Candida is a yeast, not the maple syrup-loving country in North America - although Candida
can be found in Canada as well! Candida sometimes causes a mild yeast infection, but in some
situations, can get into the bloodstream and cause severe illness. Now, there are various
types of Candida species, and over twenty of them cause disease in humans - C. albicans,
C. parapsilosis, C. tropicalis, C. glabrata, C. krusei, C. auris, the list goes on. Of
these, the most common one is C. albicans. Candida is found throughout the body; it likes
warm, moist environments like the mouth, the diaper region of babies, and in women it can
be found in the vagina.
Now, it’s normal for microbes - bacteria, fungi, and viruses - to live all over the
body, but each microbe is slightly different in terms of whether it’s colonizing the
body - in other words just living and not causing any problems, or whether it’s infecting
the body, causing some degree of tissue damage or destruction. An important factor is exactly
how much of a microbe is present. Candida is considered an opportunistic microbe. When
the amount of Candida is relatively low, it's harmless. But if a person’s immune system
is weakened or if there’s less competition for the Candida, then the amount of Candida
can increase - and that’s called Candida overgrowth.
Now, Candida can exist in multiple forms - it’s a bit like a chameleon. All candida species
can take on a round or oval shape, and these are called yeast cells. One candida species,
called Candida albicans, can also appear like pseudohyphae where it looks like long thin
filaments - kind of like a segmented cactus plant. So, these two shapes of Candida albicans
express: different protein profiles, and they give the cells different properties. In “yeast
mode”, Candida is better at moving from one part of the body to another, whereas in
“filamentous mode” it’s better at invading tissues.
Candida typically lives on the skin or mucous membranes, and when it starts to overgrow
it can damage nearby tissue. There are a few patterns of injury, the most common one is
pseudomembranous candidiasis, and it’s primarily due to a weakened immune system that allows
for Candida overgrowth. The result is destruction of the stratified squamous epithelium layer,
which is the outermost layer of the skin or mucous membranes. This causes accumulation
of the destroyed cells and the keratin protein that fills that outermost layer, forming a
white lesion, called a pseudomembrane, that looks kind of like “cottage cheese”. The
white lesions aren’t typically painful and they can be scraped away with a tongue depressor,
leaving behind a red mucosal base which sometimes bleeds. Since the underlying cause is a weakened
immune system, it’s fairly common in young infants and the elderly, both groups that
have a relatively weak immune system. It can also be related to an immunosuppressive condition
like diabetes or HIV, or from immunosuppressive medical treatments like steroids - including
inhaled steroids, as well as radiotherapy or chemotherapy.
Another pattern of injury is called erythematous candidiasis and that typically results from
a change in the levels of microbial competition keeping Candida in check. For example, a course
of antibiotics or tobacco smoking, which both selectively destroy certain bacterial populations
more than they affect Candida. The opposite is true as well, sometimes there are mechanical
devices like braces that favor Candida growth more than the growth of other microbes. In
either situation, the result is an overgrowth of Candida which causes increased blood flow
to the affected tissue with red painful lesions. In a lot of situations there’s a mixed pattern
of injury with both a pseudomembranous and erythematous component.
Candidal infections can affect various parts of the body. The most common location is in
the mouth where it causes thrush - which is usually a pseudomembranous candidiasis, so
classically it looks like white cottage cheese on the buccal and gingival mucosa. Candida
can also affect the esophagus, resulting in an esophagitis that can cause pain with swallowing.
Candida can affect the diaper region of babies, causing a diaper rash with redness surrounded
by scattered red spots that are called satellite lesions because they look like tiny red satellites
that are not contiguous with the rest of the rash. Another common one is vulvovaginitis,
commonly called a yeast infection in women, and it can cause vaginal itching and discharge
as well as pain while urinating.
Now in addition to local infections, Candida can cause more serious invasive infections,
and these often develop in individuals that have underlying immunodeficiencies. Other
risk factors for serious candidal infections include hyperglycemia and reduced stomach
acidity. The most common situation is an infection of a prosthetic device like a central venous
catheter. In these settings, Candida forms a biofilm which is where the yeast lives within
a jelly like matrix of proteins and behaves more like a large colony than like a collection
of individual cells. Candida is able to infect devices and if the infection isn’t treated,
then over time, small clumps of the yeast-laden biofilm can break away, get into the blood,
causing candidemia. From the blood, Candida can spread to other locations, which is known
as invasive candidiasis. Commonly affected sites are the heart valves, the liver and
spleen as well as the bones and joints. Candida can also cause kidney infections, either as
a complication of candidemia, or as a complication of a urinary tract infection. Candida can
also cause meningitis, in particular if it infects a device in that space - like a ventricular
shunt. What’s more is that Candida can cause particularly worrisome infections of the eye.
Diagnosis of oral candidiasis can be confirmed with a culture, and ultimately a tissue biopsy
might be done in some cases as well. For invasive infections, the gold standard of diagnosis
is a positive blood culture, but blood cultures can be negative a lot of the time. Alternatively,
antigen testing can also be done - for example one test looks for a cell wall component called
beta-D-glucan which is found in Candida and a few other fungal species. Another test is
Candida PCR which detects candidal DNA.
Treatment of candidiasis depends on the location and severity of the infection. Oral thrush
is treated with oral nystatin suspension, whereas vulvovaginitis and skin infections
are treated with topical antifungals. Resistant infections are often treated with azole antifungal
medications. In severe infections, especially when there’s a prosthetic device like a
catheter or a ventricular shunt that’s infected with a biofilm, it’s often necessary to
remove the device. Severe infections are often treated with a medication like amphotericin,
azole antifungals, or echinocandins like Micafungin.
Alright recap: Candida is an opportunistic fungus that can overgrow in situations where
there are fewer competitive microorganisms or a weakened immune system. Common sites
of infection include the mouth, the esophagus, the vulvovaginal region, and the groin in
infants. These superficial infections are usually treated with nystatin or topical antifungals.
Severe infections include those where a biofilm forms over a prosthetic device, and typically
require removal of the infected device.