The menace of disease outbreak and subsequent use of all manners of antibiotics to prevent and control diseases had resulted into humongous economic loss and inefficient disease control. The adoption of phytogenic agents is sinequanone, because the natural compounds are biodegradable, eco-friendly, no resistance to pathogens and contain various bioactive ingredients which are responsible for their antimicrobial activities. Grossly, the observed result revealed that, African catfish infected with A. hydrophila showed clinical signs as ulcerative skin, haemorrhages on body surface and fin rot, ulcers with mortality up to 30% in the control group, these could be attributed to the pathogenic effect of the bacterial organism. Similar pathology and mortality rate were observed by Zhang et al. (2020). Albeit plethora of studies have been reported in the control of aeromonas infection in fish (Abdel-tawwab & El-araby, 2021; Moustafa et al., 2020), the outbreak has not ceased to recurring and mortality is usually high. It was evident in this study that the plant additives (A. boonei and M. scaber) in different percentages enhanced the survival rate of the fish after challenged with Aeromonas hydrophila. This may not be unconnected with the antimicrobial properties of the plants. A. boonei (Ogueke et al., 2014; Opoku & Osei, 2014) and M. scaber (Adeshina et al., 2019, 2021; Ajadi et al., 2021) have been reported to contain bioactive components such as such as flavonoids, alkaloids, phenols, saponins, tannins among other aromatic compounds that play important roles in defence mechanism against microbial organisms. This is evident in this study where the fish fed with A. boonei and M. scaber had higher survival rate than the control. The protection is irrespective of the percentage of the plant present in the feed except A. boonei (0.5%) with 85% survival rate against other groups with 100% survival rate. This finding is in agreement with Adeshina et al. (2019) that reported the protective effect of M. scaber against A. hydrophila in common carp. There is dearth of information on the protective effect of dietary A. boonei in fish.
Haemato-biochemical indices are essential clinical tools employed to examine fish health status (Abdel-Tawwab & Hamed, 2020). One of the virulence factors characterized by A. hydrophila is the production of haemolysin which alters the haematological indices of the host it infects (Han et al., 2020). Chen et al. (2020) also reported the apoptosis of RBC induced by A. hydrophila infection. It is evident in this study that the erythrocyte indices of groups of fish fed with feed supplemented with A. boonei and M. scaber improved the haematological parameters more than the control group after A. hydrophila challenge. This may be attributed to numerous phytoactive components present in the plants. Akinmoladun et al. (2007) reported that A. boonei contained numerous minerals and vitamins in which vitamin C and iron were more abundant in addition to other phytochemicals. These are essential ingredients for blood formation. M. scaber has also been reported to contain several bioactive components and essential oils that may also be associated with its medicinal properties (Ali et al., 2021; Ekalu, 2021). The significant differences in the values of WBC which was higher in the control group than the treatment groups except M. scaber (1.5%) may be connected to the reaction of the fish to the bacterial challenge whereby the heterophils were mobilized against invading A. hydrophila. This is in agreement with Harikrishnan et al. (2003) that reported a significant increase in the values of WBC in common carps, 10 days after infected with A. hydrophila and the further increase, 30 days post infection. On the other hand, the reverse was the case in the groups treated with Azadirachta indica. Since haematological ratio is an important prognostic parameter for evaluation of health status in a diseased subject (Ulas et al., 2015), the lower significant values of HLR and PLR in the treatment groups than the control indicates that the prognosis is better in the former than the latter.
The higher serum total proteins in the treatment groups than the control in this study may be associated with improved immune response. The increase in the values of creatinine and BUN in treatment groups more than the control in this present study is in concomitant with the finding of Adeshina et al. (2021) who reported significant increase in fish fed plants materials-extracts based diets than the control group. The liver enzymes did not reveal much significant differences between the treatment groups and control except for AST that’s was higher in the latter than the former. The A. hydrophila might cause moderate hepatocyte damages but the fish fed with plant supplements were not affected as much as the control. Oxidative stress markers provide valuable tools in evaluating the health status of fish. Important enzymes such as SOD, GPx, GSH, MPO, MDA and CAT are important markers used in the evaluation of oxidative stress in fish ( Chen et al., 2020; Abdel-tawwab & El-araby, 2021). In this study, the dietary administration A. boonei and M. scaber to African catfish for 84 days and subsequent A. hydrophila challenged, increased the activities of SOD, GPx, GSH and MPO but reduced MPO when compared with the control. This could be attributed to antioxidant properties of the phenolic and other phytoactive compounds present in A. boonei (Obiagwu et al., 2014; Opoku & Osei, 2014) and M. scaber (Ali et al., 2021; Ekalu, 2021). These findings share similar observations with Adeshina et al. (2021) and Abdel-tawwab et al. (2021) who reported elevated SOD, CAT and GPx, and reduction in the activities of MDA due to dietary plants in Nile tilapia challenged with Gyrodactylus malalai and A. hydrophila respectively. The histological alterations of the liver observed in this study in the fish fed with control diet after A. hydrophila challenge include dilation of the sinusoid, fatty degeneration of the hepatocyte, mild congestion and leucocytic infiltration of the sinusoid. These similar findings were reported by Hal et al. (2020) who described the histological alterations of the liver of A. hydrophila group as vacuolar degeneration, hypoplasia of hepatocytes, dilatation in sinusoids and fibrosis. Histopathological findings on the kidney in the control group revealed tubular epithelial degeneration, necrosis and inflammatory cells infiltration. Similar observations were earlier reported by Hal et al., (2020) that the kidney of Nile tilapia infected with A. hydrophila revealed interstitial mononuclear cell infiltration and tubular degeneration.
Immunohistochemistry is an essential diagnostic and investigative tool that allows the identification of tissue components through antigen-antibody reaction (Ajadi et al., 2019). This present study demonstrated that specific antigen of A. hydrophila could be identified in Davidson’s fixative paraffin-embedded tissue samples of African catfish challenged with the putative organism using immunoperoxidase technique. This present finding is consistent with the report of Delghandi et al., (2020) whose study made use of immunoperoxidase technique to detect the antigens of Renibacterium salmoninarum and Mycobacterium sp. in the tissues of wild brown trout. The immunoreactivity of A. hydrophila antigen to antibody occurred in the interstitial space of the kidney and in the sinusoids of the liver with more intensity in the control group than the treatment groups. The less intensity in the groups fed with plant supplemented feed is indicative of protective potentials of the dietary plants resulting in the reduced amount of the bacterium in the sampled tissues.