Type II grass carp reovirus (GCRV-II) with high pathogenicity and infectivity causes severe hemorrhagic disease, which leads to extensive death in the grass carp and black carp aquaculture. However, the early invasion portal remains unclear. In this study, we explored the invasion portal, time, and pathway of GCRV-II by immersion infection in grass carp. Through the detection of the infected grass carp external body surface tissues, most of them could be detectable to carry GCRV-II within 45 minutes except for the skin covered by scales. Further shortening the duration of infection, we proved that GCRV-II rapidly invades through the nostril (especially), gill, and skin on head at only 5 min post immersion, rather than merely adhesion. Subsequently, visual localization investigations of GCRV-II were conducted on the nostril, olfactory system (olfactory bulb and olfactory tract), and brain by immunofluorescence microscopy and transmission electron microscopy. We found that few viruses were located in the nostril at 5 min post immersion infection, while significantly increased viruses were distributed in all these tissues at 45 min. Furthermore, semi-qRT-PCR and western blotting results of different infection time confirmed that GCRV-II invades grass carp via nostril-olfactory system-brain axis and then unfolds viral replication. These results revealed the infection mechanism of GCRV-II on the invasion portal, time, and pathway in grass carp. This study aims to understand the invasion mode of GCRV-II in grass carp, which provides theoretical support for the prevention and control strategies of hemorrhagic disease.

Small scale carp polyculture plays a key role for food supply in Bangladesh, however, factors including water pollution, limited infrastructure and inadequate disease management hinder its sustainability. This paper reports on a survey of 231 farmers across the 6 major carp producing regions in Bangladesh, analyzing factors including, farmers’ social aspects, farm characteristics, information on disease and approaches adopted to combat them, and biosecurity practices. Almost half (46.8%) of the farms surveyed experienced disease in carp species, with clear regional variations. Eighty four percent of farms reported carp mortalities during disease outbreak, with an average mortality level 10.23±11.81%. Clinical signs during outbreaks lasted between a week and a month, and there was a peak in disease outbreaks in two seasonal periods between June to July and October to December. Disease incidence was related to a range of factors including the farmer experience, ponds/farm type, stocked species, and biosecurity practice. A combination of disinfecting measures during pond preparation and measures during stocking, including discarding fingerling transport water off-farm, fingerling disinfection and check the health of fingerlings before stocking significantly reduced disease occurrence. Treatments involving the antibiotics, ciprofloxacin, erythromycin, and azithromycin were reported as ineffective, raising concerns about non-prudent use, inadequate dosing (perhaps without appropriate veterinary guidance) and the potential for further driving antimicrobial resistance in the environment. The research unveils a concerning pattern of disease high disease incidence across small-scale carp farms in Bangladesh and the significant potential for disease spread highlights the need for responsible disposal practices. The study emphasizes the need for improving training and awareness programs are vital for addressing biosecurity and disease management challenges, ensuring sustainable aquaculture and community well-being.

ccurate recording of growth indicators for aquaculture species at different stages is essential for evaluating aquaculture production effectiveness and the rationality of feed formulations. Due to their convenience and non-invasiveness, weight, length, and condition factor are commonly used to assess the growth of aquaculture species . However, fish growth indicators, can vary significantly with age structure and feed components(nutrition), and relying on a single indicator may lead to misjudgments . In this study we investigated the growth indicators of gibel carp (Carassius auratus gibelio var. CAS V) at different growth stages (juvenile and adult) and different feed formulations. Meanwhile, fish weight-length relationship () was used to assess the growth indicators. The results showed that the weight-length relationship of gibel carp varied significantly with age and feed formulations. Additionally, the condition factor calculated depended on the weight-length relationship was more consistent with weight/length changes than measured. Weight analysis indicated that both weight, length, height, back thickness, and carcass ration had higher weights in analyzing fish growth. Therefore, during aquaculture production, fish growth evaluating requires considering diverse indicators such as weight, length,body height, back thickness, carcass ration,as well as condition factor to avoid misjudging the actual growth situation. Meanwhile, the use of the condition factor should consider the sufficient amount of data and whether the assumptions (such as being in a isometric growth period) are met.