Encapsulation of Bolbophorus damnificus (Digenea: Bolbophoridae) Metacercariae in Juvenile Channel Catfish, Ictalurus punctatus, Is Linked to Delayed‐onset Mortality

Abstract

Bolbophorus damnificus (Digenea: Bolbophoridae) causes significant economic losses in catfish aquaculture in the southeastern USA. To determine mortality trends associated with B. damnificus exposure, three separate disease challenges were conducted. The first challenge exposed channel catfish, Ictalurus punctatus, fingerlings (7–13 cm) to approximately 285 B. damnificus cercariae/L, while the second exposed fish to approximately 215 cercaria/L. Fish were monitored for morbidity and mortality for 28 d. In both studies, mortality first occurred 8 d postchallenge (PC) and peaked at 9 and 10 d PC, respectively. Mortality had subsided by 14 d PC, with only sporadic mortality occurring >15 d PC. Dead and moribund fish presented with lesions consistent with B. damnificus infection. A third challenge was performed to record histological changes associated with parasite development from initial exposure up to the cessation of mortality. Catfish (2–5 cm) were exposed as described above to a range covering 0, 35, 70, 100, 150, 175, and 215 cercariae/L. Each day, two fish were sampled from each treatment. Mortality trends were consistent with previous challenges. The mortality window coincided with parasite encapsulation, neovascularization, and development of the host‐derived cyst, suggesting that morbidity and mortality are linked to host pathophysiological changes in response to metacercariae encapsulation.

Histopathology and external examination of heavily parasitized Lost River Sucker Deltistes luxatus (Cope 1879) and Shortnose Sucker Chasmistes brevirostris (Cope 1879) from Upper Klamath Lake, Oregon

“…Various helminths are recognized to cause angiogenesis (Dennis, Schubert, & Bauer, ), including metacercariae of B. damnificus (Griffin et al., ). We observed essentially identical changes as the latter report, in which excessive distinct blood vessels were intermixed within the melanized fibrous tissues encircling the encysted metacercariae.…”

“…based on wet morphology and 28S rDNA sequence (Janik, ); whereas, many black spot metacercariae infect the dermis (Williams & Jones, ), Bolphophorus spp. appear to be restricted to the muscle (Griffin, Khoo, Reichley, & Ware, ; Overstreet et al., ). However, the metacercariae often occur near the skin (Overstreet et al., ), and these are externally visible in suckers from the Upper Klamath Lake due to their large size and dark colour of associated cyst (Markle et al., ).…”

Comparative Mortality of Juvenile Channel and Hybrid Catfish Exposed to Bolbophorus damnificus Cercariae

“…On the day of collection, all snails were placed into individual scintillation vials with 10 mL of water collected from a reservoir pond that was void of fish and that had no history of pelican presence. Water was passed through a 20‐μm mesh following previously established protocols (Griffin et al 2010b, 2014, 2018). Each snail was fed spinach that was purchased from a local grocery and rinsed with distilled water.…”

“…During the challenge, water flow was suspended to each tank and cercariae were added to challenge aquaria ( n = 5 tanks/group), equating to an estimated challenge dose of about 100 cercariae/L. This dose was determined based on previous work challenging similar‐sized fish (Griffin et al 2018). One tank each for Channel and hybrid catfish received no cercariae and served as a negative control.…”

“…Encapsulation of the B . damnificus metacercariae is extremely taxing to the fish, with mortality and morbidity occurring at 7–14 d postinfection (dpi; Griffin et al 2018). The most effective strategy for managing B .…”