Dermatophilus congolensis in a feral cat

Abstract

A young adult feral cat presented to the Champaign County Humane Society with a subcutaneous mass near the stifle. The mass was aspirated. Chains of paired cocci organisms were identified, consistent with Dermatophilus congolensis. The identity of these organisms was confirmed by culture and polymerase chain reaction.

Case Report

An adult female feral cat, 1–2 years of age, presented to the Champaign County Humane Society for ovariohysterectomy at their trap–neuter–vaccinate–release program. The cat spent the majority of its time outside; however, limited history from the caretaker was available. The cat was sedated with xylazine and ketamine so a physical examination could be performed. Upon physical examination, a 2 cm subcutaneous swelling was noted caudal to the right stifle (Figure 1a). The mass was firm with irregular borders and significant alopecia. The mass had a single draining area, which had a brownish red scab covering it. Fine-needle aspiration of the mass was performed (Figure 1b). Additionally, the scab was removed and an impression smear of the draining tract was made.

Figure 1

(a) The mass is large and in the popliteal area of the right hind limb. (b) Fine-needle aspirate of the mass pictured in (a). Arrows indicate chains of Dermatophilus congolensis organisms. Magnification × 50. Wright–Giemsa staining

The cytological specimens were presented to the University of Illinois Veterinary Diagnostic Laboratory for evaluation. The fine-needle aspirate was highly cellular and contained a mixed inflammatory population predominated by degenerate neutrophils and few vacuolated macrophages. Mats and chains of bacteria were observed, including filaments of paired cocci bacteria (<1 µ) and long (2–4 µ), rod-shaped bacteria (Figures 2a,b). Red blood cells and streams of nucleoproteinaceous debris filled the background.

Figure 2

(a) Chains of paired cocci (Dermatophilus congolensis) and filamentous rod bacteria are identified. Arrows indicate chains of D congolensis organisms. Magnification × 100. Wright–Giemsa staining. (b) Chains of paired cocci in a thick mat of degenerate neutrophils and streams of nucleoproteinaceous debris. Arrow indicates a small chain of paired cocci. Magnification × 100. Wright–Giemsa staining

Cytologically, the paired cocci were consistent with Dermatophilis congolensis. Organisms were further evaluated by culture and polymerase chain reaction from the aspirate of the mass and swab of the draining tract. Culture revealed three bacterial organisms: rare, non-hemolytic Staphylococcus coagulase negative, Neisseria species and Dermatophylus congolensis. Dermatophilosis was confirmed by sequencing of the 390 base pair 16S–23S ribosomal RNA amplicon, which had highest homology with D congolensis #HQ113103.1 in the GenBank database, searched via nucleotide BLAST (http://blast.ncbi.nlm.nih.gov).

D congolensis is a Gram-positive coccobacillary actinomycyte and normal dermal organism in commonly affected species such as cattle, horses and sheep. ¹ In susceptible species, the organism is opportunistic and disease results after chronic trauma or moisture allowing for bacterial overgrowth.^2,3 In these species the lesion is an exudative skin disease involving the epidermis and is often referred to as ‘rain scald’ in horses and ‘fleece rot’ in sheep. ⁴ The infection is usually limited to the cutaneous tissues, including the hair follicles, but rare reports describe affected lymphoid tissue. ¹

D congolensis has been rarely reported in cats.^5,6 In cats, the subcutaneous tissues, muscle and lymph nodes are more commonly affected. ² The popliteal lymph node area has been reported as a typical area of infection for cats. ⁵ Cats are not considered primary hosts of D congolensis, but infection has been experimentally induced via subcutaneous inoculation. ⁶ It is theorized that infection of cats can occur through trauma, puncture wounds or insect bites. ⁴ In this case, the cat was feral and therefore limited history was available. However, it lived outdoors and was at risk for puncture wounds, various trauma and insect bites. At the time of presentation, the environment had been wet and humid, which would support the infection and likely contributed to it. The patient was treated with amoxicillin trihydrate/clavulanate potassium (Clavamox), 62.5 mg by mouth, twice daily, for 6 weeks. Because this was a feral cat, the tablets were crushed and mixed with canned food. The patient showed significant improvement and was released.

Conclusions

Dermatophilus congolensis is an uncommon infection in cats but it should be considered in patients in moist environments. The morphology of the organism is distinct regardless of the infected species. It can be easily diagnosed with cytology.