ACUTE PSEUDOMEMBRANOUS CANDIDIASIS (THRUSH)

Clinical Features. Thrush is the prototype of the oral infections caused by Candida.It is a superficial infection of the outer layers of the epithelium, and it results in the formation of patchy white plaques or flecks on the mucosal surface (Figure 5-15, A). Removal of the plaques by gentle rubbing or scraping usually reveals an area of erythema or even shallow ulceration. Because of their prevalence, characteristic appearance, and ease of removal, the lesions of thrush are easily recognized, and a diagnosis of thrush is frequently made on the basis of the appearance of the lesion. A smear demonstrating a yeast or myelin is helpful when the diagnosis is uncertain.

Thrush is seen in children and in adults of all ages whenever the number of Candida organisms in the oral cavity increases significantly. When Candida is reduced or eliminated by the administration of antifungal agents, the lesions of thrush rapidly disappear. Transient episodes of thrush may occur as isolated phenomena,with lesions that disappear spontaneously with minimal or no treatment. These episodes are usually unrelated to any recognized predisposing factor and are common in neonates and young children. Alternatively, the lesions may promptly recur following treatment, suggesting the persistence of a predisposing factor, as is often seen in adult patients with candidiasis.
The typical lesions in infants are described as soft white adherent patches on the oral mucosa. The intraoral lesions are generally painless and can be removed with little difficulty. In the adult, inflammation, erythema, and painful eroded areas are more often associated with this disease, and the typical


ACUTE PSEUDOMEMBRANOUS CANDIDIASIS (THRUSH)

pearly white plaquelike lesions are relatively inconspicuous at times. Any mucosal surface may be involved, and erythematous or white areas often develop beneath partial or complete dentures. The lesions may involve the entire oral mucosa (see Figure 5-15, B) or may involve relatively localized areas where normal cleansing mechanisms are poor.
A prodromal symptom of a rapid onset of a bad taste and the loss of taste discrimination is described by some adults. A burning sensation of the mouth and throat may also precede the appearance of the white pseudomembranous lesions. Symptoms of this type in a patient receiving broad-spectrum antibiotics are strongly suggestive of thrush or other forms of oral candidiasis. Patients with immunodeficiencies, such as those suffering from AIDS or hematologic malignancies, are also especially susceptible to this form of candidiasis.
The differential diagnosis of thrush includes food debris, habitual cheek biting, and rarely,a genetically determined epithelial abnormality such as white sponge nevus.
Causative Organism and Frequency. The yeastlike fungus that causes thrush and other manifestations of candidiasis occurs in both yeast and mycelial forms in the oral cavity. The organism grows by a budding of the yeast cells to form germ tubes and individual hyphal elements, which undergo limited peripheral branching to form a pseudomycelium. These phenomena can be demonstrated in smears and tissue sections and form the basis for confirmatory laboratory diagnostic tests for candidiasis.
Candida species are normal inhabitants of the oral flora of many individuals, but are present in the mouth of the healthy carrier in a low concentration of 200 to 500 cells per milliliter of saliva.At this concentration, the organism cannot usually be identified by direct microscopic examination of smears from the oral mucosa, and its presence can be demonstrated only by inoculation onto a selective medium such as Sabouraud agar. Saliva samples give a carrier rate of 20 to 30% for healthy young adults whereas imprint cultures, which sample colonized sites rather than detached cells and organisms in the mixed saliva, give a figure as high as 44%.Imprint cultures suggest that the papillae of the posterior oral surface of the tongue are the primary colonization site in the oral cavity of healthy dentate carriers and that other areas are contaminated or secondarily colonized from this site.
The asymptomatic carrier state is affected by a number of known factors, including the immune status of the host, the strain of Candida, the local oral environment, smoking, prior use of antibiotics, and the general health of the host. The carrier state is more prevalent in diabetic individuals, and the density of Candida at various oral sites is also increased in persons with diabetes. As reported by Guggenheimer and colleagues, diabetic patients with clinical features of candidiasis were more likely to have a longer duration of insulin-dependent diabetes mellitus (IDDM), to have poorer glycemic control, and to experience the complications of nephropathy and retinopathy.The wearing of removable prosthetic appliances is also associated with higher asymptomatic carrier prevalence rates. Importantly, simple measures to improve the oral health of the patient will reduce the rate of Candida colonization of the oral mucosa and denture.Because Candida spp are normal oral inhabitants, thrush and other forms of oral candidiasis may be classified as specific endogenous infections. A variety of species of Candida have been isolated from carriers and from patients with candidiasis. Candida albicans, Candida tropicalis, and Candida glabrata account for over 80% of medical isolates; Candida parapsilopsis, Candida guilliermondii, Candida krusei, and Candida pseudotropicalis are also recognized as pathogens. Candidiasis in HIV-positive patients is often associated with a shift in species, from Candida albicans to Candida glabrata and Candida krusei.The particular species involved with a given oral infection is generally not thought to be of any significance, but Candida albicans is most commonly found in thrush, and several subtypes of this species have been implicated as cocarcinogens in speckled leukoplakia.Severity and refractoriness of Candida infection to treatment possibly depend more on the site of involvement and on predisposing factors than on properties of the infecting species. While certain phenotypic characteristics such as tissue invasion may give strains of Candida a competitive advantage in the oral cavity, it is the host’s immunocompetence that ultimately determines whether clearance, colonization, or candidiasis occurs.Like other microorganisms involved in endogenous infections, Candida spp are of low virulence, are not usually considered contagious, and are involved in mucosal infection only where there is a definite local or systemic predisposition to their enhanced reproduction and invasion.
Predisposing Factors. The following predisposing factors for oral candidiasis have been defined by clinical observation:
1. Marked changes in oral microbial flora (due to the use of antibiotics [especially broad-spectrum antibiotics], excessive use of antibacterial mouth rinses, or xerostomia).
2. Chronic local irritants (dentures and orthodontic appliances)
3. Administration of corticosteroids (aerosolized inhalant and topical agents are more likely to cause candidiasis than systemic administration)
4. Poor oral hygiene 5. Pregnancy 6. Immunologic deficiency
—congenital or childhood (chronic familial mucocutaneous candidiasis ± endocrine candidiasis syndrome [hypoparathyroidism, hypoadrenocorticism], and immunologic immaturity of infancy)
—acquired or adult (diabetes, leukemia, lymphomas, and AIDS)
— iatrogenic (from cancer chemotherapy, bone marrow transplantation, and head and neck radiation)
7. Malabsorption and malnutrition
So important are these predisposing factors in the etiology of this infection that it is extremely rare to find a case of oral candidiasis in which one or more of these factors cannot be identified. A diagnosis of thrush should always be followed by a search for a possible undiagnosed medical disorder, a review of the patient’s medications, and a search for some locally acting predisposing factor such as a denture.
Xerostomia and chronic local irritants may alter the oral mucous membranes, predisposing them to colonization and invasion. Shifts in the bacterial flora often accompany these situations and provide an opportunity for Candida spp to increase. Radiation to the head and neck also affects the oral mucous membranes and produces xerostomia. In Sjögren’s syndrome, sarcoidosis, and other diseases of the salivary glands, xerostomia often develops gradually and is tolerated by the patient until superinfection with Candida develops. The mucosal lesions, pain, and associated symptoms of thrush then cause the patient to seek medical or dental care.
Knowledge of the ecology and epidemiology of Candida in the human mouth has increased substantially in recent years, particularly in regard to the attachment of the organism to oral mucosal surfaces.Candida colonization and infection depend on the initial ability of the organism to adhere to host surfaces. In immunocompromised hosts, adhesion varies significantly among species of Candida.Also, the quality of the epithelial cells in immunocompromised (HIV-positive) patients, including the cells’ receptivity to Candida,may play a role in increasing the oral concentration of yeast.
Histologic Features. Microscopic examination of the lesions of thrush reveals a localized superficial inflammatory reaction, with hyperparakeratosis and ulceration of the surface. The ulcer is covered with a fibrinoid exudate, in which are found large numbers of yeast and pseudohyphae. The fungi rarely penetrate below this superficial layer. This pseudomembrane imparts the characteristic white-flecked appearance to the mucosal lesions. Thrush is correctly described as an acute pseudomembranous candidiasis.