The Biology of Candida Infection
Candida albicans culture
The image above shows Candida albicans fungus cultured in a petri dish.

Candida albicans infections, commonly known as 'thrush' are fungal infections [mycosis] (from yeast relatives) is the causal agent of opportunistic oral and genital infections in humans and can be a major cause of morbidity and mortality in immunocompromised patients (e.g., AIDS, cancer chemotherapy, organ or bone marrow transplantation). In addition, hospital-related infections in patients not previously considered at risk (e.g. patients on an intensive care unit) have become a cause of major health concern.

Normally Ccandida albicans ranks amongst the normal gut flora — the many organisms which live in the human mouth and gastrointestinal tract. Under normal circumstances, C. albicans lives in 80% of the human population with no harmful effects. However, any effect that results in the overgrowth of the fungus results in diseases known as candidiasia. Candidiasis is often observed in immunocompromised individuals such as HIV-positive patients. It most commonly occurs in the mouth and genital tract but can also occur in the blood. Candidiasis, also known as "thrush", is a common condition which is usually easily cured in people who are not immunocompromised. To infect host tissue, the usual unicellular yeast-like form of Candida albicans reacts to environmental cues and switches into an invasive, multicellular filamentous form.

Candidiasis encompasses infections that range from superficial, such as oral thrush and vaginitis, to systemic and potentially life-threatening diseases. These more severe forms of Candida infections are knwn as candidemia and are usually confined to severely immunocompromised persons, such as cancer, transplant, and AIDS patients. In contrast the superficial infections of skin and mucosal membranes by Candida typically causes local inflammation and discomfort is common in many human populations.

Candida generally manifests itself in the following areas:

  • the oral cavity (oral thrush)
  • the vagina or vulva (vaginal candidiasis or thrush)
  • folds of skin in the groin (nappy/diaper rash)
  • the nipples while breastfeeding
  • the penis or foreskin
  • the armpit
  • the ear
  • the skin around the nostrils or in the nostrils

Candidiasis is a very common cause of vaginal irritation, or vaginitis, and can also occur on the male genitals. Children, mostly between the ages of 3 and 9 years, can be affected by chronic mouth yeast infections, normally seen around the mouth as white patches. However, this is not a common condition.

Symptoms

Vaginal thrush
The image above shows a severe case of vaginal thrush with Candida albicans fungal growth at the mouth and around the sides of the cervix.

Vaginal thrush, also known as candida, vulvovaginal candidiasis, or vaginal candidosis most commonly affects women in their thirties and forties, and those who are pregnant. Many women are affected by vaginal thrush at some point in their lives and, in some women, it recurs regularly.

Candida albicans is often found in the vaginas of women who do not have thrush, but the infection commonly causes irritation and swelling of the vagina and vulva. However, in the majority of cases, thrush can be treated effectively.

The symptoms of vaginal thrush include:

  • vulval itching,
  • vulval soreness and irritation,
  • vaginal discharge,
  • pain, or discomfort, during sexual intercourse (superficial dyspareunia), and
  • pain, or discomfort, during urination (dysuria) may sometimes be present.

As well as the above symptoms of thrush, you may also have vulvovaginal inflammation. The signs of vulvovaginal inflammation include:

  • erythema (redness) — of the vagina and vulva,
  • vaginal fissuring (cracked skin) - in severe cases of inflammation,
  • oedema (swelling from a build up of fluid) - also in severe cases of inflammation, and
  • satellite lesions (sores in the surrounding area) - this is rare, but may indicate the presence of other fungal conditions, or the herpes simplex virus.

Vaginal discharge

A vaginal discharge is a common symptom of vaginal thrush. It is often white and 'cheese-like', but may also be watery or purulent (contain pus). The discharge is usually odourless.

Many women mistake the symptoms of the more common bacterial vaginosis for a yeast infection. In a 2002 study published in the Journal of Obstetrics and Gynecology, only 33 percent of women who were self treating for a yeast infection actually had a yeast infection. Instead they had either bacterial vaginosis or a mixed-type infection.

Vaginal thrush
The image above shows a severe case of oral thrush with Candida albicans fungal growth coating the entire tongue.
Vaginal thrush
The image above shows a severe case of penile thrush with Candida albicans fungal growth on the head and behind the head of the penis.

The other common form of thrush is oral thrush. Most commonly Candidal growth is seen on the tongue. Bui it can also be present on the palate and at the back of the throat.

In men, symptoms include red patchy sores near the head of the penis or on the foreskin, severe itching and/or a burning sensation. Candidiasis of the penis can also have a white discharge, although uncommon. However, having no symptoms at all is common and usually, a more severe form of the symptoms may emerge later.

Symptoms

Accepted risk factors for developing candidosis include:

Antibiotics — hrush occurs in about 30% of women who are taking a course of systemic, or intravaginal antibiotics. Although using any type of antibiotics can increase your risk of getting thrush. However, to develop the condition Candida must already be present.

Pregnancy — changes in the levels of female sex hormones, such as oestrogen, make you more likely to develop thrush. During pregnancy, the Candida fungus is more prevalent, and recurrent infection is also more likely.

Diabetes melitus — If you have diabetes mellitus which is not properly controlled you are significantly more likely to develop thrush.

Immunodeficiency — having an immune system that is weakened by an immunosuppressive condition such as HIV increases the risk of developing thrush. This is because your immune system, which usually fights off infection, is unable to effectively control the spread of the Candida fungus.

Other potential risk factors:

Contraceptives — it is thought that the use of oral contraceptives, particularly combined oral contraceptives, increase the risk of thrush. Though the results of the various studies that have been conducted in this area remain inconclusive.

Sexual behaviour — whilst it is true that thrush is more common during the peak years of sexual activity (during the thirties and forties), there is little evidence to support that the condition is spread during sex, or that there is any benefit in treating sexual partners. Though oral sex may be a contributing factor.

Tight-fitting clothing — wearing tight-fitting clothing, such as tights, or using panty liners, may increase your chances of developing thrush as this keeps the genital area more moist and warm than would otherwise be the case.

Female hygiene — there is little evidence to suggest that sanitary towels are a risk factor for thrush. There is also no evidence that tampons, or vaginal douching, are risk factors for developing the condition.

Hormone Replacement Therapy and infertility treatments may also be predisposing factors.

In penile candidiasis, the causes include sexual intercourse with an infected party, low immunity, antibiotics and diabetes. However, male yeast infection is less common and the risk of getting it is only a fraction of that in women.

Diagnosis

A diagnosis of thrush is usually based on the presence of the symptoms of the condition, such as vaginal or vulval itch, discomfort, pain during sexual intercourse, and a thick, creamy, odourless discharge.

However, if the more common recommended treatments fail to relieve the symptoms or if the itch regularly recurs or if the symptoms are particularly severe then further tests may be conducted. Testing will usually involve a sample of vaginal secretion being taken for analysis in a laboratory.

Typically two methods are utilized to diagnose yeast infections: microscopic examination, and culturing.

For microscopic examination scrapings or swabs are taken from the infected area. these are placed on a microscope slide and a single drop of 10% potassium hydroxide (KOH) solution is then added. The KOH dissolves the skin cells but leaves the Candida untouched, so that when the slide is viewed under a microscope, the hyphae and pseudo spores of Candida are visible. Their presence in large numbers strongly suggests a yeast infection.

For culturing a a sterile swab is rubbed on the infected skin surface. The swab is then rubbed across a culture medium (a growth medium for the fungus) which is then ncubated for several days, during which time colonies of yeast and/or bacteria develop. The characteristics of the colonies provide a presumptive diagnosis of the organism causing symptoms.

Treatment

Candida infections are commonly treated with antifungal or anti mycotic drugs which for mild infections will typically last for 1 to 3 days. Typical Anifungal treatments include:

Oral antifungal treatment

Oral antifungal treatment is usually recommended for girls who have vaginal thrush and are between 12-16 years of age. The antifungal drugs commonly used to treat candidiasis are topical clotrimazole, topical nystatin, fluconazole, and topical ketoconazole. For example, a one-time dose of fluconazole (as Diflucan 150-mg tablet taken orally) has been reported as being 90% effective in treating a vaginal yeast infection. However, oral antifungal treatments can cause side effects that include: nausea, vomiting, diarrhoea, constipation, bloating, and an upset stomach.

Intravaginal pessaries

Intravaginal pessaries that are often recommended include clotrimazole, econazole, or miconazole. They do not cause as many side effects as oral antifungal treatments, but they can: be awkward to use; cause mild irritation, and stinging, when they are inserted, and can damage latex condoms and diaphragms. Therefore, you should use another form of contraception while you are using intravaginal pessaries.

If you are pregnant, or breastfeeding, you will not be prescribed oral antifungal treatment because it may affect your baby. Your GP will probably prescribe an intravaginal pessary, such as clotrimazole, econazole, or miconazole, to be used for at least seven days.

If you have thrush, as well as using OTC treatments, there are also a number of other things that you can do to help ease the problem. These include: washing your vaginal area using water - avoid using perfumed soaps, shower gels, vaginal deodorants, or douches; avoiding using latex condoms, spermicidal creams, and lubricants, if they cause irritation; avoiding wearing tight-fitting, synthetic clothes, and wearing cotton underwear and loose-fitting clothes, where possible.

Treatment for thrush using antifungal medication is ineffective in up to 20% of cases. Treatment for thrush is considered to have failed if the symptoms do not clear up within 7-14 days.

Depression and psychosexual problems (anxiety about having sex) can sometimes occur in women who have recurrent thrush. Your GP will be able to advise you about specialist treatments, such as counselling, if you have either of these conditions.

In rare cases, candidal balanitis (inflammation of the head of the penis) can occur in male partners of women who have thrush.

Some women, who have vaginal thrush, use probiotics, such as live yoghurts, to help treat and prevent the condition. There is no medical evidence that probiotics are effective but, equally, there is no reason to think that they could be unsafe.