Atypical caudal duplication with phenotypic sex reversal in a dog.

A male German Shepherd Dog mix was presented for chronic non-weight-bearing lameness of the left hind limb. At clinical examination, the dog's left hind limb had severe joint contractures, with the presence of what appeared to be a second paw. The dog also had 2 sets of external genitalia of opposite phenotypic sex; a complete male reproductive tract with a left retained testicle and a right descended testicle, as well as rudimentary female external genitalia including a hypoplastic vulva with a blind-end vagina and a hypertrophied clitoris. The female genitalia were located on the proximal posterior third of the deformed limb. Following amputation of the hind limb, gross pathologic analysis revealed a duplication of the fibula, tarsal, and metatarsal bones, digits, and appendices. The supernumerary structures and female genitalia were concluded to represent a parasitic twin. As conjoined or parasitic twinning of non-identical twins is thought to be impossible, the presence of genitalia of opposite phenotypic sex appeared paradoxical. Polymerase chain reaction analyses were therefore performed to determine the genotypic sex of both animals, which revealed the presence of the Y chromosome in all tissues, including the female genitalia. The non-masculinization of the external genitalia in the parasitic twin was presumed to be the result of an embryonic developmental defect. On this basis, a diagnosis of atypical caudal duplication (parasitic twinning) with phenotypic sex reversal was made.