# Avian Candidiasis: A Comprehensive Review of Pathogenesis, Diagnosis, and Control

## TL;DR

This review summarizes avian candidiasis, a fungal disease in birds caused by Candida species, covering its causes, symptoms, diagnosis, treatment, and prevention.

## Contribution

The paper provides a comprehensive review of avian candidiasis, addressing knowledge gaps and emphasizing the need for integrated surveillance.

## Key findings

- Candida albicans and non-albicans species cause avian candidiasis, with increased prevalence of non-albicans species.
- Nystatin is effective for localized infections, but azole resistance is rising, especially in non-albicans species.
- Integrated One Health surveillance is needed due to the zoonotic potential of avian Candida.

## Abstract

Candidiasis is a fungal infection that is caused by Candida yeasts, mostly Candida albicans, and affects domestic and wild birds worldwide. The disease tends to affect young chicks, stressed birds, and birds with weakened immune systems, especially under poor hygiene conditions, excessive antibiotic use, or nutritional deficiencies. Although of clinical significance, no single volume was accessible to summarize existing knowledge on this avian disease. This review fills this gap by studying the different Candida species identified in birds and the predisposing factors. Molecular pathways involving the fungus’s tissue binding, switch from yeast to invasive hyphae, toxin release, and formation of drug-resistant communities are investigated. Clinical symptoms comprising white spots in the mouth and distension of the crop to severe systemic disease are detailed, as well as diagnostic approaches, starting with simple microscopy and ending with advanced molecular methods. Treatment options are considered, though nystatin is still the choice of localized infections, whereas prevention by means of proper hygiene, proper nutrition, and responsible use of antibiotics is still central. Since birds have the potential to spread these yeasts to humans, this review highlights the importance of integrated surveillance, which is beneficial to veterinarians, poultry producers, wildlife managers, and public health professionals.

This review is a comprehensive investigation of avian candidiasis, mainly caused by Candida albicans, although the prevalence of non-albicans Candida species has increased in domestic and wild birds. Avian candidiasis causes significant economic losses in poultry production through increased mortality, cost of treatments, and reduced growth rates, particularly in young birds and intensive farming operations. The pathogenesis section provides a description of the molecular virulence factors such as adhesin-mediated attachment (ALS, Agglutinin-Like Sequence family; HWP1, Hyphal Wall Protein 1), yeast-to-hypha morphogenesis, tissue damage by Candidalysin, biofilm formation on mucosal and abiotic surfaces, and secreted hydrolytic enzymes including secreted aspartyl proteinases (SAPs) and phospholipases. The identified predisposing factors include immunosuppression, malnutrition, abuse of antibiotics, bad husbandry, and crop stasis. The diagnostic methods discussed encompass cytological analysis and fungal culture on selective media to more sophisticated methods of molecular analysis (PCR, MALDI-TOF MS, and NGS). Antifungal susceptibility investigations indicate that nystatin and amphotericin B are still very effective against most avian isolates and that resistance to the azoles is on the rise, especially with respect to the non-albicans Candida species. Nystatin is still the first-line treatment of localized infections; azoles are still used for resistant or systemic infections despite their hepatotoxicity. Sanitation, proper nutrition, and proper use of antimicrobials are essential to prevent diseases. The knowledge gaps comprise the absence of avian-specific pharmacokinetic information, poor knowledge of species-species virulence phenotypes, and the lack of point-of-care diagnostics. The need to have integrated One Health surveillance systems is emphasized by the zoonotic potential of the avian Candida reservoirs.