Clinical variation in foot and mouth disease: sheep and goats

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506 Rev. sci. tech. Off. int. Epiz., 21 (3) A5/Allier/60 A22/IRQ/24/64 A22/IND/300/94 A/SAU/16/95 17.9% A/SAU/19/95 A/SAU/24/95 A/SAU/35/94 A/TUR/3/95 18 16 14 12 10 8 6 4 2 0 Percentage nucleotide difference (nt 475-639 of VP1) Fig. 1 Dendrogram showing the close genomic relationship between foot and mouth disease serotype A isolates from India and Saudi Arabia (N.J. Knowles, personal communication) has also been isolated from goat samples submitted from Bangladesh and goats imported from this country were responsible for an outbreak of Asia 1 in Kuwait. Isolation of other serotypes is rare, but does not necessarily indicate that infection of small ruminants with these serotypes does not occur; for example, Kuwait reported isolating South African Territories (SAT 2) virus from sheep during the incursion of FMD into Saudi Arabia during 2000. However, even in East Africa, where outbreaks due to serotypes O, A, C, SAT 1 and 2 are common, predominantly serotype O virus was identified in clinically affected sheep and goats. Transmission As is the case with other ruminants, sheep and goats are highly susceptible to infection with FMD virus by the aerosol route, with as little as 20 TCID 50 (tissue culture infectious doses) being sufficient for infection. Aerosol production by infected pigs can be as high as log 10 8.6 TCID 50 per day, theoretically sufficient to infect over 20 million sheep. Aerosol production by infected sheep, however, is considerably less, and whereas there are reports of airborne virus spreading from pigs over 250 km to infect cattle (France to England in 1981) (5), aerosol PAK/1/54 TAI/2/95 IND/26/95 14.8% IND/40/95 IND/233/95 IND/43/95 SAU/39/94 SAU/40/94 IND/51/95 18 16 14 12 10 8 6 4 2 0 Percentage nucleotide difference (nt 475-636 of VP1) Fig. 2 Dendrogram showing the close genomic relationship between foot and mouth disease serotype Asia 1 isolates from India and Saudi Arabia (N.J. Knowles, personal communication) © OIE - 2002
Rev. sci. tech. Off. int. Epiz., 21 (3) 507 O1/Kaufbeuren/66 O1/Manisa/69 15.4% O/IND/6/94 O/IND/9/94 O/SAU/15/94 O/SAU/62/94 O/SAU/65/94 O/SAU/58/94 O/SAU/72/94 O/SAU/100/94 O/SAU/28/95 O/SAU/14/95 O/SAU/20/95 O/SAU/77/94 18 16 14 12 10 8 6 4 2 0 Percentage nucleotide difference (nt 475-639 of VP1) Fig. 3 Dendrogram showing the close genomic relationship between foot and mouth disease serotype O isolates from India and Saudi Arabia (N.J. Knowles, personal communication) transmission from infected sheep is unlikely to occur over distances greater than 100 metres (7, 26). Sheep are also less likely to become infected by airborne virus than cattle because of their lower respiratory volume. Sheep and goats are probably most often infected by direct contact with infected animals. The virus may infect sheep and goats through abrasions on the skin or mucous membranes, through contaminated food, as well as by the respiratory route. During the FMD outbreak that took place in the UK in 2001, disease spread was reported to occur frequently by mechanical carriage of virus between flocks by humans or vehicles. Sheep-to-sheep spread by contact appears to be restricted, to the extent that the rate of transmission within an affected flock is lower than that observed in infected pig or cattle herds. A good example of this phenomenon is illustrated by the outbreak of FMD that took place in Greece during 1994. Serological investigations showed that in many of the affected flocks not all individuals had sero-converted to the virus, indicating that the virus had not disseminated sufficiently to infect entire flocks. In some flocks affected towards the end of the outbreak, only 20% of the sheep were sero-positive (21). There was suspicion, but little firm evidence, that the 1993 outbreak of FMD in Italy had also failed to maintain itself in affected sheep flocks. Similarly, evidence from the recent UK epidemic shows considerable variation in the level of intra-flock infection rates. On one farm visited, only 5% of 237 sheep that were blood tested were sero-positive, and 3% were viruspositive, whereas 91% of the 75 cattle present were clinically affected. On a second farm tested, 8% of 148 sheep were seropositive, 24% virus-positive, whilst 98 of 100 cattle showed clinical signs (1). A recent study by Hughes (14) has provided supportive evidence for the observed difference between the dynamics of FMD transmission in sheep populations as compared with cattle and pigs. The study showed that, using the 1994 Greek outbreak strain, there was significant reduction in the level of infection and estimated transmission rates over time during serial passage through groups of sheep. These results infer that some, possibly most, strains of FMD virus may die out if they are restricted to sheep. Infection of cattle or pigs may be sufficient to increase the level of circulating virus and consequently the probability of transmission of infection to © OIE - 2002
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Clinical variation in foot and mouth disease: sheep and goats

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