Actinobacillus equuli ssp. haemolyticus in a semi-occlusively treated horse bite wound in a 2-year-old girl

Abstract

We report on the isolation of Actinobacillus equuli ssp. haemolyticus from wound smears of a 2-year-old girl who was admitted to the hospital due to partial amputation of the distal phalanx of her right middle finger caused by a horse bite. A. equuli typically causes diseases in horses and only very few reports describing human infections (mostly associated with wounds) are available in the literature. Interestingly, although the bacteria could be found in consecutive samples taken at different points in time, there were no signs of advancing infection or inflammation. Moreover, the fingertip regenerated after 74 days under semi-occlusive dressings with very pleasant results. For strain identification two automated systems were employed producing discrepant results: VITEK 2 described the pathogens as Pasteurella pneumotropica while MALDI-TOF MS analysis revealed A. equuli. Sequence analysis of 16S rDNA gene finally confirmed A. equuli ssp. haemolyticus as the isolated strain. The antimicrobial susceptibility testing was performed according to the CLSI criteria for Pasteurella spp. Additionally we conducted a test according to the EUCAST criteria.

Keywords: Actinobacillus equuli; MALDI-TOF MS; Pasteurella; VITEK 2; finger amputation; semi-occlusive dressing.

Table 1. Antibiotics tested according to CLSI criteria. The antibiotics were tested according to the CLSI-guidelines published for Pasteurella spp. Ampicillin, Amoxicillin/Clavulanic Acid, Moxicillin, Trimethoprim/Sulfamethoxazole and Tetracycline were tested to be susceptible (S) and Erythromycin resistant (R).

Table 2. Antibiotics tested according to the EUCAST criteria. The proposed test panel for Pasteurella multocida was chosen. The gained MICs were interpretated according to the EUCAST non-species related breakpoints. Ampicillin, ampicillin/clavulanic acid, cefotaxim and ciprofloxacin were tested as susceptible (S). For Trimethoprim/Sulfamethoxazole and Tetracycline there is an insufficient evidence (IE) for being a good therapeutic target.

Figure 1. Photographs of the injured finger taken on days 2 (a), 28 (b), 56 (c), at the end of the treatment on day 74 (d) and finally at around 3 months after the injury (e)

Figure 2. Growth of Actinobacillus equulii spp. haemolyticus on Columbia Blood Agar and Gram Staining. As demonstrated in a) the bacteria grow in small grey colonies with a sharp β-hemolysis while b) shows the typical Gram-staining showing the bacterial growth as both as gram-negative rods and cocci.