Location: Aquatic Animal Health Research2011 Annual Report
1a. Objectives (from AD-416)
Determine effects of parasitism by Ichthyophthirius or Gyrodactylus on fish susceptibility to Edwardsiella ictaluri and Flavobacterium columnare and develop diagnostic assays for detection of fish pathogen in cultured fish and farm water.
1b. Approach (from AD-416)
Catfish and tilapia will be used to evaluate the effect of parasitism by Ichthyophthirius multifiliis or Gyrodactylus on fish susceptibility to Edwardsiella ictaluri and Flavobacterium columnare; and determine if parasites can harbor and subsequently vector the bacterial pathogens using histology, fluorescent and molecular techniques. Rapid molecular and immunological detection assay will be developed and tested for their effectiveness in the detection of bacterial and parasite pathogens.
3. Progress Report
One of the goals for this non-funded cooperative agreement is to develop rapid detection tests for fish pathogens, especially those pathogens that cause high mortality in fish and lead to heavy aquaculture loss. Scientists of the cooperator developed a rapid polymerase chain reaction (PCR) assay for identifying Streptococcus (S.) iniae, a severe fish pathogen in cultured tilapia. The diagnostic assay was used in epidemiological studies of Streptococcus disease in Guangxi and 4 other provinces in Southern China to identify the pathogen Streptococcus iniae. The cooperating scientists collected 92 isolates of Streptococcus iniae from the epidemic area and studied seasonal occurrence of the disease. The results provided valuable information on the pathogen distribution and management strategies to prevent the disease outbreak. Another goal of this agreement is to determine effects of parasitism on fish susceptibility to pathogenic bacteria, including the impact of parasitism on fish immune function against pathogens. ARS scientists conducted studies to demonstrate that parasitism in fish increased infection and mortality following exposure to pathogenic bacteria. The mechanical injury from the parasite apparently provided a portal of entry for the bacterium. There is no information on the effects of parasitism on the vaccine efficacy against bacteria in cultured fish. ARS scientists and a visiting scientist from Brazil conducted studies to determine whether parasitism influenced vaccine efficacy against Streptococcus iniae (a gram positive bacteria) in Nile tilapia. The study results demonstrated a reduction in vaccine performance in fish infected by parasites compared to non-parasitized fish. A decrease in antibody levels, red blood cell and white blood cell counts were observed in the parasitized vaccinated fish. This study highlights the importance of monitoring parasite infections in the aquaculture to optimize vaccine efficacy. The prevention of parasite infection in fish not only reduced the direct damage by parasites but also reduces fish mortality due to bacterial infection and enhances fish immune protection against pathogens. A site visit by two ARS scientists in September 2010, provided discussion for the collaborative research in fish health. ARS scientists presented seminars “Fish vaccines against Streptococcus iniae in tilapia”, “Fish vaccines against Aeromonas hydrophila in channel catfish” and “Infection by Ichthyophthirius enhanced tilapia susceptibility to Streptococcosis” for approximate 30 research scientists and graduate students. Scientists of ARS and the cooperators have also had discussions by phone and e-mails to plan collaborative research in fish health and monitor the progress of this project.