Characterization of virulence gene profiles of Aeromonas hydrophilaand Lactococcus garvieaefrom diseased Nile tilapia in Zambia

Abstract

Fish production is threatened by frequent disease outbreaks, especially bacterial diseases that cause significant economic losses.This study characterized the key virulence gene profiles of Aeromonas hydrophila and Lactococcus garvieaeisolated from diseased Nile tilapia in southern Zambia and assessed genotype–lesion associations. A total of 163 clinically affected tilapia were examined, from which A. hydrophila(43%) and L. garvieae(22%) were recovered predominantly from brain and kidney tissues. Virulence gene screening showed that A. hydrophilaexhibited low-frequency profiles dominated by hemolysin A (hlyA) (20.5% in broodstock) and elastase (ela) (11.5% in grow-out), while aerolysin (aerA) and enterotoxin (act) were infrequently detected. L. garvieaedisplayed a hemolysin-skewed profile, with hly2being most prevalent (27.3% in broodstock; 21.4% in grow-out), hly2detected only in broodstock, and capsule gene cluster (CGC) and fibronectin-binding protein (fbp) genes occurring rarely. Significant gene–lesion associations linked aerAand elawith pale gills (r= 0.41, p <0.01; r= 0.24, p= 0.05, respectively), actwith skin discoloration (r= 0.27, p=0.02), and demonstrated inverse correlations between hlyAand fins (r= −0.4, p <0.001) or hepatic hemorrhages (r= −0.27, p= 0.02) in A. hydrophila. In L. garvieae, hly3correlated with enlarged liver (r= 0.23, p <0.001), corneal opacity (r= 0.15, p= 0.05), and gill necrosis (r= 0.21, p= 0.01), while hly2and capsule genes were associated with skin discoloration (r= 0.18, p= 0.02; r= 0.24, p <0.001, respectively). Overall, virulence determinants occurred at low frequencies and in limited combinations, suggesting the circulation of less virulent strains and underscoring the value of genotype-informed surveillance for improving disease control in tilapia aquaculture.

Keywords

Nile tilapia / Aeromonas hydrophila / Lactococcus garvieae / Virulence genes / Post-mortem lesions

Acknowledgments

We would like to express our sincere gratitude to the fish farmers in the study areas for their cooperation and support, which were crucial to the successful completion of this research. Additionally, we are grateful for the facilitation provided by the Ministry of Fisheries and Livestock through the Department of Fisheries, which was essential to ensuring the smooth execution of this study.

Funding

None.

Conflict of interest

The authors declare that they have no conflicts of interest relevant to this publication.

Author contributions

Conceptualization: Kunda Ndashe
Formal analysis: Kunda Ndashe
Investigation: Kunda Ndashe, Bernard Mudenda Hang’ombe, Katendi Changula
Methodology: Kunda Ndashe, Ladslav Moonga
Writing–original draft: Kunda Ndashe
Writing–review & editing: John Yabe, Katendi Changula, Bernard Mudenda Hang’ombe

Ethics approval and consent to participate

The study was approved by the Institutional Review Board (or Ethics Committee) of ERES Converge IRB (reference number: 2019-AUG-024).

Consent for publication

Not applicable.

Availability of data

All data analyzed have been presented in the paper.

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