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The epidemiology of Streptococcus suis type 2 in Western Australia

wa Mwaniki, Charles Gathinji (1994) The epidemiology of Streptococcus suis type 2 in Western Australia. PhD thesis, Murdoch University.

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This study was designed to determine the prevalence of infection with Streptococcus suis type 2 in pigs in Western Australia (WA), to ascertain if differences exist between isolates cultured from diseased and healthy pigs, and to investigate differences in the virulence of isolates from different genetic backgrounds.

A selective medium, that incorporated antiserum raised against S. suis type 2, was used to determine the prevalence of infection in apparently healthy pigs at slaughter. Overall 60% of pigs were identified as being infected with S. suis type 2. The bacterium was found in 57.7% of palatine tonsils and 61.5% of pneumonic lung tissue examined, but only in 27.6% of apparently healthy lung tissue. Fifty of 51 herds sampled were found to have some pigs infected with S. suis type 2, with most herds (76.5%) having over 40% of the pigs infected. The only herd free from infection was a specific pathogen free unit. The prevalence of infection in herds that had reported clinical disease caused by S. suis type 2 was similar to that of herds where the disease had previously not been reported or where disease was not a regular occurrence.

Multilocus enzyme electrophoresis (MEE) was used to examine 124 isolates cultured from healthy and diseased pigs. These isolates were subdivided into 17 electrophoretic types (ETs) of which the majority belonged to ETs 8, 1 and 7. Isolates cultured from clinically diseased pigs in WA primarily belonged to ET 1, whilst those from other states of Australia mainly belonged to ET 8. However more isolates cultured from healthy pigs in WA belonged to ET 8. Some piggeries were found to be infected with up to 5 different ETs and one pig was found to be concurrently infected with 3 strains.

Thirty three isolates of S. suis type 2 were examined using DNA restriction endonuclease analysis (REA), and 14 different patterns were recognised. REA was capable of further subdividing specific ETs. On one farm isolates cultured from diseased pigs could be separated from isolates from apparently healthy pigs, even though they belonged to the one ET. Conversely some isolates belonging to closely related ETs were grouped into one REA pattern.

When isolates were inoculated into mice, those belonging to ET 1 were found to be more capable of causing disease than other isolates, however all isolates could produce disease when administered in a sufficiently high dose.

More isolates cultured from clinically diseased pigs were found to produce two proteins [muramidase released protein (MRP) and extracellular factor (EF)] than were isolates from healthy pigs, and EF, in particular, is a useful marker of virulent strains.

It was concluded that although 5. suis type 2 is widely distributed in the Western Australian pig industry, disease is not necessarily a direct consequence of the presence of the bacterium. Disease caused by S. suis type 2 is multifactorial in nature, and requires the presence of a virulent strain of bacterium along with appropriate predisposing management and environmental conditions for it to occur.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary Studies
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Robertson, Ian and Hampson, David
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