Epidemiological significance of the domestic black pig (Sus scrofa) in maintenance of bovine tuberculosis in Sicily

Abstract

Bovine tuberculosis (bTB) is an emerging disease among wild animals in many parts of the world. Wildlife reservoir hosts may thus represent a potential source of infection for livestock and humans. We investigated the role played by the Sicilian black pig, an autochthonous free- or semi-free-ranging domestic pig breed, as a potential source of bTB infection in an area where bTB prevalence in cattle is high. We initially performed a preliminary field study to assess the occurrence of bTB in such animals. We sampled 119 pigs at abattoir and found 6.7% and 3.4% of them to be affected by gross tuberculous-like lesions (TBL) and Mycobacterium bovis culture positive, respectively. We then proceeded to investigate the dissemination and characteristics of lesions in a second field study performed on 100 animals sampled from infected herds. Here, tissues collected at the abattoir were examined macroscopically, microscopically, and by culture tests. Most pigs with TBL showed generalized lesions in both gross and histological examinations (53% and 65.5%, respectively). Head lymph nodes were the most frequently affected in both localized and generalized TB cases observed macroscopically and microscopically. M. bovis was the most frequently isolated etiologic agent. The molecular characterization of isolates from both field studies by spoligotyping and analysis of 12 mycobacterial interspersed repetitive-unit-variable number tandem repeat (MIRU-VNTR) loci, followed by their comparison to isolates of cattle origin, suggested a potential transmission of mycobacteria from domestic animals to black pigs and vice versa. Our findings, along with ethological, ecological, and management considerations, suggest that the black pig might act as a bTB reservoir in the ecosystem under study. However, additional studies will be necessary to establish the true epidemiological significance of the Sicilian black pig.

Fig 1

The geographic area under study: the Nebrodi and Madonie Natural Parks. (Modified from a map available at www.siciliainmoto.it with permission.)

Fig 2

Morphological characteristics of the Sicilian black pig. The pig has black skin entirely covered by strong bristles, a long head, a thin and lengthened snout, a small trunk with a compressed thorax, a short abdomen, a curly tail, strong and medium-length limbs, thin joints, and hard and resistant hooves.

Fig 3

Gross pathology. Shown are caseous necrotic-calcified tuberculous-like lesions in the mandibular, retropharyngeal, and parotid lymph nodes of a black pig.

Fig 4

Histopathology. Panels A to E show granulomatous lesions by type. (A) Type 1: initial granuloma composed by mononuclear inflammatory cells (*). Magnification, 20×. (B) Type 2: granuloma with a necrotic center (*) surrounded by epitheliod cells, a variable number of giant cells, macrophages, and lymphocytes. Magnification, 20×. (C) Type 3: necrotic granuloma with minimal calcifications in the center (*). Magnification, 20×. (D) Type 4: granuloma with necrotic calcified center (*) and peripheral fibroplasias. Magnification, 10×. (E) Type 5: granuloma with abundant calcifications occupying all of the necrotic area. Magnification, 10×. Hematoxylin and eosin.