## 1 Introduction  
The genus _Aerononas_ is a group of bacteria of importance to aquaculture, which is frequently isolated in aquatic environments [1, 2]. They can act as important pathogens in poikilotherm species because they are widely distributed in water bodies [3], and they are part of the normal gut microbiota of aquatic and terrestrial animals [4]. They have zoonotic potential, high pathogenic potential, and the ability to adapt to different environments [5]. Currently, 36 genetically identified _Aerononas_ species are known and isolated from aquatic environments [6]. This genus is divided into two groups: an immobile group, with _A. salmonicida_ being the main species, and a mobile group that contains the rest of the _Aerononas_ species [5]. _A. bestiarum_, _A. caviae_, _A. lydrophila_, _A. piscicola_, _A. salmonicida_, and _A. veronii_ are reported to be the species that are mostly isolated from pathological conditions in salmonids [7]. _Aerononas_ infection in different species of fish can occur at different ages of those fish that are susceptible to the disease, the presentation of stress in the fish is an important factor for the development of the disease, however the participation of external factors such as changes in water temperature, large amounts of exceta in ponds, overcrowding, and others can favor the presentation of the disease [3, 5].  
The pathology associated with the infection of _A. Salmonella, A. hydrophila, A. best-tiarum_, and _A. veronii_ in rainbow trout includes septic processes, the colonization of liver tissue and mortality [8], fish can develop exophthalmos, ocular ulceration, anorexia, the clinical signs included dermis lesion (ulcers, erosion, hemorrhage, darkened skin, loss of scales, furunculosis, and others), abdominal distension, fin rot, and in some cases, premaxilla bone exposition can be observed [1, 8, 9, 10]. Infected young fish modify their behavior, including jumping out of the pond, the darkening of the skin, abnormal swimming, low swimming, lethargy and inappetence [1, 5, 11]. Sometimes, bloody discharge from the nostrils and death occurs [12]. In the fish into the nasal cavity, the olfactory organ or olfactory rosette, or just rosette is a chemosensory system, which are composed by pseudostratified epithelium with sensory (olfactory sensory neurons) and mucosal areas (goblet cells), on the lamina propria of the rosette are localized the terminal axons [13, 14].  
However, studies focused on nasal infection in _Aeromonas_ have not been investigated. [PERSON] et al. [15] carried out an infection in rainbow trout intraperitoneally with _A. salmonicida_ and through real-time monitoring by bioluminescence, it was established that the colonization sites, were dorsal and pectoral fins; gills, stomach, intestine, anus; as well as the oral, nasal and eye cavity, these sites being the first to be exposed to the aquatic environment, as well as the skin. Currently, the immunization of fish against _Aeromonas_ is carried out by intraperitoneal injection or by immersion bath [6]. Recently, the nasal route of _Yersinia ruckeri_ was demonstrated as an entry point for bacteria to establish, infect and spread through the nervous system, which in turn causes erratic swimming and exophthalmia [16]. This suggests that pathogenic and virulent bacteria can cause nervous problems that affect behavior. On the other hand, the use of the nasal route as a vaccination route was investigated as an alternative for the immunization in teleost fish, on the case of rainbow trout, it was used as a biological model for the challenge with infectious hematopoietic necrosis virus [13].  
The aim of the present research was to identify the presence of histopathological lesions observed in the rosette and nervous tissue, to identify a relation between the nasal tissue and nervous tissue with fish behavior, in rainbow trout fingerlings, experimentally infected with different _Aeromonas_ species.