Division of Comparative Pathology, Department of Pathology & Laboratory Medicine, Miller School of Medicine, University of Miami, Miami, FL, USA
Summary
Aspergillosis is a disease of human and animals with much to be learned about the etiology of infection and the pathophysiology of disease.
Introduction
Invasive aspergillosis is a common opportunistic infection in immune-compromised human patients including bone marrow transplant recipients and now, more recently, COVID patients.1,2 Despite treatment, high mortality rates persist, and early diagnosis continues to be challenging.
Avian Aspergillosis
The process of infection and diagnosis in avian species is similarly complicated.3,4 Clinical signalment and basic testing including culture and hematology remain mainstays with radiography, other imaging options, and endoscopy. Serodiagnostic testing can provide value, but results can be difficult to interpret. There are many shared aspects in the challenges of diagnosis in animals and humans.5 This makes maintaining a knowledge base of fungal infection in animals, animal models, and developments in diagnostics a key factor in moving forward on this issue in birds as well as other species.
Antibody Detection
The first application of an enzyme-linked immunosorbent assay (ELISA) for antibody detection in birds was described in raptors 1994.6 Other studies have documented the use of ELISA technology for the detection of antibodies in penguins and an ELISA has been described for use in multiple avian species.7-9
A multiyear study was reported using the ELISA in a variety of avian species.7 Psittacine species were found to have the lowest reactivity and high reactivity was observed in birds like penguins even when clinically normal.
It appears from this collective work that measuring antibody levels may have some diagnostic value but it not an absolute test. Work is in progress to address applications of an assay directly to a recombinant fungal antigen named Afmp1p which was shown to have some applications in falcons.10
Antigen Detection
Antigen detection assays including measurement of galactomannan and beta glucan have been widely applied in the detection of aspergillosis in humans.1,2 In infected falcons, increased galactomannan was observed in naturally infected birds but not in experimental infection.9,11 In a small study of clinical cases, many of the psittacine birds with invasive infection were positive although birds with focal infections like a granuloma were negative or borderline positive.12,13
In human patients, testing for beta-glucan, a pan fungal antigen, is also part of the standard of care testing for immune compromised patients.1,2 In naturally and experimentally infected birds including seabirds and quail, results were found to be highly variable limiting the utility of this assay.14
PCR, while offered commercially, has not been studied or validated for use in avian species.
Protein Electrophoresis
Plasma protein electrophoresis has been described as a valuable tool in monitoring and detecting ongoing acute phase responses in avian species and, specifically with aspergillosis.9,12-17 In experimentally infected falcons, significant increases in alpha-2 and beta globulins were observed with decreases in prealbumin and albumin.9 This has also been observed in penguins.17 Of note, electrophoresis provides the only valid quantitation in albumin in birds.18 As albumin is a key negative acute phase protein, monitoring the A/G ratio via protein electrophoresis can provide an important prognostic tool.19
Gliotoxin and Other Biomarkers
Gliotoxin is the primary mycotoxin produced by Aspergillus fumigatus during replication and is considered a main virulence factor as it causes tissue damage and affects the immune system.20 In a study of mostly penguins, gliotoxin was detected by mass spectrometry in 86% of plasma samples from birds with confirmed aspergillosis and less than 1% of samples from clinically normal penguins.21
Hydroxybutyrate, a marker of ketosis, was described to be increased in falcons with aspergillosis.22 This finding was extended in a retrospective study of penguins where elevated levels were observed by not in tandem with inappetence.17 Notably, the use of these measures in tandem resulted in a high specificity and negative predictive value.
References
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21. Reidy L, Desoubeaux G, Cardenas J, et al. Detection of gliotoxin but not bis(methyl)gliotoxin in plasma from birds with confirmed and probable aspergillosis. J Zoo Wildl Med. 2022; in press.
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