The diagnosis and vector potential of Ornithonyssus bacoti tropical rat mites in northern Europe.

The mesostigmatid tropical rat mite, Ornithonyssus bacoti, is an important cause of disease in small rodents, and of gamasoidosis in humans when they come into contact with infestations. Most reports of O. bacoti infestations are from warmer parts of the Americas, southern Europe and Asia; and infection has only rarely been recorded in northern Europe. In 2021 and 2024, two separate cases of gamasoidosis were identified in student flats in the city of Edinburgh, UK. Further investigation highlighted the value of combining conventional morphological and 16S ribosomal DNA sequencing methods in establishing the species identity of the mites; hence confirming the diagnosis of gamasoidosis. The bacterial microbiome associated with the mites was explored by conventional culture and metabarcoding microbiome sequencing of the ribosomal16S v3-v4 hypervariable region. The results highlight the utility of the mixed approach; and show the presence of potentially pathogenic bacteria and recognised causes of opportunistic nosocomial infections, along with known mite gut and intracellular symbionts. The results indicate the potential for O. bacoti mites as vectors of bacterial infections. The clinical presentation of gamasoidosis is indistinguishable from non-specific arthropod-bite reactions; and the cause is seldom confirmed because the temporarily parasitic mites spend most of their time in the environment. The two confirmed index cases may, therefore, represent a more widespread emerging problem; putatively associated with an increase in urban rodent populations.