Immunology of fungal infections in animals

Experimental Models to Study the Pathogenesis and Treatment of Mucormycosis

Mucormycosis presents a formidable challenge to clinicians and researchers. Animal models are an essential part of the effort to decipher the pathogenesis of mucormycosis and to develop novel pharmacotherapeutics against it. Diverse model systems have been established, using a range of animal hosts, immune and metabolic perturbations, and infection routes. An understanding of the characteristics, strengths, and drawbacks of these models is needed to optimize their use for specific research aims.

Cryptococcus Infection in Captive Callitrichids in the United Kingdom

Cryptococcosis is widely reported in North America and Australia but considered rare in the UK. Disease is mainly caused by Cryptococcus neoformans and Cryptococcus gattii and affects animals and humans. Cases of cryptococcosis have been described in non-human primates, including a few in the Callitrichidae family. Six captive callitrichids from five zoological facilities in the UK were diagnosed with cryptococcosis on post-mortem examination over a 13-year period. Histological examination revealed intralesional yeast bodies consistent with Cryptococcus spp in lung, brain, lymph nodes, larynx, salivary gland and ears. C. neoformans was confirmed in two cases by culture and by culture and polymerase chain reaction (28S rRNA gene), respectively. This case series is the first report of Cryptococcus spp in captive callitrichids in the UK and reinforces the need to include this fungal disease in the differential diagnosis of sick callitrichids in zoo settings.

Ketoconazole-conjugated ZnO nanoparticles based semi-solid formulation and study their impacts on skin disease

In this study, the ketoconazole-conjugated zinc oxide (ZnO) nanoparticles were prepared in a single-step approach using dextrose as an intermediate compound. The physical parameters confirmed the drug conjugation with ZnO and their size was around 70-75 nm. The drug loading and in vivo drug release studies indicated that the -CHO group from the dextrose increase the drug loading up to 65% and their release kinetics were also studied. The anti-fungal studies indicated that the prepared nanoparticles exhibit strong anti-fungal activity and the minimum concentration needed is 10 mg/ml. The nanoparticles loaded semi-solid gel was prepared using carbopol, methylparaben, propyl paraben and propylene glycol. The in vitro penetration of the ketoconazole-conjugated nanoparticles was studied using the skin. The results indicated that the semi-solid gel preparations influenced the penetration and also favoured the accumulation into the skin membrane. The veterinary clinical studies indicated that the prepared gel is highly suitable for treatment of Malassezia.

Specific alterations in complement protein activity of little brown myotis (Myotis lucifugus) hibernating in white-nose syndrome affected sites

White-nose syndrome (WNS) is the most devastating condition ever reported for hibernating bats, causing widespread mortality in the northeastern United States. The syndrome is characterized by cutaneous lesions caused by a recently identified psychrophilic and keratinophylic fungus (Geomyces destructans), depleted fat reserves, atypical behavior, and damage to wings; however, the proximate cause of mortality is still uncertain. To assess relative levels of immunocompetence in bats hibernating in WNS-affected sites compared with levels in unaffected bats, we describe blood plasma complement protein activity in hibernating little brown myotis (Myotis lucifugus) based on microbicidal competence assays using Escherichia coli, Staphylococcus aureus and Candida albicans. Blood plasma from bats collected during mid-hibernation at WNS-affected sites had higher bactericidal ability against E. coli and S. aureus, but lower fungicidal ability against C. albicans when compared with blood plasma from bats collected at unaffected sites. Within affected sites during mid-hibernation, we observed no difference in microbicidal ability between bats displaying obvious fungal infections compared to those without. Bactericidal ability against E. coli decreased significantly as hibernation progressed in bats collected from an affected site. Bactericidal ability against E. coli and fungicidal ability against C. albicans were positively correlated with body mass index (BMI) during late hibernation. We also compared complement activity against the three microbes within individuals and found that the ability of blood plasma from hibernating M. lucifugus to lyse microbial cells differed as follows: E. coli>S. aureus>C. albicans. Overall, bats affected by WNS experience both relatively elevated and reduced innate immune responses depending on the microbe tested, although the cause of observed immunological changes remains unknown. Additionally, considerable trade-offs may exist between energy conservation and immunological responses. Relationships between immune activity and torpor, including associated energy expenditure, are likely critical components in the development of WNS.

Anti-fungal cell wall beta-glucan antibody in animal sera

beta-Glucan is a major component of the cell walls and pathogen-associated microbial patterns of fungi. We previously reported the presence of an antibody which reacts to beta-glucan, anti-beta-glucan (BG) antibody, in human sera. In livestock and domestic pets, the antibody's response to fungal cell wall beta-glucan is little understood. In this study, we examined the existence and reactivity of anti-BG antibody in various animal species. We demonstrated the presence of the anti-BG antibody in each animal's serum. Individual differences in the titer existed. The antibody was highly reactive to Candida solubilized cell wall beta-glucan (CSBG) while reacting little to grifolan (GRN) from Grifola frondosa. This suggested that the anti-BG antibody interacted with fungal cell wall beta-glucan and participated in the immune-response to pathogenic fungi.

Embryonated eggs as an alternative infection model to investigate Aspergillus fumigatus virulence

Infection models are essential tools for studying microbial pathogenesis. Murine models are considered the "gold standard" for studying in vivo infections caused by Aspergillus species, such as A. fumigatus. Recently developed molecular protocols allow rapid construction of high numbers of fungal deletion mutants, and alternative infection models based on cell culture or invertebrates are widely used for screening such mutants to reduce the number of rodents in animal experiments. To bridge the gap between invertebrate models and mice, we have developed an alternative, low-cost, and easy-to-use infection model for Aspergillus species based on embryonated eggs. The outcome of infections in the egg model is dose and age dependent and highly reproducible. We show that the age of the embryos affects the susceptibility to A. fumigatus and that increased resistance coincides with altered chemokine production after infection. The progress of disease in the model can be monitored by using egg survival and histology. Based on pathological analyses, we hypothesize that invasion of embryonic membranes and blood vessels leads to embryonic death. Defined deletion mutant strains previously shown to be fully virulent or partially or strongly attenuated in a mouse model of bronchopulmonary aspergillosis showed comparable degrees of attenuation in the egg model. Addition of nutrients restored the reduced virulence of a mutant lacking a biosynthetic gene, and variations of the infectious route can be used to further analyze the role of distinct genes in our model. Our results suggest that embryonated eggs can be a very useful alternative infection model to study A. fumigatus virulence and pathogenicity.

Immune response to fungal infections

The immune mechanisms of defence against fungal infections are numerous, and range from protective mechanisms that were present early in evolution (innate immunity) to sophisticated adaptive mechanisms that are induced specifically during infection and disease (adaptive immunity). The first-line innate mechanism is the presence of physical barriers in the form of skin and mucous membranes, which is complemented by cell membranes, cellular receptors and humoral factors. There has been a debate about the relative contribution of humoral and cellular immunity to host defence against fungal infections. For a long time it was considered that cell-mediated immunity (CMI) was important, but humoral immunity had little or no role. However, it is accepted now that CMI is the main mechanism of defence, but that certain types of antibody response are protective. In general, Th1-type CMI is required for clearance of a fungal infection, while Th2 immunity usually results in susceptibility to infection. Aspergillosis, which is a disease caused by the fungus Aspergillus, has been the subject of many studies, including details of the immune response. Attempts to relate aspergillosis to some form of immunosuppression in animals, as is the case with humans, have not been successful to date. The defence against Aspergillus is based on recognition of the pathogen, a rapidly deployed and highly effective innate effector phase, and a delayed but robust adaptive effector phase. Candida albicans, part of the normal microbial flora associated with mucous surfaces, can be present as congenital candidiasis or as acquired defects of cell-mediated immunity. Resistance to this yeast is associated with Th1 CMI, whereas Th2 immunity is associated with susceptibility to systemic infection. Dermatophytes produce skin alterations in humans and other animals, and the essential role of the CMI response is to destroy the fungi and produce an immunoprotective status against re-infection. The resolution of the disease is associated with a delayed hypersensitive response. There are many effective veterinary vaccines against dermatophytoses. Malassezia pachydermatis is an opportunistic yeast that needs predisposing factors to cause disease, often related to an atopic status in the animal. Two species can be differentiated within the genus Cryptococcus with immunologic consequences: C. neoformans infects predominantly immunocompromised hosts, and C. gattii infects non-immunocompromised hosts. Pneumocystis is a fungus that infects only immunosupressed individuals, inducing a host defence mechanism similar to that induced by other fungal pathogens, such as Aspergillus.

Disseminate Cryptococcosis in a guenon (Cercopithecus ascanius)

A 23-year-old captive-bred red-tailed guenon (Cercopithecus ascanius) with a brief history of inappetence, lethargy, and seizures was submitted for necropsy. On postmortem examination, multiple cryptococcomas were identified in brain and heart. Cryptococcus neoformans organisms were also identified microscopically in kidney, eye, and pancreas. Fungal yeast formed rare pseudohyphae. The histologic diagnosis of cryptococcosis was confirmed by a positive test for C. neoformans antigen in a serum sample. Immunohistochemical staining confirmed that macrophages were the principal inflammatory cell in brain lesions and often contained phagocytosed yeast. As disseminate cryptococcosis is often associated with immune suppression, serology and immunohistochemical staining for simian immunodeficiency virus were performed but showed no evidence of SIV infection.

Distribution ofMalasseziaorganisms on the skin of unaffected psittacine birds and psittacine birds with feather-destructive behavior

OBJECTIVE

To ascertain whether Malassezia organisms can be detected via cytologic examination and fungal culture of samples from the skin surface of psittacine birds and determine whether the number of those organisms differs between unaffected psittacines and those that have chronic feather-destructive behavior or differs by body region.

DESIGN

Prospective study.

ANIMALS

50 unaffected psittacines and 53 psittacines that had feather-destructive behavior.

PROCEDURE

Samples were collected by use of acetate tape strips from the skin of the head, neck, proventer, propatagium, inguinal region, and preen gland area of each bird; 0.5-cm(2) sample areas were examined microscopically for yeast, and samples were also incubated on Sabouraud dextrose agar. Polymerase chain reaction assays specific for Malassezia spp, saprophytic fungi, and Candida albicans were performed on DNA prepared from cultured colonies; nested PCR evaluation for Malassezia pachydermatis was then performed.

RESULTS

Microscopically, 63 of 618 (10%) tape-strip samples contained yeast. Thirty cultured colonies were assessed via PCR assays, and all yielded negative results for Malassezia spp; C albicans was identified in 2 colony samples. The numbers of yeast identified microscopically in psittacines with feather-destructive behavior and in unaffected birds did not differ significantly, and numbers did not differ by body region.

CONCLUSIONS AND CLINICAL RELEVANCE

Yeast were identified infrequently via cytologic examination of samples from the skin surface of unaffected psittacine birds or those that had chronic feather-destructive behavior. If yeast are identified on the skin of birds with feather-destructive behaviors, fungal culture of skin samples should be performed to identify the organism.

West Nile virus outbreak in North American owls, Ontario, 2002

From July to September 2002, an outbreak of West Nile virus (WNV) caused a high number of deaths in captive owls at the Owl Foundation, Vineland, Ontario, Canada. Peak death rates occurred in mid-August, and the epidemiologic curve resembled that of corvids in the surrounding Niagara region. The outbreak occurred in the midst of a louse fly (Icosta americana, family Hippoboscidae) infestation. Of the flies tested, 16 (88.9 %) of 18 contained WNV RNA. Species with northern native breeding range and birds >1 year of age were at significantly higher risk for WNV-related deaths. Species with northern native breeding range and of medium-to-large body size were at significantly higher risk for exposure to WNV. Taxonomic relations (at the subfamily level) did not significantly affect exposure to WNV or WNV-related deaths. Northern native breeding range and medium-to-large body size were associated with earlier death within the outbreak period. Of the survivors, 69 (75.8 %) of 91 were seropositive for WNV.

Fungal diseases of columbiformes and anseriformes

Because of the high incidence of aspergillosis, fungal disease is an important condition in the waterfowl. Although this is generally a disease of individual birds, epizootics have been reported when overwhelming spore loads are present. By contrast, the occurrence of fungal disease is quite sporadic in the pigeon. In both groups of birds, however, the zoonotic potential of fungal diseases such as cryptococcosis, histoplasmosis, and blastomycosis is clinically significant. The abundant creatinine levels in droppings serve as a nitrogen source for these organisms. This allows their numbers to increase dramatically, which in turn increases the risk of disease transmission. Because pigeons often live in close proximity to people, their role in disease transmission is considered particularly important.

Effect of macrophages and serum of fish susceptible or resistant to epizootic ulcerative syndrome (EUS) on the EUS pathogen, Aphanomyces invadans

Epizootic ulcerative syndrome (EUS) is one of the most destructive diseases of fresh and brackish water farmed and wild fish in the Asia-Pacific region. The in vitro germination and growth of the propagules of the EUS pathogen, Aphanomyces invadans (= A. piscicida), were assessed in the presence of the head-kidney macrophages, serum, and serum heated to inactivate complement proteins, of three EUS-susceptible and one resistant fish species. The susceptible species were: striped snakehead (Channa striata), giant gourami (Osphronemus gouramy) and silver barb [Barbodes (= Puntius) gonionotus], and the resistant species was Nile tilapia (Oreochromis niloticus). Fish of all species were acclimatised to either low temperature (20 degrees C +/- 1.6) at which EUS is known to occur, or to high temperature (32 degrees C +/- 5.0) at which EUS does not occur, except for giant gouramis which were only studied at low temperature. The respiratory burst of the macrophages was assessed in the presence of A. invadans or the stimulant phorbol myristate acetate (PMA), and compared to that of controls. Anti-A. invadans antibody concentrations were assessed in all species except silver barbs. All assays were carried out at the same temperature, regardless of the temperature that the fish were kept at. Macrophages of all species other than snakeheads inhibited fungal germination at both temperatures, though only silver barb and gourami macrophages could inhibit germling growth. PMA increased the respiratory burst in nearly all cases. The respiratory burst in the presence of A. invadans was consistently lower than that of controls, though the difference was only significant in the case of snakeheads. The respiratory burst of all macrophage treatments was higher at a low temperature. Except in the case of PMA-stimulated macrophages, regressions between respiratory burst and inhibitory action were only found in susceptible species, suggesting that the respiratory burst is important in those species, but is unable to prevent the proliferation of A. invadans. Serum inhibited fungal germination in all cases other than low temperature tilapia, indicating that the EUS resistance of tilapia is not due to the serum. Inhibition of germling growth by serum only occurred in silver barbs and gourami. Heated serum did not inhibit germination in any case except that of high temperature snakehead, and in fact stimulated germination in the case of tilapia. Heating serum did not affect the growth inhibiting activity of silver barbs and gouramis, but it stimulated growth in some groups. At high temperatures snakeheads had high anti-A. invadans antibody concentrations, which may explain the inhibitory activity of their heated serum. A role for complement and antibodies in defence against A. invadans in susceptible species is suggested.

Lactoferrin peptide increases the survival of Candida albicans-inoculated mice by upregulating neutrophil and macrophage functions, especially in combination with amphotericin B and granulocyte-macrophage colony-stimulating factor

To develop a new strategy to control candidiasis, we examined in vivo the anticandidal effects of a synthetic lactoferrin peptide, FKCRRWQWRM (peptide 2) and the peptide that mimics it, FKARRWQWRM (peptide 2'). Although all mice that underwent intraperitoneal injection of 5 x 10(8) Candida cells with or without peptide 2' died within 8 or 7 days, respectively, the survival times of mice treated with 5 to 100 microg of intravenous peptide 2 per day for 5 days after the candidal inoculation were prolonged between 8.4 +/- 2.9 and 22.4 +/- 3.6 days, depending on the dose of peptide 2. The prolongation of survival by peptide 2 was also observed in mice that were infected with 1.0 x 10(9) Candida albicans cells (3.2 +/- 1.3 days in control mice versus 8.2 +/- 2.4 days in the mice injected with 10 microg of peptide 2 per day). In the high-dose inoculation, a combination of peptide 2 (10 microg/day) with amphotericin B (0.1 microg/day) and granulocyte-macrophage colony-stimulating factor (GM-CSF) (0.1 microg/day) brought prolonged survival. With a combination of these agents, 60% of the mice were alive for more than 22 days. Correspondingly, peptide 2 activated phagocytes inducing inducible NO synthase and the expression of p47(phox) and p67(phox), and peptide 2 increased phagocyte Candida-killing activities up to 1.5-fold of the control levels upregulating the generation of superoxide, lactoferrin, and defensin from neutrophils and macrophages. These findings indicated that the anticandidal effects of peptide 2 depend not only on the direct Candida cell growth-inhibitory activity, but also on the phagocytes' upregulatory activity, and that combinations of peptide 2 with GM-CSF and antifungal drugs will help in the development of new strategies for control of candidiasis.

Malassezia pachydermatis with special reference to canine skin disease

A review of the diseases caused by Malassezia pachydermatis has led to the conclusion that the yeast is an opportunistic pathogen that depends on predisposing host factors and different immune suppressive mechanisms for clinical manifestation. Until recently, the role of M. pachydermatis in seborrhoeic dermatitis and otitis externa in dogs has been largely unrecognized. The clinical manifestation, aetiology, diagnosis and treatment of disease conditions in dogs caused by M. pachydermatis are reviewed. Human Malassezia furfur infections are briefly described.

Production and function of cytokines in natural and acquired immunity to Candida albicans infection

Host resistance against infections caused by the yeast Candida albicans is mediated predominantly by polymorphonuclear leukocytes and macrophages. Antigens of Candida stimulate lymphocyte proliferation and cytokine synthesis, and in both humans and mice, these cytokines enhance the candidacidal functions of the phagocytic cells. In systemic candidiasis in mice, cytokine production has been found to be a function of the CD4+ T helper (Th) cells. The Th1 subset of these cells, characterized by the production of gamma interferon and interleukin-2, is associated with macrophage activation and enhanced resistance against reinfection, whereas the Th2 subset, which produces interleukins-4, -6, and -10, is linked to the development of chronic disease. However, other models have generated divergent data. Mucosal infection generally elicits Th1-type cytokine responses and protection from systemic challenge, and identification of cytokine mRNA present in infected tissues of mice that develop mild or severe lesions does not show pure Th1- or Th2-type responses. Furthermore, antigens of C. albicans, mannan in particular, can induce suppressor cells that modulate both specific and nonspecific cellular and humoral immune responses, and there is an emerging body of evidence that molecular mimicry may affect the efficiency of anti-Candida responses within defined genetic contexts.

Sequential pathological studies in the udder of goats intramammarily infected with Aspergillus fumigatus

Intramammary inoculation of goats with Aspergillus fumigatus spores resulted in the development of mastitis with characteristic gross and microscopic lesions. The mastitis and the lesions were restricted to the infected udder halves only and there was no dissemination of infection to other tissues of the body. The experiment was continued for 45 days. Gross changes in the infected udder were observed up to the 45th day post-infection. The lesions, in general, included variable sized abscesses in the first 15 days followed by development of varying sized greyish-white nodules in the infected udders. Microscopic changes consisted of granulomatous reaction with well developed granulomas in the infected udders. Hyphae and spores of Aspergillus fumigatus could be demonstrated in sections of the infected udders up to 45 days after infection. Reisolation of the fungus consistently was achieved up to 45 days. It is concluded that intramammary inoculation of Aspergillus fumigatus spores in goats leads to chronic granulomatous mastitis.

Sequential pathological studies in goats infected intratracheally with Aspergillus fumigatus

Intratracheal inoculation of goats with Aspergillus fumigatus spores resulted in the development of characteristic gross and microscopic lesions. The lesions were restricted to lungs and there was no dissemination of infection to other tissues of the body except liver in one goat 16 days after infection. The experiment was continued for 37 days. Gross changes in lungs were observed up to the 24th day post-infection. The lesions, in general, included congestion and oedema in the first 6 days followed by the development of varying greyish-white nodules in the lungs. Microscopic changes consisted of granulomatous reaction with well developed granulomas in lungs. Hyphae and conidiophores with fruiting bodies of Aspergillus fumigatus could be demonstrated in sections up to 24 days of infection. Reisolation of the fungus consistently was achieved up to 24 days. It is concluded that intratracheal inoculation of Aspergillus fumigatus spores in goats leads to pulmonary aspergillosis up to 24 days.

Murine subcutaneous granulomatous zygomycosis induced by Absidia corymbifera

Subcutaneous zygomycosis developed in 90% of mice killed from 1 to 9 weeks following subcutaneous inoculation of spores from Absidia corymbifera. Initially the inflammation was characterized as pyo-granulomatous, but after five to six weeks of infection the dominant cells were mononuclear, and giant cells and more regular conglomerates of granulomata were formed. Within the granulomata necroses also became more abundant with time. In animals developing mycosis fungal elements were restricted to the site of infection and were never observed in regional lymph nodes or internal organs examined by conventional and immunohistochemical staining methods. The localized extension of infection was accompanied by development of specific antibodies against somatic antigens from A. corymbifera as assessed by counterimmunoelectrophoresis.

Sequential pathological studies in Japanese quails infected experimentally with Aspergillus fumigatus

Intratracheal inoculation of young quail chicks with Aspergillus fumigatus spores resulted in the development of characteristic gross and microscopic lesions. The lesions were restricted to respiratory tract and there was no dissemination of infection to other tissues of the body. Gross changes in lungs and air sacs were observed within 24 hours and continued up to 20 days while in trachea these were noticed from the 3rd to the 9th day post-infection. The lesions, in general, included congestion and focal haemorrhages in the first 2 days followed by the development of varying-sized greyish-white nodules in the lungs, air sacs and trachea. Microscopic changes consisted of congestion, haemorrhages and a diffuse cellular infiltration in the first 2 days followed by granulomatous reaction with well developed granulomas in lungs, air sacs and trachea. Spores and developing hyphae of Aspergillus could be demonstrated in sections from 24 hours to 20 days of infection. Reisolation of the fungus was consistently achieved from the lungs, air sacs and trachea up to 14 days.

The genetics of medically important fungi

The present review is concerned with recent progress in basic genetic investigations with a variety of fungi which are pathogenic for man and animals. The principles and strategies involved in undertaking genetic investigations of sexual species and of asexual species are discussed. Progress in genetic analysis of Cryptococcus neoformans made possible by the discovery of its sexual phase is described in detail, as is progress in development of parasexual methods of analysis in Candida albicans. The genetic bases of virulence and drug resistance are discussed for those few species in which these phenotypes have been investigated. Suggestions for future research, including the application of recent advances in molecular biology to the study of pathogenic fungi, are presented.