This session reviewed the characteristics of HC viruses isolated in the USA and persistent congential infection of swine with this virus; the behaviour of BVD virus in swine; the potential role of insects in the transmission of HC and the pathogenesis of HC in relationship to its spread by direct contact.
Eight of 135 (c. 6 percent) field isolates made in the USA from 1965 to the present caused inapparent or immune tolerant infections lasting up to 152 days. During this period, the pigs had high titres of virulent virus in their blood but were relatively free from clinical infection. Of the 135 isolates, approximately 45 percent were of high virulence, 27 percent of low virulence, 22 percent were avirulent and 6 percent caused the persistent infections described. The most recent outbreak in the USA occurred in garbage-fed herds located in four eastern states of that country and it has been attributed to the surreptitious use of HC vaccine. An outbreak that occurred in Texas in 1975 did not appear to spread despite the fact that it was caused by a fully virulent virus. Two other outbreaks in 1974 were caused by an avirulent virus (Mississippi) and one of low virulence (Puerto Rico).
Natural infection of a pig with BVD virus has been detected in a herd in which pigs showed only fever and some evidence of clonic spasms. Immunofluorescence was detected in cryostat sections of the tonsils of this pig by routine tests but BVD virus was isolated and identified after isolation in PK 15 cells. The BVD isolate inoculated into a calf produced a neutralizing antibody titre of 256 but not detectable HC antibody. When gilts at 38, 53 and 54 days of gestation were inoculated with BVD virus and killed at about 50 days after infection no virus was detected in any of the normal foetuses or one mummified foetus. From this it was concluded that BVD virus does not cause transplacental infection in swine as readily as does HC virus although it was noted that the period of gestation in which this could occur might have been different from the times used.
It was noted that at least two years after HC had been eradicated from Australia some pigs had HC neutralizing antibody but the titre was always higher against BVD virus in these sera. The percentage of positive sera was greater in pigs over 12 months of age. It was noted that it is now accepted that BVD virus has a wider host range than previously recognized. When pigs were exposed to a non-virulent strain of HC virus, they produced neutralizing antibody to this virus only, and following challenge with virulent HC virus these pigs reacted with an increase in HC antibody and only a very low or no titre to BVD virus. Pigs inoculated with the TVM2 strain of BVD virus did not show any clinical signs of infection but virus antigen was detected by immunofluorescence in a wide range of tissues of the pigs killed one to six days post infection. It was observed that inoculation of pigs with high tissue culture passage levels of the TVM2 virus gave greater protection against challenge with virulent HC virus than did lower passage levels. This is at variance with results reported from Australia with a number of isolates of BVD isolated in that country. There was some indication that survival was greater following challenge with the Brescia HC strain than occurred following challenge with the Ames strain. This result may indicate some antigenic difference between these two HC viruses since they are considered to be equally virulent for pigs.
Sows experimentally infected with a Dutch low-virulent strain of HC virus produced piglets with a prolonged period of viraemia. These piglets exhibited a runting syndrome at seven weeks of age, and they had a viraemia until death which occurred between 38 and 189 days (average: 93 days), after birth. Virus was excreted by these animals throughout their life.
The role of insects in transmission of HC: Field epidemiological observations have suggested that farmyard flies may be involved in the spread of HC virus Experimental studies with tabanids, Stomoxys calcitrans, Musca domestica, Musca autumnalis and Aedes aegypti have demonstrated transmission of HC virus to susceptible swine by allowing these insects to feed on an infected pig. There was no evidence of multiplication of the virus in the insects, but the results suggest that insect suppression on infected premises might help to reduce the spread of HC to other susceptible swine.
Pathogenesis of HC in relationship to its spread: Pathogenesis studies using fluorescent antibody have clearly demonstrated that infected reticulo-endothelial cells are responsible for the lesions in HC infection and that infected epithelial cells contain a large amount of viral antigen. The epithelial cells of the respiratory, gastro-intestinal and urinary tracts, as well as epithelial cells of the sebaceous, sudoriferous glands of the skin, pancreas and salivary glands of infected pigs contain very large amounts of fluorescent antigen. It is considered that large amounts of infectious virus are disseminated from infected pigs in expired air, saliva, faeces, urine, dandruff and other skin debris and that virus from these sites does not usually come in contact with antibody in an infected pig.
Infection of swine is probably most frequently acquired by the ingestion of contaminated material which enters the host by way of the epithelial cells of the oral and pharyngeal tissues. Inhalation of virus is a likely factor in the spread of HC among closely confined pigs but may play little part in herd to herd spread of the disease.
Papers presented in this session covered inactivation of HC virus in tissues by gamma irradiation and thermal treatment; transplacental HC infection in immune and non-immune sows; characteristics and advantages of safe and effective live HC virus vaccine such as the Chinese strain and two different “cold” strains; and prophylactic measures taken in Bulgaria.
As the HC eradication programme has reached the final phase in the USA, the presence of the virus in laboratories could be suspected as a possible source of infection. Gamma iŕradiation can be used for inactivation of HC virus without significantly altering the viral antigens in tissues for fluorescent antibody tests. Complete inactivation of the virus at 6 M.rad was confirmed by inoculation of 300 samples into 25 susceptible animals.
For the development of a safe method for the thermal processing of waste food products, an attempt has been made in the USA to determine the time (in seconds) required for any given temperature or the temperature required for any given time. Preliminary findings on a thermal inactivation curve for HC virus were presented.
Transplacental HC infection: Studies were performed in susceptible and immune sows, involving the use of attenuated virus vaccines, field strains of low virulence and a highly virulent strain. Transplacental infection occurred frequently in susceptible sows and infrequently in so-called immune sows. In the latter sows, transplacental infection occurred only in those animals having a severe clinical reaction after infection. In addition, transplacental infection did not occur in sows having a high level of serum neutralizing antibodies before inoculation. The most important factors seem to be the virulence of the strain and the level of immunity in sows at the time of inoculation. These results suggest that vaccination performed in young sows before puberty could induce high immunity levels able to protect foetuses of subsequent litters against transplacental infection.
The Chinese strain vaccine: Lapinized Chinese “C” strain has been used in Hungary, Poland and the Netherlands and the safety and immunogenicity of the vaccine have been confirmed. In three areas of the Netherlands with a high incidence of HC, all pigs over two weeks of age have been vaccinated with the C-strain vaccine. The results of the vaccination campaigns in the Netherlands suggest that HC can be eradicated from enzootic areas by a strict vaccination regimen pursued for one year.
In Hungary, the safety and potency of the Chinese strain have been compared with those of other attenuated strains. The C-strain proved safe for pregnant and nursing sows as well as for newborn and older piglets. Small quantities of the virus were excreted from some of the vaccinated pigs. This live virus vaccine (C-strain) was considered an excellent tool in preparing the ground for the complete eradication of HC, now achieved in Hungary.
In Poland, studies on the C-strain vaccine have shown that the virus is nonpathogenic for pregnant sows, foetuses and newborn piglets. Attempts to reverse the virulence of the strain by passage through suckling piglets failed. The results of laboratory experiments and mass vaccination in the field over a period of 12 years have proved that the Chinese strain of HC vaccine is very safe and has excellent immunogenicity.
Experience in the prophylaxis and control of HC in Bulgaria was reported. General prophylactic measures and the vaccination scheme with lapinized strain K in Bulgaria were summarized. Experience over the last two years has proved the effectiveness of the scheme from the epizootiological and economic viewpoints. Bulgaria is free of HC at present.
Cell culture vaccines: Live virus cell culture vaccines were developed in two different laboratories in Europe. In Czechoslovakia, a line of avirulent virus was isolated, using the terminal dilution technique, from the virus which had been passaged in cell cultures at 30°C. This selected virus was avirulent for pigs of various ages and retained its immunogenicity. Pigs vaccinated subcutaneously with 1,000 immunizing doses were protected against contact infection 24 hours post-inoculation and against parenteral challenge 5 days post-inoculation. The immunity lasted at least one year and vaccinal virus did not spread by contact. Pigs exposed to aerosol vaccination for at least 20 minutes developed immunity. Approximately, 50,000 weaning pigs were successfully vaccinated with this new vaccine.
In France, a clone called “Thiverval” derived from the virulent Alfort strain was isolated in cell cultures maintained at low temperatures (29° - 30°C). This strain of HC virus had lost its virulence for pigs even for pregnant sows while retaining its immunogenicity. In contrast to virulent strains which were “hot” and “solid”, the “Thiverval” strain was characterized as “cold” and “fragile”. Experiments performed in the laboratory and also in the field have proved its safety and genetic stability. This new live virus vaccine is successfully used in France at the present time.
The three papers presented in this session illustrated the results of different systematic programmes to detect, prevent and eradicate HC.
In Italy, systematic vaccinations of the entire pig population with C-strain vaccine and a stamping-out policy of all infected herds were started in 1968 resulting in a marked reduction in the number of reported outbreaks. During the same period, African swine fever occurred in the country, posing a serious risk to the Italian pig population, but the vaccination programme against HC facilitated the surveillance for ASF and the disease was later eradicated.
From 1968 to 1975 approximately 57 million pigs were vaccinated against HC with an annual average of over 7 million animals. In 1974 and 1975 there were no outbreaks of HC in peninsular Italy but Sardinia remained infected. This result illustrates the effectiveness of the strategy of vaccination and control measures taken by the Government of Italy.
In the next paper, the organization and function of the emergency programmes in the USA were described. It was obvious that a nationwide eradication and control scheme had to make use of modern resources in its planning and performance. The necessity of careful organization of the field operations in case of emergency as well as the need for satisfactory financial compensation to the owner, when herds had to be slaughtered out, was stressed.
The epidemiological survey of HC in the EEC member states illustrated the way to plan and consider eradication of this disease by coordinating different national programmes. Regular data collection was necessary to develop and adjust the control strategy. Furthermore, the economic analysis of costs in existing programmes compared with prospective alternative plans create a necessary tool for decision making. It was discussed how better cooperation among all European countries could be organized.
Diagnosis, immune responses, vector transmission and airborne transmission were among the topics discussed in the session.
The various laboratory diagnostic methods were reviewed and evaluated in relation to the changing nature of the disease on the Iberian Peninsula. Among the methods based on the detection of virus are the haemadsorption test, direct immunofluorescence test, and pig inoculation. While the haemadsorption test continues to be the most sensitive of the methods of virus detection and is routinely applied to all tissue samples submitted, its results are not as quickly forthcoming as desired. As the numbers of subacute and chronic infections have increased, so too have the occasions increased where one or more subpassages in buffy coat cultures are required in order to obtain definitive results.
Direct immunofluorescence is a rapid and simple method, but the detection rate appears to be declining as the disease continues to modify.
Pig inoculation is still to be recommended in countries where ASF is suspected for the first time.
Diagnostic methods based on detection of antibodies have become more and more useful as the numbers of subacute and chronic infections encountered have increased. The direct immunofluorescence test as applied in Spain is especially useful because of its speed and excellent sensitivity. The immuno-electro-osmophoresis test should prove to be especially useful in large surveys.
In examining the immune responses in ASF, Fluctuations in T and B cell numbers and the development of delayed hypersensitivity have been noted. The pig immune system does not appear to be impaired by the infection. However, the production of virus neutralizing antibodies has still not been convincingly demonstrated.
With the clear demonstration of trans-stadial, transovarial, and sexual transmission of ASF virus in the argasid tick Ornithodoros moubata porcinus and its proven ability to transmit the virus to swine, the importance of an arthropod vector in the maintenance and transmission of ASF is clearly established.
Airborne transmission of ASF has now been demonstrated and some of the conditions required for this type of transmission have been also defined.
