In healthy individuals, fungi rarely cause disease; nevertheless, opportunistic and pathogenic fungi can cause severe disease when immunosuppression or other debilitating conditions affect the host (1). Aspergillosis is the most common fungal disease in birds, mostly caused by Aspergillus fumigatus and A. flavus, although the genus includes more than 300 known species (1–4). In birds, Aspergillus spp. tends to initially colonize the lower respiratory tract and the infection can develop as a focal aspergilloma or disseminate to different organs (5). Another very common fungal disease in birds is candidiasis, which is caused by fungi of the genus Candida that are frequently found in the microbiota of healthy humans and animals and, thus, considered commensal and facultative pathogens. Infections in the oral cavity and gastrointestinal system are the most common forms of candidiasis in birds (1, 6).

Cryptococcus species can also cause invasive diseases of the upper respiratory system of parrots kept in captivity (7–11). This can result in clinical signs referable to mycotic rhinitis or involve anatomic areas closely connected with the nasal cavity, like the retrobulbar space, palate, choana, sinuses, and beak (7, 8). In some instances, the infection involves bones causing osteomyelitis (9, 10).

More rarely than the previous orders of fungi, Mucorales represents a group of fungi living as saprotrophs in soil, decaying organic and dead plant material, and able to cause disease in a wide variety of vertebrates (12). Although the order Mucorales was considered problematic for the nomenclatural stability due to a satisfactory identification using mostly the microscopic morphology, a clear phylogenetic classification was recently proposed using DNA barcoding, introducing taxonomic nomenclatural changes in this group (13).

Likewise, to candidiasis and aspergillosis, a deficiency of the immune system or metabolic disorders can act as predisposing factors to Mucorales infection (1). Few reports are published on the mucormycosis in avian species, in which the respiratory and gastrointestinal tract are mainly involved (14–16). Alongside that most important fungal genus, additional groups of fungi can cause disease in humans and animals, such as the opportunistic mold Alternaria spp. The latter represents a parasite of plants (17), but it is also often involved in dermatomycosis in animals (18), resulting in cutaneous nodular lesions in horses (19). In addition, the genus Fusicladium, usually not involved in vertebrate pathology, could be an unintentional discovery in animal clinical cases. The present article describes four pathological cases in which opportunistic fungi are identified as the cause of death in psittacine birds.

This study describes unusual fungal findings in four parrots. The molecular characterization confirmed the histological description and identified two Mucor species (M. racemosus and M. circinelloides) as most likely true pathogens in the two lovebirds. The evidence of Alternaria and Fusicladium in Blue and Gold Macaw, suggests that saprophytic fungi can act as opportunistic pathogens in some instances, and are mostly linked to an immunodeficiency of the host.

Fungi are ubiquitous organisms both as saprophytes in the environment or commensal in the host (27). “Commensalism” is the state of infection that causes no damage or clinically unapparent damage to the host, though it can arouse a response of the immune system (28). Although evolution has provided both the parasites and the host's immune system with similar mechanisms of selection and defense (29), sometimes the interplay with some microorganisms is deleterious for the host and, consequently, opportunistic infections can take place (30). Several fungal genera, including Alternaria spp., Aspergillus spp., Mucor spp., and Venturia spp. were isolated from migratory birds, confirming the role of birds as a possible vector of fungi and the commensalism of fungi with their avian host (31–33).

Most species of the numerous fungi groups are saprobic in several environments (34). M. racemosus and M. circinelloides are ubiquitous fungi belonging to the phylum Zygomycota and order Mucorales. Like most fungi, they can be found in healthy birds (35), but are considered the primary pathogen of severe diseases, known as zygomycosis or mucormycosis, in case of immunodeficiency of the host (36). M. circinelloides and M. racemosus, which were identified as the cause of death in two lovebirds in this study, were isolated in different types of soils in Mexico, France, and Iran (34). Interestingly, excreta of herbivore animals, including most of the psittacine birds, are rich in digested plant material and can host a wide variety of fungal strains (34). For this reason, for example, poultry-house litter shows a severe increase in fungal presence after the completion of the birds' growth cycle (37). It is, therefore, reasonable to hypothesize that poor hygiene in the cage of captive-bred birds can cause strong contamination of fungal elements in their living environment and increase the possibility of infection. Lovebirds are traditionally kept in small cages and usually, the breeding pair fills a wooden nest with plant material, spending all the night and most of the day inside this nest (38). All the aforementioned elements promote the overgrowth of fungi. Furthermore, predisposing factors, as well as the debilitation of each individual, are fundamental for the development of infection by fungal flora. Thus, efforts should be directed to avoid stress in the aviaries or cages and strict hygiene should be encouraged (27).

In humans and animals, there are three basic exposure pathways to pathogenic fungal infection: inhalation, ingestion, and direct contact, in case of disruption of the cutaneous barrier (34). In human medicine, mucormycosis became the third most common invasive mycosis in order of importance, after candidiasis and aspergillosis (36). In birds, the most common mycosis is aspergillosis, caused by Aspergillus fumigatus in 95% of cases. Infection is favored by the host species, with a predilection for turkeys, penguins, raptors, and waterfowl, as well as by environmental conditions, immunosuppression, physical stress (migration), and administration of exogenous corticosteroids (5). Fungi of the genus Candida spp. are the second most common agents of mycosis in birds, typically affecting the gastrointestinal tract (6, 39). Other fungi are rarely described as pathogens in birds.

The M. racemosus has been identified as a pathogen in immunocompromised human patients (40), as well as in birds (16), where it has been associated with circovirus infection, which normally induces immunosuppression. However, in the 4 cases described in this article, no inclusion bodies or other lesions referable to viruses were found, and the isolated fungi were identified as a primary pathogen, actually causing the death of the birds. Mucorales species have a vascular tropism, causing tissue infarctions, and they can lead to cutaneous, rhinocerebral, and sinopulmonary diseases, which in turn can evolve in disseminated and fatal infections, especially in immunocompromised hosts (36). In our cases, the fungi most likely entered through the respiratory tract, given the constant involvement of the lungs and air sacs found at the necroscopic examinations. Then, the pathogens generalized through the bloodstream, causing vascular lesions and widespread areas of inflammation at the level of highly vascularized organs with filter function, such as the liver and kidneys. Only in the case of the Blue and Gold Macaw male, in which the gastrointestinal tract was diffusely affected with large ulcerative lesions and the involvement of the vessels of the submucosa, it is possible to hypothesize a different mechanism of entry of the fungi, with a primary localization to the liver through the portal route. The predilection of fungi for the respiratory entry route in birds and some classes of reptiles can certainly benefit from the particular anatomy of the respiratory system of these animals due to the presence of air sacs. This, and some other anatomical features, preclude the mechanisms of ejection of inhaled fungal spores that remain in areas of low turbulence of the airflow, such as parabronchi and air sacs, where constant temperature and humidity favor spores' germination. In addition, the absence of resident macrophages within airway lumens and the dependence on heterophils (that use cationic proteins, hydrolase, and lysozyme rather than catalase and myeloperoxidase) could also be liable for the increased susceptibility of birds to develop lesions of mycotic origin that can be devastating (41, 42).

Alternaria spp. are plant parasites, causing leaf spots on adult plants and wilt diseases of sprouting seeds. Some species, such as A. alternata and A. tenuissima, are saprophytic on a wide range of decaying plant tissues (17) and consequently are very common on every animal farm, where they can contaminate the animals' skin with spores (19). In horses, A. alternate causes cutaneous nodular dermatitis (19, 43, 44). Alternaria spp. can also infect cats (45–48) and less commonly dogs. A case report in the latter species describes A. infectoria as the cause of multiple cutaneous lesions in a patient under immunosuppressive therapy for immune-mediated hemolytic anemia (49). In birds, infection of Alternaria spp. is rarely reported. In Indian jungle bush quail (Perdicula asiatica), A. alternata causes a season-dependent lung invasion, causing severe disease when the lowest immune status occurs (50). In psittacine birds, only a case is reported, describing a dermal infection of A. alternata in an Indian ring-necked parakeet (Psittacula krameri), with feather picking and a skin lesion on the wing (51). A. alternata was isolated in decaying nests of wild birds, made up of twigs, leaves, and bark, which are all substrates decomposed by fungi (52, 53). This observation reinforces the hypothesis that poor hygiene in the management of the captive birds included in this study could have had an important role in the pathogenesis of mycosis.

Fusicladium spp. (anamorph of Venturia) are phytopathogens causing worldwide significant economic loss to crops (54). According to molecular analysis, ascomycetes, such as Dothideales / Capnodiales (for example, Pseudocladosporium, Fusicladium), are anamorphs and morphologically similar to Cladophialophora, were reclassified, with the dothidealean species Venturia hanliniana classified as the teleomorph of Fusicladium brevicatenatum (55). Another distinction between Chaetothyriales and Dothideales/Capnodiales is found in their ecology, with opportunistic infections and lesions in humans. Some of these organisms, attributed to Cladophialophora and predominantly saprobic or plant-associated, were never associated with infection in vertebrates.

To our knowledge, this report is the first one describing M. racemosus and M. circinelloides infections in lovebirds, and A. alternata and Fusicladium spp. in macaws. Although infections caused by atypical fungi in parrots are rare, exposure to these pathogens should always be considered by clinicians during clinical practice and aviary management.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/supplementary material.

LG, CF, PD, and GR conceived the study. LG, CZ, LB, and GR performed necropsies and histological analysis and wrote the manuscript. CF and PD performed molecular analysis and wrote the manuscript. SB, A-RA, and GR reviewed the article and provided critical suggestions and comments. All authors discussed the results and approved the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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