In Bhutan, small ruminants are an integral component of the traditional agricultural system. Government initiatives aimed to increase the quality of village flocks by providing superior rams. But in 1990 at Bhutans's National Sheep Development Project there was an outbreak of footrot associated with imported stud rams. Nevertheless rams continued to be distributed to village flocks.
Using information available from research in Australia, a similar footrot problem in Nepal was successfully managed by the application of specific vaccine. A similar approach was tried with the flock at Bhutan's National Sheep Breeding Centre (NSBC) and substantial progress was made in the first year of the project. The strain responsible for the disease there was identified and vaccine produced from it. Its use in a pilot trial and subsequently on the whole flock, demonstrated again the efficacy of that approach.
The NSBC had records that showed the villages that had received rams distributed during the five years preceding the ACIAR program. Logically footrot was likely to have been introduced into those villages. However, it was possible that footrot due to strains of Dichelobacter nodosus not originating from NSBC might also be present in Bhutan. It was recognised that before vaccine could be prepared for use in all sheep it would be necessary to determine whether strains other than those from NSBC were present, and this was the purpose of this project. The preferred approach was to use the animal health infrastructure in Bhutan to survey a representative sample of village flocks to determine whether or not footrot was present in the two categories of village flocks - those with known contact with NSBC and those with no record of contact. Armed with these data a specific vaccine could be prepared and tested.
This small project comprised three objectives: establish distribution of virulent footrot (VFR) and virulent strains of D. nodosus in Bhutan; isolate and characterise strains of D. nodosus for inclusion in a specific vaccine; establish the role in flock diagnosis of footrot for an ELISA test that had been designed to determine whether or not sheep apparently free of disease have had virulent footrot in the past.
The scientists surveyed districts for lameness in sheep and attempted to correlate it with the presence of the footrot organisms. A sample of village flocks was investigated clinically for evidence of footrot. Where there was evidence, samples were collected for bacteriology.
Using bacteriological techniques well established at the Regional Veterinary Laboratory in Bumthang multiple samples of D. nodosus from clinical cases were isolated and transferred to the University of Sydney. Samples were characterised for (a) serogroup, (b) virulence and (c) genotype. Only strains classified virulent were considered for inclusion in a vaccine. A specific vaccine was then prepared, based on these virulent strains.
This specific vaccine was used in a pilot trial in Bhutan. Approximately 20 sheep with footrot were purchased from village flocks and brought to Bumthang. They were vaccinated and their response to vaccine observed. Where scientists detected a high immune response to the confirmed strains but persistent symptoms they concluded this was evidence of additional serogroups. These were then isolated for inclusion in a modified specific vaccine.
It is sometimes difficult to decide whether the disease in a flock is virulent or benign. This would become easier if the ELISA test proved it had the potential to exclude many flocks from the expense of an unnecessary footrot eradication program.
The national survey revealed a wide variation in both the prevalence and severity of footrot across the country. The disease was most severe in the Bumthang district. By contrast, in many districts, few cases were seen in the sample examined and often only mild infections were recorded. In small groups like these it was not possible to arrive at a clinical diagnosis from the data collected. Because most of the evidence gathered in this investigation indicate that NSBC was the source of footrot throughout Bhutan it is surprising that there was such a wide variation in expression observed.
The national survey established that footrot was widely distributed throughout Bhutan and that this disbursement corresponded with the receipt of sheep from NSBC. Characterisation of all Bhutanese isolates suggested that for all practical purposes the strains of D. nodosus from such cases were identical with those from Bumthang and NSBC.
The earlier development of a specific vaccine targeting this strain led to the elimination of footrot from sheep at the National Sheep Breeding Centre and also from animals subsequently distributed throughout Bhutan by the Centre. Continued surveillance of the flock at NSBC did not reveal further cases of footrot two years after vaccination had ceased. The techniques for vaccine preparation were transferred to the Royal Veterinary Epidemiology Centre in Bhutan so the country can now deal with any outbreak in the future.
Aspects of the research done in this project continue to be applicable to the Australian sheep industry. Footrot is an ongoing problem in Tasmania and parts of NSW and Victoria. Targeted vaccination programs could make a valuable contribution to its management.
The ELISA test was successfully trialled and now has several potential uses, not least of which is application to sheep offered for sale or export. Its use could prevent future accidental importation of diseased animals, as experienced in Bhutan.
Links:
[1] http://www.aciar.gov.au/country/Bhutan
[2] http://www.aciar.gov.au/country/Nepal
[3] http://www.aciar.gov.au/programarea/Animal Health