Septicaemic pasteurellosis (SP) in pigs associated with P. multocida serotype B:2 has been reported, however knowledge of the prevalence and distribution of this disease in pigs is limited. The aims of this study were to determine the extent of specific antibody to P. multocida serotype B:2 in pigs in defined regions of Vietnam; to characterise pig strains of P. multocida and to study the pathology and pathogenesis of the disease caused by P. multocida serotype B:2 in pigs in Vietnam.
A total of 141 isolates of P. multocida, (of which 80 were from healthy pigs in Vietnam, 57 from healthy pigs m Australia and 4 from diseased pigs in Vietnam) were characterised by carbohydrate fermentation profiles and assigned to capsular groups by a multiplex-PCR capsular typing method. Protein profiles of 18 capsular group B isolates from Vietnam were compared
by polyacrylamide gel electrophoresis (PAGE), and found to be indistinguishable. Specimens of serogroup B were recovered both from diseased pigs and clinically healthy pigs in Vietnam, while this capsular group was not isolated from Australian pigs. Eight REP types from Australian pigs (designated Al through A8) and 10 REP types from Vietnamese pigs (designated VI through VIO) were recognised. Analysis based on REP type and protein profile by SDS-PAGE showed similarity of P. multocida type B:2 obtained from pigs, cattle and buffalo in Vietnam.
Antigenic relationship of P. multocida B:2 isolates from pigs and cattle/buffalo was demonstrated by protection tests in mice and pigs. The results demonstrated that P. multocida type B:2 strains isolated from pigs, cattle and buffalo possess cross-reacting,
protection-inducing antigens. The similarity of immunoreactivity of serotype B:2 antigens from various strains was demonstrated by immunoblotting assays. When compared with non-immune pig sera, there was strong reactivity of immune pig sera, to bands of about 18 and 48 kDa. However, it is not known if these particular proteins have any role in protective immunity.
A total of 406 serum samples of pigs from 9 provinces in Vietnam were tested for naturally acquired antibody to P. multocida B:2 by ELISA and PMPT, and 74 of 406 (18%) pig sera were positive. A relationship between recent incidence of SP in the vicinity, and the proportion of pigs possessing antibody, was demonstrated.
It was shown that experimental disease in pigs susceptible to P. multocida type B:2. (SP) was peracute or acute, with death
occurring from 6 to 48 h after onset of clinical signs. There were non-specific pathological changes in pigs with SP. However, in most cases, swelling of throat regions of diseased pigs was observed. Histopathological lesions were not consistently present in all pigs with SP.
Concentration of bacteria measured in blood from pigs with SP at 1-2 h prior to death was commonly from 104-105 cfu/ml, while levels in liver, spleen and lung collected at necropsy, ranged from 106-107 cfu/g. These results were similar regardless of whether the method of exposure was intramuscular or intranasal. Changes in leucocyte counts in pigs with SP were also observed, with total white blood cell counts increased at the early stage and decreased at the terminal stage.
An attempt was made to
detect toxin in pig serum at the moribund stage. Neither mice nor PK 15 cells were affected when inoculated with filtered sera taken from diseased pigs. Interestingly, some mice (25%) that were inoculated with these sera were protected against a challenge of P. multocida B:2, 15-21 days after serum inoculation.
Pigs showing clinical signs of SP excreted P. multocida in both their nasal secretions and faeces. P. multocida was also found in the nasopharynx and tonsils of asymptomatic pigs following experimental exposure to P. multocida B:2 by different routes. Vaccination of pigs with P. multocida serotype B:2 bacterin prevented clinical disease, but did not prevent animals from either shedding bacteria in their nasal secretions, or carrying the organisms in their
tonsils. The period of excretion of bacteria in the nasopharynx of these animals was short, ranging from 2 to 22 days after exposure.
Neither serotype A:3 nor serotype D:3,4 produced SP in pigs, however a chicken strain of P. multocida serotype A:1 (VP 161) was able to produce systemic disease in pigs. The time to death and clinical signs were similar to those caused by serotype B:2, except there was no swelling of the throat. Gross lesions produced by VP 161 included marked pulmonary oedema and a sero-fibrinous exudate in pleural and abdominal cavities.
The conclusions from the experiments in this thesis are:
multocida can be isolated from the nasopharynx of a high proportion of healthy pigs in Australia and Vietnam. Most isolates in both countries were either capsular type A or D, but a small proportion of isolates from Vietnam were type B. Biochemical, PAGE, and REP-PCR profiles of the pig isolates of P. multocida serotype B:2 isolated in this study are indistinguishable from isolates obtained from cattle/buffalo with HS.
2. Commercial Haemorrhagic Septicaemia vaccines for cattle/buffalo protected pigs from virulent challenge by P. multocida serotype B:2 isolates from both pigs and cattle.
3. In a survey of 406 pig sera from 9 provinces of Vietnam, 18% were positive for antibodies to an ELISA using heat-extracted antigens of
Pasteurella multocida B:2. Fifty nine (74%) of the ELISA-positive sera also gave a positive PMPT.
4. Pasteurella multocida type B:2 isolated from either pigs or cattle can produce acute or peracute disease in pigs, with relatively non-specific lesions and there are no differences in the disease caused. A strain of P. multocida type A isolated from an avian cholera outbreak in chickens was pathogenic for pigs, but the disease it caused had a longer incubation period and the lesions were consistently different.
5. Pigs with fulminating SP shed P. multocida in nasal secretions and faeces from about 12 hours after challenge, while apparently healthy pigs which had either been deliberately
inoculated with virulent P. multocida or been in contact with diseased pigs, are capable of carrying the organism in the nasopharynx for at least 24 days, with some shedding in nasal secretions for at least 12 days. Carriage may be maintained despite seroconversion.
6. Multiplex PCR, which had previously been applied to isolates from cattle and buffalo, is a useful technique for rapid identification of P. multocida recovered from pigs.