A.P. van Nieuwstadt, N. Stockhofe-Zurwieden, W. Loeffen, E. Kamp, J.C.de Jong

Institute for Animal Science and Health (ID-DLO), P.O.Box 65, NL-8200 AB LELYSTAD, The Netherlands

Animal Health Service, BOXTEL, The Netherlands

National Influenza Centre, Erasmus University, Rotterdam

Introduction.

Swine influenza is an acute respiratory disease caused by influenza virus type A, subtype H1N1 or H3N2. Outbreaks of respiratory disease due to influenza virus have occurred in the swine population of most West European countries since 1979. European H1N1 isolates are distinct from American strains, which are named classical swine influenza virus, and are more closely related to the virus that caused the severe influenza epidemic in humans in 1918, called the Spanish flu. Antgenically and genetically, European swine influenza virus isolates are closely related to the H1N1 isolates from ducks. Since 1984, infections with influenza virus subtype H3N2 also have been associated with outbreaks of respiratory disease in pigs. These H3N2 virus isolates from swine were antigenically very similar to the human Port Chalmers/1/73 strain. Serological surveys revealed that the H3N2 virus circulated in the swine population already at least ten years before clinical influenza in swine due to H3N2 virus infections was reported.

During the winter of 1995/96 we conducted a survey in 40 holdings of fattening pigs with a history of recurring respiratory disease for diagnosis of causative infectious agents. Sixteen outbreaks of respiratory disease were reported. Results showed that 7 outbreaks were caused by influenza virus and 5 were caused by Actinobacillus pleuropneumoniae. No causative infectious agent could be unambiguously diagnosed in the other outbreaks. The results indicated that H1N1- and H3N2 virus infections were a major cause of acute respiratory disease in fattening pigs, which had not been expected. The results did raise questions about (1) extensiveness of circulation of influenza virus among fattening pigs; (2) whether influenza virus infections caused disease on their own without complicating secondary bacterial infections; (3) whether an antigenic drift of H1N1- or H3N2 virus had occurred, which would require an updating of virus strains in the vaccine.

Forty herds with a history of recurring respiratory disease in fattening pigs were monitored during the winter of 1995/96. In these herds pigs were housed together at an age of 10 weeks in groups of 60 to 100 pigs. Stockholders were requested to notify early signs of respiratory disease such as increase of coughing, laboured breathing, and decrease of food intake. In this way 16 outbreaks of acute respiratory disease were reported to the veterinarian. From each outbreak 4 diseased pigs from a group with clear clinical signs were slaughtered for post mortem examination of the lungs, and 2 clinically healthy pigs of comparative age and origin from another group with no signs were slaughtered and served as controls. Lungs were examined for gross pathological lesions and tissue specimens were collected for histological examination and isolation of bacteria, viruses, and mycoplasmas. Acute and convalescent blood samples were collected from 10 groupmates in each outbreak. A 3rd blood sample was collected from the same pigs at the end of the fattening period. In addition, one or two other groups from the 16 holdings with a reported outbreak of respiratory disease were monitored serologically for influenza virus infections (27 groups in total). Blood from 10 pigs from each group was collected at the beginning and the end of the fattening period. Farmers who had reported a first outbreak of respiratory disease in their livestock were not requested to notify further outbreaks, so no clinical observations were available from those latter groups.

Seven out of 16 outbreaks of acute respiratory disease were diagnosed as influenza. These influenza outbreaks occurred 5 to 10 weeks after pigs were grouped together for fattening. In 5 outbreaks H1N1 virus was isolated from pneumonic lung lesions of all 4 diseased pigs that were necropsied and diagnosis was confirmed by a seroconversion of all 10 groupmates in a HAI test with H1N1 virus. In 2 other groups a H3N2 virus infection was diagnosed by virus isolation from the lungs and diagnosis was confirmed serologically. First signs of an outbreak were fever, prostration, apathy, and anorexia in 10 to 30% of pigs in the affected group. At necropsy a multifocal catarrhal pneumonia was observed, which was frequently associated with parenchymal haemorrhages and affected 10 to 50% of the lung tissue. Histological examination revealed necrotizing endobronchitis/bronchiolitis accompanied by broncho-interstitial inflammation and sero-fibrinous exudation and haemorrhages into the alveolar space. Similar lesions were observed in 7 of 14 control pigs, but lesions were much less severe. Influenza virus was isolated from lungs of 5 control pigs. Pathogenic bacteria were isolated from 15 out of 28 lungs of diseased pigs, but we never isolated the same species from all 4 pigs of the same outbreak.. Species isolated were Haemophilus parasuis, Bordetella bronchiseptica, Pasteurella multocida, Streptococcus suis, and (from one pig only) Actinobacillus pleuropneumoniae. In 13 pigs influenza was not complicated by a secondary bacterial infection, as no pathogenic bacteria could be isolated from the lungs.

Serological examination revealed extensive circulation of H1N1 virus during the fattening period in 20 of 43 groups (46.5%; the 16 groups with a reported outbreak were included) and of H3N2 virus in 9 groups (21%). Because no HAI antibodies were detected in blood samples collected at the beginning and the end of the fattening period, we concluded that no H1N1 virus circulated in 4 groups, and no H3N2 virus circulated in 11 groups. In the remaining groups, a few pigs or not any showed a significant rise of HAI antibody titre, but most pigs had antibodies in the first blood sample either indicating an earlier infection during the breeding period or a subclinical infection in the first few weeks of the fattening period.

Recent isolates of H1N1 virus [Infl A/swine/Neth/Best/96 (H1N1)] and H3N2 virus [Infl A/swine/Neth/St.Oedenrode/96 (H3N2)] were used as a HA antigen in the HAI test. A few convalescent sera were also tested with older isolates of H1N1- and H3N2 virus from swine, and HAI antibody titres were not much different. However, with the human Port Chalmers/1/73 (H3N2) strain antibody titres were significantly lower than with the St. Oedenrode/96 strain.

Influenza virus infections are a major cause of respiratory disease in fattening pigs. Influenza virus infections are often complicated by secondary bacterial infections, but an influenza virus infection can be the primary and single cause of a pneumonia.

Clinical signs of influenza are not pathognomic, and an infection with Actinobacillus pleuropneumoniae has to be considered in clinical differential diagnosis. At gross pathological examination, the two infections can be distinguished, however. An infection with Actinobacillus pleuropneumoniae causes a necrotizing haemorrhagic fibrinous pleuropneumonia.

There are indications of an antigenic drift of porcine H3N2 virus, by which antigenic relatedness with the human Port Chalmers/1/73 strain was lost.

This study was financially supported by Intervet International, Boxmeer, The Netherlands