Effects of parasites coinfection with other pathogens on animal host: A literature review

A parasite-host relationship is complicated and largely remained poorly understood, especially when mixed infections involving pathogenic bacteria and viruses are present in the same host. It has been found that most parasites are able to manipulate the host’s immune responses to evade or overcome its defense systems. Several mechanisms have been postulated that may explain this phenomenon in different animal species. Recent evidence suggests that coinfections involving many parasitic species alter the host’s vulnerability to other microorganisms, hinder diagnostic accuracy, and may negatively impact vaccination by altering the host’s immune responsiveness. The objective of this review was to provide a comprehensive summary of the current understanding of how parasites interact with other pathogens in different animal species. A better understanding of this complex relationship will aid in the improvement efforts of disease diagnosis, treatment, and control measures such as novel and effective vaccines and therapeutics for infectious diseases.


Introduction
Parasites possess many mechanisms by which they manipulate or evade the immune response to establish infection [1]. Most studies regarding host-parasite relationships have been conducted in laboratory animals, primarily rodents, under a controlled environment [1]. Infection with multiple pathogens within the same host is common, as different infectious agents are obtained through similar routes of exposure (inhalation, oral, ingestion of contaminated feed, and water) [2]. The coinfecting agents interact with one another; their interaction may include direct and resource competition and immune-mediated interactions [3]. Each interaction can alter the dynamics of every single pathogen [4]. In the event of a coinfection, the way the two or more infectious agents interact can lead to different outcomes, including inhibition, activation, or competition between the coinfecting agents [5].
Coinfection between pathogens can affect susceptibility to other pathogens and, therefore, can alter host immune responsiveness [1]. In addition, coinfection can hinder accurate diagnosis and influence disease pathology development [6][7][8][9][10]. A previous study regarding parasites coinfection with other pathogens showed an apparent influence on disease susceptibility [10].
Therefore, the objective of this review was to summarize the complex relationships between parasites in a single host in different animal species.

Literature Search
Data search was conducted using popular web search engines such as PubMed, Research Gate, and Google Scholar to extract all published papers related to the effects of parasites coinfection with other pathogens on host immunity in different animal species. Only published papers in scientific and refereed journals between the years 2000 and 2021 were considered. In total, 28 research papers published between 2000 and 2021 fit the inclusion criteria and were included in this analysis (Table-1) .

Mechanisms of Parasites-pathogens Interactions within a Single Host (Coinfection)
The net result of multiple infecting pathogens within a single host could be characterized as synergistic (intensifying adverse effects of individual pathogens) or antagonistic (lessening of adverse effects of individual pathogens) [39]. The potential mechanisms of this interaction remain speculative and many theories have been suggested to explain this complex relationship. Among the most widely accepted theories are the community ecology theory (competitive interactions between pathogens sharing similar resources or     location in a host) and pathogen-host immune modification theory, where a pathogen modifies the host immune system and a new infecting parasite or pathogen finds an altered immunoenvironment created in response to previous or current infections [39]. The antagonistic effect of coinfecting pathogens has been suggested to occur when one pathogen inhibits the invasion, development, or reproduction of another pathogen within the host [40]. Within this context, certain species of nematodes and bacteria could block the transmission of some trematodes by biologically controlling intermediate hosts and relieving symptoms by suppressing infections by other pathogens, including viruses, bacteria, and parasites [40]. Parasites, especially helminths, are known to produce a robust Th2-type response which can downregulate the effects of a secondary T-helper (Th) 1-dependent parasitic challenge [40]. This typically results in a rapid proliferation of secondary pathogens due to T-cell inhibition. Furthermore, severe immunopathological damage could ensue due to uncontrollable inflammatory reactions [40].
On the other hand, strong clinical and scientific evidence exists that suggest that certain parasite infections may protect the host by reducing the density of pathogens and mitigating immunopathological injury [40]. It was found that infection with certain species of parasites may dampen or inhibit the maturation of dendritic cells (DC) and promote the propagation of regulatory T (Treg) cells [40]. Several studies have indicated that DCs induce an adaptive immune response in the early phases of parasite infections [40]. In addition, some parasites produce toxic compounds that directly eliminate or hinder the development of pathogenic competitors within the host [40]. Interference competition can also occur when certain cestodes produce some substances termed crowding factors [40]. This results when end metabolites accumulate in the microenvironment of the host, acting as a growth inhibitor of competing pathogens [40].

Examples of Parasite-pathogen Coinfection Interactions Bovine tuberculosis (TB) and Fasciola hepatica coinfection
Bovine TB, caused by Mycobacterium bovis, is a zoonotic disease and is considered one of the most important diseases of cattle throughout the world. At present, diagnosis of TB in cattle is based on the single intradermal comparative cervical tuberculin test and the in vitro interferon-gamma (IFN-γ) assay [41].
In England and Wales, an epidemiological study was conducted on bovine dairy herds [12]. It was found that as the incidence of F. hepatica increased, more animals were detected as TB negative reactors [11]. Similar findings were obtained from a naturally concurrent F. gigantica infection with bovine TB in African cattle [42]. Meanwhile, experiments performed in bovine herds confirmed that bovine TB coinfection with F. hepatica affects the sensitivity of bovine TB tests by reducing the host Th1 immune responses against bovine TB [43]. In a recent review, it was concluded that liver fluke infection may affect the diagnosis of bovine TB [44].

Bovine TB and gastrointestinal nematodes
There are many studies that addressed the coinfection relationship between gastrointestinal nematodes and bovine TB [1,45,46]. A compilation of experimental, cross-sectional, and modeling approaches has been utilized to study the nature of this coinfection relationship. Most of these studies reported that nematode infection decreases host's Th1 response which may result in an increased parasite susceptibility [13,47]. It was also found that animals infected with TB showed less severe coinfection with nematodes. This relationship was correlated with weakened Th1 responses against TB antigen.
Available at www.veterinaryworld.org/Vol.15/October-2022/7.pdf These results clearly indicate that TB-infected animals may express stronger anti-nematode defenses associated with lessened control over the concurrent TB infection [47]. It was also concluded that TB coinfected animals may develop a rapid onset, leading to increased infectiousness potential [13]. A study conducted in Portugal explores the effects of coinfection of wild boars with TB and Metastrongylus spp. (Swine lungworm), a direct and strong correlation was found between the severity of TB and Metastrongylus spp. coinfection [13].

Human TB
Mycobacterium tuberculosis is the causative agent for human TB, a chronic disease affecting onethird of the human population worldwide [48,49]. The bacterium mainly targets the lung with most of the infected individuals are asymptomatic with latent disease. It was noted that TB coinfected individuals with parasites often had more severe disease [48,49]. The previous studies [48,49] of TB coinfection with parasitic diseases in humans have shown significant modulation of host's immune responses in which coinfected people suffered more severe symptoms of TB. It has been shown that coinfection with leishmaniasis, lepromatous leprosy, and TB downregulated the Th1 cell response significantly [49]. Coinfection with TB and malaria was also reported to decrease or suppress immune responses against TB, resulting in a more protracted and chronic course of TB in infected people [49]. These results may suggest a competitive antagonist effect TB and malaria parasites. Furthermore, coinfection between TB and echinococcosis was reported to increase the chronicity of TB infection in which the immune responses were changed from a Th1 to Th2 response [49].

Trematodes and Pregnancy Toxemia in Sheep
In the advanced stages of pregnancy in sheep, pregnancy toxemia occurs due to abnormal carbohydrate metabolism. Pregnancy toxemia is characterized by increased blood ketone bodies. One of these ketone bodies (B-hydroxybutyrate) is used as an indicator that can assess the development of pregnancy toxemia in sheep. It was reported that increased hyperketonemia with such a ketone body was present in trematode-infected ewes more than in their non-infected ewes [14]. Mavrogianni et al. [14] hypothesized that B-hydroxybutyrate could modulate the immune response by which the ketone bodies negatively impact the immunity in the udder. Therefore, the udder would be at risk of infection. The same authors postulated that the use of anthelmintic drugs could decrease the incidence of udder infection in sheep [14].

Blood Flukes (Schistosomiasis)
Schistosomes or blood flukes in humans and animals could cause chronic inflammation [1]. Similar to other parasites, schistosomes induce a Th2 immune response, which allows them to induce a long-lasting infection with the host [1]. It was demonstrated that gastrointestinal nematode infections in mice could have an impact on the pathogenesis of schistosome infection in the host [15]. Mice with an established Trichuris muris infection had enhanced survival and migration of Schistosome mansoni infection [15]. In another mice study, it was shown that differences in the coinfecting species can have variable effects on schistosome infections [16]. Heligmosomoides polygyrus specifically induces infection in the duodenum of murine species. It was reported that coinfection of mice with Schistosome spp. and H. polygyrus infections ameliorated the hepatic pathology induced by schistosomes' eggs [16]. The phenomenon seemed to correlate with the decreased expression of pro-inflammatory cytokines [16]. Therefore, the above-mentioned studies demonstrated that gastrointestinal helminths may impact schistosomiasis. However, these data do not predict whether they increase or decrease disease pathogenesis. Some experimental work suggested that the large intestinal mucosa allowed the gastrointestinal helminth to initiate chronic infections [50,51]. This phenomenon can increase the host's susceptibility to other infections with helminth parasites [46]. It was reported that African buffalo positive for Cooperia fuelleborni nematode exhibited increased Schistosoma mattheei burden than those animals negative for the nematode species [46]. Similar results were seen in cattle infected with Cooperia oncophora by which the infection enhanced their susceptibility to lungworms (Dictyocaulus viviparus) infection [51]. Furthermore, it was shown that calves infected with Ostertagia ostertagi expressed an increased susceptibility to lungworm infestation [51]. The mechanisms that mediate these effects in coinfected animals are still not understood and could just represent variability in different gastrointestinal helminthic infections [1].
An experimental study was done in Rhesus macaques to examine if helminth parasite coinfection would create higher viremia loads of simian-human immunodeficiency virus (SHIV) clade C [17]. In that study, a group of monkeys was coinfected with S. mansoni cercariae and SHIV. By week 5 after infection, coinfected monkeys were reported to significantly shed more eggs in their feces, had eosinophilia, and increased mRNA expression of Th type 2 cytokine ([Il]-4) than monkeys without schistosomiasis infection [17]. Coupled with that, viral replication was profoundly increased in coinfected monkeys compared to controls. Therefore, S. mansoni coinfection resulted in increased viral replication and induced T-cell subset alternations in monkeys with chronic SHIV clade C infection [17].

Trypanosomes
In an experimental study, it was reported that the severity of malaria was increased when mice were coinfected with Plasmodium berghei and Trypanosoma brucei [18]. The mice with the coinfection had lower survival rates, greater parasitemia loads, and severe anemia. The authors were uncertain about the mechanism of such effects on the increased expression of IFN-γ in the coinfected mice [18]. In addition, trypanosome infection was also found to ameliorate the vulnerability of the host to bacterial infections [19]. In a study conducted in chronically infected mice with either Brucella melitensis, B. abortus, or B. suis, it was noted that bacterial loads in the spleen were decreased when they coinfected with T. brucei [19]. The authors demonstrated that coinfection with T. brucei increased the immune control of chronic Brucella infection and eliminated infection by enhancing CD4+ T cells dependent Th1 immune response [19]. On the other hand, T. brucei infection failed to induce similar effects as those seen in Brucella spp. infections study in mice when coinfected with Mycobacterium tuberculosis [1,19]. In another study conducted in mice, it was found that there was a significant alteration in the mechanism of disease as well as the susceptibility of mice to T. cruzi infection that was previously infected with S. mansoni [20]. The mice were unable to control T. cruzi infection, and there was tremendous inflammation in their livers due to increased parasitemia [20]. T. cruzi infection was found to be associated with Th1 immune response and the production of nitrous oxide (NO) which is needed to clear the parasite [20]. Therefore, increased parasitemia and its associated inflammation were seen due to reduced production of IFN-γ and NO.

Eimeria spp. and Toxoplasma gondii Coinfection
Eimeria spp. is a genus of apicomplexan parasites that lead to the disease coccidiosis in a variety of animal species, including cats, dogs, poultry, and ruminants. Eimeria tenella is a notable pathogen that causes intestinal coccidiosis in chickens and causes high morbidity and mortality. Toxoplasmosis is caused by T. gondii infection. The disease is an important cause of reproductive failure and abortion in animals with significant zoonotic potential. In normal animals, toxoplasmosis is frequently subclinical and chronic in immunocompetent individuals. In poultry, it is common to have E. tenella and T. gondii coinfection [21]. In different experimental studies that addressed the coinfection relationship between E. tenella and T. gondii, it was noted that the immunopathology responses following coinfection had no differences from those chickens infected with Eimeria alone [1,21,22]. Moreover, the coinfection showed no effect on the burden of tissue samples with T. gondii or its clinical course [1,21,22]. Therefore, those coinfections in chickens did not show any detrimental effects on disease development or pathology [1,21,22]. On the contrary, a study conducted on wild rabbits in Scotland showed that rabbits with coinfection of T. gondii and E. stiedae had higher burdens of E. stiedae [22].

Toxoplasma gondii and H. polygyrus Coinfection
A coinfection study was performed to study the immunological relationship between the enteric nematode H. polygyrus in mice with a previous infection of T. gondii [23]. It was found that T. gondii parasitemia was not affected by concurrent helminth infection [23].

Salmonella spp.
Salmonella is a Gram-negative bacterium and is an important cause of diarrhea worldwide. Salmonella coinfection with different parasites has been shown to aggravate the disease process of salmonellosis [1]. An experimental study of mice coinfected with H. polygyrus and Salmonella demonstrated that H. polygyrus infection had enhanced the pathogenesis of Salmonella enterica serovar Typhimurium [24]. It was shown that infection with H. polygyrus can disrupt metabolic profile in the small intestine, thereby affecting the invasiveness of Salmonella Typhimurium [24]. The parasite appeared to enhance the expression of Salmonella pathogenicity island 1 genes [24]. The study provided awareness of how parasites could modulate the susceptibility of the host to other bacterial infections [24].

Bordetella bronchiseptica and H. polygyrus Coinfection
In an experimental study conducted to study the impact of coinfection of H. polygyrus and B. bronchiseptica in laboratory mice, it was found that coinfection led to the presence of higher bacterial loads early in the infection (first 5 days) that could lead to host mortality [25]. Coupled with that, coinfection led to the development of "super-shedders," which are individuals that will shed helminth eggs chronically with larger than average amounts [25]. Lass et al. [25] proposed that coinfection can be an underlying factor in how parasites can have inconsistency in their parasitic load and shedding rates, and this should be taken into consideration for the control and management of the disease [25].

Pneumonia and Gastrointestinal Parasitism
A study was done on goats in Nigeria to investigate if there was any relationship between occurrence of pneumonia and gastrointestinal parasitism [26]. Based on the data obtained in the study, a significant correlation was present between the presence of gastrointestinal parasitism and granulomatous pneumonia. The authors suggested that gastrointestinal parasites could have decreased the immunity in the lung, thereby allowing other pathogens to invade the lungs resulting in pneumonia [26].

Viruses and Parasites Coinfection
There are some studies exploring the coinfection relationship between parasites and viruses [27,28]. In those scenarios, a Th2 immune response from the parasites impeded the development of Th1 immune response from the viral pathogen, thereby hindering effective antiviral immunity. An experimental study revealed that mice coinfected with Trichinella spiralis or H. polygyrus and with Mouse norovirus had increased viral loads and reduced amounts of specific CD4+ T cells expressing IFN-γ and TNF-α when compared to the mice infected with Norovirus alone [27]. Another experimental study showed that mice infected with H. polygyrus or S. mansoni could encourage the reactivation of latent γ-herpesvirus infection [27].
Another experimental study demonstrated that coinfection of T. spiralis led to reduced levels of human necrosis factor in bronchoalveolar lavage fluid and inhibited cellular recruitment into the airways of mice coinfected with influence A virus [28]. The authors stated that T. spiralis ameliorated the pulmonary pathology, causing less CD4+ and CD8+ lymphocytes and neutrophils recruitment and suppressed the virus from the increasing effect of vascular permeability in pulmonary tissue [28].

Scrapie and Teladorsagia circumcincta Coinfection
In an experimental study conducted to explore the impact of abomasal parasite, T. circumcincta infection, on the host susceptibility to scrapie using sheep naturally exposed to scrapie, chosen by their genotype at the PrP gene, it was found that the onset of prion disease was shortened in coinfected sheep [29].

Escherichia coli (O157)
Escherichia coli O157 is an important zoonotic bacterium of worldwide public health significance. It causes hemorrhagic diarrhea in infected humans. The main reservoir for human infection is cattle, and the animals are generally asymptomatic. A study was performed in 2018; on 14 British farms showed that coinfection with the liver fluke (F. hepatica) might increase the risk of E. coli (O157) shedding [30]. The study proposed that controlling F. hepatica in cattle could have a beneficial effect by lowering the shedding of E. coli (O157) [1,30].

Feline Leukemia Virus (FeLV) and Feline Immunodeficiency Virus (FIV) Coinfection with Mycoplasma haemofelis Formerly (Haemobartonella felis)
Feline infectious anemia is caused by an epicellular bacterial parasite of the erythrocytes of cats that can cause hemolytic anemia. The causative agent is M. haemofelis (formerly H. felis). A study was conducted to explore the effects of a pre-existing FeLV infection or FIV on the pathogenicity of M. haemofelis. The results of the experimental study showed that cats coinfected with FeLV and M. haemofelis developed more critical anemia than cats infected with M. haemofelis alone. It was, however, unclear how the two infectious agents acted together to produce a more severe disease [11].

Coinfections in Fish
Many experimental studies displayed the synergistic relationship between various parasitic spp. and bacterial infections in fish. Parasitic infections are thought to increase to decrease the disease resistance of fish to other secondary bacterial infections. In addition, certain parasites may harbor pathogenic bacteria that infect the host while the parasites are feeding [5]. Mixed infections are common in fish, leading to increased morbidity and mortality. On the other hand, most studies focused on the dynamics of a single pathogen. A coinfection experimental study was done on Nile tilapia [31]. The fish in the study were infected with Gyrodactylus niloticus (monogenean helminthic ectoparasite) and then were later infected with Streptococcus iniae bacteria. The study demonstrated that increased mortality rates were observed in coinfected group (42.2%) when compared to fish infected with S. iniae only group (6.7%). No deaths were reported in the fish only infected with G. niloticus group. A study conducted by Xu et al. [31] proposed the ectoparasite facilitated the entry of pathogenic bacteria through mechanical damage to fish epithelium [31]. Xu et al. [32] conducted an experimental model of coinfection in N. tilapia. Nile tilapia had a coinfection of I. multifiliis and S. iniae, and the coinfection relationship had a negative impact on developmental size. There was an increase in mortality and parasite loads. When the time between exposures was increased, the coinfection permitted extra time for I. multifiliis to produce large, well-developed trophonts that allowed for more damage to the epithelium of fish and thus more bacterial invasion.
Another study conducted on Goldfish (Carassius auratus) demonstrated that goldfish with Dactylogyrus intermedius infection demonstrated increased susceptibility to Flavobacterium columnare bacteria [33]. The coinfection interaction led to higher mortality rates, and higher bacterial loads compared to fish infected with F. columnare alone. The parasite monogenean D. intermedius resulted in enhanced bacterial invasion after induction of host immune suppression. There was downregulation of immune genes such as TGF-B and complement 3 in gills and kidneys. Thus, D. intermedius modulates host's immune response [33].
Shoemaker et al. [34] performed an experimental study to investigate the effects of Ichthyophthirius multifiliis (ciliated ectoparasite fish protozoan) parasitism on survival and bacterial burden of channel catfish exposed 1 day before to Edwardsiella ictaluri (pathogen causes enteric septicemia of catfish). The coinfected group of catfish had increased mortality rates and increased bacterial loads in different organs (71.1%) when compared to single infection groups [34]. Xu et al. [35] executed another experiment using I. multifiliis and E. ictaluri in channel catfish. First, the fish were infected with I. multifiliis, and then, the fish were challenged 5 days later with E. ictaluri. There were increased mortality and higher bacterial burden in internal organs, and the results were similar to the previous studies [5,36]. High mortality rates were observed in N. tilapia in the 2009 summer season in Egypt. The significant high mortality rates were the result of F. columnare and myxosporean parasite, Myxobolus tilapiae coinfection. Damage to the fish skin may have allowed for more F. columnare invasion [37].
A study was done on rainbow trout to investigate if coinfection of Myxobolus cerebralis and Yersinia ruckeri resulted in higher mortality rates [38]. Rainbow trout in the study was chronically infected with M. cerebralis and 12 months post-exposure were challenged with Y. ruckeri. The coinfected rainbow trout had higher mortality rates and the onset of death was faster than in rainbow trout without M. cerebralis infection. Authors in the study suggested that M. cerebralis has immunomodulatory effects such as suppression of lymphocytes to four mitogens. This, therefore, allowed for a more destructive invasion by secondary infection with Y. ruckeri [38].

Conclusion
It is common to have two or more pathogens coexisting in a single host. The host-pathogen relationship is complex and remains largely unexplored. This review highlighted the impact of parasites on the susceptibility or disease pathogenesis of coinfecting agents in different hosts. Some studies demonstrated that a parasitic coinfection exacerbated disease and led to rapid or more severe onset of disease, while other studies showed that coinfection status could lead to increased shedding of infectious agents. In addition, some studies showed that coinfection could alter the sensitivity of some diagnostic tests for a particular disease. A few studies determined that a coinfection of bacterial pathogen and parasite could lead to the reduction of parasite egg-induced pathology, and the phenomena correlated with the expression of pro-inflammatory cytokines responsible for induction of egg-induced immunopathology. A better understanding of how infectious disease pathogenesis can be influenced by concurrent parasite infections will improve or revolutionize existing diagnostic and control strategies for infectious diseases. However, further studies are still required to explore the nature and immunological interactions between coinfecting agents in a single host.