DAO 62:27-34 (2004)  -  doi:10.3354/dao062027

Susceptibility of channel catfish fry to Channel Catfish Virus (CCV) challenge increases with age

Larry A. Hanson*, Mary R. Rudis, Lora Petrie-Hanson

Department of Basic Sciences, College of Veterinary Medicine, PO Box 6100, Mississippi State University,Mississippi 39762, USA

ABSTRACT: Susceptibility of channel catfish to Channel Catfish Virus Disease (CCVD) has been generally considered to be inversely related to age. However, in experimental immersion challenges, we found that channel catfish fry, 3 to 8 d post hatch (dph), are most resistant to CCV and susceptibility increases with age. Initial studies involved 2 spawns that had high CCV carrier percentage. To determine if the resistance seen in the fry was related to the CCV carrier status of the parents, we selected 4 spawns from CCV negative parents and 2 spawns from CCV positive parents and immersion challenged them at 8, 23, 36 and 60 dph with 0, 2.5 × 104 or 2.5 × 106 plaque forming units (PFU) of CCV l–1. Survivors of the low-dose exposed groups were rechallenged at 120 dph with 2.5 × 106 PFU CCV l–1. Each brood demonstrated increasing susceptibility to CCVD with age and only the fish that were initially exposed at 60 dph developed protective immunity. Time course assays evaluating tissue levels of virus in channel catfish exposed to CCV at 7, 21 and 42 dph suggested that the resistance was an early event in the infection process. The resistance in fry was most pronounced in fish from CCV positive spawns and was correlated to neutralizing antibody titers in the maternal parent in the 8 dph challenge. However, other factors may be involved because all groups displayed the initial resistance and subsequent susceptibility to CCVD. The age effect may be an important influence on the progression of CCVD outbreaks and indicates the need to consider age for experimental challenges. Additionally, we documented the level of vertical transmission of CCV. Fry from the 4 positive spawns had a CCV prevalence of 40 to 75%.

KEY WORDS: Disease susceptibility · Ictalurus punctatus · Teleost · Ictalurid herpesvirus · Herpesvirus latency · Herpesvirus vertical transmission · Maternal antibody

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