Finfish aquaculture in northern Nigeria: Characterisation of potential bacterial pathogens, disease management practices and steps towards improvement

Abstract

Nigeria is the largest aquaculture producer in Sub-Saharan Africa but has a fish demand-supply deficit of over 1 million tons. Aquaculture has been promoted as a viable pathway towards bridging this gap and achieving self-sufficiency in fish supply. The sector however, faces infectious disease challenges that threaten both food security and the socioeconomic well-being of the people especially in northern Nigeria, a region with a higher poverty rate and lower food security. Insufficient capacity for fish health management has led to many fish farmers using antibiotics sometimes inappropriately as the primary means of disease control and prevention. This poses a threat to both aquaculture sustainability and the One Health mantra. Despite these challenges, there is still a dearth of information on prevailing pathogenic bacterial strains as well as a poor understanding of general disease management and prevention strategies within the aquaculture sector especially in northern Nigeria. This thesis, therefore, aimed to address these knowledge gaps and characterise prevailing pathogenic bacteria of catfish, conduct a survey of infectious disease outbreaks and associated management practices and propose interventions towards improved biosecurity practice in aquaculture in northern Nigeria.

A bacteriological survey was conducted in the field of 55 catfish (Clarias gariepinus) displaying gross symptoms of infectious disease at nine sampling sites which included seven fish farms and two fish markets distributed across three states of northern Nigeria between June and August 2020. Forty-two bacterial isolates were successfully recovered in the UK from the original 100 isolated bacteria in Nigeria. Sixteen of the recovered bacterial isolates sampled from skin ulcers and kidney of catfish at six different sites were identified as Aeromonas spp. (including A. hydrophila, A. veronii, A. dhakensis, and A. jandaei) through a combination of morphological, biochemical, and molecular techniques (16S rRNA and whole genome sequencing and MLST typing). A further twelve suspected Pseudomonas spp. isolates were also cultured from catfish mainly associated with one market location.

The 16 Aeromonas spp. isolates displayed phenotypic virulence traits that included haemolysis, protease, DNase, gelatinase and lipase enzyme activities in addition to bearing genes that code for these traits. Antibiotic resistance profiles of these 16 Aeromonas spp. against commonly used antibiotics in the study area determined using the microbroth dilution method of the Clinical and Laboratory Standards Institute (CLSI VET03) revealed that all 16 were wild type (WT) against gentamicin and erythromycin and non-wild type (NWT) against oxytetracycline and sulfamethoxazole, while the majority 15/16 (93.75%) were NWT against both amoxycillin and oxolinic acid. A smaller proportion of 12 (75%), 11 (68.75%) and 2 isolates (12.50%) were NWT against colistin, enrofloxacin and florfenicol respectively. Both chromosomal and plasmid-borne antibiotic resistance determinants were also detected in these Aeromonas spp. field isolates.

A structured questionnaire to inform on current aquaculture husbandry practices, disease occurrence, disease management, and biosecurity practices was administered via telephone interview to 45 catfish farmers across 10 of the 19 states that make up northern Nigeria between September 2021 and February 2022. The survey of fish farms revealed that 25 (56%) farms poorly implemented biosecurity on their farms and another 29 farms (64%) reported disease outbreaks. Only five (11.1%) farms consulted a veterinarian while self-medication with antibiotics was the most common (40/45; 88.89%) method of disease control and prevention. An online training workshop on biosecurity was designed, implemented and its impact on biosecurity practices of participating fish farmers assessed. A significant difference in biosecurity score (p < 0.05) was observed between the cohort that received online biosecurity training and the control group of fish farmers that did not attend the workshop.

This thesis revealed that biosecurity on participating fish farms was poorly implemented, however, an online training workshop on biosecurity significantly improved this outcome. Infectious disease outbreaks were common and disease management commonly involved self-prescribed antibiotics. The most frequently isolated bacteria from diseased catfish were Aeromonas spp. and found to carry multiple virulence and resistance genes. Therefore, the evidence presented here suggests the potential role of aquaculture in the emergence and dissemination of antibiotic resistance. The economic and public health (zoonotic) significance of Aeromonas spp. makes this a significant finding especially in the context of northern Nigeria, a region with an emerging aquaculture sector. A concerted effort is therefore required to improve farms management practices and reduce inappropriate antibiotic use, to ensure sustainable aquaculture production and food security.