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This article was published in 1962
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.

1961 Swine Fever Outbreak

N. K. GOLDING, B.V.Sc., Principal Veterinary Officer, Division of Animal Industry

INTRODUCTION

A paper dealing with the field and administrative aspects of this outbreak has been prepared for presentation to the Annual Meeting of the Australian Veterinary Association at Melbourne in May, and this paper will be printed in the April issue of the Australian Veterinary Journal. The information given here is therefore, almost a repetition of that paper with an emphasis on a few other aspects which have been brought to light by subsequent investigations

Detection of Outbreak

Diagnosis was finally established in May, 1961. However, there were serious mortalities in pigs reported and investigated in the outer metropolitan areas of Sydney during 1960. An attempt was made to transmit the disease early in 1960 from material submitted from one of those outbreaks in order to eliminate Swine Fever. Unfortunately, the mild febrile reaction given following the inoculation was overshadowed by the detection of Salmonellosis in all organs sampled, and the disease was then thought to be due to salmonella infection. At that stage, the possibility of a mild strain of Swine Fever virus was overlooked because of the experience in the two previous outbreaks with virulent strains in 1927-1928 and 1942-1943.

In February, 1961, a most intensive investigation was commenced on a piggery at Wetherill Park, where losses in young pigs and in adults had been over fifty (50) per cent over a few months. Specimens secured from this outbreak indicated the possibility of Swine Fever, although the pigs on which transmission attempts were made did not actually die. About this time, attention was directed to two heavy mortalities in pigs in the Newcastle area, and, as a result of the investigation into these two outbreaks, combined with that of the Wetherill Park outbreak, it was postulated that a mild strain of Swine Fever was present.

Further transmission trials at Glenfield confirmed this, and support was received from Dr. Loomis, of the University Veterinary School, when he examined specimens of brain tissue histologically. Representative samples of different tissues were forwarded also to Weybridge Laboratories in England and Cornell University, America, where the diagnosis was confirmed.

Action Taken

The usual action associated with an outbreak of exotic disease was instituted. Staff were alerted and transferred to deal with the main centre of infection. A quarantine area (Sydney Newcastle Quarantine Area) was gazetted, and store sales prohibited within this area. Movement of pigs and pigmeats, except canned meats, out of this area was prohibited, and close investigation made of all establishments feeding food refuse. Stock were allowed to be taken to three centres for sale for slaughter, viz: Homebush, Waratah and Maitland pig saleyards.

Shortly afterwards, the North Coast Protected Area was established, as it was realised that no Swine Fever was present in that area. Stimulus was given to this action because of the heavy concentration of pigs in that area. There are about 450,000 pigs recorded in statistical information as present in New South Wales; over 140,000 of them being situated within this Protected Area.

Concurrently with these legislative measures, slaughter-out policy was instituted where the disease was found to be present. The first herd was slaughtered out early in June, and in that month twenty-six (26) other properties were dealt with, In July, twenty-four (24) positive herds were diagnosed; almost all of these being situated within the Quarantine Area. The three which were outside at that stage were practically adjacent to the area, and there was good reason to believe that the disease had been, for all practical purposes, contained within the Quarantine Area, Subsequent events proved, however, that this was incorrect. In August, nine (9) properties were slaughtered out; in September-three; October-one; November-eleven; December-four; January-six; February-four; and March-six: making a total of ninety-four (94) outbreaks. The total number of pigs on these properties was a little over 13,000, and it is estimated that a further 2,000 pigs were slaughtered subject to compensation; whilst diagnosis was being made on these properties.

The position after two months "slaughter-out" was that only three (3) infected properties had been found outside the Quarantine Area; but in the remainder of the period, a further twenty-nine (29) affected properties were detected in various parts of the country outside the established Quarantine Area.

Centres of infection are present in the Narrabri, Tamworth and Inverell areas; in the Dubbo, Coonabarabran and Coonamble area; the Bathurst District; the southern Condobolin-northern Narrandera area; and in the Urana-Corowa Districts. All thirty-two (32) outbreaks in areas outside the Quarantine Area had been associated with these five sections of the State. Mortalities on 111 properties within the North Coast Protected Area were investigated, after the intensive campaign was instituted, and in no case was it possible to demonstrate Swine Fever. All except two (2) have had no suspicion directed towards them; except from clinical and post-mortem signs. Bacteriologically and histologically the vast majority were negative. There were two properties on which suspicion rested because of histological lesions in brain tissue. A transmission experiment was negative in one and a gel diffusion test of survivors was negative in the other.

Diagnostic Criteria

It can be seen from the number of properties dealt with in each of the months mentioned above that there was a very heavy accumulation in the first two months; following which the number of cases diagnosed decreased. The nature of the virus was such that reliance could not be placed on transmission experiments, as these were not always successful or easily able to demonstrate infection. To be certain whether transmission had occurred, the pig to which material had been given had to be kept for at least three weeks, challenged with virulent virus, and kept for another two or three weeks before it could be stated with certainty that it had resisted challenge. Such a method was obviously impracticable, and within the Quarantine Area it was decided that where the history, clinical aspects and post-mortem lesions were indicative of the disease, confirmation would not be obtained from the laboratory before slaughter out was instituted.

It is possible, therefore, that some herds which are included as outbreaks in the records were not infected; but it also is probable that the majority were. As the pressure on the laboratory eased, more reliance tended to be placed on the examination of brain tissues for outbreaks within the Area. For outbreaks outside the Quarantine Area, it was decided that infection actually must be demonstrated in some way or other. This generally took the form at that time of challenge of recovered field cases, as this tended to short circuit the time necessary to establish a diagnosis. However, as a perusal of the paper presented by Messrs. Keast, Littlejohns and Helwig will show, this had some unsatisfactory features also. The pigs presented for challenge, although showing clinical signs resembling the disease, sometimes failed to show resistance to the virulent strain or else died within the expected period after exhibiting no temperature changes.

About this time, the gel diffusion precipitin test started to show promise and now reliance tends to be placed rather heavily on this test; firstly as a means of selecting animals for challenge and secondlyly, where there is a history of known contact with infected properties, as a final diagnostic method.

CLINICAL SIGNS

Inspectors are aware of the clinical signs, which have been well described in circulars setting out the action required of them in the outbreak. Briefly, these are fever, inappetence, inco-ordination, scouring and pneumonia; together with haemorrhages or erythema of the skin. Much can be said about these signs. They are not characteristic and have been seen in most of the common infectious diseases causing septicaemias in pigs.

POST-MORTEM LESIONS

Information concerning these also has been circulated in detail. Again, briefly, the lesions most commonly seen are haemorrhages in the lymph nodes, particularly towards the peripheral; apical haemorrhages in the kidneys; haemorrhages or ulceration of the mucous membrane of the caecum and colon, the contents of which often contain free blood; haemorrhages in the heart; acute broncho-pneumonia involving apical and cardiac lobes, generally in the stage of red hepatisation; and petechial haemorrhages on the surface of the lungs. Less common lesions are haemorrhages in the urinary bladder and gall bladder, and on the larynx.

These lesions also are not specific for Swine Fever. Further more, they are not present in all pigs examined, and it is quite common to find practically no lesions at all in pigs which die of the disease or are slaughtered in the terminal stages. It is also common to find most of the lesions present in some of the pigs examined, and, of course, there can be any variation between these two extremes. The picture describes haemorrhages from a general consideration of the clinical aspects and the post-mortem lesions: where mortality is continuing and a series of post-mortem examinations is made. Mortality may be negligible to severe, depending on the conditions and concurrent infections with other organisms. Some assistance occasionally is derived from consideration of the history of the outbreak, particularly with respect to the failure of antibiotic and commonly used drugs to control the disease. Any mortality investigated which has some features which fit into this pattern needs to be regarded with the utmost seriousness; and this aspect cannot be emphasised too strongly.

Attention has been drawn to the introduction of legislation which will give greater control of boiling of food refuse; in the elimination of store pig sales in country areas, except the North Coast Protected Area; and in the fencing of properties suspected of being infected. Appropriate notification has been made by the Minister requiring all owners of pigs to fence the land sufficiently to prevent the ingress or egress of pigs, and this means that if Inspectors consider that a pig should be properly enclosed, an appropriate notice to fence should be issued. This power, however, should not be exerted unless there is a real need to contain the pigs.

Mention was made also of the possibility of infection persisting In lungworm larvae, which have the earthworm as an intermediate host. Four (4) properties have been re-infected, but greater suspicion rests on the areas from which the pigs were obtained than on the persistence of infection on the properties concerned.

RESULTS ACHIEVED

Judging by the records of condemnations for Septicaemia at metropolitan and country abattoirs, Swine Fever now must be at a low ebb. From late 1959 until the middle of 1961, when eradication measures were instituted, graphs showed a very steep rise in the number of pigs condemned for this condition. Many of them would have been infected with Swine Fever, and this is in marked contrast to the current figures. There has been no outbreak in the Maitland Pastures Protection District. i.e. the northern half of the Sydney-Newcastle Quarantine Area, since August 1961, and this is indeed a cause for satisfaction. Pig mortality generally is now very light, but it is essential that Inspectors should not relax their efforts in the slightest. Indeed, now is the time to make an all-out endeavour to eradicate. Such would be assisted greatly by the more widespread use of a satisfactory laboratory test. Nevertheless, we must push on whether this is available or not. Much has been achieved, but much still remains to be done.


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