Pilot study to evaluate the role of Mycoplasma species in cat bite abscesses

Abstract

Mycoplasma species are common inhabitants of the feline oral cavity, and so likely contaminate many cat bite abscesses. The objectives of this study were to determine whether Mycoplasma species are common contaminants of cat bite abscesses and whether they are are associated with β-lactam-resistant clinical disease. Twenty-six privately owned cats with clinical evidence of an abscess suspected to be from a cat bite were included in the study. Samples from each cat were evaluated by aerobic and anaerobic culture, as well as Mycoplasma species culture and polymerase chain reaction (PCR). All cats were initially treated with appropriate wound management and were administered an antibiotic of the β-lactam class (amoxicillin, amoxicillin clavulanate or cefovecin sodium). Mycoplasma species DNA was amplified by PCR from 4/26 samples (15.4%); one of these cases was concurrently culture positive. Adequate DNA for sequencing was present for 2/4 positive PCR samples; one was most homologous with Mycoplasma felis, and the other was most homologous with Mycoplasma equigenitalium and Mycoplasma elephantis. Of the 26 cats, 25 responded to the initial treatment by day 7. The cat that failed initial treatment was positive for M equigenitalium or M elephantis DNA on days 0 and 12, and ultimately responded to administration of enrofloxacin and clindamycin. The results suggest that while Mycoplasma species can contaminate cat bite abscesses, routine wound management and β-lactam antibiotic therapy is adequate for treatment in most cases of abscess. However, as Mycoplasma species infections do not respond to β-lactam class antibiotic therapy, these organisms should be on the differential list for cats with abscesses that fail treatment with this antibiotic class.

Short Communication

Cat bite cellulitis and abscesses are common in cats and are among the most common infectious diseases presented to veterinary practices each year. ¹ The penetrating wounds induced by the feline canine teeth inoculate high numbers of microbes into the subcutaneous tissues, while providing crush injury that results in tissue devitalization. The wounds are predominately closed immediately after the canine tooth exits, and even the most delicate microbes can thrive in this environment. Abscesses often develop and are generally treated with surgical drainage, with or without antibiotic therapy.

The oral cavity of cats has a rich population of bacteria that has been shown to be similar in composition to the population found in cat bite abscesses. ² The vast majority of these organisms are obligate and facultative anaerobes, including Actinomyces species, Pasteurella species, Bacteroides species and Fusobacterium species. ² These organisms are usually susceptible to β-lactam antibiotics like amoxicillin, which is why this antibiotic class is frequently prescribed empirically by veterinarians for the treatment of this syndrome in lieu of performing aerobic and anaerobic culture and susceptibility.

Mycoplasma species are cell wall-deficient organisms that are also normal flora of the cat mouth. ³ These organisms have been isolated most frequently from the pharynx, bronchus and conjunctiva.^3,4 It is possible that Mycoplasma species are commonly associated with cat bite cellulitis or abscesses; however, to our knowledge, this hypothesis has not be assessed in a large number of cases. In a single case report, two Mycoplasma species were isolated from wounds, induced by a dog, on a 25-year-old cat. ⁵ These wounds resolved approximately 15 days after the administration of ciprofloxacin, an antibiotic with known anti-Mycoplasma activity. ⁵ Mycoplasma species were also suspected to be involved in several chronic abscesses in cats of Canada. ⁶ Mycoplasma species have resulted in local tissue infections in humans after being bitten by cats.^7,8

In general, Mycoplasma species are non-pathogenic organisms that reside on the surface of mucous membranes. However, the pathogenic potential of various Mycoplasma species may vary, and some may be more capable than others of being primary pathogens. For example, intravenous inoculation of Mycoplasma gatae can been associated with polyarthritis in cats. ⁹ In addition, Mycoplasma species are sometimes isolated from clinically ill cats with no evidence of underlying diseases. ⁴ Because Mycoplasma species are cell wall-deficient, β-lactam antibiotics are not effective therapeutic agents. Thus, these organisms may be more common in cats with abscesses that fail to totally resolve after administration of this class of antibiotics.

In this pilot study, we hypothesized that a variety of Mycoplasma species commonly grow within cat bite abscesses and may persist within the tissues of some cats, causing persistent disease. The objectives were to determine the prevalence of Mycoplasma species within cat bite abscesses before and after treatment with β-lactam antibiotics and to determine the infecting Mycoplasma species by polymerase chain reaction (PCR) and genetic sequencing.

The study was approved by the Institutional Animal Care and Use Committee at Colorado State University, and all owners signed an informed consent form. Client-owned adult cats (>6 months of age) with open or closed cutaneous abscesses, presumably from cat bites, were included (n = 26); cats with an abscess caused by other means, such as a penetrating foreign body or untreated external wound, were excluded. Cases were included from the Veterinary Teaching Hospital at Colorado State University and select small animal clinics in Larimer County. Both open and closed abscesses were included, and so material from within the abscess pocket was collected onto three sterile swabs. One swab was placed in an aerobic/anaerobic bacterial transport medium, one swab was placed in Amies medium with charcoal for Mycoplasma species culture, and one swab was placed in 0.5 ml of sterile phosphate-buffered saline solution for Mycoplasma species PCR and genetic sequencing of positive samples.

Aerobic bacterial culture, anaerobic bacterial culture, Mycoplasma species culture and aerobic bacteria antibiotic susceptibility testing were performed at the Veterinary Diagnostic Laboratory at Colorado State University immediately after collection (www.dlab.colostate.edu). The Mycoplasma species PCR assay was performed in the laboratories of the Center for Companion Animal Studies (www.csuvets.colostate.edu/companion) at Colorado State University using previously validated techniques. ¹⁰ Mycoplasma species DNA from positive samples were purified using a commercially available kit (Qiagen Gel Purification Kit; Qiagen) and submitted for sequencing at a commercial laboratory (Macromolecular Research Core Laboratory, Colorado State University). Resultant sequences were analyzed in comparison to sequences in GenBank using the BLAST program on the National Institutes of Health’s website (http://www.ncbi.nlm.nih.gov).

Surgical drainage and wound lavage was provided as indicated by the clinician in charge of each case. All cats were administered either amoxicillin at 20 mg/kg PO q12h, amoxicillin clavulanate (Clavamox; Zoetis) 15 mg/kg PO q12h for at least 7 days or cefovecin sodium (Convenia; Zoetis) 8 mg/kg SC once. All cats were rechecked at least once between days 3 and 7, and cultures and PCR repeated as described if drainage was still present. If there was incomplete response at recheck, it was recommended that an antibiotic with an anti-Mycoplasma species spectrum be prescribed (enrofloxacin at 5 mg/kg PO q24h, marbofloxacin at 2.55 mg/kg PO q24h or clindamycin at 10 mg/kg PO q24h).

Based on culture, samples collected from 23/26 cats entered into the study were positive for at least one bacterium. Twenty-six aerobic, anaerobic and Mycoplasma species cultures were performed: 15 samples grew anaerobic and facultative anaerobic bacteria; five samples grew aerobic and anaerobic bacteria; two samples grew aerobic bacteria alone; and one sample grew Mycoplasma species (graded as 2+ growth) and an anaerobic bacterium. The most common aerobic bacteria isolated were Bacillus species (four cases) and Corynebacterium species (two cases). The most common anaerobic and facultative anaerobic bacteria isolated were Pasteurella species (12 cases), Actinomyces species (five cases), Fusobacterium species (six cases) and Peptostreptococcus species (five cases).

Of the four cats that were PCR-positive for Mycoplasma species DNA, one cat grew Mycoplasma species and an anaerobe, two cats grew anaerobic bacteria alone and one cat grew no bacteria. Of the four samples that were Mycoplasma species PCR-positive with adequate DNA for sequencing, one was the most homologous to Myocplasma felis, and the other was most homologous to Mycoplasma equigenitalium and Mycoplasma elephantis. The cat with the DNA most consistent with M equigenitalium or M elephantis was also Mycoplasma species culture-positive.

Of the 26 cats, 25 responded to the initial treatment by day 7, including 3/4 Mycoplasma species PCR-positive cats. Thus, a follow-up culture and PCR analysis was only available for one cat. The cat that failed initial treatment with a β-lactam (amoxicillin clavulanate) was positive for M equigenitalium or M elephantis on both days 0 and 12, and ultimately responded to administration of enrofloxacin and clindamycin. In this cat, non-haemolytic Streptococcus and Staphylococcus species (coagulase-negative) were also isolated from the day 0 sample; non-haemolytic Streptococcus species, Pasteurella dagmitis, Alcaligenes faecalis, Actinomyces species and Bacillus species were isolated from the day 12 sample.

For the microbiological cultures, it would have been optimal to have sterilized the skin surface prior to collection, used sterile needles to collect the samples, capped the syringe to lessen the oxygen content to support the growth of anaerobic bacteria, and delivered the samples to the laboratory within minutes. However, cases were accepted into the study even if the wound was already open, and so swabs were used. It is possible that some of the samples were falsely negative for anaerobes, and positive results in some cats could have resulted from contamination from the skin surface, the oral cavity or the environment.

Mycoplasma species are difficult to culture, depending on the transport and culture media used; therefore, they may be overlooked as a causative agent in cat bite abscesses. For example, even though an appropriate transport medium was used in this study, Mycoplasma species were cultured from only 1/4 PCR-positive cases. These results suggest that PCR may be a more accurate way of detecting the presence of Mycoplasma species in clinical samples. Future studies could also evaluate other PCR assay techniques. For example, quantitative PCR assays could be developed and used to determine whether the Mycoplasma species DNA load might correlate to treatment response.

While bite wound infections with Mycoplasma species appear to be relatively rare in cats, there are several cases reported of cats with an abscess that failed to respond to β-lactam therapy, and Mycoplasma species was either identified or thought to be the causative agent. Three reports are from cat bites ⁶ and one from a dog bite. ⁵ In the three cases of the cats with chronic abscesses from cat bites, bacteria could not be cultured, organisms resembling Mycoplasma species were identified in 2/3 cases and all three of these cases responded to tetracycline therapy. Mycoplasma canis and Mycoplasma spumans were identified in the case of the cat with the abscess from the dog bite. ⁵ In the study described herein, genetic sequencing was used to prove M felis in one case, and infection with either M equigenitalium or M elephantis was proven in another case. The other two cases that were PCR-positive for Mycoplasma species could not be sequenced. The cat with M felis, as well as the two other Mycoplasma species PCR-positive cats, responded to abscess drainage and β-lactam therapy. As Mycoplasma species are resistant to β-lactam drugs, these cats show that control of co-infecting bacteria and appropriate local management of the abscess with drainage can be effective, even if bacteria with specific antimicrobial resistance patterns are present within the wound. The cat with M equigenitalium or M elephantis DNA in the wound required an antibiotic class switch from a β-lactam to enrofloxacin and clindamycin before having an apparent clinical response. In this case, it was the clinician’s preference to use dual therapy.

For the cat that failed β-lactam therapy, it cannot be determined definitively whether the Mycoplasma species were primary pathogens, whether the co-infections were playing a role, if the cat was immune compromised or if there were other factors that may have induced the failure to respond to the initial therapy. In human medicine, one report identified that patients with impaired humoral immunity were more likely to have Mycoplasma species infection. ⁸ The cat in this report was feline leukemia virus antigen-negative, feline immunodeficiency virus antibody-negative, and was otherwise normal, suggesting that immune compromise was unlikely. However, in this case, the same organisms were detected on days 0 and 12, supporting the hypothesis that the Mycoplasma species could have been pathogens. We did not have the chance to inoculate healthy cats with this strain of M equigenitalium or M elephantis, as was described for M gatae, and so have no other information as to whether they could be pathogenic strains. ⁹

Conclusions

The results of this study show that while Mycoplasma species can contaminate cat bite abscesses (15.4%), routine wound management and antibiotic therapy is adequate for the control of most cases. However, as Mycoplasma species infections do not respond to β-lactam antibiotics, these organisms should be on the differential list for cats with abscesses that fail treatment with this antibiotic class.