FISH VACCINATION- FACTORS TO CONSIDER

1David Wise, 1Suja Aarattuthodiyil, 1Todd Byars, 1Matt Griffin, 1Lester Khoo, 1Terrence Greenway, and Ganesh Kumar
1Thad Cochran National Warmwater Aquaculture Center, Mississippi State University, Stoneville, MS 38776
 

Aquaculture industry is seriously impacted by several diseases. Although vaccination can minimize mortality due to some of these diseases, the effectiveness of a vaccine is dependent on the cross protection offered against multiple isolates. For example, in the case of Enteric Septicemia of Catfish (ESC), caused by the bacteria, E. ictaluri, traditionally, losses have been controlled by withholding feed from fish to reduce the oral route of infection combined with medicated feeds. Recently, a live attenuated ESC vaccine is delivered orally to catfish. Under laboratory conditions, the vaccine was shown effective against the parental wild-type strain and proved to be safe at 10 times the applied target dose. While live vaccines are very effective in providing long lasting immunity against disease, vaccine safety and efficacy could be compromised, if delivered to animals in suboptimal health or under stressful conditions. In a compromised animal, attenuated vaccine can cause infections leading to morbidity and mortality. Another key factor in field vaccinology is antigenic variation among pathogenic species, where immunization with a vaccine derived from one strain does not provide protection against genetic variants of the same species. Vaccinated and control fish were challenged with the wild type E. ictaluri isolate 30 days post-vaccination. Low oxygen stress did not induce any post-vaccination mortality in any of the vaccinated treatments. Similarly, all groups of vaccinated fish, regardless of stress treatment, were protected against virulent E. ictaluri infection. Data indicated that acute oxygen deprivation, before or after vaccination, does not alter vaccine safety and efficacy, however the effects of chronic long term stress have not been evaluated.  Therefore, short acute stressors are unlikely to influence vaccine safety and efficacy and provides valuable insight in developing commercial vaccination protocols. Additional trials were conducted to determine if the attenuated isolate afforded protection against 23 archived field isolates collected over a time span of twenty years (1997-2016). Vaccination followed by bacterial challenge with archived isolates were conducted over a three year period. In all trials, vaccination was shown to protect catfish against all challenge isolates, regardless of host species, geographic region (state and farm location) or isolation year. While on farm vaccination greatly improved survival, yield and fish net-value, limited mortality was observed in vaccinated pond populations. Results indicated that mortality observed in farm vaccinated fish populations was not related to antigenic variations among isolates. The most likely cause of on-farm mortality was related to unequal distribution of vaccine laden feed to individual fish, an inherent problem with mass delivery of oral vaccines to large populations of animals. In order to differentiate between the 23 isolates, their clonal relation were determined. The PCR profile indicated relative homogeneity among the isolates dating back to 1997. This further confirmed the results which indicated no significant difference between the isolates. The clonal nature of E. ictaluri isolates demonstrated by our data negates the need to develop multivalent vaccines or construct new vaccines to account for antigenic variation occurring over time. Commercial vaccination trials are showing net economic benefit of $2000 to $3000/acre for channels and hybrid fingerling production phase.