Coronavirus Disease 2019 (COVID-19) is an ongoing pandemic with global confirmed cases rising once again. In Iran, a country that started COVID-19 vaccination in February 2021, is now (August 2022) involved in the 7th wave of this infection. Bacterial and fungal co-infections are among the assorted factors that lead to comorbidity and mortality in COVID-19 patients (Gangneux et al., 2020; Macauley and Epelbaum, 2021). Most of these patients have risk factors making them prone to fungal infections, such as hospitalization in the intensive care unit (ICU), high prescription rates of broad-spectrum antibiotics, corticosteroid therapy, use of various catheters, underlying diseases, and immunodeficiencies. Among the opportunistic fungal infections, (e.g., Aspergillosis, Mucormycosis, and Candidiasis) the most common fungal co-infections during previous influenza pandemic outbreaks are now the most common fungal co-infections in COVID-19 patients (Meijer et al., 2020; Salehi et al., 2020; Fortarezza et al., 2021; Kayaaslan et al., 2021; Kayaaslan et al., 2022; Vaseghi et al., 2022).

Due to the undefined standard treatment for COVID-19, side effects of medications, aggressive treatment methods, and combinations of treatment regimens, especially those requiring long-term treatment that suppresses the immune system, some oral disorders such as ulcers, blisters, necrotizing gingivitis, salivary gland alterations, white and erythematous plaques, gustatory dysfunction and oral candidiasis (OC) are widely reported in these patients (Brandini et al., 2021).

Oral candidiasis may be the possible cause of these oral disorders due to excessive colonization of Candida species and tissue invasion. As a result of this infection, the patient’s quality of life is also affected due to discomfort and local pain, change in the sense of taste, burning sensation in the mouth, and difficulty breathing and swallowing. It also affects the absorption of liquids and foods consumed by the patients. In addition, OC can progress and involve the esophagus and digestive tract, especially as it can become invasive and spread throughout the bloodstream, causing systemic infections. Timely and accurate diagnosis of OC and accurate identification of its etiological factors in patients suffering from COVID-19 are important to optimize and improve effective treatment (Salehi et al., 2020).

Although Candida albicans is the predominant Candida species found in patients with OC, there is an increasing incidence of oral colonization and infections caused by non-albicans Candida species. Management of candidiasis by non-albicans Candida species is challenging due to the resistance pattern of these species to common antifungal agents. Some of these species, including the emergence of multidrug-resistant C. auris, are sporadically reported in all continents and cause outbreaks in some cases, presenting a serious global health threat. Also, fluconazole resistance is common in non-albicans Candida species such as C. glabrata, and C. krusei, which are now frequently identified as human pathogens, making treatment of these infections arduous (Laudenbach and Epstein, 2009; Aslani et al., 2018; Saris et al., 2018; Ahangarkani et al., 2020; Arastehfar et al., 2020; Carolus et al., 2021; Chatzimoschou et al., 2021). Moreover, candidiasis caused by mixed Candida strains is of great clinical importance, since susceptibility to antifungals differs dramatically among Candida species. Limited data on the characterization of OC and the Candida species profile in patients with COVID-19 are available. This study characterizes clinical conditions, the prevalence profile of Candida species, and outcomes of COVID-19 patients with OC.

Among the 4133 patients with COVID-19 admitted during this study, 120 (2.90%) suffered from OC. Demographic characteristics, comorbidities, and clinical features of patients are shown in Table 1 . Based on age group patients were distributed in three groups including 49.16% (n=59) less 50 years old, 21.66% (n=26) in the range 50-65 years and 29.16% (n=35) more than 65 years old which are illustrated in Figure 1 .The mean age of the patients was 56.55 ± 15.56 years in the range of (24-96 years old) (n=64; 53.3%) of patients were female and (n=56; 46.7%) were male. The majority of patients had multiple underlying disorders. The most common underlying diseases were diabetes (n=35; 29.2%), hypertension (n=31; 25.8%), and cardiovascular disease (n=26; 21.7%). Also, hyperglycemia during time of admission was seen in (n=17; 14.2%) patients. The most common symptoms were dyspnea (70%), myalgia (65.8%), and fever (55.8%). The most common concomitant medications in patients were Enoxaparin 82.5% and Remdesivir 71.7%. In total, 8.3% of patients were admitted to the ICU, all requiring invasive mechanical ventilation. The average hospitalization stay of patients was 8.22 ± 3.95 days.

Oral candidiasis clinical manifestations, comorbidities, oral hygiene, and healthcare-associated factors for higher risk of OC in patients are illustrated in Table 2 . The onset of signs and symptoms of OC in patients was on average (2.92 ± 3.596 days) with a range of 1-29 days.

White or yellow macules were present on the buccal surface (n=79; 65%), the tongue (n=62; 51.66%), soft palate (n=33; 27.5%), gums (n=8; 6.66%), lips (n=4; 3.33%) and on the pharynx (n=1; 0.83%). Other presentations included, xerostomia (n=39; 32.5%), irritation (n=26; 21.66%), atrophy of the tongue (n=14; 11.66%), erythematous patches on the tongue (n=10; 8.33%) and perioral fissures (n=1; 0.83%). A majority of patients did not observe oral hygiene (n=100; 83.33% mouthwash, n=77; 64.16% teeth brushing) before OC presentation. Also n=57; 47.5% patients had at least one decayed and tooth (n=40; 33.33%) had dentures. Moreover, n=16; 13.33% of patients were cigarette smokers. Healthcare-associated factors for higher risk of OC were: corticosteroids (n=102; 85%), nasal tube oxygen therapy (n=74; 61.66%), oxygen mask (n=70; 58.33%), broad-spectrum antibiotics (n=66; 55%) and corticosteroid spray use (n=36; 30%). Nystatin suspension (n=102; 85%), mouthwash containing nystatin (n=14; 11.66%), and fluconazole (n=2; 1.66%) were administrated for patients, while (n=2; 1.66%) didn’t receive antifungal therapy. Oral candidiasis was cured in (n=103; 85.83%) of patients at discharge, while (n=17; 14.16%) of patients were not cured at time of discharge.

In total, n=47; 39.16% of patients suffered from OC caused by multiple Candida strains (with 2 or more Candida spp.). Distribution of Candida species causing OC based on single or multiple Candida strains is illustrated in Figure 2 .

In this study, 175 strains of Candida species isolated from 120 patients were diagnosed as causative agents of oral candidiasis. The most common Candida species were C. albicans (n = 106; 60.57%), followed by C. glabrata (n = 30; 17.14%), C. tropicalis (n = 20; 11.42%), C. kefyr (n = 13; 10.83%) and C. krusei (n = 6; 3.42%). Oral candidiasis clinical manifestations, comorbidities, and healthcare-associated factors were not significantly different between patients infected by single Candida species vs multiple Candida strains. Oral candidiasis caused by single Candida strains was significantly higher in patients with decayed teeth (OR 2.17, 95% CI 1–4.707, P = 0.046). In addition, OC caused by multiple Candida strains was considerably more in patients with dentures (OR 1.171, 95% CI 0.986–1.391, P = 0.034). Significantly, OC caused by multiple Candida strains was more predominant in patients under corticosteroid therapy (OR 4.185, 95% CI 1.047–17.714, P <0.0001), broad-spectrum antibiotics (OR 4.078, 95% CI 2.238–7.429, P = 0.028), and those using inhaled corticosteroid sprays (OR 2.361, 95% CI 1.63–3.419, P <0.0001). Notably, the majority of patients which recovered from OC at the time of discharge from the hospital were patients with candidiasis by single Candida species (OR 3.005, 95% CI 0.912–9.895, P = 0.049). Also, the frequency of Candida species causing OC in patients undergoing corticosteroid and antibiotic therapy versus non-users is shown in Table 3 .

Oral candidiasis is unusual in healthy adults. People at higher risk for getting OC include neonates and individuals with at least one of the risk factors, such as wearing dentures, diabetes, cancer, HIV/AIDS, taking antibiotics or corticosteroids, taking medications that cause dry mouth, and smoking. The occurrence of OC should be anticipated in patients with COVID-19, especially those with moderate to severe forms of infection who are hospitalized and may be on long-term use of broad-spectrum antibiotics or prolonged corticosteroid therapy.

In the current study, during one year, 2.9% of 4133 patients admitted to the COVID-19 referral center in the north of Iran suffered from OC at time of admission. Consistent with other studies, C. albicans was the predominant, isolated species in these patients (Aslani et al., 2018; Salehi et al., 2020). However, it is interesting that 39.16% of these patients suffered from OC caused by two or more Candida spp. The compromising host immune status that affects the natural defenses that suppress the growth of invading pathogens results in multiple Candida species infections. Also, it has been seen in immunocompromised patients that the frequency of infections involving mixed fungal genera would be similar to that of disorders involving mixed fungal species due to severe immunosuppression, which accelerates colonization of Candida strains and boosts the ability of independent pathogens to penetrate tissue Soll, 2022.

In this study, 89% of patients were under corticosteroid therapy, and 55% used broad-spectrum antibiotics. It was found that corticosteroid therapy and use of broad-spectrum antibiotics were significantly higher in patients with OC caused by two or more Candida spp. In addition, the duration of corticosteroid therapy was greater in these patients compared to patients with OC caused by one Candida species. Similar to our findings, Xia et al. reported corticosteroid therapy caused an increased prevalence of OC by non-albicans strains (Xiao et al., 2020). Moreover, Nambiar et al. noted that the development of OC in COVID-19 patients could be due to prolonged mechanical ventilation in the ICU and the long-term use of broad-spectrum antibiotics. (Nambiar et al., 2021). It is noted that the possibility of a potential of OC caused by empirical broad‐spectrum antibiotics prescription in a mild or moderate form of COVID‐19 case should also be considered (Riad et al., 2022). Ahmed et al., in a review article, reported a direct correlation between the development of candidiasis with the use of antibiotics and corticosteroids in COVID-19 patients (Ahmed et al., 2022).

The profile of Candida species causing OC in our patients did not changed during the COVID-19 pandemic, and albeit of antibiotics overuse and corticosteroids in patients, emerging uncommon Candida species was not observed in our study (Aslani et al., 2018; Shokohi et al., 2018; Arastehfar et al., 2020). Candida glabrata, C. tropicalis, C. kefyr and C. krusei were the most prominent non-albicans Candida species isolated from patients with OC caused by multiple Candida species. Notably, these non-albicans Candida species are intrinsically azole resistant or low susceptibility to azole antifungals (Shokohi et al., 2018; Ahangarkani et al., 2019; Ahangarkani et al., 2020). Colonization of oral mucosa with C. glabrata is common in cancer patients (Aslani et al., 2018). The prevalence of C. glabrata in our study was higher compared to the Khalil et al. study in Egypt and Salehi’s study in Iran. However, C. tropicalis prevalence was consistent with the study by Khalil et al., and the prevalence of C. krusei was similar to the findings of Salehi’s study (Salehi et al., 2020; Khalil et al., 2022).

As OC is an indirect indicator of cell-mediated immunodeficiency and has a high predictive value for invasive candidiasis in immunocompromised patients, and the lack of fungal identification methods to species level in low-income countries such as Iran, invasive candidiasis caused by these species in patients exposed to high-risk medications such as patients with moderate to severe form of COVID-19 is noteworthy. The current study has some limitations. Since this observational study was performed on COVID-19 patients with OC, ideally, to obtain insights into the epidemiological status, it was better to compare COVID-19 patients with OC with a control group, such as COVID-19 patients without any co-fungal infection. Moreover, the occurrence of OC in patients with a mild and moderate form of COVID-19 should be investigated. Furthermore, several OC caused by multiple Candida species were observed, antifungal susceptibility testing should be performed for all isolates deemed clinically significant. Also, this study endorses the involvement of dental practitioners among the treatment teams dealing with COVID‐19 patients.

Although the profile of Candida species causing OC in our patients did not changed during the COVID-19 pandemic, overconsumption of corticosteroids and antimicrobial therapy in COVID-19 patients could result in OC by multiple Candida strains.

The original contributions presented in the study are included in the article/Supplementary Materials, further inquiries can be directed to the corresponding author/s.

The study was approved by the Ethics Committee of the Mazandaran University of Medical Science (IR.MAZUMS.REC.1400. 8977), Sari, Iran and was performed in compliance with the Declaration of Helsinki. The patients/participants provided their written informed consent to participate in this study.

FB, MR, FA and AK designed the project, collected data, wrote and performed the critical review of the manuscript. FB, MR, AK, RA-N, NN, AA, AH, LA, SK, KA, AD, ZD and FA contributed to clinical data collection. AA and FA carried out statistical interpretation. All authors contributed to the article and approved the submitted version.