Fulfilling Koch’s postulates confirms the mycotic origin of Lethargic Crab Disease

Abstract

In the northeast region of the Brazilian coast, a disease has been causing massive mortalities of populations of the mangrove land crab, Ucides cordatus (L.) since 1997. The clinical signs of this disease, which include lethargy and ataxia, led to the disease being termed Lethargic Crab Disease (LCD). Evidence from a variety of sources indicates that there is an association between LCD and a new species of black yeast, Exophiala cancerae de Hoog, Vicente, Najafzadeh, Badali, Seyedmousavi & Boeger. This study tests this putative correlation through in vivo experiments. Disease-free specimens of U. cordatus were experimentally infected with Exophiala cancerae (strain CBS 120420) isolate. During the 30-day experimental period, only a single death was observed within the control crabs. However, at the end of this period, crabs that were inoculated once or three-times with mycelial elements and hyphae of E. cancerae had a 60% and 50% mortality rates, respectively (n = 6 and n = 5). These results support that the fungal agent is pathogenic and is the causative agent of LCD. Species-specific molecular markers confirm the presence of E. cancerae (strain CBS 120420) in recovered colonies and tissue samples from the infected animals. The experimentally infected crabs manifested signs (lethargy, ataxia and tetany) that were consistent to LCD-affected animals in the environment. These results fulfil Koch's postulates and the hypothesis that the tested strain of Exophiala cancerae is a causative agent of LCD is accepted.