1. INTRODUCTION

 

1.1 Scrapie and Scrapie freedom

 

The main purpose of this publication is to make a risk assessment for the occurrence of scrapie in sheep, goats and moufflon in Argentina. It is recognised that because of the long incubation period of the disease, and the current absence of simple, practical, validated and internationally-accepted methods to detect infected live animals, whether clinically infected or not, considerably restricts the ability of authorities to show that scrapie infection is absent from a country. Currently the only document available internationally that could be used as a guide to conduct a risk assessment for scrapie in a country is the updated draft chapter on scrapie prepared for the OIE International Animal Health Code (OIE, 1999b). This has been selected as the best available approach for achieving recognition as a scrapie-free country even though at the present time it has not been approved for use by the International Committee of the OIE.

 

A secondary aim is to make a risk assessment for the occurrence of other transmissible spongiform encephalopathies (TSE) in sheep and goats in Argentina. This need stems from the fact that experimental challenge of Cheviot sheep and of goats by the intracerebral or oral routes with the agent of bovine spongiform encephalopathy (BSE) (see below) results in transmission of a scrapie-like disease in some challenged animals (Foster, Hope and Fraser 1993; Foster et al., 1996). The BSE agent is biologically different from any of the known strains of scrapie. The experimental disease, so-called ‘BSE in sheep’, cannot easily be distinguished from natural scrapie either clinically or pathologically. The agents that cause the two diseases can be readily distinguished by biological strain typing (see below) but this is expensive and takes over a year to accomplish so is not practical in the field situation. Recently developed molecular strain typing methods (Collinge et al., 1996; Hill et al., 1998 and Kuczius, Haist and Groschup, 1998), offer some hope of discriminating the different agents much more quickly and cheaply in the future though Hope et al. (1999) express some reservations. It is very important to emphasize that BSE in sheep has not been reported as a natural disease in sheep, goats or moufflon anywhere in the world. The importance of the BSE in sheep, should it ever occur naturally is because the biological strain of agent that causes BSE is indistinguishable from that of an agent that has caused a small number of cases a new variant form of Creutzfeldt-Jakob disease in relatively young people in the United Kingdom (see below), a country that has had a high incidence of BSE in cattle, though no known case of BSE in sheep. The next chapter will provide some more detail on the experimental disease, BSE in sheep. In the meantime there is sufficient evidence available that a system of surveillance that is effective in detecting scrapie is likely also to detect BSE in sheep and in goats should it occur as a natural disease. In the following pages we demonstrate that the system of surveillance for TSE in sheep and goats in Argentina is effective for detecting such an occurrence.

 

Based on our current knowledge, clinical scrapie whether caused by one of the several known scrapie agents, the BSE agent or indeed any other TSE agent is likely to be easy to detect by clinical observation by those aware of the clinical signs. In Argentina this has been until recently, the main means of surveillance.

 

Recent development of potentially useful (but as yet not validated) tests means that surveillance, as conducted to-day, falls short of the standard that is aimed for in the years to come. However, the current evidence from various sources suggests that scrapie is absent from Argentina and that any risks for its introduction are low. The ensuing chapters evaluate the risks for endogenous and exogenous (imported) scrapie in Argentina. The assessment is set within the framework of the OIE International Animal Health Code, draft chapter 3.3.8 (OIE, 1999b) on scrapie which is the only available international guideline relating to country freedom.

 

Scrapie is a progressive, fatal, neurodegenerative, transmissible spongiform encephalopathy of sheep. The disease has been known for at least 250 years. In flocks and countries where scrapie is established, the disease usually becomes endemic and is extremely difficult to eradicate. This is because the agent, or agents, that cause the disease are extraordinarily resistant to chemical, physical and natural methods of destruction, infected healthy animals cannot be identified and natural transmission can occur between sheep and between sheep and goats.

Scrapie can be transmitted from affected ewes to other related or unrelated sheep via maternal and especially horizontal, transmission from affected placenta (Pattison et al., 1972, 1974) and possibly from other sources (Hadlow, 1991). Hourrigan et al. (1979), showed also that there was an increasing incidence of scrapie in the offspring of affected ewes the longer the lambs were kept with the ewe after parturition. Infection may arise from contact with scrapie-affected sheep or goats (Brotherston et al., 1968; Pattison, 1972). The role of a contaminated environment is largely unknown (Detwiler, l992). The incubation period is presumed to equate with the mean and modal age of occurrence which is about 3.5 years in Suffolk sheep (Parry, 1983), but with a wide variation in this and other breeds. The disease is more frequently reported in countries of the northern hemisphere than the southern. It occurs particularly in Europe where severe epidemics occurred in the XVIIIth and XIXth centuries and from where many sheep breeds originated to populate southern hemisphere countries. Scrapie can also affect goats though the disease is much rarer than in sheep. Although moufflon are naturally susceptible to scrapie (Wood, Lund and Done, 1992), the disease is exceptionally rare in this species.

 

New Zealand (1997) and Australia are generally assumed to be scrapie-free despite many, but not all, foundation stock having originated in Europe and particularly, Great Britain. Within the British Isles, the incidence of scrapie is much more frequently recorded and reported in Great Britain than in Ireland, north and south which have similar low incidences. This is typical of the bizarre differences that appear to exist in scrapie incidence between neighbouring countries. The existence of scrapie in a country can seriously affect trade, especially in breeding sheep though throughout the centuries of its existence there is no known danger to man or to any other species, other than rare occurrences in goats and in moufflon.

 

The incidence and prevalence of scrapie in the world is incompletely known and, even in countries with the disease, the incidence is unclear as quantitative data are lacking. The scrapie status of many countries in the world is unknown.

 

The recent emergence in Europe of Bovine Spongiform Encephalopathy (BSE) (Wells et al., 1987), a new transmissible spongiform encephalopathy (TSE) of cattle, its natural and experimental transmission to a wide range of related and unrelated species (Bradley, 1996, 1997) and the occurrence of a new variant form of Creutzfeldt-Jakob Disease (v-CJD) of man (Will et al., 1996) has caused a concern to both animal and public health authorities world-wide. This concern has been exacerbated by the recent findings that the agent that causes v-CJD is biologically indistinguishable from that which causes BSE (Bruce et al., 1997). This agent is unlike any historical or contemporary isolate from sheep or goats with scrapie (Bruce et al., 1994). Furthermore molecular strain typing of isolates based on the fragment sizes and the glycosylation patterns of prion protein in Western blots following proteinase K treatment supports the view that v-CJD and BSE have the same cause (Collinge et al., 1996; Hill et al., 1998 and Kuczius, Haist and Groschup, 1998). However, there is still some difference of opinion (Somerville et al., 1997; Parchi et al., 1997; Hope et al., 1999) in regard to the standardization of methodology, practical value and use of molecular typing of isolates except to distinguish v-CJD from other forms of CJD.

 

As mentioned above although there is no evidence that BSE has been naturally transmitted to sheep and goats, it is experimentally possible to do so via the oral route (Foster, Hope and Fraser, 1993) which adds a further concern. This risk assessment takes account of this possibility. Some of these aspects are dealt with in the sister publication, BSE Risk Factors in Argentina (SENASA/INTA, 1998, SAGPyA-IICA-SENASA, 1999, in preparation). However, this volume covers surveillance for the occurrence of any TSE, including BSE in sheep, goats and moufflon in Argentina.

 

1.2 The objectives of this report

 

The objectives of this report are twofold. First to establish that ARGENTINA is a scrapie and TSE free country so far sheep, goats and moufflon (small ruminants) are concerned, determined so far as is possible by the criteria set in the OIE International Animal Health Code draft chapter (OIE, 1999b) on scrapie. The second objective is to prepare a comprehensive account of the sheep industry in Argentina in so far as it relates to the risk of having scrapie or other TSE in small ruminants or introducing scrapie into these species by importation.

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