In recent years, some Ecuadorian shrimp farmers have reported several cases of muscle necrosis in P. vannamei
grow out cultures, associated to low mortalities at harvest. This abnormal condition is characterized by focal to extensive
necrotic areas in striated tail muscle tissues, displaying a white, opaque appearance. Furthermore, histological analysis from
suspected samples with macroscopic lesions revealed a loss of sarcomeric structure accompanied by coagulative muscle
necrosis along with hemocytic infiltration. Two viruses, not reported in Ecuador, are described as etiological agents of
muscle necrosis in P. vannamei: infectious myonecrosis virus (IMNV) and Penaeus vannamei nodavirus (PvNV). In this
study, the etiology of the muscle necrosis cases found in the Pacific white shrimp P. vannamei cultured in Ecuador was
examined. This muscle necrosis was hypothesized to be caused by an infectious agent. Three sequential challenge tests,
using diseased P. vannamei with macroscopic lesions (opaque, whitish discolorations in the abdominal muscles) as starting
material (crude inoculum), were carried out. Essentially, histological examination confirmed that most of the challenged
shrimp had lesions in skeletal muscle, including multifocal necrosis, fibrocytic inflammation and phagocytosis. In the first
trial, 7 challenged shrimp (out of 10) presented multifocal necrosis and hemocytic infiltration in the skeletal muscle. In the
second trial, 14 challenged shrimp (out of 20) exhibited coagulative muscle necrosis and hemocytic infiltration. In the third
trial, 48 challenged shrimp (out of 69) showed muscle necrosis and hemocytic infiltration. There were no mortalities
throughout the all four-week trials. All challenged shrimp tested by RT-PCR assay were negative for IMNV. In our
experimental procedures, muscle necrosis could be consistently reproduced through three sequential trials, confirming that
the disease has an infectious etiology. Our results suggest that the etiological agent of this disease could be a new infectious
agent or a different strain of IMNV.
Key Words: Muscle necrosis, hemocytic infiltration, infectious etiology, Penaeus vannamei.
Introduction
Infectious diseases have led to severe mortalities
of cultured