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Case History: Four juvenile gulls were collected from a vacant lot in Wisconsin where approximately 50 were found dead. Gull carcasses were found in varying states of decomposition throughout the area, and four were selected with the best postmortem condition.

The lot was described to have rat traps, pools of green algae, and several containers of rat poison with at least one appearing broken open. 

Gross Findings: A juvenile, female 320-g gull of unknown species presented for necropsy in fair nutritional condition and fair postmortem state with negligible external findings. On internal examination, the air sac was tan and moderately thickened. The pericardium was thickened and opaque, with multifocal tan plaques, and adhered to the heart. There were also multifocal loose adhesions between the viscera, ribcage, and air sac. The lungs contained abundant dark red fluid, were diffusely dark red, and when put in formalin floated low in the container, suggesting increased density. The ventriculus was mostly empty but contained small amounts of green material and a 3-mm diameter piece of plastic. All four individuals had similar lesions of varying degrees.

Histopathological Findings: Multifocally within the superficial myocardium, pericardial fat, and diffusely throughout visceral pericardium, filling pericardial sac and adhering to the parietal pericardium there is necrosis, lymphoplasmacytic inflammation with macrophages, granulocytes and foci of basophilic bacterial coccobacilli admixed with necrotic debris, edema, and fibrin. The pericardium is expanded at least ten times normal by fibrin and edema. Affected cardiomyocytes are shrunken and hyper-eosinophilic, swollen, and pale, or completely replaced by previously described inflammatory processes. Within the pectoral muscle there was a similar inflammatory process creating streaking lesions along the fascial plane. The sacs and the serosa of various organs were also multifocally, similarly affected. 

Images of tissues from a gull showing lesions and bacteria.
Figure 1: Tissue from a gull (Larus sp) from Wisconsin. (A) Diffusely the pericardium (star) is greatly expanded by fibrin, edema and necrotic debris (H&E). (B) Lymphoplasmacytic myositis (arrows) creating linear lesions along fascial planes of the pectoral skeletal muscle (H&E). (C) Large colony of basophilic coccobaccili (star) within the pericardium surrounded by lymphocytes, plasma cells and macrophages admixed with fibrin, edema, and necrotic debris (H&E). (D) Superficial myocarditis with gram negative coccobaccili and mixed inflammation (arrow). Inset: gram negative bacteria oil immersion.

Morphologic Diagnosis/es:

  1. Heart, pericardium, epicardium: peri and epicarditis, necrotizing, lymphoplasmacytic with histocytes and granulocytes, diffuse, severe, chronic with large colonies of gram-negative coccobacilli and mild multifocal myocarditis and steatitis in a gull of unknown species(Larus sp). 
  2. Skeletal muscle (pectoral): myositis, lymphoplasmacytic, multifocal to coalescing, mild to moderate, chronic with few coccobacilli. 
  3. Air sac: air sacculitis, necrotizing, granulocytic and histiocytic, multifocal to focally extensive, severe with basophilic coccobacilli and steatitis.
  4. Proventriculus, ventriculus: serositis, necrotizing, lymphoplasmacytic, granulocytic, focally extensive, moderate to severe with basophilic coccobacilli and steatitis. 

Disease: Bisgaard Taxon 40 

Etiology: Mergibacter septicus, an encapsulated gram-negative bacillus or coccobacillus bacterium in the pasteuerellaceae family. Five strains have been discovered so far.

Distribution: Reported in North America and Europe, but likely worldwide given seabirds capacity for long-distance migrations. 

Seasonality: There is no known seasonality, although large outbreaks seem to correlate with extreme weather or environmental events.

Host range: Primarily seabirds such as gulls, auklets, terns, egrets, but likely other avian species. 

Transmission: Likely an opportunistic pathogen as it is found as part of the respiratory flora in healthy individuals in these avian species. 

Clinical signs: Usually birds are found dead, moribund, or in respiratory distress with significant weakness. With prolonged epizootics, birds may present with lethargy and signs suggestive of neurologic involvement.

Pathology: Currently it is unknown if M. septicus can cause primary disease or if it is only opportunistic after an immunosuppressive disease or event. Organs affected may vary between species. Overall, the disease is necrotizing to fibrino-necrotizing and most commonly affects air sacs, pericardium, myocardium, pectoral muscles, and lungs.

Diagnosis: Although molecular techniques are important to identify this organism, agent identification must coincide with gross and histologic lesions. Since this and related bacteria can be normal flora, it is important to not rely solely on molecular diagnosis. Other disease processes may need to be ruled out. It is also worth considering a primary immunosuppressive disease such as that caused by circovirus.

Public health concerns: None reported.

Wildlife population impacts: This may be an emerging disease that can lead to large, wide-scale epizootics resulting in high mortality in susceptible avian species. This was exemplified by the 2016 mass mortality event of 300 Rhinoceros auklets which were found washed ashore in Washington (Knowles et al., 2019). New events are still being documented and studies are ongoing to understand overall impacts to wildlife. 

Management: As this is a newly recognized disease, management guidelines have not yet been developed. One possibility for treatment of individual birds is to use antibiotics, as M. Septicus exhibits low minimum inhibitory concentrations to several common antibiotics approved for veterinary use. It is important to consider M. septicus when outbreaks occur in the face of environmental stressors, such as harmful algal blooms or hurricanes in at risk populations of congregating flocks. 


  • Abdel-GLIL, M. Y., S. Braune, S. Bouwhuis, Andl. D. Sprague. 2023. First Description of Mergibacter septicus Isolated from a Common Tern (Sterna hirundo) in Germany. Pathogens 12.
  • De Luca, E., S. Álvarez-Narváez, G. Maboni, R. P. Baptista, N. M. Nemeth, K. D. Nierdringhaus, J. T. Ladner, J. M. Lorch, G. Koroleva, S. Lovett, G. F. Palacios, Ands. Sanchez. 2021. Comparative Genomics Analyses Support the Reclassification of Bisgaard Taxon 40 as Mergibacter gen. nov., With Mergibacter septicus sp. nov. as Type Species: Novel Insights Into the Phylogeny and Virulence Factors of a Pasteurellaceae Family Member Associated With Mortality Events in Seabirds. Front Microbiol 12: 667356.
  • Knowles, S., B. L. Bodenstein, B. M. Berlowski-Zier, S. M. Thomas, S. F. Pearson, Andj. M. Lorch. 2019. Detection of Bisgaard Taxon 40 in Rhinoceros Auklets (Cerorhinca monocerata) with Pneumonia and Septicemia from a Mortality Event in Washington, USA. J Wildl Dis 55:246-249.
  • Nierdringhauss, K. D., L. A. Shender, A. Dinuovo, L. J, Flewelling, G. Maboni, S. Sanchez, P. J. Deitschel, J. Fitzgerald, Andn. M. Nemeth. 2021. Mortality in Common (Sterna hirundo) and Sandwich (Thalasseus sandvicensis) Terns Associated with Bisgaard Taxon 40 Infection on Marco Island, Florida, USA. J Comp Pathol 184: 12-18.

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