3. ORIGIN OF SHEEP AND SHEEP PRODUCTION METHOD IN ARGENTINA
3.1 The origin of sheep in Argentina
The sheep of Argentina were derived largely from Europe commencing in the XVIth century. Significant imports, including from Great Britain, followed in the XIXth century, a time when scrapie was well known in some of the exporting countries. However, at this same period exports from Europe, especially Great Britain were made to Australia, New Zealand, South Africa and North America. None resulted in reports of scrapie in their new location. The reason for the non-occurrence of scrapie may just have been good fortune but more likely it was due to breed selection. Merinos from whatever source, Romney Marsh from Kent in England, Corriedale and Lincoln are seldom or never reported to have scrapie. Some other breeds from Great Britain were not suited to Argentine conditions and were culled. Black-faced sheep, such as the Suffolk, were not favoured because in Argentina the black wool diminished the value of the fine wool fleeces which was the raison d’être for the Argentine sheep industry.
Scrapie has never been recorded or reported in Argentina nor in any breeding sheep exported from Argentina to other countries without scrapie. According to Parry (1993) who had a specialised knowledge of Suffolk sheep scrapie, countries which imported sheep which subsequently developed scrapie were mostly of the Suffolk breed but with some incidents in Cheviots and Dorset and Hampshire breeds. However, none of these were favoured by Argentinean importers. Australia (in 1952) and New Zealand (in 1952-1954 and 1976-1977) imported Suffolk sheep that developed scrapie but in each case eradication was achieved by draconian slaughter of all affected and in-contact stock. The disease has never reappeared. By contrast Suffolk sheep imported into the USA from Great Britain, some via Canada, and reared intensively developed scrapie (USDA, 1991). Moreover, the disease has spread and has been difficult to control and eradicate. The majority of cases in the USA have been in Suffolks (88%). This breed which is strongly associated with the introduction of scrapie into countries previously free of the disease, has no importance in Argentina.
Parry, (1993) provided a list of breeds in which scrapie has or has not been found. Gordon (1966) and later, Pattison (1972) reported the famous 24 breed experiment. A variety of British sheep breeds were experimentally challenged with scrapie by the efficient intracerebral route of inoculation. There was a considerable variation in the incidence of disease between breeds as a result. At the time of these two reports the influence of the PrP gene on disease occurrence was unknown. In the 24 breed experiment scrapie negative animals could have been due to the unknown presence of resistant (long incubation period) alleles. Nevertheless, because between 30 and 57 sheep of each breed were inoculated, the experiment may have given some indication of the prevalence of such alleles should they have been responsible for the resistance to scrapie. Certainly, the results of these two studies fit well with the observations from Australia, New Zealand and Argentina that some breeds may be less likely to develop scrapie than others, even when imported from countries with scrapie.
Currently, the main breeds of sheep in Argentina are Merinos, mostly of Australian origin, Corriedale, Romney Marsh and crosses with Lincolns (APPENDIX A). Since 1990, there has been no importation of live breeding sheep into Argentina from a country that has reported the occurrence of scrapie to the OIE. Before this time, breeds were selected that have fine wool, white faces and points, and in which scrapie had either never been recorded, or was rare. In any event no scrapie case has ever been recorded or reported in Argentina.
3.2 Sheep production systems
Historically, with no known occurrence of scrapie, the Argentinean national sheep flock is regarded as having a low risk of harbouring scrapie. Importation criteria (see below) maintain the risk of scrapie introduction to a low level.
Endogenous risks of scrapie transmission are low for a number of reasons:
The future of the sheep industry is uncertain. Some diversification to meat production is occurring but the national flock is declining in size.