Streptococcus agalactiae infection is one of the major factors limiting the expansion of tilapia farming globally. In this study, we investigated the serotype distribution, virulence and antimicrobial resistance of S. agalactiae isolates from tilapia farmed in Lake Volta, Ghana. Isolates from 300 moribund fish were characterised by Gram staining, MALDI-TOF/MS and 16S rRNA sequencing. Serotype identification was based on multiplex polymerase chain reaction (PCR) amplification of the capsular polysaccharide genes. Detection of virulence genes (cfb, fbsA and cspA) and histopathology were used to infer the pathogenicity of the isolates. The susceptibility of isolates to antibiotics was tested using the Kirby-Bauer disk diffusion assay. All 32 isolates identified as S. agalactiae were of serotype Ia. This was notably different from isolates previously collected from the farms in 2017, which belonged to serotype Ib, suggesting a possible serotype replacement. The prevalence of the pathogen was related to the scale of farm operation, with large-scale farms showing higher S. agalactiae positivity. Data from histopathological analysis and PCR amplification of targeted virulence genes confirmed the virulence potential and ability of the isolates to cause systemic infection in tilapia. Except for gentamicin, the majority of the isolates were less resistant to the tested antibiotics. All isolates were fully sensitive to oxytetracycline, erythromycin, florfenicol, enrofloxacin, ampicillin and amoxicillin. This study has improved our understanding of the specific S. agalactiae serotypes circulating in Lake Volta and demonstrates the need for continuous monitoring to guide the use of antimicrobials and vaccines against streptococcal infections in Ghanaian aquaculture systems.

Environmental barriers influencing the movement of insect vectors can govern adaptive gene flow, including the dispersal of insecticide resistance mechanisms that compromise population control. We sought to understand population connectivity of the major malaria vector, An. coluzzii , across the different bioclimatic zones of West Africa using SNPs from whole genomes and inversion karyotypes previously associated with environmental adaptation. We identified restricted gene flow between populations from northern savannah and southern forested regions. Using Ghana as a case study, we found marked differences in insecticide resistance profiles across the different bioclimatic zones suggesting that population connectivity impacts on adaptive allele sharing. Greater evidence for target site pyrethroid and metabolic cross-resistance in the North reflects differences in insecticide use across the country. We also observed distinct resistance mechanisms in the coastal region of Greater Accra which may result from intense urban agricultural activity. Overall, findings suggest that environmental conditions restrict An. coluzzii gene flow to impact the geographical distribution of molecular insecticide resistance.