Discussion

Based on the gross, histopathological, and bacteriological findings, a diagnosis of pyogranulomatous pneumonia by A. hyovaginalis was established. In our search, we found no histopathological descriptions of pyogranulomatous pneumonia associated with A. hyovaginalis in wild boars. A similar pyogranulomatous pattern with necrotic centers, similar to the pattern observed in the present case, is occasionally induced by A. hyovaginalis infection in domestic pigs (Liljegren et al. 2003; Aalbæk et al. 2003; Reichel and Wragg 2007). Moreover, bronchiolar ectasia is rarely associated with this infection in domestic pigs (Liljegren et al. 2003), but this finding was not present in the lung sections of the boar.

As in many countries worldwide, wild boars are one of the most important invasive animals in Brazil, and boar hunting has been permitted in the country since 2013 as a population control measure (Rosa et al. 2018). This suid species can harbor multiple pathogens, posing a threat to commercial pig farming, other livestock, humans, and native biodiversity (Rosa et al. 2018; Lopes et al. 2021b; Silva Andrade et al. 2022; Kmetiuk et al. 2023). Although the commercialization of boar meat and by-products is prohibited, their consumption and handling by hunters are allowed (Kmetiuk et al. 2023). Moreover, wild boars may serve as reservoirs for several porcine pathogens, including important respiratory agents, such as Mycoplasma hyopneumoniae (Biondo et al. 2021; Souza et al. 2021; Ninkovic et al. 2024). Therefore, when lesions are identified in carcasses of wild boars, it is essential that hunters submit tissue samples for histopathological and microbiological analysis, as was done in the present case, given the importance of identifying and reporting circulating agents in this population.

The source of A. hyovaginalis infection in domestic pigs remains unknown (Aalbæk et al. 2003; Schumacher et al. 2009). Some strains were isolated from uterine cervix swabs of healthy gilts and sows, suggesting that this bacterium may be a part of the normal reproductive tract microbiome (Broes et al. 2019; Kellerman et al. 2022). Additionally, A. hyovaginalis was isolated from the tonsils and the nasal conchae of healthy piglets before and after weaning, indicating that it may also be a component of the nasal and tonsillar microbiome (Baele et al. 2001). In wild boars, the source of A. hyovaginalis infection is likewise unknown, as there is only one previous report describing this infection in this suid species (Alssahen et al. 2020). Because wild boars and domestic pigs are subspecies of the same species, it is plausible that this agent may also act as a commensal organism within the normal wild boar microbiome, as hypothesized for domestic pigs. As A. hyovaginalis is considered a possible commensal organism in wild boars, immunosuppressive factors may have facilitated the development of pneumonia in this case. Porcine circovirus 2 (PCV2) infection, which is frequently detected in Brazilian wild boars (Dal Santo et al. 2022), could have been implicated as an immunosuppressive factor for this wild boar; however, although the available samples were not tested for PCV2 by PCR, infection was considered unlikely, as neither the lymph node nor the lung samples contained histologic lesions consistent with PCV2-associated disease. Likewise, a concurrent Metastrongylus spp. infection cannot be completely excluded, since this parasite is frequently reported in Brazilian wild boars (Oliveira et al. 2023) and only selected lung fragments, rather than the entire lungs, were submitted for examination. Pulmonary infection by Metastrongylus spp. may predispose animals to secondary bacterial pneumonia, a well-recognized association in domestic pigs (Brewer and Greve 2019). However, neither adult nematodes within the bronchi nor histologic lesions consistent with metastrongylosis were observed in the examined tissues. Despite the severity of the lesions observed in the present case, it seems unlikely that A. hyovaginalis represents a significant pathogen transmissible from free-ranging wild boars to domestic pigs; however, further studies are needed to clarify this possibility. Furthermore, A. hyovaginalis likely does not pose a zoonotic risk, as there are no reports of human infections caused by this bacterium.

In wild boars, the main differential diagnosis for this pattern of gross and histologic lesions is tuberculosis (Zanella et al. 2008; Lopes et al. 2021b). This differential was considered unlikely due to the presence of Gram-positive bacterial colonies within necrotic areas, the absence of acid-fast bacilli in Ziehl-Neelsen stain, and the absence of regional lymph node involvement (Zanella et al. 2008); however, a co-infection of A. hyovaginalis and Mycobacterium spp. cannot be totally excluded, as no culture or PCR could be performed. Embolic suppurative bacterial pneumonia, as described in domestic pigs (Piva et al. 2020), mainly associated with Staphylococcus aureus infection (Kruse et al. 2015), was also considered a gross differential diagnosis; however, it was excluded histologically because S. aureus-induced lesions are typically suppurative (Piva et al. 2020). Excluding both conditions is important because they may represent zoonotic risks for consumers of game meat. Regarding embolic pneumonia in wild boars, only one case has been reported, consisting of suppurative pneumonia caused by bacterial embolism of Staphylococcus hyicus in a case of exudative epidermitis (Pérez et al. 2013). Rarely, in domestic pigs, Actinobacillus pleuropneumoniae serotype 2 has been associated with granulomatous pleuropneumonia; however, the granulomatous inflammatory response caused by this agent typically exhibits the Splendore-Hoeppli phenomenon (Ohba et al. 2009). Although Actinomyces, which was isolated in our case, may also induce the Splendore-Hoeppli phenomenon, this feature was not observed. Additionally, Rhodococcus equi infection has been reported to cause pyogranulomatous pneumonia in wild boars (Vargas et al. 2013). Nevertheless, these differential diagnoses were ruled out following the pure culture growth and identification of A. hyovaginalis.

This report describes a case of pyogranulomatous pneumonia caused by A. hyovaginalis infection in a free-ranging wild boar, highlighting the gross, histopathological, and microbiological findings. Although not previously reported in wild boars, A. hyovaginalis infection should be considered a differential diagnosis for pyogranulomatous pulmonary lesions in this suid species, as in domestic pigs.