The study was conducted in animals on a farm where the employees had observed abscesses on the heads of the mink, followed by progressive deaths. Information obtained from the breeder revealed that after the lesions appeared on the neck and head, the animals stopped feeding and gradually became weak, and after a few days deaths followed. At the same time, in a few individuals in which the abscess was opened spontaneously or mechanically due to scratching by the animals or veterinary intervention, the individual’s health improved and it was completely cured. No relapse was observed in these animals. In a few individuals the abscesses healed with no intervention and the animals’ health returned to normal. The abscesses appeared suddenly and frequently in the herd, and the employees noted from 30 to 50 new cases over the following week. The disease ultimately affected about 900 animals from a foundation stock of 4,000 females. Antibiotic treatment with amoxicillin and clavulanic acid added to the feed for 14 days was not successful in resolving the symptoms or preventing new cases. At the same time, the occurrence of new cases of the disease was observed to slow down, which indicated that it was bacterial.
To diagnose the problem, the breeder was asked to deliver several newly deceased animals to the Department, where post-mortem examination was performed. During the necropsy, organs were sampled for histological analysis. Following fixation in 10% neutral formalin, microscope slides were prepared of the samples by the paraffin technique and stained with haematoxylin and eosin (HE) and by special methods: periodic acid-Schiff (PAS), Gomori methenamine-silver (GMS) and Ziehl-Neelsen acid-fast (ZN) staining. Before the organ samples were taken, the lesions in the head of the animals were cut and samples were taken for microbiological testing. The material was plated on an agar medium with 5% sheep blood, MacConkey agar, and Sabouraud agar with chloramphenicol, and then incubated for 24 hours at 37 °C. The resulting colonies were identified using bioMerieux API biochemical assays.
A complete necropsy with histological and microbiological examinations was performed on 6 randomly selected dead mink. External examination revealed dehydration and a decline in the animals’ body condition. Multifocal, variously shaped, coalescing, hairless areas of about 0.5 cm2 to about 12 cm2, covered with yellow-brown crusts, were observed on the scalp and dorsal surface of the neck (Fig. 1).
Removal of the crust and compression of the skin revealed fistulas penetrating the skin and subcutaneous tissue, from which a purulent exudate oozed to the surface. The surrounding skin was significantly swollen. In 4 mink, moderate bilateral enlargement of the submandibular lymph nodes was evident. In addition, moderate splenomegaly, pulmonary oedema and congestion, and congestion of the liver and kidneys were found. No other macroscopic lesions were observed in internal organs. Microscopic examination of skin lesions revealed multifocal and coalescing nodular aggregates of numerous viable and degenerate neutrophils, moderate numbers of macrophages, and fewer lymphocytes and plasma cells admixed with cellular and karyorrhectic debris infiltrating the dermis and subcutis. Similar inflammatory cells were scattered between dermal collagen bundles and around the adnexal structures (Figs. 2 and 3). The overlying epithelium was focally ulcerated and covered with a serocellular crust. The additional staining methods (PAS, GMS and ZN) revealed no evidence of parasites, fungi, or mycobacteria. Gross and microscopic lesions were consistent with the diagnosis of focally extensive suppurative dermatitis and panniculitis with purulent fistulas.
Microscopic lesions in internal organs were limited to the liver. The periportal areas were multifocally infiltrated by low numbers of lymphocytes, plasma cells, fewer neutrophils, and macrophages, whereas multiple hepatocytes of centrilobular areas contained one to few clear cytoplasmic nucleus-displacing vacuoles (vacuolar change, lipid-type).
The microbiological analysis showed a very large increase in Escherichia coli in all samples taken from the abscess material. The results indicate the migration of intestinal bacteria through disturbance of the permeability of the intestinal barrier and their transfer to the blood. The interview with the breeder indicated that the feed ration was correctly balanced in terms of energy. Attention was drawn to the introduction of a feed component in the form of a 30% share of ground turkey bones, which coincided with the onset of disease among the animals and could have caused mechanical damage to the intestinal barrier.