Mycoplasma bovis antimicrobial susceptibility testing at the AHL

Hugh Cai, Jeff Caswell

Supported by OMAFRA and the Animal Health Strategic Investment (AHSI) program, the AHL examined temporal changes in the in vitro minimum inhibitory concentrations (MIC) of antimicrobials for 210 M. bovis isolates collected from 1978-2009, and found the following changes in MIC50 levels1 (Table 1):

1. The MIC50 levels of clindamycin, spectinomycin, and tulathromycin were low in the 1980s, increased in the 1990s to 8 μg/mL (clindamycin) and 32 μg/mL (spectinomycin and tulathromycin), then decreased again in the 2000s.

2. MIC50 levels for tetracyclines, tilmicosin, and tylosin tartrate were low in the 1980s, then increased in the 1990s and remained high.

3. Enrofloxacin, danofloxacin, and tiamulin MIC50s remained low (0.25 μg/mL) from the 1980s to 2000s.

MIC testing for M. bovis was implemented at the AHL in 2014. However, the tests were under-used for clinical isolates except for those from academic research. Although the number of field isolates tested is low, the test results indicated that the MIC for most antimicrobials had increased (Table 1).  For example, MIC50s of a recent M. bovis isolate was 2 for chlortetracycline, 8 for oxytetracycline, >64 for tilmicosin, >32 for tylosin tartrate, >64 for tulathromycin, >16 for clindamycin, 2 for tiamulin, >1 for danofloxacin, >2 for enrofloxacin, 8 for florfenicol, and 64 for spectinomycin, which are mostly much higher than the isolates from the 1980s to 2000s.

Temporal changes in MIC levels of various antimicrobials show the importance of monitoring the susceptibility of mycoplasmas to antimicrobials. Fluoroquinolones, e.g., danofloxacin and enrofloxacin, are category I antibiotics (very high importance), and are considered essential for the treatment of serious bacterial infections in humans, and there is limited or no availability of alternative antimicrobials for effective treatment in case of emergence of resistance to these agents,2 and therefore they should not be used as first line antimicrobials. “The drug that has evidence-based efficacy and is in the category of least importance in human medicine should normally be selected as the first choice.3   AHL


1. Cai HY, et al. Changes in antimicrobial susceptibility profiles of Mycoplasma bovis over time. Can J Vet Res 2019;83:34-41.



MIC50 = the lowest concentration of an antimicrobial at which 50% of isolates are inhibited.

Table 1. AHL Mycoplasma bovis MIC50 (µg/mL) for bovine field isolates.

AHL Mycoplasma bovis MIC50 (µg/mL) for bovine field isolates

Chronic Erysipelothrix rhusiopathiae infection in lambs

Maria Spinato, Durda Slavic, Rex Crawford

Ten East Friesian lambs in a group of 80 animals developed ataxia and hindlimb weakness at ~ 2 wk of age. The lambs were treated with antibiotics and injected twice with selenium. A few animals improved slightly and there were no deaths. Three 5-wk-old intact male lambs were euthanatized and submitted to the AHL for complete postmortem evaluation. All 3 lambs had bilateral stifle synovitis characterized by increased amounts of cloudy synovial fluid and edematous synovial membranes; one lamb also had a small clump of organizing fibrin (pannus) within the joint. Vertebral columns were sectioned to examine bones and spinal cord. Two lambs had suspected diskospondylitis characterized by absence of L7-S1 intervertebral disks and associated irregular expansion of the intervertebral space which was filled with dark red fluid. Tails were not docked and omphalitis was not evident. No gross abnormalities were noted in viscera or the central nervous system. As an infectious etiology was suspected, joint and bone swabs were submitted for bacterial culture. Histologic evaluation was performed. Liver samples were submitted for a tissue mineral panel to rule out copper deficiency and selenium toxicity or deficiency.

Histologic examination of stifle synovia revealed marked lymphoplasmacytic villus hyperplasia (Fig. 1). Subsynovial stroma was infiltrated by a broad zone of lymphocytes, plasma cells and neutrophils. The affected intervertebral disk spaces were filled by an irregular mass of degenerate neutrophils and fibrin surrounded by hemorrhagic and organizing fibrovascular stroma. Focal chronic glomerulitis present in all 3 lambs was typified by dilation of urinary spaces by proteinaceous fluid, epithelialization and thickening of the parietal layer of Bowman capsules, and periglomerular fibrosis (Fig. 2). Amyloid deposits were present in the spleen of one lamb only. Hypomyelination was not observed in sections of brain, and there were no digestion chambers consistent with compressive myelopathy noted in the lumbar spinal cord segments. A single colony of Erysipelothrix rhusiopathiae was isolated from the joint swab of one lamb; no other significant bacterial pathogens were isolated from other swabs of affected joints or disks. Copper and selenium levels were within normal reference intervals.

Chronic polyarthritis and diskospondylitis caused by Erysipelothrix rhusiopathiae septicemia was diagnosed as the most probable cause of ataxia and hindlimb lameness in this group of affected lambs. Many cases of septicemia are subclinical, with infection only identified following localization in typical sites, including joints, kidneys, bones, brain, and heart valves. It can be difficult to confirm this etiology in chronic stages, because of the paucity of bacteria within affected tissues. Chronicity and/or antibiotic therapy may have played a role in the failure to isolate more than a single colony of E. rhusiopathiae from multiple lambs/swabs in this case, as also reported in other cases of chronic erysipelas polyarthritis in lambs and pigs. Even when synovial membrane scrapings rather than joint fluid swabs are cultured, only 1-2 colonies may be isolated.1 PCR is more sensitive for detection of bacteria in affected joints and tonsils of chronically affected lambs.1

Erysipelas septicemia is a not uncommon infection in the neonatal period, particularly in swine and sheep. Swine, birds and other wildlife are considered to be the usual carriers/shedders, and contamination of the environment is widespread. Persistent infection and associated antigenic stimulation are suspected to be the cause of lymphoplasmacytic synovitis and amyloidosis of the liver and spleen. Endocarditis is also an occasional sequela diagnosed in juvenile sheep.   AHL


1. Ersdal, C, et al. Acute and chronic Erysipelothrix rhusiopathiae infection in lambs. Vet Pathol 2015;52:635-643.

Marked proliferation of stifle synovium with extensive lymphoplasmacytic infiltrates.

Figure 1. Marked proliferation of stifle synovium with extensive lymphoplasmacytic infiltrates.

Chronic glomerulitis with epithelialization

Figure 2. Chronic glomerulitis with epithelialization and thickening of Bowman capsules and periglomerular fibrosis in affected glomeruli (arrows). Compare to normal glomeruli (stars).

Unusual intestinal lesion in a goat

Jan Shapiro

Formalin-fixed tissues were submitted from a 4-y-old male Boer goat from a herd of 200. The goat had been losing weight for a few weeks, and had not responded to a course of antibiotics and dewormers. A field postmortem showed that the goat was emaciated, and had 2 segments of the jejunum that were stenotic because of intramural thickening by white firm tissue (Fig. 1), and other intestinal segments had clusters of raised white foci on the serosa. The wall of the stenotic segments of jejunum was 5-6 times thicker than the unaffected segments.

Jejunal histopathology confirmed the presence of a very scirrhous intramural tumor which appeared to originate in areas of the mucosa with dysplastic crypts. Tumor cells formed acinar or glandular structures which infiltrated the full thickness of the jejunal wall, and onto the serosal surface, and were surrounded by marked fibroblastic response (Fig. 2). Tumor clusters were seen in a few vascular structures. The tumor diagnosis was intestinal adenocarcinoma.

Histopathology of other sections of the intestine and the ileocecal lymph node revealed underlying granulomatous enteritis and lymphadenitis typical of paratuberculosis (Johne’s disease), which was confirmed by a positive PCR test for Mycobacterium paratuberculosis. Paratuberculosis was likely a major factor in emaciation, given that intestinal adenocarcinoma in small ruminants is often an incidental finding at slaughter.

Unlike sheep, intestinal adenocarcinoma is not common in goats. The mid-jejunum is a common site, forming dense white polypoid or intramural masses extending from 0.5 to several cm along the gut segment, and causing annular constriction of the lumen. Weight loss and sometimes ascites are clinical signs. The tumor is highly infiltrative and can metastasize to local nodes and distal sites, as well as spreading through the peritoneum. In sheep in New Zealand, where the disease is common, there may be an association with exposure to bracken fern or other unidentified carcinogens, heavy use of  certain fertilizers, and pastures with the weed Cyanosaurus cristatus. There may also be a genetic predisposition. The risk factors for this tumor in goats are not known, likely because of the small number of reported cases.   AHL


Munday JS, et al. Tumors of the alimentary system. In: Tumors in Domestic Animals. Meuten DJ, ed. 5th ed. John Wiley, 2017:564-568.

   Uzal FA, et al. alimentary System. In: Pathology of Domestic Animals, Maxie MG, ed. 6th ed. Elsevier, 2016, vol 2:103-104.

Section through formalin-fixed jejunum showing segmental transmural thickening.

Figure 1. Section through formalin-fixed jejunum showing segmental transmural thickening.

Histologic section through stenotic jejunal segment, arrows showing tumor infiltrates (mucosa at bottom of photo).

Figure 2. Histologic section through stenotic jejunal segment, arrows showing tumor infiltrates (mucosa at bottom of photo).