Infection with a low virulent Mycoplasma hyopneumoniae isolate does not protect piglets against subsequent infection with a highly virulent M. hyopneumoniae isolate
Introduction
Mycoplasma hyopneumoniae (M. hyopneumoniae) is the primary cause of enzootic pneumonia in pigs, a chronic respiratory disease that causes major losses to the pig industry worldwide [39]. The infection pattern and the severity of the disease are largely determined by management practices and housing conditions [24], but also the virulence of the M. hyopneumoniae isolate is important [42], [25]. Experimental infections with a highly virulent isolate consistently resulted in clinical disease and lung lesions, whereas infection with a low virulent isolate was associated with no or very mild clinical symptoms and lung lesions [26], [42].
Previous studies showed that M. hyopneumoniae isolates originating from the same batch of slaughter pigs were very similar at genomic level, whereas isolates originating from different batches showed major diversity at genomic [37] and also at proteomic level [7]. Since infected pigs are commonly transferred from one herd to another, it can be expected that different M. hyopneumoniae isolates are circulating within the same herd. This would imply that under field conditions, pigs may become infected with different M. hyopneumoniae isolates before they reach slaughter age. The effect of multiple infections with different M. hyopneumoniae isolates on the severity of the disease is, however, not known.
The currently available inactivated whole-cell vaccines do not offer complete protection against lung lesions, they do not prevent colonization [39] and they are not able to significantly reduce the transmission of M. hyopneumoniae[26]. Consequently, they are not suitable to eliminate M. hyopneumoniae from infected pig herds neither to end up in a sustainable control of the disease. Attenuated vaccines might be a good alternative. Indeed, for other Mycoplasma sp. such as M. synoviae in chickens, live attenuated, temperature sensitive vaccines have been used successfully to control virulent infection (synovitis and respiratory disease) in commercial chicken flocks [19]. For M. hyopneumoniae it is not known if infection with low virulent isolates may induce protection against highly virulent isolates.
The present study aimed to investigate the effect of infection of pigs with low virulent isolates of M. hyopneumoniae on a subsequent challenge infection with a highly virulent isolate.
Section snippets
Animals
Fifty-five, 3-week-old, cross-bred (Rattlerow Seghers®, Buggenhout, Belgium) piglets were weaned and moved to the animal facilities of the Faculty of Veterinary Medicine, Ghent University, Belgium. The piglets were obtained from a herd that was free of M. hyopneumoniae and PRRSV. The herd had been monitored repeatedly during the last 8 years by means of clinical, pathological and serological parameters for M. hyopneumoniae and PRRSV in sows and pigs of different age categories, and none of the
Clinical parameters
Three out of the 55 piglets died during the study: one from the LV1–HV group (at D14), one from the LV2 group (at D18) and one from the control group (at D18). Necropsies were performed to establish the causes of death. The piglet from the LV1–HV group died due to a congenital stenosis of the rectum, the two other piglets died due to meningitis caused by Streptococcus suis serotype 2 infection.
During D0–D28, coughing was absent in the groups that were inoculated with sterile culture medium
Discussion
The present study showed that infection with a low virulent M. hyopneumoniae isolate does not protect piglets against subsequent infection 4 weeks later with a highly virulent isolate. It appeared, based on clinical parameters and macroscopic and histopathological lesions, that a previous infection with a low virulent isolate may increase the severity of an infection with a highly virulent isolate.
The two low virulent isolates did only induce very mild clinical symptoms during the first 4 weeks
Acknowledgements
The study was financially supported by IWT project number 050642. The authors would like to thank Hanne Vereecke for all the help, effort and laboratory work dedicated to the project.
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