Abstract
Direct detection of Mycoplasma hyopneumoniae through molecular tools is a growing trend for early diagnosis, highlighting the importance of knowing M. hyopneumoniae dynamics in the respiratory tract upon infection. This study focused on monitoring the infection level and its effects in different anatomic sites of the respiratory tract of experimentally infected swine in four time-points post-infection. To this end, 24 pigs were allocated to either non-inoculated group (n = 8) or inoculated group (n = 16). On day 0 post-infection (dpi), animals of the inoculated group were intratracheally inoculated with M. hyopneumoniae. Nasal swabs were collected weekly for qPCR detection of bacterial shedding. At 14, 28, 42, and 56 dpi, four animals from the inoculated group and two from the control group were necropsied. Bronchoalveolar lavage fluid (BALF) and samples from three different anatomical tracheal sections (cranial - CT, medium - MT, lower - LT) were collected for qPCR and histopathology. Bacterial loads (qPCR) in tracheal samples were: 4.47 × 102 copies∕μL (CT), 1.5 × 104- copies∕ μL (MT) and 1.4 × 104 copies∕μL (LT samples). M. hyopneumoniae quantification in BALF showed the highest load at 28 dpi (2.0 × 106 copies∕ μL). Microscopic lesions in LT samples presented the highest scores at 56 dpi and were significantly correlated with the pathogen load on 14 dpi (0.93) and 28 dpi (0.75). The greatest bacterial load of M. hyopneumoniae in CT samples and BALF was registered at 28 dpi, and it remained high in BALF and LT throughout the 56 dpi. The pathogen was able to persist during the whole experimental period, however higher estimated quantification values were registered in the lower parts of the respiratory tract, especially at 56 dpi. These findings are important for improving diagnostics, treatment, and control measures of M. hyopneumoniae infection in swine herds.
Highlights
Mycoplasma hyopneumoniae is the primary causative agent of enzootic pneumonia (EP) [1], a chronic disease characterized by dry, non-productive cough, high morbidity, low mortality, and losses in daily weight gain [2, 3]
M. hyopneumoniae is notorious for its specific membrane proteins that adhere to the tracheal epithelial cilia during infection [10], inducing tissue inflammation and cell damage in vitro [11]
Considering the aforementioned fact, this study focused on reporting the infection dynamics upon experimental M. hyopneumoniae challenge and the associated effects in different respiratory tract sections of swine during a period of 56 dpi
Summary
Mycoplasma hyopneumoniae is the primary causative agent of enzootic pneumonia (EP) [1], a chronic disease characterized by dry, non-productive cough, high morbidity, low mortality, and losses in daily weight gain [2, 3]. M. hyopneumoniae is known for having a tropism for lower parts of the swine respiratory tract [6,7,8,9]. The population of this bacteria was shown to be 100 times higher in these sites when compared to the upper respiratory tract [6]. Collection of tracheal secretions, using tracheobronchial swabs (deep tracheal catheters) and laryngeal swabs, have been proven to be sensitive and reliable diagnostic samples to detect M. hyopneumoniae infection in live animals [12, 13]. It is very likely that the trachea is an infection and multiplication site for M. hyopneumoniae [14]. As far as we know, there are no studies that looked into the distribution of the pathogen in the trachea throughout infection
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