Introduction

Candidosis has been described since the times of Hippocrates (circa 460–370 BC), being referred to as 'a disease of the diseased', which highlights the opportunistic nature of the infection and the primary role of a reduced host defence in its development. Candida is a type of fungus (yeast), that is frequently encountered in the mouth of healthy individuals and as such this microorganism is considered to be a member of the normal oral microbiome. The incidence of oral candidal carriage within the general population has traditionally been reported to be between 35%-80%, depending on the specific cohort studied.1,2 However, these findings have historically been based on cultural studies. More recently, the use of molecular detection methods would suggest that Candida species are in fact present in the mouths of all individuals.3 In addition, other fungal species, including Saccharomyces, Geotrichum and Cryptococcus species have on rare occasions been isolated from the mouth (Table 1), but their numbers are generally limited and they have not been implicated in specific oral infection.4

Table 1 Fungal species recovered from the human mouth

It is important to recognise that oral candidosis is not a single infection entity, and traditionally, four distinct forms of primary oral candidosis are described based on clinical presentation.5 The reasons why a particular individual patient may present with one form of infection as opposed to another are unclear, especially since all forms are seemingly caused by the same fungal species. It is likely that a combination of host factors and microbial factors will ultimately determine the occurrence of a particular form of oral candidosis. What is also evident is that Candida albicans, which is most often the cause of oral candidosis, is an extremely heterogeneous species, whose strains differ markedly, both phenotypically and genotypically. Thus, Candida strain variation could also be an influencing factor on whether an individual actually manages to clear the colonising strain or whether it is retained as a commensal. It is conceivable that strain variation could promote pathogenesis through elevated expression of virulence determinants and by affecting the nature of host immune responses. On rare occasions Candida can enter the blood stream and be disseminated to many organs with resultant life threatening infection.5 Such systemic candidal infection is beyond the scope of this article.

While Candida species are normally harmless residents of the mouth, when conditions alter to an environment that favours proliferation of Candida, a shift to a 'pathogenic' relationship with the host may occur. Infection with Candida is described in the literature as a candidosis (candidoses, pl) or candidiasis (candidiases, pl). Both terms are widely used, although candidosis is preferred due to its consistent use of the 'osis' stem with the terminology for other fungal infections. The term Candida originates from the Latin word 'candidus', meaning white.

Identity and pathogenicity

The most prevalent Candida species recovered from both healthy and infected human mouths is C. albicans, and it is estimated that this species is responsible for over 80% of oral fungal isolates. In terms of prevalence, C. albicans is followed in descending order by C. glabrata, C. krusei, C. tropicalis, C. guilliermondii, C. kefyr and C. parapsilosis. Even less frequently encountered are C. inconspicua, C. lusitaniae, C. norvegensis and C. rugosa. In recent years, the importance of these non-Candida albicans Candida species in human disease has increasingly been recognised. While these species typically lack the range of virulence factors encountered with C. albicans, they have come to prominence due to their enhanced resistance to antifungal agents. Candida dubliniensis was first described in 1995, following its co-isolation with C. albicans from cases of oral candidosis in HIV-infected individuals.7 Since C. dubliniensis shares a number of unique phenotypic characteristics with C. albicans, its mis-identification as C. albicans has probably occurred frequently.

The transition of Candida from a harmless commensal to a pathogen is complex and most likely relates to local environmental changes in the host that promote either increased growth of Candida or altered expression of its virulence factors. It is becoming increasingly acknowledged that as well as interaction between host factors and Candida, the bacterial component of the oral microbiome is also involved in the development of oral candidosis.8 When considering Candida virulence factors, it is important to note that a number of these do not directly induce damage to host tissues, but will influence lifestyle of Candida, which indirectly promote pathogenicity.9

In order to cause oral candidosis, Candida has to be retained within the mouth. Consequently, a key virulence attribute of Candida is its ability to adhere to host surfaces. In the mouth, this allows Candida to resist removal from the effects of salivary flow and swallowing. Adherence can be to the oral epithelium or surfaces of prosthetic devices including dentures and orthodontic appliances. In the case of oral epithelium, as cells are sloughed off during the constant replacement of the oral mucosa, Candida will be removed and swallowed. Therefore, its ability to grow on the surface at a rate that at least is in equilibrium to loss of cells is vital to its persistence. Furthermore, mucosal surfaces can readily be recolonised from chronic reservoirs of Candida on intra-oral devices, in particular acrylic dentures, the surfaces of which obviously lack the ability to overcome colonisation by renewal. Once attached to host surfaces, Candida, and in particular C. albicans, can switch from yeast to filamentous forms, which may facilitate epithelial penetration. This property, coupled with an increased resistance to phagocytosis, will promote persistence of Candida in the mouth.

Destruction of host tissues by Candida might occur due to the physical effect of filamentous growth into tissues. However, the invading Candida release extracellular hydrolytic enzymes into the local environment which also lead to tissue damage.10 The enzymes most frequently implicated in the virulence of C. albicans are secreted aspartyl proteinases (SAPs). In addition to SAPs, enzymes categorised as phospholipases (PLs) are also viewed as virulence factors.

Isolation and identification of Candida from the oral cavity

A variety of techniques that enable recovery of Candida from the oral tissues,4 including swab,11 oral rinse12 and imprint13 (Davenport 1970) have been developed (Table 2). Each sampling method for microbial culture has its own particular advantages and disadvantages and as such the most appropriate technique for an individual patient is dependent on the lesion type and suspected clinical form of candidosis.4

Table 2 Sampling methods for recovery of Candida from the mouth

When there is a defined lesion a direct sampling approach such as use of a swab or imprint is preferred, since both these methods will provide information on the organisms on the surface of the lesion itself. A plain microbial swab can be directly applied to lesion tissue (Fig. 1) and sent for culture. Uptake of the swab is improved if it is dampened first and rotated 360° when on the tissues. The imprint culture technique involves placement of a sterile foam–pad (ca 2.5 cm2), which has been dipped in sterile saline, onto the lesional tissues or intra-oral appliance for 30 seconds (Fig. 2). The imprint foam pad is then placed directly onto an agar plate in the clinic before being sent to the laboratory.

Figure 1
figure 1

Plain swab sampling of left angle

Figure 2
figure 2

Imprint sampling of hard palate

Where no obvious mucosal lesion is present, then indirect sampling involving collection of whole saliva in a sterile universal container or an oral rinse method is more appropriate. The oral rinse technique involves the patient holding 10 ml of sterile phosphate buffered saline (0.01 M, pH 7.2) in the mouth for 60 seconds. The rinse is then collected in a sterile universal container, which is then labelled and sent for culture (Fig. 3). The sample is concentrated (10-fold) by centrifugation before inoculation onto agar using a spiral plating system. Growth is expressed as candidal colony forming units per ml (cfu/ml) of the rinse. The quantitative nature of the oral rinse also permits differentiation between the level of up to 100 cfu/ml, which is considered to be 'commensal carriage', and higher levels which can be regarded as 'pathogenic'.

Figure 3
figure 3

Oral rinse sampling of mouth

Samples for Candida detection are traditionally inoculated onto Sabouraud dextrose agar (SDA) and incubated aerobically at 37 °C for 24-48 hours.13 The vast majority of candidal species cultured on SDA will appear as white/cream colonies and therefore subsequent tests are required to identify isolates to species level (Fig. 4). Chromogenic media (Fig. 5), on which various Candida species grow as different colony colours are now available. These differential media provide presumptive identification of certain Candida species based on colony appearance and colour following primary culture. The advantage of such media is that the presence of multiple Candida species in a single sample can be determined, which may be important in selecting subsequent treatment options, in particular choice of antifungal therapy.

Figure 4
figure 4

Incubated Sabouraud dextrose agar plate inoculated with oral rinse using a spiral plating system and yielding a heavy growth of Candida species

Figure 5
figure 5

Incubated CHROMagar Candida plate yielding a mixed growth of C. albicans (green), C. tropicalis (blue), C. krusei (pale) and C. glabrata (mauve)

Definitive identification of Candida can be made through a variety of supplemental tests usually involving evaluation of morphological and physiological characteristics of an isolate. Biochemical identification of Candida is primarily based on carbohydrate assimilation, with a range of commercial systems available to facilitate such tests. Identification is determined by the profile of carbohydrates able to support growth of the test isolate. Increasingly, molecular-based methods are being employed with a number of species-specific polymerase chain reaction (PCR) approaches for Candida being used.4

The presence of Candida on the mucosal surface of the mouth or an intra-oral appliance can easily be confirmed for the vast majority of cases of pseudomembranous candidosis and erythematous candidosis by use of one of microbial culture based techniques. In contrast, a tissue biopsy is required for definitive diagnosis of chronic hyperplastic candidosis, since it is necessary to demonstrate invasion of the epithelium by Candida by histological staining using the periodic acid-Schiff (PAS) technique or Gomori's methenamine silver stains. Molecular techniques have also been used to identify candidal species within formalin fixed paraffin embedded tissue.15

Antifungal agents

Relatively few antifungal drugs are available compared with the wide range of antibiotics for bacterial infections, which probably reflects the difficulty involved in developing an agent with activity against a eukaryotic cell type without inherent problems of associated host toxicity. Antifungal drugs can be classified into four types according to mode of action and target:

  1. 1

    Disruption of fungal cell membranes, as in polyene antifungals (nystatin and amphotericin)

  2. 2

    Inhibition of ergosterol synthesis, in the azole group of antifungals (fluconazole and itraconazole)

  3. 3

    Inhibition of β-1,3 D-glucan synthetase enzymes in the echinocandin antifungals (caspofungin)

  4. 4

    Interference with RNA synthesis and DNA replication (flucytosine).

Polyenes were developed in the 1950s and were the first true antifungal agents. These drugs are fungicidal and act by inducing cell membrane porosity following interaction with the ergosterol component of the membrane, and the subsequent effect of loss of cytoplasmic content. Polyenes have a broad spectrum of antifungal activity, but due to their poor absorption through the gut their use in dentistry is limited to topical delivery. Intravenous delivery of amphotericin is an option for serious life threatening forms of systemic candidal infection, but is associated with significant side effects and toxicity. Until the advent of the azoles, polyenes were the only practical option for treatment of all forms of oral candidosis. The efficacy of topical polyenes in the mouth is limited due to difficulty in maintaining sufficient levels of the antifungal agent in the environment of the candida. This is further complicated by the unpleasant taste of polyenes, which stimulates salivary flow, thereby diluting salivary levels of the drug below therapeutic levels. Pharmaceutical companies have now ceased manufacturing topical forms of amphotericin and all but one format (suspension) of nystatin. Despite widespread use over the past six decades, the incidence of resistance to polyenes by Candida is rare, but can sometimes arise through a reduction in the ergosterol content of cell membranes.16

Azole antifungals became available for clinical use in the 1980s. These drugs are fungistatic and act through interference with a fungal enzyme, lanosterol demethylase, which is key to the biosynthesis of ergosterol component of the cell membrane. Miconazole is available in cream or ointment format and in combination with a steroid anti-inflammatory. All these formats of miconazole can only be used topically. In contrast, other azoles including ketoconazole, fluconazole and itraconazole, are well absorbed through the gut and were the first antifungals that could be administered systemically through oral administration. However, ketoconazole is associated with significant toxicity, which restricts its clinical use and as such this drug will not be considered any further in this article. Itraconazole and fluconazole are relatively safe and rarely associated with adverse reactions. However, there is an important drug interaction between warfarin and azole antifungal agents, including miconazole even when delivered topically.17 Warfarin is potentiated and concomitant use can result in serious bleeding and even death. It is possible to use topical nystatin suspension in patients taking warfarin but, as described above, the clinical efficacy of this antifungal preparation is poor. Consideration can be given to approaching the medical staff prescribing the warfarin to see if the patient can be provided with the newer alternative oral anticoagulant drug, dabigatran, since it does not interact with fluconazole. However, the use of dabigatran instead of warfarin may not be appropriate or possible, which does create a dilemma when trying to manage significant oral candidal infection in a patient taking warfarin.

Another commonly encountered potential drug interaction involving azole antifungals is with statins. A patient prescribed fluconazole should be instructed to refrain from taking their statin for the duration of antifungal therapy, usually only seven days. Fluconazole is particularly appropriate for use in the treatment of oral candidosis since it is secreted in saliva at levels equivalent to those achieved in the blood. Fluconazole is routinely prescribed for out-patients in capsule form, but is also available as a suspension if this format of delivery is required. In addition, fluconazole can be administered intravenously.

Unfortunately, acquired resistance to azole antifungals has emerged in recent years and certain Candida species are also inherently resistant to these agents. There are several mechanisms of azole resistance reported and include 1) an alteration in the chemical structure of demethylase enzyme, 2) removal of the azole from the cell by multidrug transporter pumps, 3) compensation by other sterol synthesis enzymes in membrane biosynthesis. Even in the absence of a defined resistance mechanism, the in vitro susceptibility of a given Candida strain often does not often correlate with subsequent clinical outcome for patients with oral candidosis. One possible explanation for this could relate to the phenotypic differences between planktonic and biofilm cultured cells, as it is the former that are most frequently used for in vitro antifungal susceptibility testing.

Echinocandin antifungals and flucytosine do not have a role to play in the outpatient treatment of oral candidosis and will not be considered here.

Oral candidoses

The four distinct primary forms of oral candidosis comprise: pseudomembranous candidosis, acute erythematous candidosis, chronic erythematous candidosis (CEC) and chronic hyperplastic candidosis (CHC). Pseudomembranous candidosis (thrush) was previously classified as being either acute or chronic based on duration of signs and symptoms. However, more recently, distinction between the two has been dropped. Any form of oral candidosis can be accompanied by the presence of angular cheilitis. Each of these forms of candidal infection is associated with characteristic clinical signs and symptoms and a range of host predisposing factors (Box 1).

The clinical presentation, diagnosis and management of each type of oral candidosis is described in detail below. However, before describing specific issues related to each individual clinical form of candidosis, some general principles that apply to all need to be considered. Identification and subsequent correction of any underlying predisposing host factor is crucial in the management of oral candidosis.

The medical history will reveal any already recognised predisposing factor. However, assessment of the patient should routinely involve the following haematological tests: full blood count (FBC), serum ferritin, vitamin B12, folate. Any abnormality in the FBC or haematinic deficiency needs to be investigated further and the cause identified. Iron deficiency anaemia or pernicious anaemia are found relatively frequently in patients with candidosis and correction usually leads to resolution of infection. The basis of this association is not clear but may be due in part to the involvement of iron or vitamain B12 in epithelial cell formation in the oral mucosa or antimicrobial aspects of the host defence systems. Undiagnosed or poorly controlled diabetes is often an underlying problem in candidal infection and therefore blood glucose and haemoglobin A1c (HbA1c) need to be measured. Establishment of good glycaemic control and maintenance of blood glucose (<10 mmol/L) is key in the management of oral candidosis, but unfortunately many patients have difficulty achieving this. Tobacco smoking is associated with oral candidosis, in particular chronic hyperplastic candidosis. Any patient with a tobacco habit should be advised on smoking cessation.

In some instances, a recognised predisposing factor cannot be eliminated, such as the essential use of immunosuppressive drug therapy in solid organ transplantation or presence of HIV. In such circumstances the role of antifungal therapy in management of candidosis has special relevance.

Pseudomembranous candidosis

Pseudomembranous candidosis is synonymous with the term 'oral thrush' and is characterised by the presence of superficial white plaques that are easily removed by gentle scraping of the lesion18 (Fig. 6). The ability to remove these plaques is an accepted diagnostic feature that differentiates pseudomembranous candidosis from the other forms of white patches that develop in the mouth. Microscope examination of a stained smear taken from these plaques reveals fungi in yeast and filamentous forms, together with epithelial cells. Alternatively, a swab from the white patches can be sent for culture and this can be used to identity the Candida species present.

Figure 6
figure 6

Pseudomembranous candidosis presenting as removable white plaques in the soft palate

The pseudomembranous candidosis form of candidosis is most frequently associated with use of steroid therapy. It is also found relatively frequently in neonates. There is a direct relationship with immunodeficiency, and pseudomembranous candidosis is a recognised presenting feature and ongoing complication of leukaemia and HIV infection. In such instances, long-term use of antifungal agents is often successful.

The vast majority of cases of pseudomembranous candidosis seen in primary dental care will be due to the use of inhaled steroids. Antifungal therapy in the form of fluconazole, 50 mg, once daily for seven days, with appropriate considerations (see above), should be prescribed. This will reduce the candidal load to normal levels. In addition, the patient should be instructed on the need to rinse their mouth with water following inhaler use. This advice is now included on the inhaler product information leaflet.

Acute erythematous candidosis

Acute erythematous candidosis tends to develop as a consequence of a reduction in the levels of the bacterial component of the oral microflora following the receipt of broad spectrum antibiotics. Unsurprisingly the condition is therefore often referred to as 'antibiotic sore mouth'. Concomitant use of steroid therapy, particularly in inhaler form, may be an additional contributing factor as this can lead to local immunosuppression with a resulting overgrowth of Candida.

A reduction of bacterial numbers in the oral cavity results in a lowering of microbial competition with Candida in terms of nutrition and adherence sites. The clinical signs and symptoms of acute erythematous candidosis are therefore a direct consequence of an ecological shift from the normal homeostatic balance of the microbial community. Acute erythematous candidosis presents as a painful reddened lesion on the dorsum of the tongue (Fig. 7). Confirmation of the presence of Candida and their identity can be made by taking a swab or imprint for culture. Cessation of antibiotic therapy results in a return to normal levels of bacteria, which subsequently resolves the candidosis without intervention. If symptoms are significant then systemic fluconazole, 50 mg once daily for seven days can be prescribed.

Figure 7
figure 7

Acute erythematous candidosis presenting as a red atrophic area on the dorsum of the tongue

Chronic erythematous candidosis

Chronic erythematous candidosis is commonly referred to as Candida-associated denture stomatitis and presents as a reddening of the mucosa beneath the fitting surface of a denture (Fig. 8). The infection may develop under any acrylic denture or indeed intra-oral appliance, but is most often encountered on the palatal rather than the mandibular mucosa. Principal host factors associated with this condition are inadequate oral hygiene, continuous wearing of the denture or the presence of a poor fitting denture.19 Chronic erythematous candidosis is the most prevalent form of oral candidosis, with up to 75% of denture wearers having clinical signs of this condition, although often the individual is unaware of infection.20

Figure 8
figure 8

Chronic erythematous candidosis presenting as red areas of the palatal mucosa

Imprints or swabs taken from the fitting surface of the denture and the palatal mucosa can be used to confirm the presence of Candida following culture. It is important to appreciate that the Candida are colonising the denture and not the mucosa. Therefore, while the sample from the denture should yield candidal growth, sampling of the erythematous mucosa may be negative.

The management of this form of candidosis is primarily focused on eradication of colonisation of denture. As such, miconazole can be applied topically to the fitting surface of the denture and placed in the mouth. The denture, if there are no metal components, should be removed while sleeping and placed in dilute hypochlorite (usually overnight). Dentures with metal components should not be placed in hypochlorite, but soaked in chlorhexidine instead to avoid tarnishing of the metal. While of primary importance in the management of chronic eryhtematous candidosis, the denture hygiene measures described above should be instituted as an aspect of the treatment of all forms of oral candidosis since candidal colonisation of the acrylic componenet of the denture acts as a chronic reservoir of Candida species that will lead to recurrence of infection within the mouth.

Chronic hyperplastic candidosis (CHC)

Characteristically, CHC appears as a thickened white plaque, most frequently at the commissural region of the mouth or on the dorsum of the tongue (Fig. 9). A smoking tobacco habit is almost universally present in patients with CHC. Of concern with this infection is the proposed link with malignant change at lesional sites, although the role of Candida in carcinogenesis or the development of epithelial dyspasia remains unclear.21,22 It has been suggested that it may be helpful to provide a seven day course of systemic antifungal therapy before taking a biopsy of a suspicious lesion since any epithelia dysplasia subsequently observed in the biopsy material can be interpreted as 'true' dysplasia rather than be due to the presence candida.

Figure 9
figure 9

Chronic hyperplastic candidosis presenting as bilateral adherent white patches in the left (a) and right (b) labial commissure regions

Two clinical types of CHC have been described based on the lesion encountered. Homogeneous CHC is described as having smooth and white lesions which are notably distinct from those of heterogeneous CHC where areas of erythema occur resulting in a nodular, speckled appearance. It has been suggested that heterogeneous lesions have the greater likelihood of malignant transformation.23 In contrast with pseudomembranous candidosis, the white patch lesions of CHC do not rub off with gentle scraping. The diagnosis of CHC is dependent on the histological examination of lesional biopsy material which will reveal invading Candida hyphae.24 There will also be a chronic inflammatory response in the underlying connective tissues. Uncertainty remains over whether candidal invasion is the primarily ecological factor of CHC or whether Candida infection is secondary to the presence of an altered epithelium.

Management of CHC should involve the prescribing of fluconazole 50 mg daily over a period of 7-14 days depending on the severity of the lesions (Fig. 10). In addition, the patient must stop their tobacco habit. Failure to achieve smoking cessation will inevitably result in recurrence of infection. Patients should be informed of the risk of malignant transformation.

Figure 10
figure 10

Appearance of chronic hyperplastic candidosis in the left labial commissure at initial presentation (a) and after 11 days of systemic fluconazole (b)

Secondary forms of oral candidosis

Angular cheilitis presents as erythematous lesions at the angles of the mouth (Fig. 11). This form of candidosis may also be accompanied by the presence of Staphylococcus aureus or streptococcal species and therefore the exact role that Candida itself plays in the infection is difficult to ascertain.25 Often, angular cheilitis occurs in patients with a pre-existing primary form of oral candidosis. The elevated numbers of Candida within the oral cavity results in direct spread and colonisation of the angles of the mouth. Treatment should involve identification and eradication of the cause of the oral candidosis. If this is achieved, then the angular cheilitis should resolve. Symptomatic treatment of the angles should involve the use of topical miconazole alone or in a combination format with hydrocortisone.

Figure 11
figure 11

Angular cheilitis presenting as bilateral painful erythematous areas at the angles of the mouth

Median rhomboid glossitis is a chronic condition with a distinct clinical presentation involving a symmetrically shaped lesion on the midline of the dorsum of the tongue. Candida can be recovered from the surface and biopsy material. Tobacco smoking and the use of steroid inhalers seem to be predisposing host factors. The role of Candida in this mucosal condition is supported by the observation that provision of systemic fluconazole often results in resolution of the clinical signs and symptoms.

Chronic mucocutaneous candidosis (CMC) is a particular condition where a range of superficial Candida infections of mucous membranes skin and nails are encountered. The principle predisposing factor for CMC is congenital impaired cellular immunity against Candida.