Does Herd Immunity Exist in Aquatic Animals?

Isaac F Standish,Mohamed Faisal,Travis O Brenden

doi:10.3390/ijms17111898

Abstract

Viral hemorrhagic septicemia virus genotype IVb (VHSV-IVb) is presently found throughout the Laurentian Great Lakes region of North America. We recently developed a DNA vaccine preparation containing the VHSV-IVb glycoprotein (G) gene with a cytomegalovirus (CMV) promoter that proved highly efficacious in protecting muskellunge (Esox masquinongy) and three salmonid species. This study was conducted to determine whether cohabitation of VHSV-IVb immunized fishes could confer protection to non-vaccinated (i.e., naïve) fishes upon challenge. The experimental layout consisted of multiple flow-through tanks where viral exposure was achieved via shedding from VHSV-IVb experimentally infected muskellunge housed in a tank supplying water to other tanks. The mean cumulative mortality of naïve muskellunge averaged across eight trials (i.e., replicates) was significantly lower when co-occurring with immunized muskellunge than when naïve muskellunge were housed alone (36.5% when co-occurring with vaccinated muskellunge versus 80.2% when housed alone), indicating a possible protective effect based on cohabitation with vaccinated individuals. Additionally, vaccinated muskellunge when co-occurring with naïve muskellunge had significantly greater anti-VHSV antibody levels compared to vaccinated muskellunge housed alone suggesting that heightened anti-VHSV antibodies are a result of cohabitation with susceptible individuals. This finding could contribute to the considerably lower viable VHSV-IVb concentrations we detected in surviving naive muskellunge when housed with vaccinated muskellunge. Our research provides initial evidence of the occurrence of herd immunity against fish pathogens.

Highlights

Hedrich [1] introduced the concept of “herd immunity” following research involving measles outbreaks in humans
In most trials, positive shedding by infected muskellunge was detected within the first week of initiation, and in most cases decreased to near zero or below detectable limits by weeks 3 and 4 post infection, at which point all infected muskellunge had succumbed to infection
Within two weeks following the initiation of each trial, signs of viral hemorrhagic septicemia virus (VHSV)-IVb infection were observed in muskellunge held in the downstream tanks, in muskellunge from the all naïve treatments

Summary

Introduction

Hedrich [1] introduced the concept of “herd immunity” following research involving measles outbreaks in humans. As part of that research, it was determined that epidemics declined when 68% of children under 15 years of age developed immunity against measles [1]. Since this initial research, herd immunity and the associated critical vaccination threshold necessary to illicit this immunity has been investigated for both human and veterinary practices. In the case of measles, upwards of 92% to 95% vaccination coverage was predicted as being necessary for eradication of the disease [4] This critical threshold has been investigated for numerous terrestrial pathogens; little work has been conducted on aquatic pathogens apart from simple simulation exercises [5]. Whether the concept of herd immunity is even applicable in an aquatic setting is not presently known

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