## 4 Discussion
[30] identified that the olfactory bulb has an important role in the detection of pathogens, as well as in the regulation of the immune response in addition to identifying the genes that are responsible for the secretion of inter-leucine (IL) 1\(\beta\) and IL-8, present during the inflammation process. [PERSON] et al. [31] identified that part of the inflammatory response due to _A. lydrophila_ infection in the intestine is closely related to the expression of IL8 and IL1\(\beta\) levels. This suggests that infection with _Aeromonas_ species can infect the nasal route, and the nasal route has an important role in the early stages of infection and triggers the immune response, however more studies are needed to accept this hypothesis. The production of vaccines in the aquaculture industry is focused on the stimulation of humoral immunity, by stimulating the mucosal immune response present in the mucosal tissue distributed in the body, while the intranasal application of vaccines in fish has generated, step by step, a fertile field as an alternative route of immunization in aquaculture [14]. In the case of _Aeromonas_ spp., active and inactive vaccines were administered by immersion bath or intraperitoneally [32]. Therefore, due to the clinical importance of some _Aeromonas_ species, they could be candidates for use in the generation of bacterins. As in the case of infectious hematopoietic necrosis and enteric red mouth disease [13]; however, more studies are needed to support this hypothesis, so this work will serve as a basis for future studies. The presence of sensory neurons in the rosette, and their connection between the olfactory bulb and the brain, suggest they can induce alterations in feeding behavior, reproductive behavior and alarm reactions [29]. The histopathological findings in telecephalon consisted of moderate multifocal congestion for fish infected with _A. media_, and a moderate-to-severe degree for the groups infected by _A. salmonicida_, _A. bestarium_, _A. axiae_, _A. hydrophila_, and _A. lusitana_, which presented moderate focal congestion; and, in the oblongata medulla were focal and multifocal congestion, and moderate to severe degrees of infection with _A. bestarium_ and _A. veronii_. No significant histopathological lesions (\(p>0.05\)) were observed for the groups infected with the species _A. allosacharophilia_, _A. axiae_, _A. hydrophila_, _A. lusitana_, _A. media_, _A. popoffii_, _A. sobria_, and the control group. The telencephalon in teleost participates in appetite regulation factors and the regulation of hormone inducing genes, such as corticotrophin releasing hormone 1a, which causes an anoretic effect; and adrenergic receptors, which participate in sex-specific swimming behavior [30]. On the other hand, the medulla oblongata participates in progressive hearing loss, in balance disorders and in the feeding role of fish [33]. [PERSON] et al. [16] demonstrated that experimental infection with _Yersinia ruckeri_ O1 biotype 2 causes lesions in the nasal epithelium and nervous system of fish infected via an immersion bath; this shows that bacterial pathogenesis begins with direct contact of the pathogen and the nasal epithelium close to the nervous system, causing problems in coordination and abnormal behavior. In the control group, the presence of congestion in the nasal epithelium was observed in two small fingerlings, because the hierarchy behavior of trout at feeding or the social status in the aquarium cause injuries in some subordinated rainbow trout; this induces stress by the biogenesis of cortisol [34]. The color change in teleost can be caused by multiple factors, stress being one of the main ones; continuous stress favors an increase in the secretion of the hormone cortisol, adrenocortic hormone and alpha melanophore-stimulating hormone (Msh), which increases the number of chromophores and favors the presentation of skin darkening in stressed fish [35]. In the present study, hyperpigmentation or darkened skin was observed in fish infected with the species of _A.