Ectopic (Subcutaneous) Paragonimus miyazakii Infection in a Dog

Abstract

Ectopic infection with Paragonimus miyazakii was determined to be the cause of a subcutaneous inguinal mass in a 15-month-old, male, boar-hunting dog. On histologic examination, the mass comprised granulomatous panniculitis, intralesional adult trematodes and eggs, and lymphadenitis. Extrapulmonary paragonimosis in animals is rare. This appears to be the first report in a dog of ectopic P. miyazakii infection with mature trematodes and eggs that involved the inguinofemoral lymphocenter and surrounding subcutis.

Paragonimosis is a parasitic disease caused by trematodes of the genus Paragonimus and is an important food-borne endemic zoonosis worldwide. ^6,8 Adult trematodes reside in the lungs of definitive hosts (humans and various wild and domestic animals, including dogs). ^6,8 Ectopic paragonimosis with larvae and/or adults is well recognized in humans; ^3,6,8 however, only a few cases of natural or experimentally induced ectopic paragonimosis were reported in dogs. ^1,2,5,7,14 To the best of our knowledge, this represents the first report of extrapulmonary P. miyazakii infection in a dog with mature trematodes and eggs in an inguinal mass that involved lymph nodes and surrounding subcutis.

A 15-month-old, setter-type, male dog that had been used for boar hunting was presented to a private animal hospital for examination of a palpable inguinal mass. Reportedly, the dog occasionally ate raw wild boar meat. Lymphoma was suspected clinically, so a core biopsy of the inguinal mass was performed. After unidentifiable fragments of a parasite were detected on histologic examination, excisional biopsy was performed for diagnosis and therapy.

On gross examination, the formalin-fixed subcutaneous mass was gray and lobulated, approximately 16 × 10 × 10 cm. The cut surface of the mass was mottled yellow-brown to dark-red and contained trematodes and enlarged lymph nodes. By stereomicroscopy, the trematodes, encapsulated in cysts, were whitish, had a thick ellipsoidal body and a small, reddish, crater-shaped acetabulum. Eggs were operculate, irregularly barrel shaped, and averaged 77.7 µm × 48.0 in length and width.

On histologic examination, the subcutaneous mass comprised granulomatous panniculitis with cysts that contained adult trematodes and eggs, and lymphadenitis associated with the eggs. Dense, broad bands of fibrous tissue encapsulated the mass and dissected between the cysts and nodules (Figs. 1, 2). Cysts contained single or paired adults, scattered mature eggs, mixed inflammatory cells, red blood cells, and necrotic debris. Nodules consisted of numerous mature and degenerated eggs in granulomatous inflammation (Fig. 2). Some nodules contained a central mass of eggs and a thin peripheral rim of fibrosis; in others, solid nodules of eggs were separated by fine fibrous septa. Infiltrating leukocytes included a variable number and mixture of plasma cells, lymphocytes, neutrophils, eosinophils, and macrophages, with a few multinucleated giant cells. The affected lymph nodes had both small and massive clusters of eggs scattered throughout the nodal sinuses (Fig. 3). Some egg clusters elicited granulomatous response. Follicles were hyperplastic with prominent germinal centers; nodal fibrosis was severe and diffuse.

Fig. 1.

Inguinal subcutaneous mass; dog. Granulomatous panniculitis and lymphadenitis associated with adult trematodes and eggs. HE. Bar  =  1 mm.

Fig. 2.

Inguinal subcutaneous mass; dog. Clusters of trematode eggs in granulomatous inflammation. HE. Bar  =  100 µm.

Fig. 3.

Lymph node of the inguinofemoral lymphocenter; dog. Granulomatous lymphadenitis with trematode eggs. Lymphoid follicles have prominent germinal centers. HE. Bar  =  400 µm.

On histologic examination, the adult trematodes had tegument, with a single spine, well-developed oral and ventral suckers, uterus, ovary, and intestine. Numerous mature eggs were in the uterus; lobes of the ovary were moderately branched. Testes and vitelline ducts were also fully developed and contained mature sperm and vitelline cells, respectively.

Microscopic features of the adult parasites and stereomicroscopic features of the eggs were characteristic of Paragonimus spp., so ectopic paragonimosis was diagnosed. After approximately 2 weeks, the excision site swelled again to approximately two thirds of its size before surgery. Oral praziquantel treatment (10 mg/kg, once a day, for 10 days) was administered. The swelling subsided after the treatment, and the dog has remained free of clinical disease, without further treatment, for more than 2 years. Additional clinical examinations, including chest radiography or fecal examination for Paragonimus eggs, were not performed.

The Paragonimus sp. was identified from the formalin-fixed specimen by polymerase chain reaction (PCR) linked restriction fragment length polymorphisms (RFLPs) and DNA sequencing. ¹² Restricted fragment length polymorphisms of the second internal transcribed spacer (ITS2) from nuclear ribosomal DNA were identical to those of P. miyazakii (Fig. 4), and the sequences of ITS2 were identical to those deposited in the GenBank/EMBL/DDBJ nucleotide database (accession number U96912) for P. miyazakii. The species was thus identified as P. miyazakii.

Fig. 4.

RFLP analysis of ITS2 PCR products. The DNA from the sample (S), P. westermani (diploid type) (Pw), P. miyazakii (Pm), and Paragonimus ohirai (Po) was amplified with ITS2-specific PCR primers and digested with endonucleases, BsoBI, and BssSI. The restricted fragments of the sample are identical to those for P. miyazakii.

Paragonimosis is an important food-borne parasitic zoonosis caused by trematodes (genus Paragonimus), which infect the lungs of humans and various other animals. ^6,8 At least 28 species of Paragonimus have been identified, ⁸ and 10 species are recognized as causing human disease. ⁶ Paragonimus spp. are mainly parasites of cats, dogs, and various mammals that eat freshwater crabs and crayfish (the second intermediate host), as well as the raw meat of wild boars (the paratenic host). ⁹ Occasionally, humans become accidental hosts. ⁸ With approximately 200 million people at risk and 22 million people infected worldwide, paragonimosis still is an important public health threat. ⁶ In the Far East, including Japan, the prevalence of human parasitic diseases has been greatly reduced as living standards improved, but pulmonary paragonimosis remains an important endemic parasitic disease. ¹¹ Dogs and cats, as more likely definitive hosts, generally play a greater role than humans in the Paragonimus life cycle. ⁶

In veterinary medicine, 2 Paragonimus spp., Paragonimus westermani and Paragonimus kellicotti, are of particular interest. ⁴ P. westermani is the best-known species in Asia; P. kellicotti occurs in North America. In addition, in Japan, P. miyazakii is another important species responsible for human and animal paragonimosis. ^3,6,10

Paragonimus trematodes usually infect the lungs of the mammalian host. ³ However, aberrant migration may be more likely in accidental hosts, ⁸ and, because humans are less suitable than other mammals as a definitive host for Paragonimus, they may be more commonly affected by ectopic paragonimosis. In human extrapulmonary paragonimosis, various sites and tissues, including the brain, spinal cord, abdominal cavity, or subcutis, may be involved; ^3,6,8 pathologic effects are principally caused by the presence of adult trematodes and eggs, the movement of trematodes through tissues, and the metabolites produced by trematodes. ³ Some species differences exist in the frequency and effect of ectopic paragonimosis. Subcutaneous masses associated with parasitic migration reportedly occur in 20–60% of Paragonimus skrjahbini infections, compared with approximately 10% with P. westermani and 2.4% with P. miyazakii (2/82). ^6,10 For those Paragonimus spp., such as P. miyazakii and P. skrjahbini, for which humans are an unsuitable host, the trematodes are unlikely to mature, so eggs are rarely found in extrapulmonary sites. ³

In dogs, ectopic paragonimosis is rare. ^1,2,5,7,14 In reported cases, extrapulmonary lesions were mostly produced by migration of immature trematodes or by eggs; ^5,14 exceptional cases had P. westermani cysts and eggs in trachobronchial lymph nodes ⁷ or spleen. ^2,5 In contrast, the present case developed an inguinal subcutaneous mass that contained encysted solitary or pairs of adult trematodes. The trematodes had matured in this ectopic location, forming cysts and producing eggs. Granulomatous panniculitis and lymphadenitis were mainly attributed to the presence of eggs. Because dogs are definitive hosts for P. miyazakii, the incidence of canine extrapulmonary paragonimosis should be low. However, once P. miyazakii settles in an extrapulmonary site, maturation and egg laying may occur, as in the present case.

In this dog, the trematodes may have migrated to an inguinal lymph node, mated, deposited eggs, and provoked lymphadenitis and regional panniculitis. In human medicine, lymphadenitis because of the presence of metazoan parasites, including Paragonimus or their eggs, is rare but has been described. ¹³ In dogs, such lymphadenitis is also rare but has been described with P. westermani ⁷ and P. kellicotti infection. ¹ However, lesions in those cases were much less severe than in the present case.