and-mouth disease virus (had virus replicating in them) and those which have not, whether or not the animals have been vaccinated.(J69.20S2.w1, J69.20S2.w2, J70.17.w2, J71.142.w1, J71.143.w1, J71.145.w1) However, these are not yet accepted by the OIE for the purposes of international trade.

• Literature Reports: Clinical-Pathological Test Findings (including serology)

• Literature Reports: Virus Detection and Identification Techniques

Definitive Host Species (Agent undergoes final stage of replication for transmission)

Foot-and-Mouth Disease is primarily a disease of domestic and wild cloven-hoofed animals (Artiodactyla); that is cattle, antelope, sheep, goats, deer, pigs and camels. However, a wide range of mammal species have been reported to have been infected with the Foot-and-Mouth Disease virus and reports of clinical disease have been recorded for at least 12 orders of animals.

The major host species for FMD virus are widely recognised to be domestic cattle (Bos taurus - Domestic cattle), sheep (Ovis aries - Domestic sheep), goats (Capra hircus - Domestic goat) and pigs (Sus domesticus - Domestic pig), whilst Syncerus caffer - African buffalo is considered to be the definitive host for FMDV serotypes SAT1, SAT2, SAT3.

Natural infection with FMD virus can cause serious disease in other species, including elephants (Elephantidae) and hedgehogs. A wide variety of species, including many rodents, have been experimentally infected by injection, but considerably fewer species have been infected by natural contact (either experimentally or in the field / wild).

Horses are accepted to be totally resistant to Foot-and-Mouth Disease virus and in general carnivores are resistant.

CARRIER STATUS

Animals have been recorded to "carry" the FMD virus without appearing clinically ill under the following conditions:

• after recovery from clinical disease.
• after undetected subclinical disease.
• on exposure of vaccinated animals to virus.

In all these situations, only a percentage of animals become carriers. The duration of the carrier state varies with species and has been recorded in Syncerus caffer - African buffalo for at least five years, cattle for up to 3 years, sheep and goats for up to 9 months. Some deer and antelope have been recorded to carry the virus for several months. Pigs have never been reported to become carriers.

Reports of carrier animals infecting susceptible in contact animals are EXTREMELY RARE.

Different strains of FMD have different capacities to establish persistent infection and the proportion of animals which become carriers following exposure to virus varies depending on the severity of the challenge, but does not appear to be influenced by the age or sex of the host.

The establishment of carriers is reduced in endemic areas if routine vaccination is used; this is probably related to reduced clinical cases resulting in less virus being present in the environment.

N.B. Vaccination does not make animals into FMD virus carriers: "Properly inactivated [killed] vaccine, when injected into animals, does not of itself give rise to the carrier state." (P5.40S.w2)

(B22.35.w7, B47, B58, B207, B209, B213.w1, B214.3.9.w1, B214.3.17.w7, B214.3.11.w4, B214.3.13.w5, B214.3.18.w8, B214.3.19.w9, B214.3.22.w10, B216, B217.38.w38, B218, D36.Para44, D36.Appendix II, J3.75.w3, J3.84.w1, J3.99.w2, J3.102.w4, J3.104.w2, J3.113.w1, J3.131.w1, J12.60.w1, J12.74.w1, J19.45.w1, J19.66.w1, J19.73.w1, J19.111.w1, J19.114.w1, J19.124.w1, J21.16.w1, J35.149.w1, J35.158.w1, J42.50.w1, J42.77.w1, J42.79.w1, J42.81.w1, J42.82.w1, J42.84.w1, J42.84.w2, J42.85.w2, J42.89.w1, J42.118.w1, J62.53.w2,   J64.7.w3, J64.7.w2, J64.15.w1, J65.11.w1, J65.12.w1, J71.57.w1, J72.41.w1, J74.46.w1, J75.20.w1, J76.47.w1, J77.14.w1, J78.3.w1, P5.40S.w2, P21.78.w1)

• Literature Reports: Virus Occurring in Specific Definitive Host Species