A review on aeromoniasis in poultry: A bacterial disease of zoonotic nature

Aeromonas spp. are widely distributed in surface water, sewage, untreated and chlorinated, drinking water, as well as meats, fish, shellfish, poultry, and their products. A disease caused by Aeromonas spp. is designated as aeromoniasis. It can affect different aquatic animals, mammals, and birds in different geographic regions. Moreover, gastrointestinal and extra-intestinal disease conditions may be provoked in humans as a result of food poising with Aeromonas spp. Some Aeromonas spp. have been identified, however, Aeromonas hydrophila (A. hydrophila ), A. caviae , and A. veronii bv sobria may be of public health significance. Aeromonas spp. are members of family Aeromonadaceae and genus Aeromonas. They are Gram-negative rod-shaped, facultative anaerobic, and oxidase and catalase-positive bacteria. The pathogenicity of Aeromonas in different hosts is mediated by several virulence factors such as endotoxins, cytotoxic enterotoxin, cytotoxins, hemolysins, adhesins, and extracellular enzymes such as proteases, amylases, lipases, ADP-ribosyltransferases, and DNases. Most avian species are susceptible to either natural or experimental infections with Aeromonas spp. Infection usually arises through feacal-oral route. Traveler’s diarrhea as well as other systemic and local infections are the clinical picture of food poisoning associated with aeromoniasis in humans. Despite Aeromonas spp. being sensitive to various antimicrobials, multiple drug resistance has been commonly reported worldwide. Accordingly, this review highlights aeromoniasis in poultry regarding Aeromonas virulence factors epidemiology, pathogenicity, zoonosis, and antimicrobial resistance.


Introduction
Aeromonas spp.have a wide geographical distribution, being able to cause a wide spectrum of diseases in humans [1].Aeromonads are considered as ubiquitous inhabitants of aquatic and terrestrial coldblooded animals such as fish [2].The clinical disease caused by Aeromonas spp. is referred to as aeromoniasis and is globally distributed.The disease in poultry has been reported in many parts of the world causing localized or systemic infections either alone or in combination with other infections [3,4].Aeromonas spp.have the ability to adapt to different ecological sites in the host [5] and have astonishing characteristics that permit their capabilities to survive and flourish under diverse environmental conditions [6,7], thus allowing their cosmopolitan occurrence in nature.The droppings of living birds [8], poultry carcasses, and poultry plant processing water [9] are important sources of infection for the pathogenic types of Aeromonas spp.
Aeromonads have been implicated in food-borne disease outbreaks, particularly in developing countries where hygiene is a challenge [12].Strains of A. hydrophila, A. sobria, and A. caviae have been shown as emergent food-borne pathogens implicated in human gastroenteritis and extra-intestinal diseases [13].However, the pathogenesis and virulence factors associated with aeromonads in different hosts are not fully understood [14].Aeromonas spp.found in food can produce different exotoxins, some of which are enterotoxins [15].
Little information regarding aeromoniasis in poultry is available as most of the researches deal with the infection of aquatic animals and the presence of Aeromonas spp. in retail animals and poultry products.Based on currently available knowledge about aeromonads, this review article silhouettes at investigating aeromoniasis in poultry regarding Aeromonas virulence factors, epidemiology, pathogenicity, zoonosis, and antimicrobial resistance.

Etiology
Aeromonas spp.are members of the family Aeromonadaceae and genus Aeromonas, Gramnegative and asporgenous short rod-shaped bacteria.They are facultative anaerobe microorganisms that can grow over a wide range of environmental conditions as pH values from 4.0 to 10.0 and salt concentrations up to Most members of Aeromonas genus are mesophilic that can grow at an optimal growth temperature of 28 ºC, while some members can grow at temperatures ranging from 4 ºC to 42 ºC.They are non-lactose fermenter, but oxidase, catalase, indole, glucose fermenter, and nitrate reduction positive [25].These organisms are able to grow on sheep blood agar and produce β hemolysis.According to the product information from the manufacturer, the Microbact TM 24E system (Oxoid) can identify the following species, A. hydrophila, A. veronii bv sobria, A. veronii bvveronii, and A. caviae [26].

Virulence factors
The pathology and virulence of Aeromonas spp.may result from stress responses and heat shock proteins [27].Moreover, intestinal and systemic infections are mediated by numerous virulence factors [28,29] including endotoxins, cytotoxic enterotoxin (act), cytotoxins, hemolysins, adhesins, and extracellular enzymes such as proteases, amylases, lipases, ADP-ribosyltransferases, and DNases.The diversity in virulence among Aeromonas spp.isolates and the relationship of virulence markers reveal great variation according to the survival needs in the environment.The expression of Aeromonas virulence factors has been linked with gene regulation cascades associated with interactions of the pathogen with the environment [30].
Species of Aeromonas harbor different virulence genes such as haemolysin (hlyA), aerolysin (aerA), and extracellular deoxyribonuclease (exu) [31].Each of these genes has an essential role in the pathogenicity related to diarrheal diseases [32,33].For instance, the exu gene codes for an extracellular DNase that blocks the antibacterial host defenses [34].Its existence is correlated with the bacterial ability to invade, colonize, and survive in the host immune system [28].This gene is prevalent in 96% of environmental Aeromonas spp.isolates worldwide [35].Moreover, many toxin genes have been reported among Aeromonas spp.isolates [30,36].In cases of intestinal infections, the heat-labile cytotonic enterotoxin (alt) gene has been associated with loose feces, alt plus heat-stable cytotonic enterotoxin (ast) with watery feces, while cytolytic enterotoxin (act) with bloody feces [37].The alt gene causes excessive secretion of fluid inside the host's cell [30].Besides, aerA gene is the major virulence contributor in pathogenic Aeromonas spp.[38].It is a pore-forming toxin that binds to the receptors of the host's cell membrane.Following proteolytic activation, aerA gene causes pores that lead to the destruction of membrane permeability, osmotic lysis, and cell death [39].It is important to note that aerA gene is the most prevalent in marine mammals and food sources [14].Genes associated with Aeromonas virulence are not only present in clinical cases, but also they are present in different food sources such as water and fish [40].Cytotonic enterotoxin is incriminated in the triggering of inflammatory response in host cells, disorders of the plasma membrane, and degeneration of the intestinal villi causing bloody diarrhea [33].Biofilm formation is another vital virulence factor and plays a potential role in the initial bacterial attachment, adhesion, and colonization of the host's surface epithelium and intestinal villi, as well as reduced susceptibility to antibiotics and recognition by the immunologic system [41,42].The presence of biofilm-forming Aeromonas isolates in poultry and poultry workers was reported [9].Igbinosa [43] assessed the presence of biofilm-forming Aeromonas isolates in chicken droppings and found that 42.1% (8/19), 31.6%(6/19), 10.5% (2/19), and 15.8% (3/19) were moderate, weak, non, and strong producers of biofilm, respectively.The biofilm genes may include polar flagellin (fla) and lateral flagellin (lafA).

Susceptibility and infection
Aeromonads infections were frequently reported in some avian spp.Aeromoniasis were represented as high mortality in chicks [17], septicaemia in turkey [53], salpingitis in ducks [54,55], epidemic deaths in Mallard ducks [56], watery diarrhea in waterfowls [27], necrotizing enteritis and septicemia in ostriches [49], conjunctivitis in pet parrots [57], haemorrhagic septicaemia in captive ground-hornbill [58], and [47] African grey parrot Both Burkholderia cepacia and A. hydrophila were detected in brain, lung, liver, kidney, and heart samples of birds showed neurological and respiratory signs and lesions.[48] California Ostriches Aeromonas spp.were identified microscopically from intestine, liver, lungs, and trachea of 10-years-old male ostrich had neurological signs, severe necrotizing enteritis, and septicemia.However, vitamin A deficiency might have predisposed the case to the Aeromonas infection. [49]

Nigeria Commercial chickens
Two thousand oro-pharyngeal swabs and samples from bone marrow, heart, liver, lung and spleen were collected from 400 apparently healthy and diseased chickens for isolation of Aeromonas spp.From the bone marrow, heart, and liver of the diseased chickens, a total 11 (0.5%) A. hydrophila isolates were identified.However, Aeromonas bacterium was not isolated at all from the apparently healthy chickens.
The horizontal mean of aeromonad's infection through fecal-oral route is common in birds [50].Rosskopf and Woerpel [63] demonstrated that exposure of birds to A. hydrophila infection may occur via their food and transmission is primarily via oral routes and fecal shedding into the environment.This may reflect a disturbance in the intestinal ecology which has permitted the pathogen growth to high numbers [64].Contaminated drinking water and un-hygienic contaminated feed particularly fish meal are sources of aeromonads infection in poultry.Fish and shellfish may harbor pathogenic A. hydrophila.For example, Farag [65] demonstrated that poorly processed fish or shellfish in poultry ration could lead to Aeromonas infection.
Some suitable environmental conditions such as increased humidity and temperature as well as poor hygienic conditions in hatcheries may provoke A. hydrophila infection via eggshell penetration [18,66].Musgrove et al. [67] isolated A. hydrophila and other enterobacteria from the eggs shell of chickens.However, aeromoniasis is not a congenitally transferred disease, as the ovary and oviduct have no role in dissemination of A. hydrophila infection [18].

Pathogenicity
The pathogenicity of Aeromonas spp. is complex and multifactorial as it is associated with many virulence factors [68].It has been documented that A. hydrophila either alone or in combination with other pathogens may cause localized and systemic infections in poultry [69].Concomitant Aeromonas infections with other diseases such as salmonellosis [70] and fowl cholera [3] have been reported.
Depression, ruffled feathers, severe diarrhea, emaciation, and congested friable livers were observed 2 days post-experimental infection of Japanese quails with A. hydrophila [27].Additionally, A. hydrophila challenge in 2 and 5-day-old chicks induced 60% to 100% mortality, gastrointestinal disturbance, and congestion of the liver, spleen, lungs, kidneys, and intestine [71].Moreover, petechial haemorrhages on the liver, omphalitis, enteritis, and nephrosis have been reported following experimental inoculation of chicks with A. hydrophila [17].Mahmoud and Tanios [72] detected a mortality rate of 52.5% following subcutaneous (S/C) injection of a dose of 3.5 × 10 7 of A. hydrophila, while the mortality was decreased to 35% following inoculation of 1.5 × 10 9 of A. hydrophila.Hatched chicks from A. hydrophila infected eggs showed mortalities reached 13.3% and 1.7% during 1 st and 2 nd week post-hatching, respectively, and the chicks exhibited omphalitis, enteritis, unabsorbed yolk sac, distended gall bladder, and congested liver and heart (18).In the study of Girh et al. [73], the results showed that S/C inoculation of 2-weeks-old Fayoumi chicks with Aeromonas spp.isolates induced mild pathogenicity with a long course of diarrhea and enteritis.The pathogenicity test for A. hydrophila, A. trota, A. caviae, and A. schubertii was reported in dayold chicks and the results revealed mortality rate of 13.3% in A. hydrophila, 20% in A. trota, 13.3% in A. caviae, and 6.7% in A. schubertii infected chicks [4].Off-food, pasty vent, diarrhea, enteritis, unabsorbed yolk sac, distended gall bladder, generalized congestion, enlarged spleen and kidney, congested lungs, and air sac turbidity were also seen.Besides, A. schubertii infection induced more marked adverse effects on body weight than that of A. caviae, A. trota, and A. hydrophila.

Zonoosis
The risk of food-borne disease, due to Aeromonas infection has been increased due to the ability of the organism to grow at low temperatures and produce toxins [74].Many predisposing factors are involved in human's aeomoniasis including ingestion of contaminated food and water [75], presence of other diseases conditions as diabetes [76], and immunosuppression and age of humans [77].Depending on Aeromonas virulence and antibiotic resistance profiles, numerous spp.have been reported as important zoonotic pathogens [22].Chickens carcasses, heart, and liver could be potential sources for the spread of Aeromonas infection and they present a possible threat to public health [78].
Aquatic environment as well as different food including fish, seafood, and raw and cooked meat and chickens can be a potential vehicle for human's infections with aeromonads [15, [87][88][89][90][91]. Examined 563 samples of fish, raw and cooked meat, and pre-prepared salads revealed the presence of mesophilic Aeromonas spp. in 287 samples as most of contaminated samples were offals (84.3%) and chickens (79.3%) [92].Aeromonas salmonicida was isolated from paddlefish [93].Moreover, Aeromonas spp.were isolated from frozen fish intended for human consumption in Mexico City [94].Therefore, the presence of Aeromonas organisms in the raw meat samples can represent a serious potential risk for public health.In India, out of 154 food samples represented chickens, fish, and readyto-eat sprouts, 22 (14.28%)isolates were Aeromonas spp.and the highest percentage of isolation was from chickens (28.6%), followed by fish (20%), and sprout (2.5%) [95].Moreover, 53 (57.6%) and 27 (17%) of aeromonads isolates were characterized in 92 chickens and 158 minced meat samples, respectively.The isolation rate of aeromonads was significantly higher in chicken than in minced meat samples [96].Mailafia et al. [97], in Nigeria, reported on isolation of A. hydrophila from diarrheic patients in a rate of 6.8%.

Antimicrobial resistance
Multiple drug resistance of Aeromonas spp.strains have been commonly reported worldwide due to frequent administration of antibiotics besides the classical resistance to β lactam group [98].Considering the risk to human health, the incidence of antimicrobial resistance is alarming especially among A. hydrophila, A. caviae, and A. sobria, which are responsible for infections in both animals and humans [99].Antibiotic resistance occurs either by carrying intrinsic genes or by acquiring resistance markers from other pathogens [15,74].Aeromonads could possess integron that enables bacteria to acquire and transfer antibiotic resistance genes, giving rise to the risk of resistant bacterial infections [9,100].Aeromonads were initially shown as susceptible to tetracycline, chloramphenicol, cephalosporins, aminoglycosides, and quinolones [22].However, chromosomal inducible β lactamase class C has been detected as a major mechanism of Aeromonas spp.resistance to cephalosporins and cefoxitin [101].Moreover, Sinha [102] found high levels of intrinsic resistance to antimicrobials among Aeromonas isolates due to gyr gene of chromosomal origin and qnr gene of plasmid origin.It has been reported that Aeromonas spp.revealed resistance to quinolones in domestic and free-living animals, hospital effluents, and wastewater [103][104][105].Aeromonads may also become a reservoir of gene encoding resistance to some antimicrobials such as tetracycline.Stratev and Odeyemi [106] reported on the spread of tetracycline-resistant plasmids between A. hydrophila and Escherichia coli and between human and aquaculture in different locations.Besides, pathogenic Aeromonas spp.are capable of transferring the genes responsible for antimicrobial resistance to other pathogenic organisms in humans and throughout the food chain which is a risk to human and animal health [31].Biofilm formation also enhances aeromonads to be more resistant to antimicrobial agents and host defenses.Bacteria may express more virulent phenotypes as a result of gene activation via bacterial communication (quorum sensing) or gene transfer [41].It has been reported that A. hydrophila is the most resistant spp. to antimicrobials followed by A. caviae, A. trota, and then A. schubertii [4].Moreover, A. hydrophila isolates of fish and poultry origins are more resistant to antibiotics than those of water origin [107].

Conclusions
Aeromonas spp.are frequently discovered in food, animals, and birds.The potential risk of Aeromonas gastro-enteric human's infections and the dissemination of the pathogen to animals, poultry, or humans with close contact and the wider community have been proven.Therefore, periodical regular screening of poultry flocks in different geographical locations is essential.Moreover, public health awareness and enlightenment of the hazard associated with Aeromonas infection is necessary.
6.5% [16].Setta [17] isolated A. hydrophila from cloacal samples of experimentally infected chicks for up to 16 days post-infection.The long persistence rate of A. hydrophila in the droppings explains its possible importance on health especially when it occurs in broilers associated with carcass contamination at processing [18].Moreover, A. hydrophila could persist in chicken crates, droppings, ration, and sawdust and straw for 11, 9, 23, 22, and 17 days, respectively [18].In addition, Kelley et al. [19] isolated A. hydrophila with other bacteria from litter during reutilization as a bedding supplement of growing broiler chickens.The genus Aeromonas consists of two groups; one group is non-motile psychrophilic (A.salmonicida) and the other one comprising of three mesophilic motile A. hydrophila (A.caviae and A. sobria) [20].Motile aeromonads are ubiquitous and autochthonous aquatic bacteria that are present in fresh, sewage, and brackish water [21] as well as in chlorinated and non-chlorinated drinking water [22,23].The completely identified

Table 1 .
The incidence of Aeromonus spp. in poultry flocks and environment of different countries (1990s-2020s).