Septicemic pasteurellosis causing peracute death and necrotizing myositis in a beef heifer calf (Bos taurus) in Alberta, Canada

Abstract

Septicemic pasteurellosis is an acute and fatal bacterial disease of cattle and wild ungulates caused by certain serotypes of Pasteurella multocida. Here we report a single case of septicemic pasteurellosis in a 6-month-old, Red Angus heifer from a cow-calf operation in Alberta, Canada. Postmortem examination revealed necrotizing and hemorrhagic myositis, fibrinous pericarditis and multisystemic bacterial emboli. Pasteurella multocida was isolated from muscle in pure culture, and the capsular antigen group was identified as serogroup B using polymerase chain reaction. To the best of our knowledge, this is the first reported case of septicemic pasteurellosis in beef cattle in Canada.

Key clinical message:

Veterinary practitioners and diagnosticians should include septicemic pasteurellosis on their list of differential diagnoses when they encounter similar presentations of peracute death and severe necrotizing myositis in cattle in Canada.

Septicemic pasteurellosis is a highly fatal bacterial disease that mainly occurs in wild ruminants in North America and is caused by several serotypes of Pasteurella multocida. Cases of the disease have been reported in the United States in free-ranging bison (Bison bison), elk (Cervus canadensis), and pronghorn antelope (Antilocapra americana) (1–3). Hemorrhagic septicemia is a clinically indistinguishable disease that occurs in domestic cattle (Bos taurus) and water buffalo (Bubalis bubalis) and is enzootic to tropical countries in Africa and Asia. Hemorrhagic septicemia is most commonly caused by P. multocida serotypes B:2 and E:2, but serotypes A:1, A:1,3, A:3, A:4, B:1, B:2,5, B:3,4, E:2,5, F:3, F:3,4 have also been implicated (4,5). Septicemic pasteurellosis is rarely reported in Canada save for recent cases of the disease in 2 farmed elk (C. canadensis) in 2016, and a non-peer-reviewed case report in a wild cow-elk in 1999 (6,7). In the former case, P. multocida serogroup B, classified according to the Carter-Heddleston system, was identified as the cause of septicemic pasteurellosis in the elk (8). The disease is characterized by an initial phase of pyrexia, a phase of respiratory involvement, and a terminal phase of septicemia and rapid prostration leading to death (9). Once animals exhibit clinical signs, mortality is typically 100%. To our knowledge, this is the first report of septicemic pasteurellosis in Canadian beef cattle.

Case description

A 6-month-old, Red Angus, heifer calf from southwest Alberta was submitted for necropsy to the Diagnostic Services Unit (DSU) at the University of Calgary Faculty of Veterinary Medicine (UCVM). The calf was found deceased by an Alberta Conservation Officer making the rounds of the foothills grazing reserve where a herd of 100 cow-calf pairs were grazing for the summer. The deceased calf was found amongst a group of 10 apparently healthy cow-calf pairs and the dam. There was no known history of illness in the calf; however, an estimated 5 other animals from other ranches using the same grazing allotment had died over the summer months in the geographical area and had not been submitted for necropsy. This grazing reserve is located on the eastern slopes of the Rocky Mountains and the domestic animals share this environment with wild ungulates and carnivorous predators. It is standard procedure to ensure all calves receive an 8-way clostridial bacterin before turnout onto this grazing reserve.

At necropsy, the carcass was in good nutritional condition with no evidence of dehydration and moderate autolysis. There was extensive subcutaneous edema over the right lateral thorax and abdomen. There was marked subcutaneous edema extending over the proximal aspect of the right hind limb. On section, the gracilis and semitendinosus muscles were dark red and dry, and the fascia was expanded by edema, hemorrhage, and fibrin consistent with necrotizing and hemorrhagic myositis (Figure 1). Similar changes were noted in the right forelimb and involved the long head of the triceps. Neither emphysema nor a butyric acid (rancid butter) odor was a prominent feature. There was diffuse pulmonary congestion and mild fibrinous pericarditis. Sections of skeletal muscle, heart, diaphragm, lung, liver, kidney, spleen, lymph node, abomasum, ileum, spiral colon, thyroid gland, and brain were fixed in 10% buffered formalin for 48 h and processed routinely for histological examination; slides were stained with hematoxylin and eosin (H&E). Fresh sections of muscle were sent for routine culture and fluorescent antibody testing (FAT) for Clostridium chauvoei, C. novyi, C. septicum, and C. sordellii.

Figure 1.

Gross appearance of skeletal muscle from the affected calf. Diffusely the skeletal muscle is pale and dry (necrosis) with dark red foci (hemorrhage) and expansion of the fascia with edema, hemorrhage and fibrin.

Microscopically, there was severe and acute coagulative necrosis of the myocytes characterized by hypereosinophilia, loss of cross striations, loss of nuclear detail, and flocculation of the sarcoplasm (Figure 2). The endomysium and perimysium were expanded by marked edema, neutrophilic inflammation, hemorrhage, fibrin, and innumerable colonies of small coccobacilli which were Gram-negative on Gram stain. Blood vessels contained thrombi and demonstrated features of vasculitis including fibrinoid change, karyorrhectic debris, and neutrophilic infiltration in the tunica media. Additional microscopic lesions included multisystemic bacterial emboli (Gram-negative coccobacilli) and diffuse alveolar damage consistent with septicemia.

Figure 2.

Microscopic appearance of skeletal muscle from the affected calf. The photomicrograph shows myocyte necrosis and expansion of the epimysium with degenerate neutrophils, fibrin and large colonies of coccobacilli (arrow). Hematoxylin and eosin. Bar = 50 μm.

Muscle tissue submitted for bacteriology testing was inoculated onto 5% sheep blood agar plates (Oxoid, Nepean, Ontario) and incubated at 37°C for 18 h in an environment containing 5% CO₂, and onto MacConkey agar plates (Oxoid) incubated as described under atmospheric conditions. Bacterial identification was done using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS; Bruker Daltonics, Billerica, Massachusetts, USA). Tests for histotoxic clostridia (C. chauvoei, C. septicum, C. sordellii, and C. novyi) were negative. Pasteurella multocida was isolated in pure culture and in large numbers (4+), and the capsular antigen group was identified as serogroup B using polymerase chain reaction (PCR), as described previously (6,10). Gene sequencing was conducted to indirectly assess somatic type (serovar) and for future reference. The results confirmed the identification of serogroup B and further classified the isolate as either serotype B:2 or B:5. That is to say, the outer lipopolysaccharide (LPS) core locus identified in the current isolate is consistent with either somatic type, as previously described by Harper et al (11). Thus, a diagnosis of septicemic pasteurellosis was made based on the isolation of P. multocida serogroup B. The complete genome sequence of the isolate is available through GenBank, chromosome accession CP048402 and plasmid accession CP048403.

Discussion

To the best of our knowledge, this is the first peer-reviewed report of septicemic pasteurellosis caused by P. multocida serogroup B in beef cattle in Canada. This disease generally occurs in cattle in tropical regions in an epidemic form, although 2 cases were recently reported in farmed elk in Canada (6). In cattle in North America, P. multocida, specifically serotype A:3, is commonly associated with pneumonia and is recognized as an important component of bovine respiratory disease complex (12). Additionally, other serogroups of P. multocida have been isolated from Canadian poultry, snow geese, rabbits, and there have been sporadic human infections following dog or cat bites (13–16).

In septicemic pasteurellosis, P. multocida is likely transmitted between cattle by ingestion or inhalation during direct contact with an affected animal or contaminated feed or water. However, it is also believed that P. multocida can survive for hours or days in decomposing carcasses and in damp soil or water, thus contact with these infected materials could represent another possible route of infection (9). Pasteurella multocida can reside in the tonsils of the nasopharynx of apparently healthy cattle and buffalo; therefore, some species may act as a reservoir of bacteria that cause septicemic pasteurellosis (17). Furthermore, wildlife reservoirs of P. multocida can allow for cross-species transmission of the disease to farmed animals. It is likely that animals carrying a pathogenic or new strain are responsible for the introduction of septicemic pasteurellosis into vulnerable populations (9). The source of infection is unknown in the present case, but the calf was kept at pasture in an isolated area with ample potential exposure to wild ruminants.

The clinical signs of septicemic pasteurellosis include increased body temperature, loss of appetite, depression, excessive salivation, edema of the head, neck, and brisket, and severe respiratory distress with foamy nasal discharge, leading to death (5,9). The disease occurs in acute, sub-acute, and chronic forms, and in the acute form of septicemic pasteurellosis death will occur in less than 24 h (5). Treatment with antibiotics is possible in the early stages, but since clinical signs of acute sepsis manifest so quickly, mortality is nearly 100% after onset.

At necropsy, some of the most common gross pathological findings are subcutaneous edema in the mandibular and brisket regions, pericarditis, and widespread petechiae to ecchymoses (9,18). Common histopathological changes include thickened alveolar septa of the lungs with increased lymphocytes and macrophages, swelling and pyknosis of tubular epithelial cells of the kidney, and subepicardial and subendocardial hemorrhages of the heart (18). However, gross pathological and histopathological findings can differ depending on the duration of the disease from acute onset to death (9).

Necrotizing myositis was the predominant gross finding in this case. Myositis is not typically described in cases of hemorrhagic septicemia or septicemic pasteurellosis in cattle, but has been described in the cases from farmed elk (6). Given the rarity of septicemic pasteurellosis in North America, this disease was not initially considered as a differential diagnosis. Given the history of peracute death and the presence of necrohemorrhagic myositis and fibrinous pericarditis, the first differential diagnosis considered at necropsy was blackleg. The true diagnosis was only revealed following histopathology, culture, and negative fluorescent antibody test for histotoxic clostridia. In retrospect, the myositis observed in this heifer did not show the emphysema or the butyric acid odor that typifies infection with C. chauvoei (19). However, to the inexperienced, a diagnosis of blackleg rendered on gross examination in the absence of histopathology and ancillary testing would have led to misdiagnosis and failure to identify this rare pathogen.

In conclusion, this is an unusual presentation of septicemic pasteurellosis caused by P. multocida serogroup B. Based on the literature, this disease is rarely seen in North America, and to our knowledge, this is the first peer-reviewed report of this disease in beef cattle in Canada. Veterinary practitioners and diagnosticians should include septicemic pasteurellosis on their list of differential diagnoses when encountering similar presentations of peracute death and severe necrotizing myositis in cattle. Furthermore, this case confirms the need for continued submission of cases for routine necropsy and ancillary testing to detect new and potentially emerging diseases in production animals in Canada. CVJ