Histopathological findings in the central nervous system of Montezuma swordtail (Xiphophorus montezumae Jordan & Snyder, 1899) fry with early mortality associated with severe neurological signs

Introduction: Neurological syndromes in fry may be associated with internal or external hydrocephalus caused by a variety of injuries, including toxic, nutritional, infectious, and genetic/heritable factors. However, limited information concerning histopathological features of nervous tissue of fish with neurological signs is available. Materials and Methods: A total of 33 live, swim-up fry were produced in an aquarium colony of the live-bearer Xiphophorus montezumae, composed by 3 females and 1 male. About fifteen days after birth, fry showed severe neurological signs ranging from anorexia and lethargy to swimming difficulties, whirling and loss of equilibrium. No gross abnormality was evident. Immediately after spontaneous death, 31 fry were formalin-fixed and routinely processed for histology. Two fry survived without clinical signs. Adult fish were not affected. Results: Histological examination of all affected fry revealed various degrees of external hydrocephalus characterized by increased space between the meninges and the brain, with presence of scant protein-like material in the sub-arachnoid space. Variable, usually mild, vacuolation of the nervous tissue was also observed. Brain was only covered by the skin on the dorsal part of chondrocranium. Conclusions: This is the first description of neurological disease associated with histological features of external hydrocephalus in the Montezuma swordtail. Water quality analysis and absence of pharmacological treatments allowed to exclude toxic causes. Vitamin deficiencies were also unlikely, since a well-balanced and integrated, commercial diet specific for swim-up fry was provided. Based upon the lack of evidence for an infectious, especially viral, etiology, a congenital, possibly genetic, condition may be supposed.