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This article was published in 1963
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.

Swine Fever Developments During the Past Twelve Months and the Future Outlook

N. K. GOLDING, B.V.Sc., Principal Veterinary Officer, N.S.W. Department of Agriculture.

At the end of March, 1962, the Swine Fever eradication campaign had been in progress in New South Wales for almost a year. During that time, the disease had been detected, according to the criteria adopted for diagnosis, on ninety-four properties, with a pig population of a little over 13,000. All pigs on properties where the disease had been diagnosed were destroyed.

At that time, the outbreak appeared to be settling down quite satisfactorily, from the point of view of eradication. The number of cases was declining, and the severity of the outbreak had subsided quite markedly. Whereas in the earlier part of the campaign it was not uncommon for mortalities of the order of 10% to 20% to occur, it was apparent at this date that, even on properties where diagnosis had been confirmed, the mortality was light.

By June, 1962, sufficient confidence had been developed in the accuracy of the gel diffusion precipitin test to utilise that test, solely, in arriving at a diagnosis, but pigs, on occasions, were still being challenged with a virulent virus.

In this month also, positive diagnosis was made for the first time in the North Coast Protected Area, when a reactor to the gel diffusion precipitin test was detected in serum samples submitted for Brucellosis testing. Follow-up tests on the remainder of the herd revealed further reactors, but, subsequently, all properties to which pigs had been distributed, and from which the pigs had been secured, were examined by the gel test, without detection of reactors.

In July, an outbreak was confirmed at Crookwell. This property had been one of the first suspected in May 1961, but, in spite of transmission and challenge tests, which were all that were then available, and intermittent investigation of mortalities, confirmation could not be obtained until sera were submitted during the month. In view of the history, pigs on the property were destroyed and the usual action taken.

Very few cases were detected in July, August and September. In September, however, it was decided that the incidence of mortality, in piggeries in which positive diagnosis had been established by the gel test, was so light, or completely absent, that there was no justification for continuing a slaughter-out policy in all cases. This idea was precipitated by a positive diagnosis in a Government herd at Kenmore, where there had been evidence of a light septicaemic mortality over several years. In view of its situation and ownership, it was decided that it would be a suitable herd on which to conduct further observations. Slaughter-out was therefore not instituted, and a series of observations were commenced, to which reference will be made later.

In October, 1962, the first evidence was produced that the disease was present in feral pigs, two such pigs having reacted to the gel test. One of them was submitted to challenge with virulent virus, and completely resisted. The other was destroyed for transmission tests, which were negative. In this month, the abattoir survey commenced in apparently healthy bacon pigs slaughtered at abattoirs in the Protected Area, including those slaughtered in two abattoirs operating in the Northern Tablelands.

In view of the seriousness of the situation, and the apparent widespread incidence of the disease in New South Wales, a meeting of the Consultative Committee, i.e. the heads of the veterinary services in the States and Commonwealth, was held in Sydney towards the end of October, 1962, and it was decided that the abattoir survey would be extended not only to pigs slaughtered at other abattoirs in New South Wales, but also those slaughtered at abattoirs in other States.

In November, the last piggery in which serious mortality occurred, presumably from Swine Fever, was slaughtered out. At any rate, diagnosis was made following the history, clinical investigation and the high proportion of positive reactors to the gel test in a screening which was put on the piggery.

Abattoir surveys continued in December, 1962, January and February, 1963, when it was evident that the incidence of reactors in slaughter pigs of bacon age was something of the order of eight per cent.

As far as possible, all piggeries, from which doubtful and positive reactors were found in slaughter pigs, were traced and placed in quarantine. Those where doubtful reactions only were obtained in slaughter pigs were screened about a month later, and those which had positive reactors were maintained in quarantine without screening. It was intended that all properties in which positively reacting pigs had been found should be screened prior to release, or, in the event of mortality of a significant nature developing in the meantime, that they should be slaughtered out. The latter provision, however, has now been abandoned, and it is intended to release the piggeries at the expiry of six months without further testing, provided no untoward sequelae develop.

In February, further administrative action was taken to remove the General Quarantine Area restrictions, and this was done by cancelling the original notification and re-gazetting the General Quarantine Area to include the Sydney - Newcastle Area. Later in the month, the North Coast Protected Area was modified by removing the Tenterfield District from the Area, as it was found impossible to police the movement of pigs and pigmeats into and through this portion of the Area.

In February, also, there was a further meeting of the Consultative Committee, and the situation within Australia was discussed. At the time of writing, it is not possible to provide details of the discussion as this Committee must report to the Standing Committee on Agriculture, and the report may, or may not, be made public.

So now, at the end of March, 1963, the campaign has been in progress for almost two years, and some 201 positive diagnoses have been established, involving a little over 22,000 pigs. Approximately sixty properties are being held in quarantine in various parts of the State So far, mortality of a significant nature in any of them, with a few possible exceptions, has not occurred. In any event, no property has yet been slaughtered out. The last mortality of a significant nature occurred in November, 1962, and it appears from the clinical point of view that Swine Fever has been eradicated. This, of course, is not true in fact, as further diagnoses continue to be made, and will continue to be made in the ensuing months.

As mentioned previously, abattoir surveys were commenced in August, 1962, and subsequently were extended to abattoirs in other areas. Figures are available for 1,633 pigs, of which 87 gave positive and 49 doubtful reactions (8.3%). There were 14 reactors in 513 pigs slaughtered at abattoirs within the Protected Area (2.6%). At other abattoirs in New South Wales there were 122 reactors in 1,090 pigs (11.2%), approximately four times as many as the number of reactors found within the Protected Area.

From feral pigs, some 205 samples have been tested, of which 11 have reacted. Reactors have come from Bathurst, Canonba, Deniliquin, Hay, Hillston and Moree Districts, so it can be seen that infection is widely distributed throughout the feral pig population.

The position in other States also gives cause for concern. Queensland and Western Australia have admitted publicly that Swine Fever caused by a virus of low virulence is present in both States. It has also been stated that no other State will furnish a certificate to the effect that Swine Fever is not present in the particular State concerned. However, no statement has been issued from any State except Queensland and Western Australia that Swine Fever is present in it.

As a result of the experiences in the field, experiments in the transmission of the virus have been commenced in three Government herds, and it is hoped that these will reveal information which will enable the disease to be combatted more effectively.

In the first herd, the disease was discovered when the herd of 15 breeder pigs was screened following a light mortality. Three Sows reacted. The experiment to be conducted on this property is to leave the sows in the herd and make no changes in the normal management. The herd is to be bled every three months, and any mortalities investigated fully to determine the cause, if possible. Up to the present time, the herd has been bled twice, but no further reactors have been found. The third bleeding will be carried out in the next few weeks.

In the second herd, a diagnosis of Swine Fever was established following the submission of a brain specimen from pig which died in August, 1962. The histopathology was typical of Swine Fever and, accordingly, the piggery was screened. Three reactors were detected among thirty sows, which were screened. The herd was then bled in September, two doubtful reactors being obtained. It was bled again in November, when a positive and a doubtful reactor were found; in January, when no reactors were found; and in March when one doubtful was found. In this experiment reactors are removed from the herd as soon as possible, and all mortalities are investigated. The three positive reactors detected at the first test were submitted to challenge with virulent virus, and resisted; the two doubtful reactors obtained at the next test were included in transmission trials, with negative results; and the positive reactor and doubtful reactor from the test in November were taken to Glenfield and stressed to see whether virus could be obtained. This was unsuccessful.

Ninety deaths have been investigated in this herd, suckers numbering 49. Of the rest, 25 were weaners, 12 porkers, 3 baconers, and one was an adult. Most of the deaths were caused by an infection with one organism or another, Salmonella cholerae-suis being found in 22 pigs, Salmonella typhi-murium in two, and the rest split up amongst Haemolytic Streptococci, Staphylococci, and various coliforms. No organisms of significance were recovered in twenty-two cases, and specimens were not submitted from three. The preponderance of Salmonellosis is the factor of note. Since September, 1962, only two brains have shown lesions of the ninety exhibited. One was suggestive and the other typical. The latter was from case No. 90, the last one examined.

The laboratory investigations were concentrated in an attempt to recover the virus from any pigs dying during the observations. Up to the middle of January, spleen or blood from sixty-five cases was used for transmission trials, with negative results in each case. In this work, pooled material, from as many as ten cases, was inoculated into single transmission pigs. In one case where a suspicious febrile reaction occurred following inoculation, the ten samples comprising the pool were used on ten individual pigs, again with negative results. Four gel reactions have been examined at Glenfield, two of them being stressed with heated serum, but it has not been possible to isolate the virus from any of these animals.

In the initial investigation in this property, the dairy herd was examined by gel test, samples from fifty-four cattle giving nineteen positive, eight doubtful, and twenty-seven negative reactions. Faecal samples were collected from the fifty-four members of the herd and given orally to two calves, which showed no febrile, clinical or serological response over a period of twelve weeks.

In the third herd, mortality of a light nature has been experienced from time to time for a year or two without suspicion being directed towards Swine Fever. Mortality is mainly in the sucker classes. After screening, three reactors were found.

In this herd ,it was decided to study the extent of transmission to two known non-infected pigs and their progeny. Accordingly, two sows from Hawkesbury Agricultural College, which were known to be free of infection, were reacting sows, which were then maintained as a group.

This group will be followed through by gel testing regularly. The first general bleeding showed that, in the rest of the herd, one of the progeny of one of the reacting sows gave a positive reaction, but this pig was not included in the group as it had been weaned prior to the formation of the transmission group. No further reactors were detected in the remainder of the herd.

Further experimental work is being carried out at the Veterinary Research Station on the transmission and study of the virus generally.

Previous work during and subsequent to the 1942-1943 outbreak had shown that the virus then present was still viable in chilled pork after five years; even then, the end point had not been reached, but in bacon it could not be recovered after approximately one month. Further studies were undertaken with the virulent strain obtained from overseas, and it was shown that this virus persisted for periods up to eighty-five days in mild cured ham held in tins at 40 degrees F. when the source of the material was unheated.

The virus could not be demonstrated in one pound cans sealed after heating at 180 degrees F. for either fifty seventy-five or one hundred minutes, heating processes consistent with commercial processing.

The viability of the virus in sausage casings was also examined. In commercial processing, where immediate use of casings is required, the immersion of the casings in brine, or their storage in salt may be omitted. Casings prepared without salting from a pig recently infected with virulent virus were infective, when 18 inches of the casings were fed. When the casings were soaked in a 20% brine solution at 40 degrees F. for twenty hours, then soaked in water for four hours, they were not infective. In both these experiments, adequate controls were kept.

The examination of the results of the gel diffusion precipitin test and those given by the serum neutralisation tissue culture test carried out by C.S.I.R.O., at Parkville, using a cytopathogenic strain of Swine Fever virus obtained from Dr. Baker, at Cornell, has shown that both tests are accurate in that few, if any, false positives are given. Both tests, however, give some false negatives. The serum neutralisation tissue culture test is costly and somewhat cumbersome, whereas the gel diffusion precipitin test is practical, but, at the present time, costly. The cost of the latter test has been estimated to be, at the moment, 10s. Od. per pig. If it were necessary to purchase all supplies of disease-free pigs, This is the reason why the research work on the test is concentrated towards recovering increased quantities of antigen. If this can be achieved, it will progressively lower the cost. Some progress has already been made with the harvesting of a soluble antigen, and when further equipment is received from overseas, this work should receive additional emphasis.

Isolation of new strains of the New South Wales virus, and their comparison with the strain previously isolated in 1961, also have received high priority, but, despite the intensive investigation, particularly in two of the three Government herds under close observation, has been, so far unsuccessful.

Following the detection of positive reactors to the gel test in cattle, it was decided to ascertain whether the New South Wales strain of Swine Fever virus was transmissible to calves. Two pigs inoculated with 1 ml. of infected blood sickened and died on the twenty-second day; both gave a suspicious gel reaction the day before they died. Two calves, aged five to six months, maintained in close contact with these pigs, failed to show clinical or serological response up to ninety-one days, when the experiment terminated.

Two calves, four and six months of age were inoculated subcutaneously with 1 ml. of infected blood (N.S.W. strain) but showed neither clinical nor serological response up to ninety-one days. Two pigs maintained in close contact with the calves over the whole period similarly failed to react.

This brings up the question of the identity of the virus operating in New South Wales. The facts that mortality has now subsided, and positive reactors to the gel test have been found in high incidence in cattle, have led to the supposition that Swine Fever virus is not present, but that some other virus primarily affecting cattle, and also transmissible to pigs, is present. Many countries have now recognised the presence of Swine Fever virus of low virulence. Dr. Baker has demonstrated that there is a cross reaction between the virus of Virus Diarrhoea and Swine Fever, but so far there is no evidence that natural transmission between cattle and pigs occurs. It is true that pigs feeding among the droppings of cattle in feed lots in America do react to Virus Diarrhoea, but no transmission has been demonstrated from pig to pig. Indeed, there is no evidence anywhere in the world that Swine Fever will spread from cattle to pigs.

It is the opinion of Dr. French and other eminent scientists in the C.S.I.R.O. that the virus which causes the development of reactions both to the tissue culture serum neutralisation test and the gel diffusion precipitin test is that of Swine Fever. While admitting the slight possibility that a new virus has been found, Dr. French and his colleagues state that the vast preponderance of evidence points to Swine Fever.

This then brings up the question of what lies in the future for New South Wales, and perhaps the rest of Australia with respect to this disease. With the tools which are now available, it is possible, by serum test, to ascertain whether a herd has been exposed to infection. These tests, however, do not show whether the virus is active, or whether it has passed through the herd and has disappeared. The disease appears to be one of low virulence and low transmissibility in the field, if the experience in the three Government herds under close observation is any criterion. It is possible therefore, for the disease to enter a herd, infect a few pigs and die out. This obviously does not happen in all cases, for if it did, the reactor which developed in the progeny of one of the reacting sows in the third Government herd, referred to above, cannot be explained, nor can such happening explain the presence of a positive histopathology developing in the second Government herd after ninety cases had been investigated, and in the presence of a complete, negative serological test two months previously.

Some authorities consider that the disease will persist in earth worms following the ingestion of lung worm eggs derived from lung worms in pigs infected with Swine Fever. There is no proof that this occurred in New South Wales in the two previous virulent outbreaks eradicated. Although the periods involved in these outbreaks were not extensive, they were of sufficient duration to allow this factor to operate. It would seem, therefore, that this is not important under Australian conditions.

The future is most uncertain. On the one hand, it is obvious that a full scale eradication is likely to fail, because of the presence of the disease in the feral pig population and the difficulty of diagnosing transient infections in commercial piggeries. The seriousness of the disease, as manifested over the last six months, makes it doubtful whether it is economically feasible to insist on a full-scale eradication. At the same time something must be done where piggery is found to be infected and is experiencing mortality which could well be due to Swine Fever. Vaccination has been suggested as a possibility, but it is costly, and, at this stage, not worth the cost involved in manufacturing the vaccine. The experimental work, particularly in the second Government herd referred to, is such as to offer some hope that blood testing may control the disease in infected herds. In any case, it is impossible to foresee what action will be taken in other States, and unless there is a uniform policy throughout Australia, a full-scale eradication cannot be carried out in one State alone. The present policy operating in New South Wales is a "wait and see" policy. A forecast may be possible in another twelve months.

It is desired to emphasise that all the work recorded in this article was done by others in the field and in the laboratory. Opinions expressed by officers of the C.S.I.R.O. and other state Departments have also been included, and it is trusted that this will serve as a grateful acknowledgement of the assistance and cooperation of all concerned.


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