The diagnosis of caseonecrotic bronchopneumonia by M. bovis was based on pathological features and confirmed by immunohistochemistry (IHC), bacteriological culture and PCR in the two lungs analyzed. To our knowledge, this is the first report of pulmonary mycoplasmosis in marsh deer. Although we cannot confirm that mycoplasmosis was the primary cause of death in the 11marsh deer, we strongly suggest its crucial role as both a primary and/or secondary pathogen, facilitating opportunistic infections by other bacteria like T. pyogenes²⁷isolated from one lung. This interaction between M. bovis and T. pyogenes could be synergistic, with M. bovis impairing the respiratory defense system and facilitating T. pyogenes infections, as seen in other ruminants^8,21.

The pathological findings in the lungs, caseonecrotic bronchopneumonia, are characteristic of mycoplasmosis, as thoroughly reported in cattle and bison^3,6,7,19. Caseonecrotic nodules are distinctive lesions with caseous necrosis that fill small bronchioles, alveoli or interlobular septa, where leukocytes undergo a distinctive form of necrosis maintaining their ghost-like cellular outlines^3,19. Also, the abundant M. bovis detected in necrotic foci of both affected lungs via IHC and distribution, evidences strong influence of the bacteria in the pneumonic process, associated to fatal mycoplasmosis in cattle^3,11. Although, M. bovis pneumonia is well documented in cattle^3,11, cases in wildlife are rare^6,7,8, often leading to an underestimation of its potential impact. In deer, pneumonia is a significant disease associated to a range of bacterial, parasitic, and viral pathogens^22,23. While M. bovis has been rarely reported in these animals, its role in pneumonia outbreaks may be overlooked. Nevertheless, it has been detected from both captive white-tailed deer, free-ranging pronghorn and free-ranging mule deer (Odocoileus hemionus)^{6,7,9,23,28,29}. Alongside our findings, this evidence highlights the importance of considering M. bovis in the differential diagnosis of respiratory diseases in deer.

T. pyogenes is likely a secondary opportunistic pathogen³⁰ commonly associated with subacute to chronic pneumonia in ruminants³¹however, its presence correlates positively with the isolation of Mycoplasma spp. in bovine respiratory diseases. This interaction suggests a possible synergistic effect, exacerbating lung disease^3,12. Particularly, in wildlife, T. pyogenes is an important emergent pathogen associated with pneumonia and extrapulmonary abscesses^29,30. However, no such findings have been reported in marsh deer³⁰. Dyer et al.⁶ described an outbreak of respiratory disease in white-tailed deer coinfected with M. bovis and T. pyogenes as herein. Additionally, in cattle, most cases of M. bovis pneumonia involve coinfections with other pathogens, including T. pyogenes, such as P. multocida, E. coli, M. haemolityca and H. somni²⁷. These coinfections intensify the severity of respiratory disease in cattle, where M. bovis causes degeneration and impairment of ciliated respiratory epithelial cells, predisposing the lungs to secondary infection^32,33, in which the typical caseonecrotic foci of M. bovis infection develop into abscesses with a fluid purulent center in coinfection with T. pyogenes³. The latter could be the case herein, exacerbating lung lesions with subsequent high mortality.

Other pathogens such as viral and parasitic agents are commonly involved in respiratory diseases in co-infection with M. bovis^3,27. Unfortunately, these tests were not carried out as samples were not conserved after bacteriological analysis.

The host species of origin in this outbreak is unknown. Given, that in areas where marsh deer are distributed, such as national or provincial parks of Argentina, cattle and marsh deer often cohabit. In this outbreak, although cattle and marsh deer did not share the same paddocks, they were separated only by a fence. Given the high prevalence of asymptomatic infections in cattle and the rare detection of M. bovis in wildlife, transmission from a livestock reservoir to marsh deer seems likely. Cattle is known to act as carrier in clinically healthy animals, with variable disease expression, and intermittent shedding, maintaining M. bovis in populations^3,34. This situation generates a possible scenario where cattle could transmit the disease to deer, implicating potential risk of transmission to wildlife with subsequent outbreaks as described. Mycoplasmosis, in other species, such as bison, has been shown to cause outbreaks with high mortality³⁵, and for this reason, monitoring marsh deer in Argentina is important. Serological studies monitoring M. bovis prevalence in marsh deer and nearby cattle could provide valuable insights into the presence of subclinically infected deer and help identify potential risk factors associated with elevated antibody levels and subsequent outbreaks. Specifically, determining the prevalence of M. bovis in coexisting cattle and deer would enable the implementation of preventive measures based on the findings. Whenever possible, these efforts should be complemented by postmortem examinations and laboratory testing to enhance diagnostic accuracy.

This report highlights the circulation of M. bovis in animal species other than cattle. A high incidence of M. bovis has been recently reported in cattle in Argentina^4,19,36, reinforcing the high circulation of the bacteria, which could easily reach other species, such as the marsh deer described herein. Further studies are necessary to gain knowledge of M. bovis behaviour as primary or secondary agent in wildlife species, together with detection of other agents, such as viruses and parasites, potentially involved in respiratory diseases as co-infections.

The occurrence of M. bovis-associated pneumonia in wildlife highlights the importance of pathogen surveillance, as identifying infectious agents provides valuable insights into the ecoepidemiology of infectious diseases. This is especially critical herein for marsh deer, an endangered species, where eradication programs are essential for its conservation.