New concerns about' scrapie risk

Press, 23 June 1978, Page 16

New concerns about' scrapie risk

New voices have been added to the call for the cessation of breeding experiments at present being carried out by the Ministry of Agriculture based on the importation of exotic sheep breeds from the United Kingdom in 1972.

They are those of three veterinarians. who recently attended a course on management and diseases of sheep held under the auspices of the British Council in Edinburgh. They are Dr Helen Chapman, New Zealand-born but now working in Perth, and Mr D. J. M. Wood, who is in veterinary practice at Cheviot, and Mr Brian Mason, of Edendale. They all signet a letter to the “New Zealand Veterinary Journal” in which they express their concern about the risk to which the New Zealand sheep industry is being subjected and suggest “the termination of the breeding experiments now being carried out by the Ministry and fie complete elimination of all imported sheep, their contacts and their offspring.” Mr Wood said this week that they were hopeful that the Veterinary Association might add its weight to their call. Scrapie was first seen in New Zealand in Suffolk sheep in Mid-Canterbury in 1952, and there was a further outbreak of the disease in Southland in 1954 in South Suffolks from the same property. In 1972 Finnish Landrace, East Friesian, Oxford Down and Oldenburg sheep were imported into New Zealand from the United Kingdom. In 1974 crossbreeding began on Mana Island and in June 1976 in-lamb Romney ewes and their 1974 crossbred progeny were transferred to the Crater block in the Rotorua area. Soon afterwards an imported East Friesian ewe on Mana died from what was thought to be scrapie, and its mother also died suddenly 18 months later without displaying any unusual symptoms, but its brain was not in a condition for satisfactory laboratory examination.

Subsequently all East Friesian pure and crossbred sheep were destroyed and all Finnish Landrace pure and crossbred sheep, I which had originated from the same property in the United Kingdom. In all this : involved the destruction of j more than 700 sheep. Scrapie is a sub-acute : or chronic disorder of the central nervous system, I though it derives its name : from the rubbing and scratching of the skin, which is an obvious feature of the disease in most, but not all affected sheep. Speaking to the annual conference of the Peren-

dale Sheep Society of New Zealand recently, Mr Wood said the agent causing the disease was described as a “slow virus,” because of the prolonged incubation period, but in fact the agent had never been identified and could withstand boiling for 18 hours and formalin, ultra violet and ionising radiation and various enzyme treatments. There was no blood or other test to identify infected or carrier sheep. Mr Wood said that in the United Kingdom it was not uncommon for 30 per cent of sheep on a farm to die or be killed because of the disease, and in the Swaledale breed alone the cost of the disease was estimated at SI.SM a year. In their letter to the Veterinary Journal, the veterinarians suggest that the sheep were imported into New Zealand against the advice given by scrapie experts at the Moredun and Compton research institutes. The conference that the three attended was addressed by an eminent export on the problem, Dr J. T. Stamp, formerly director of the Moredun institute. They recalled that Dr G. H. Adlam, the director of the Animal Health Division of the Ministry of Agriculture, had earlier claimed in the journal that scrapie was not highly contagious, but Dr Stamp now stated that it was much more contagious than previously thought, with up to 50 per cent of contacts being affected. In his talk to the Perendale conference Mr Wood said work at the Mission Research Centre in Texas indicated a possible 25 per cent infection rate from grazing non-in-fected sheep on pasture where infected sheep had grazed, and in Iceland the indications were that infection either persisted for a very long time on pasture or in an intermediate host other than sheep or goats. The three veterinarians say that while the majority of deaths probably occur in sheep between three and five years old and that the -most important means of spread is vertical from the ewe to her offspring, cases could occur even up to 11 years old “and indeed it has been suggested that in many sheep the incubation period may be longer than the natural life span of the sheep. Thus an infected sheep may die of old age having spent its life dis-

seminating the disease without ever showing overt signs of infection.” If overt signs of scrapie were seen, or a post mortem diagnosis made, the present quarantine and recording system might be adequate to control vertical transmission, but as the route of lateral transmission was not known it would be impossible to be certain that this means of spread could be controlled. The placenta, or after birth, from ewes was known to be a source of infection, but as rams were frequently thought to be infective in the field and non-pregnant ewes had been shown experimentally to be contagious, Dr Stamp had suggested that the placenta was not always the source of infection. The disease was known to be able to enter the animal around the eyes, by contamination of a wound and orally, and the possibility of biological transmission had been put forward.

Dr Adlam had said that having imported many thousands of sheep in hundreds of shipments for over 100 years before the first case of scrapie appeared, the risk of introducing the disease was not very high. But in fact the risk had greatly increased since the early importations as it had become much more widespread in the United Kingdom as a result of greater stock movement.

The fact that a case had occurred in an East Friesian ewe on Mana Island must also indicate that the risk had become greater. The possibility that the sheep imported into New Zealand came from scrapie-resistant stock was of no comfort either, as after generations of breeding in a scrapie-free environment there was no guarantee that “resistance” had persisted, or

that the same strains of scrapie occurring at the time of importation were those likely to be introduced with present day imports. It had been shown that resistance to one strain of scrapie in both sheep and mice had entailed greater susceptibility to other strains. Since the sheep had been purchased in the United Kingdom, Mr Wood said this week that scrapie had appeared on one of the source farms. It did not necessarily follow that because departmental action eradicated the disease in 1952 and 1954 similar results or a similar pattern would occur in another outbreak for the following reasons

(1) A different strain or strains of the scrapie agent might well be involved, to which New Zealand sheep could be more susceptible. (2) If carriers showing no symptoms were released, years might elapse and the disease might be endemic before diagnosis was made. (3) There was no means available of detecting carriers.

(4) Symptoms of scrapie were often considered to be specific and this was frequently the case, but in some sheep the clinical signs were not typicalFor example Pruritus, or itchiness or scratching,

and debilitation, or loss of body condition, might not be present with affected sheep merely showing inco-ordination of movement. Other affected sheep might die quickly after collapse and paralysis. (5) Pruritus and subsequent skin damage was very likely to introduce secondary infection, which could be mistaken as the primary cause. Thus Contagious ecthyma, or scabby mouth, and Dermatophilus, or lumpy wool, were seen in the ewe born on Mana Island.

It had also been suggested that several other diseases might be caused by the same or a similar agent, and if in the future a relationship was established between these diseases there was a further reason for not importing sheep from the United Kingdom. However, the important point was that following this suspicion the United States Department of Agriculture had banned the use for human food not only of sheep affected with scrapie but also related or contact sheep or goats.

"Obviously if scrapie were to become endemic in this country, or probably even in the event of a single outbreak outside a quarantine station, we would be very likely to lose our sheep' meat trade with the United States.”