Our study reports the first molecular detection of the protozoan pathogen Histomonas meleagridis, the causative agent of histomonosis, in the poultry red mite Dermanyssus gallinae. Mite samples were collected from a commercial poultry farm and a backyard flock in Hungary and analyzed using PCR assays that targeted the COX1 gene in mites and 18S rRNA gene in H. meleagridis. Phylogenetic analysis confirmed the identity of D. gallinae and revealed distinct genetic lineages between farm and backyard isolates. H. meleagridis DNA was detected in 10 of 17 mite samples, representing multiple variants within Genotype 1. No histomonosis was detected in the examined poultry, although one hen harbored Simplicimonas sp. The findings suggest that D. gallinae could serve as a carrier for H. meleagridis, potentially facilitating the persistence and transmission of the pathogen. This novel host–pathogen association highlights the need for further research into the epidemiological role of poultry ectoparasites in the spread of histomonosis.

We investigated a case of increased mortality in juvenile hybrid African catfish ( Clarias gariepinus × Heterobranchus longifilis ) reared at an intensive aquaculture facility in Hungary. Affected fingerlings exhibited severe intestinal distension, multifocal petechial haemorrhages, and extensive enteritis. Bacteriological analyses consistently yielded a dominant haemolytic bacterial colony morphotype comprising Gram-negative, rod-shaped bacteria, which were identified as Aeromonas dhakensis by 16S rRNA and rpoD gene sequencing, whereas Aeromonas veronii and Shewanella xiamenensis were isolated only sporadically. Phylogenetic analysis clearly separated the A. dhakensis strains from those of A. hydrophila , supporting their distinction as separate species. Antimicrobial susceptibility testing showed that A. dhakensis was susceptible to several antimicrobials used in aquaculture, and treatment with medicated feed containing neomycin and oxytetracycline was followed by a rapid decline and subsequent cessation of mortality. The predominance of A. dhakensis in the affected intestinal tissues, together with the clinical and pathological findings and the favourable response to antimicrobial treatment, supports its potential involvement in the observed enteritis. However, a direct causal relationship could not be conclusively established. To the best of our knowledge, this is the first report of A. dhakensis associated with hybrid African catfish and the first documented occurrence of this bacterium in Hungary.