The Impact of Fusarium Mycotoxins on Human and Animal Host Susceptibility to Infectious Diseases

Contamination of food and feed with mycotoxins is a worldwide problem. At present, acute mycotoxicosis caused by high doses is rare in humans and animals. Ingestion of low to moderate amounts of Fusarium mycotoxins is common and generally does not result in obvious intoxication. However, these low amounts may impair intestinal health, immune function and/or pathogen fitness, resulting in altered host pathogen interactions and thus a different outcome of infection. This review summarizes the current state of knowledge about the impact of Fusarium mycotoxin exposure on human and animal host susceptibility to infectious diseases. On the one hand, exposure to deoxynivalenol and other Fusarium mycotoxins generally exacerbates infections with parasites, bacteria and viruses across a wide range of animal host species. Well-known examples include coccidiosis in poultry, salmonellosis in pigs and mice, colibacillosis in pigs, necrotic enteritis in poultry, enteric septicemia of catfish, swine respiratory disease, aspergillosis in poultry and rabbits, reovirus infection in mice and Porcine Reproductive and Respiratory Syndrome Virus infection in pigs. However, on the other hand, T-2 toxin has been shown to markedly decrease the colonization capacity of Salmonella in the pig intestine. Although the impact of the exposure of humans to Fusarium toxins on infectious diseases is less well known, extrapolation from animal models suggests possible exacerbation of, for instance, colibacillosis and salmonellosis in humans, as well.

variable: co-exposure of low doses of DON, T-2 and ZEN reduces the number of goblet cells in pigs [21], but ZEN given alone at higher doses increases the activity of goblet cells [22]. Several mycotoxins are also able to modulate the production of cytokines in vitro and in vivo [9,23]. For example, DON increases the expression of TGF-β and IFN-γ in mice and fumonisins decrease the expression of IL-8 in an intestinal porcine epithelial cell line (IPEC-1) [9].  [9]). Fusarium mycotoxins can cross the intestinal epithelium and reach the systemic compartment [20,24], affecting the immune system. Exposure to these toxins can either result in immunostimulatory or immunosuppressive effects depending on the age of the host and exposure dose and duration [20,25]. Mycotoxin-induced immunomodulation may affect innate and adaptive immunity by an impaired function of macrophages and neutrophils, a decreased T-and B-lymphocyte activity and antibody production [23,25,26]. In addition to the effect of Fusarium mycotoxins on the animal or human host, these mycotoxins may alter the metabolism of the pathogen, which may alter the outcome of the infectious disease [27,28].
A wealth of research papers clearly indicate a negative influence of Fusarium mycotoxins on the intestinal function and immune system. Since the intestinal tract is also a major portal of entry to many enteric pathogens and their toxins, mycotoxin exposure could increase the animal susceptibility to these pathogens. Furthermore, mycotoxin-induced immunosuppression may also result in decreased animal or human host resistance to infectious diseases.
This review attempts to summarize the impact of Fusarium mycotoxin exposure on the animal and human host susceptibility to infectious diseases. More specifically, the effect of Fusarium mycotoxins on enteric, systemic and respiratory infectious diseases in livestock animals and animal models for human diseases are highlighted.

Coccidiosis
Intestinal protozoa, including the coccidia (Eimeria, Isospora, Cryptosporidium and Sarcosporidia) and flagellates, are important infectious agents. Coccidiosis in poultry generally refers to the disease caused by the Eimeria species, and is still considered one of the most important enteric diseases affecting performance. These obligate intracellular parasites have an oral-fecal life cycle with developmental stages alternating between the external environment and the host [29].
Seven species of Eimeria (E. acervulina, E. brunetti, E. maxima, E. mitis, E. necatrix, E. praecox and E. tenella) are found in chickens [29]. The physical and biological characteristics, pathogenicity and immunogenicity depend on the species. Immunity to Eimeria is complex, multifactorial and influenced by both host and parasite [30].
Cell-mediated immunity, mainly evoked by the intraepithelial lymphocytes (IEL) and lymphocytes of the lamina propria, is the major protective immune component against avian coccidiosis [31,32]. The CD4 + T-lymphocytes, IEL and macrophages are involved in the response against primary exposure to Eimeria [31], while CD8 + T-lymphocytes and IFN-γ are important in the protective immune response against Eimeria infection [33]. Girgis et al. [34,35] showed a negative impact of diets naturally contaminated with Fusarium mycotoxins on the cell-mediated immune response against coccidiosis in broilers (Table A2). Following primary infection of broilers with Eimeria, Fusarium mycotoxins decreased the percentage of CD4 + and CD8 + T-cells in the jejunal mucosa [35]. In addition, feeding on a mycotoxin-contaminated diet lowered the blood levels of CD8 + T-cells and monocytes, which could suggest an increased recruitment at the intestinal site of coccidial infection or a delayed replication necessary to replenish these subsets in the circulation [34,35]. Additionally, feeding on a Fusarium mycotoxin-contaminated diet increased IFN-γ gene expression in the cecal tonsils of Eimeria-challenged birds, however, without being linked to the apparent resistance to coccidial infection in terms of changes in oocyst yield [34]. The cecal tonsils constitute a lymphoid tissue in the cecum belonging to the gut-associated lymphoid tissue (GALT). Resistance to Eimeria infection is related to the expression of a set of interleukins rather than only IFN-γ and the up-regulation of the gene may not necessarily be associated with functional secretion [34]. Furthermore, it was shown that moderate levels of Fusarium mycotoxins negatively affect intestinal morphology and interfere with intestinal recovery from an enteric coccidial infection, indicated by a lower villus height and apparent villus area (Table A2) [36]. Although Girgis et al. [34,35] demonstrated that Fusarium mycotoxins impair the Eimeria-induced immune response, no effect was seen on fecal oocyst counts. Similarly, Békési et al. [37] showed no impact of a T-2 and ZEN-contaminated diet on Cryptosporidium baileyi oocyst excretion in broilers.
Research investigating the influence of mycotoxins on the animal susceptibility to infectious diseases focuses mainly on exposure to single major mycotoxins. Limited information about the impact of mycotoxin co-occurrence and plant metabolites of mycotoxins on this interaction is available. Nevertheless, Girgis et al. [34,35] showed that the combination of DON, 15-acetylDON (15-AcDON), ZEN and fumonisins alters the Eimeria-induced immune response. Interestingly, mycotoxin contamination of broiler feed may reduce the efficacy of the anti-coccidial treatment with lasalocid [38].
To conclude, Fusarium mycotoxins negatively affect the innate and adaptive cellular immune response against Eimeria, though without changing the oocyst yield. Further data of clinical coccidiosis lesion scoring is still needed in order to evaluate the effect of Fusarium mycotoxins on the severity of the disease.

Salmonellosis
Salmonellosis is an infection with the Gram-negative Salmonella bacterium, a facultative anaerobic, facultative intracellular microorganism of the Enterobacteriaceae family. The host-Salmonella interaction is complex, with a broad array of mechanisms used by the bacteria to overcome host defenses. Two important disease manifestations are differentiated, i.e., gastroenteritis and enteric fever, caused by nontyphoidal and typhoidal Salmonella serovars, respectively [39].
Nontyphoidal Salmonella strains, such as Salmonella serovar Typhimurium and Salmonella serovar Enteritidis strains, infect a wide range of animal hosts, including pigs and poultry, without causing clinical symptoms in these animals. Infection in slaughter pigs and poultry can cause meat and egg contamination [39,40].
An infection with Salmonella generally occurs in three stages: the adhesion to the intestinal wall, the invasion of the gut wall and the dissemination to mesenteric lymph nodes and other organs. Via bacterial-mediated endocytosis, Salmonella invades the intestinal epithelial cells, after which the bacterium becomes enclosed within an intracellular phagosomal compartment (the Salmonella-containing vacuole (SCV)). After crossing the epithelial barrier, the bacterium is located predominantly in macrophages in the underlying tissue [39].
Feeding pigs a Fusarium mycotoxin-contaminated diet influences the intestinal phase of the pathogenesis of Salmonella Typhimurium infections as illustrated in Figure 2. Non-cytotoxic concentrations of DON and T-2 enhance intestinal Salmonella invasion and increase the passage of Salmonella Typhimurium across the epithelium (Table A1) [28,41]. Chronic exposure of specific pathogen-free pigs to naturally fumonisin-contaminated feed had no impact on Salmonella Typhimurium translocation [42]. Once Salmonella has invaded the intestinal epithelium, the innate immune system is triggered and the porcine gut will start to produce several cytokines [28,43]. Both Fusarium mycotoxins and Salmonella affect the innate immune system. Vandenbroucke et al. [27] showed that low concentrations of DON could potentiate the early intestinal immune response induced by Salmonella Typhimurium infection. Co-exposure of the intestine to DON and Salmonella Typhimurium resulted in increased expression of several cytokines, for instance, those responsible for the stimulation of the inflammatory response (TNF-α) and T-lymphocyte stimulation (IL-12) ( Table A2). The authors suggested that the enhanced intestinal inflammation could be due to a DON-induced stimulation of Salmonella Typhimurium invasion in and translocation across the intestinal epithelium [27].
Fusarium mycotoxins also affect the systemic part of the Salmonella Typhimurium infection in pigs. After the intestinal phase of the pathogenesis, Salmonella can spread to the bloodstream using the host macrophage to establish the systemic infection. However, in pigs the systemic part of Salmonella Typhimurium is poorly documented and colonization is mostly limited to the gastrointestinal tract [44]. After bacterial uptake by the macrophage, Salmonella can survive and even proliferate in this cell. Exposure of macrophages to non-cytotoxic concentrations of DON and T-2 promotes the uptake of Salmonella Typhimurium (Figure 2, Table A1). Salmonella entry in host cells involves a complex series of actin cytoskeletal changes. Macrophage invasion coincides with membrane ruffling, followed by bacterium uptake and formation of Salmonella-containing vacuole [41]. Vandenbroucke et al. [41] showed in vitro that DON enhances Salmonella Typhimurium engulfment, since low concentrations of DON modulate the cytoskeleton of macrophages through ERK1/2 F-actin reorganization resulting in an enhanced uptake of Salmonella Typhimurium in porcine alveolar macrophages (PAM) (Figure 2, Table A1). Non-cytotoxic concentrations of the Fusarium mycotoxins DON and T-2 did not affect the intracellular proliferation of Salmonella Typhimurium in porcine macrophages ( Figure 2) [28,41].  (3) in porcine alveolar macrophages. The Salmonella invasion of macrophages coincides with membrane ruffling, caused by actin cytoskeletal changes. Activation of host Rho GTPases by the Salmonella pathogenicity island (SPI)-1 type 3 secretion system (T3SS) effector proteins SopB, SopE, SopE2 and SopD leads to actin cytoskeleton reorganization. After Salmonella internalization has occurred, the bacterium injects the effector protein SptP which promotes the inactivation of Rho GTPases. The bacterium can also modulate the actin dynamics of the host cell in a direct manner through the bacterial effector proteins SipA and SipC. The mycotoxin DON enhances the uptake of Salmonella in macrophages through activation of the mitogen-activated protein kinases (MAPK) extracellular signal-regulated kinases (ERK1/2) pathway, which induces actin reorganizations and membrane ruffles. DON and T-2 do not affect intracellular bacterial proliferation (4) (based on [41,44]).
In addition to the effects of Fusarium mycotoxins on the host susceptibility to a Salmonella Typhimurium infection, these mycotoxins also modulate the bacterial metabolism. Although no effect of DON or T-2 on the growth of Salmonella Typhimurium is detected, DON and T-2 modulate the Salmonella gene expression [28,41]. The enhanced inflammatory effect following exposure to DON is more likely a result of the toxic effect of the mycotoxin on the intestine than on the bacterium [27].
Only high concentrations of DON increase the bacterial expression of regulators of Salmonella pathogenicity island (SPI)-1 and SPI-2, respectively hilA and ssrA. SPI-1 consists of genes coding for bacterial secretion systems necessary for invasion, while SPI-2 genes encode essential intracellular replication mechanisms [41]. For T-2 the toxic effects on the bacterium itself are probably more pronounced than the host cell-mediated effects resulting in a reduced in vivo colonization in pigs. Low concentrations of T-2 cause a reduced motility of Salmonella and a general down regulation of genes involved in Salmonella metabolism, genes encoding ribosomal proteins and SPI-1 genes [28].
Only limited information is available concerning the interaction between Fusarium mycotoxins and Salmonella Typhimurium infection in other animals. The currently available publications mainly focus on the interaction of T-2 and the systemic phase of a Salmonella Typhimurium infection. In T-2-challenged broiler chickens and mice an increased level of Salmonella Typhimurium-related organ lesions or mortality was seen (Table A2) [45][46][47][48]. Infection of mice with Salmonella Typhimurium results in systemic infection and a disease similar to that seen in humans after infection with Salmonella Typhi [49]. Increased mortality might be explained partly by the synergistic effects of bacterial lipopolysaccharide (LPS) and T-2 during the late phase of murine salmonellosis [50]. In addition to Salmonella Typhimurium, DON reduces the resistance to oral infection with Salmonella Enteritidis in mice by promoting translocation of Salmonella to mesenteric lymph node (MLN), liver and spleen (Table A2) [51].
Mouse and pig models are important animal models to investigate the impact of mycotoxins, infectious diseases and their combination on animal health [52,53]. Infection of mice with Salmonella Typhimurium is an important host-pathogen interaction model to investigate typhoid fever in humans. Moderate to high concentrations of T-2 have shown to increase Salmonella-induced mortality [46,47,50]. The pig is very similar to humans in terms of anatomic and physiologic characteristics such as size, digestive physiology, kidney structure and function, pulmonary vascular bed structure, coronary artery distribution, respiratory rates, cardiovascular anatomy and physiology, and immune response, and has been used to study various intestinal pathogens, including Salmonella and Escherichia coli [53]. The interaction between mycotoxins and Salmonella Typhimurium studied in a porcine model of infection, gives us relevant information concerning the impact of this interaction on human intestinal inflammation and immune response [27].
In conclusion, the exact outcome of co-exposure to Fusarium mycotoxins and Salmonella Typhimurium is difficult to predict. Published data show an influence of mycotoxin exposure on the bacterium, the host cells and the host-pathogen interaction. Depending on the characteristics of the mycotoxin exposure, one of these effects will determine the outcome of the interaction between Fusarium mycotoxins and Salmonella Typhimurium.

Colibacillosis
Escherichia coli is a Gram-negative, non-sporulating rod-shaped bacterium of the family Enterobacteriaceae. Although this bacterium is considered to be a normal component of the intestinal microbiota, it is frequently associated with both intestinal and extra-intestinal infections in humans and animals. A certain number of these strains possess particular combinations of virulence factors which enables them to cause disease. Clinical syndromes resulting from infection with these pathotypes include enteric/diarrheal disease, urinary tract infections and sepsis/meningitis. The pathogenesis of E. coli infections depends on the pathotype involved and may include colonizing the intestinal mucosa, evasion of host defenses, multiplication, and induction of host damage [54,55].
Fusarium mycotoxins may influence the pathogenesis of E. coli infections in different animal species by stimulating intestinal colonization and translocation and negatively affecting the immune response. Feeding a diet contaminated with a moderate level of FB1 to pigs enhanced intestinal colonization and translocation of a septicemic E. coli (SEPEC) strain from the intestine to the systemic compartment. FB1-treatment resulted in a higher bacterial translocation to the mesenteric lymph nodes and lungs, and to a lesser extent to liver and spleen (Table A2) [56]. It was shown in vitro that DON increased the translocation of SEPEC over the intestinal epithelial cell monolayer (IPEC-1) (Table A1) [14].
Mycotoxins increase the calf susceptibility to shiga toxin or verotoxin-producing E. coli (STEC)-associated hemorrhagic enteritis. Recently, Baines et al. [57] showed that exposing calves of less than one month old to the combination of aflatoxin and fumonisins promoted STEC-associated hemorrhagic enteritis (Table A2) [57].
Feeding a FB1-contaminated diet to pigs negatively affects the mucosal immune response against an infection with enterotoxigenic E. coli (ETEC). Devriendt et al. [58] showed a prolonged intestinal infection of E. coli in pigs administered fumonisins for 10 consecutive days and subsequently challenged with E. coli (F4 + ETEC) (Table A2). Antigen-presenting cells (APCs) have an important role in the mucosal immune system by connecting the innate and adaptive immune response, through uptake of antigen in lamina propria, maturation and migration to GALT, and interaction with T cells. FB1 negatively affected the function of intestinal APCs by a reduced up-regulation of the major histocompatibility complex class II (MHC-II), cluster of differentiation (CD) 80/6 and IL-12p40 cytokine gene expression [58]. This altered function of APCs could therefore influence the E. coli-induced adaptive immune response [58,59]. Additionally, moniliformin and FB1 delayed systemic E. coli (avian pathogenic E. coli, APEC) clearance in broilers and turkeys after intravenous administration (Table A2) [60,61].
The results of these studies may also be valid for human infections since the gastro-intestinal tract of pigs and humans are very similar [58]. Infant diarrhea caused by enteropathogenic E. coli (EPEC) is known to be of major concern in developing countries and, for instance, enterohemorrhagic E. coli (EHEC) infections are a major worldwide public health hazard.

Necrotic Enteritis in Broilers
Necrotic enteritis (NE) is a disease in broilers caused by Clostridium perfringens. This Gram-positive spore-forming bacterium occurs naturally in the environment, feed and gastrointestinal tract of chickens and other animals [62,63]. NE is a complex, multifactorial enteric disease with many known and unknown factors influencing its occurrence and the severity of the outbreaks. The best-known predisposing factor is mucosal damage caused by coccidial pathogens [64]. Only C. perfringens strains expressing the NetB toxin are capable of inducing NE in broilers [65]. C. perfringens is auxotrophic for several amino acids, thus availability of these amino acids would allow extensive bacterial proliferation [63].
The intake of DON-contaminated feed is a predisposing factor for the development of necrotic enteritis in broiler chickens due to the negative influence on the epithelial barrier, and to an increased intestinal nutrient availability for clostridial proliferation. Recently, we [66] showed in an experimental subclinical NE infection model that chickens fed a diet contaminated with DON for three weeks were more prone to develop NE lesions compared to chickens on a control diet (Table A2). The negative effects of DON on the small intestinal barrier can lead to an impaired nutrient digestion and leakage of plasma amino acids into the intestinal lumen, providing the necessary growth substrate for extensive proliferation of C. perfringens [66].

Edwardsiella ictaluri Infection in Catfish
Edwardsiella ictaluri is a Gram-negative bacterium of the Enterobacteriaceae family. Bacillary Necrosis of Pangasianodon (BNP) caused by E. ictaluri is the most frequently occurring infectious disease in catfish [67]. Besides the Vietnamese freshwater production, also the American channel catfish (Ictalurus punctatus) industry suffers massively from E. icatluri infections which have been termed Enteric Septicemia of Catfish (ESC). BNP is characterized by multifocal irregular white spots of varying sizes on several organs including liver, spleen and kidney [68]. ESC in channel catfish may occur in an acute form characterized by enteritis and septicemia with rapid mortality, or in a chronic form, which is characterized by meningoencephalitis, open lesions on the cranial region and exophthalmia [69]. Mortality associated with the co-occurrence of Fusarium mycotoxins and E. ictaluri is difficult to predict in juvenile channel catfish. T-2 increased E. ictaluri-associated mortality [70], while moderate contamination of DON improved the survival of the channel catfish [71] (Table A2). Mycotoxin sensitivity differs between fish species. Rainbow trout, for example, are extremely sensitive to DON, while channel catfish are rather resistant [71,72]. Important data concerning the toxicity of the mycotoxin on the bacterium are lacking. Further investigation of the interaction between Fusarium mycotoxins and E. ictaluri will be necessary to evaluate the outcome.

Swine Respiratory Disease
Respiratory disease in pigs is often caused by the combined effects of multiple pathogens and predisposing factors [73]. Primary infections with bacteria such as Actinobacillus pleuropneumoniae, Mycoplasma hyopneumoniae, Bordetella bronchiseptica or viruses such as influenza virus and Porcine Reproductive and Respiratory Syndrome Virus (PRRSV), can predispose pigs to secondary pathogens such as Pasteurella multocida and Trueperella pyogenes [74]. Respiratory symptoms can vary depending on the pathogens involved. M. hyopneumoniae is the principal etiological agent responsible for enzootic pneumonia in pigs [75]. M. hyopneumoniae is an obligate symbiotic and host-specific bacterium, which is lacking a cell wall. This pathogen affects the respiratory mucosal clearance system by disrupting the celia on the epithelial surface and modulates the immune system of the respiratory tract. Consequently, M. hyopneumoniae predisposes animals to concurrent infections with other respiratory pathogens [75]. Dietary exposure to fumonisins induces pulmonary edema and may facilitate M. hyopneumoniae infection (Table A2) [76].
The progressive form of porcine atrophic rhinitis is often due to a combined infection with B. bronchiseptica and toxigenic P. multocida [73,77]. Dietary exposure to FB1 of piglets infected with both bacteria increases the risk of pneumonia and the severity of the pathological changes [73]. P. multocida type A is the most frequently occurring secondary pathogen that can cause pneumonic pasteurellosis [78]. Halloy et al. [74] showed that inoculation of piglets with P. multocida combined with an oral bolus of FB1 induced a cough and a lung inflammatory process characterized by an increased number of total cells, macrophages and lymphocytes in broncheo-alveolar lavage fluid (BALF). Lung lesions were more severe in these animals and consisted of subacute interstitial pneumonia [74].

Effect of Fusarium Toxins on Fungal Diseases
Aspergillosis Aspergillus fumigatus is an ubiquitous saprophytic fungus found in soil, plant debris, and the indoor environment, including hospitals. This fungus is also an opportunistic pathogen. Inhalation of its conidia can cause life-threatening infections in the respiratory system of immunocompromised animals and humans. Respiratory macrophages are the first line of defense against inhaled Aspergillus conidia. T-2 impaired the phagocytotic activities of macrophages against A. fumigatus conidia in chickens and rabbits (Tables A1 and A2) [79,80]. However, the pro-inflammatory response of A. fumigatus infected chicken macrophages was increased by T-2 (Table A1) [80]. The effect of T-2 on the innate immune response against Aspergillus conidia is dual, which suggests that depending on the characteristics of the mycotoxin exposure and the animal, one of these effects will determine the outcome of this interaction.

Reovirus
Reovirus is a non-enveloped double-stranded RNA virus that has been isolated from the gastro-intestinal tract and respiratory tract of both humans and animals [81,82]. Enteric reoviruses cause mostly a mild and self-limiting infection [82]. Nevertheless, reovirus infections can be more severe, affecting, for example, the central nervous system in mice and rats [81]. Viral arthritis is the most frequent reovirus-associated disease in poultry, which is characterized by lameness and swellings affecting primarily tarsometatarsal joints and the feet [83][84][85].
Fusarium mycotoxins negatively affect the intestinal virus clearance in mice. Li et al. [82,86] showed that high concentrations of DON and T-2 suppress the host immune response to reovirus as evidenced by the inability to clear the virus from the intestine as well as by increased fecal shedding of the virus (Table A2). Trichothecene exposure increased the intestinal viral load, which could increase inflammation and discomfort to the host during the infection process. The increased fecal shedding could enhance virus dissemination among individuals [86]. Both mycotoxins decreased the cell-mediated viral clearance by suppressing the gene expression of IFN-γ in Peyer's Patches (PP) [82,86]. DON enhanced Th2 cytokine expression prior to and after reovirus infection, which potentiates the IgA and IgG responses to reovirus [82]. In contrast, T-2 suppressed reovirus-induced immunoglobulin responses [86]. The lack of a similar effect of Th2 cytokines by T-2 suggests inherent differences between both mycotoxins in their capacity to modulate cytokines during viral infection, although both mycotoxins belong to the class of trichothecenes [86]. Nevertheless, the intestinal clearance of reovirus was less efficient after T-2 exposure compared to DON [86]. Since reovirus infection in mice is used as a model for several enteric and respiratory viral infections in humans and other animals [81], these results could assume an impact of mycotoxins on host susceptibility to more virulent viruses.

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV)
Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is an enveloped single-stranded RNA virus belonging to the family Arteriviridae, within the order Nidovirales [87]. Currently, PRRS is one of the most economic significant diseases in swine production [87,88]. The clinical symptoms, respiratory or reproductive, vary with the viral strain, the immune status of the herd, and management factors [88,89]. PRRSV is a highly infectious virus that replicates within the monocytes or macrophages with the lung being a predominant site of viral multiplication [89]. Exposure of piglets to FB1 increased the risk for PRRSV disease [90]. More severe histopathological lesions were observed when pigs were exposed to FB1 and subsequently inoculated with PRRSV. The authors suggest that FB1 causes immunosuppression, facilitating PRRSV to induce more severe lesions [89]. Given the importance of PRRSV in worldwide swine production and the frequent occurrence of fumonisins, research should be performed investigating this interaction also at lower doses of FB1.

Discussion
In recent years, research investigating the effects of Fusarium mycotoxins on the intestinal and immune functions has made substantial progress. However, only limited information is available on the interaction between mycotoxins and infectious diseases. The aforementioned literature data indicates that Fusarium mycotoxins may influence the animal and human host susceptibility to enteric, systemic and respiratory infectious diseases. Depending on host, pathogen and mycotoxin characteristics, exposure to Fusarium mycotoxins can generally exacerbate infectious diseases. On the other hand, T-2 has been shown to decrease the colonization capacity of Salmonella in the pig intestine. Fusarium mycotoxins may influence the host-pathogen interaction by negatively affecting the intestinal barrier function and the innate and adaptive immune response [9,23,26]. Fusarium mycotoxins affect the morphology and the barrier function of the intestinal layer [9], leading to increased translocation of different bacterial species including Salmonella enterica and E. coli, to the systemic compartment. The negative influence of these mycotoxins on the function of macrophages results in impaired phagocytosis of bacterial and fungal pathogens. However, also the adaptive immune response is targeted, demonstrated by the effect on gene expression of several cytokines, leading to an altered Th1 and Th2 response.
The economic impact of mycotoxins on animal production is generally considered to be mainly due to losses related to direct effects on animal health and trade losses related to grain rejection [91]. It is clear, however, that the indirect influence of myocotoxins on animal health, by enhancing infectious diseases, should also be taken into account. These effects, as reviewed here, occur even at low to moderate mycotoxin contamination levels of feed [8]. Some publications showed that these effects can even occur at contamination levels below the European guidance levels, suggesting that the legislation may not cover all deleterious health effects of mycotoxins.
Fusarium mycotoxins have various acute and chronic effects on humans [92]. DON could play a role in diseases such as inflammatory bowel disease (IBD) [20,93]. Taken into account conditions such as environmental, socio-economic and food production, it seems plausible that the risk for food-associated mycotoxin exposure is even higher in developing countries [94]. Besides the risk for acute mycotoxicosis in developing countries [95], results obtained in animals suggest that low to moderate concentrations of these mycotoxins could also influence human susceptibility to infectious diseases.
The effect of multi-mycotoxin contamination and of less well-known or emerging mycotoxins on the human or animal susceptibility to infectious diseases is rather unknown. Multi-mycotoxin contamination of feed is frequently occurring, raising the question on the impact on animal toxicity of this phenomenon [3]. Several in vitro and in vivo studies demonstrated an enhanced toxicity and more severe immune suppression compared to single mycotoxin contamination [96][97][98]. In addition, plant metabolites of mycotoxins may also be present in feed and are known as masked mycotoxins [99]. Fusarium fungi and infected plants may produce conjugated forms of, for instance, DON, such as 3 -AcDON (3-acetylDON), 15-AcDON and DON-3G (DON-3-glucoside). Furthermore, mycotoxins can also be conjugated by certain food-processing techniques. These conjugated forms could have a direct toxic effect, or may be hydrolyzed to their precursor mycotoxin in the digestive tract of animals, resulting in higher exposure levels [100][101][102]. The influence of mycotoxin co-occurrence and masked mycotoxins on human and animal susceptibility to infectious diseases will be an important research question in the future.
Global warming and increasing world population of humans are further important issues. Climate changes may affect the global distribution of mycotoxigenic fungi and their mycotoxins [103,104], but also the distribution of infectious diseases [105]. Livestock farming will remain an important component of the global food supply in the future. Animal health, including the impact of mycotoxins and susceptibility to infectious diseases, will be important future topics to produce enough safe food for the entire human population.
In conclusion, Fusarium mycotoxins may alter the human and animal susceptibility to infectious diseases by affecting the intestinal health and the innate and adaptive immune system. Further research will be necessary to investigate the impact of mycotoxins on infectious diseases and to develop practical, economically justified, solutions to counteract mycotoxin contamination of feed and food, and its effects on human and animal health.

Acknowledgments
G. Antonissen was supported by a PhD fellowship from Biomin GmbH, Herzogenburg, Austria.

Conflicts of Interest
The authors declare no conflict of interest.

DON
unprocessed cereals other than durum wheat, oats and maize 1250 unprocessed durum wheat and oats 1750 unprocessed maize, with the exception of unprocessed maize intended to be processed by wet milling 1750 cereals intended for direct human consumption, cereal flour, bran and germ as end product marketed for direct human consumption, with the exception of foodstuffs listed in (1) . 750 pasta (dry) 750 bread (including small bakery wares), pastries, biscuits, cereal snacks and breakfast cereals 500 (1) processed cereal-based foods and baby foods for infants and young children 200 feed materials: cereals and cereal products with the exception of maize by-products 8000 maize by-products 12,000 complementary and complete feedingstuffs: all animal species with the exception of (2) 5000 (2) complementary and complete feedingstuffs for pigs 900 (2) complementary and complete feedingstuffs for calves (<4 months), lambs and kids 2000 ZEN unprocessed cereals other than maize 100 unprocessed maize with the exception of unprocessed maize intended to be processed by wet milling 350 cereals intended for direct human consumption, cereal flour, bran and germ as end product marketed for direct human consumption, with the exception of foodstuffs listed in (2) 75 refined maize oil 400 bread (including small bakery wares), pastries, biscuits, cereal snacks and breakfast cereals, excluding maize snacks and maize-based breakfast cereals 50 (2) maize intended for direct human consumption, maize-based snacks and maize-bases breakfast cereals 100 (2) processed cereal-based foods (excluding processed maize-based foods) and baby foods for infants and young children 20 (2) processed maize-based foods for infants and young children 20 feed materials: cereals and cereal products with the exception of maize by-products 2000 maize by-products 3000 complementary and complete feedingstuffs: complementary and complete feedingstuffs for piglets and gilts (young sows) 100 complementary and complete feedingstuffs for sows and fattening pigs complementary and complete feedingstuffs for calves, dairy cattle, sheep (including lamb) and goats (including kids) 250 500 unprocessed maize with the exception of unprocessed maize intended to be processed by wet milling 4000 maize intended for direct human consumption, maize-based foods for direct human consumption, with the exception of foodstuffs listed in (3) 1000 (3) maize-based breakfast cereals and maize-based snacks 800 (3) processed maize-based foods and baby foods for infants and young children 200 feed materials: maize and maize products 60,000 complementary and complete feedingstuffs: complementary and complete feedingstuffs for pigs, horses (Equidae), rabbits and pet animals 5000 complementary and complete feedingstuffs for fish 10,000 complementary and complete feedingstuffs for poultry, calves (<4 months), lambs and kids 20,000 complementary and complete feedingstuffs for adult ruminants (>4 months) and mink 50,000 Sum T-2 and HT-2