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Molluscum contagiosum is a viral, self-limited infection of the skin caused by the molluscum contagiosum virus, which is a pox virus. This condition is quite common, and it accounts for approximately one percent of all diagnoses in dermatology. The causative virus can be transmitted by casual or sexual contact, via infected fomites, and also by self-inoculation.
The disease has a predilection for children, younger sexually active adults, as well as immunocompromised individuals. Molluscum lesions typically present as white or skin-colored papules that have an umbilicated center, and may affect any part of the body. Other morphologic variants of molluscum contagiosum may be observed (such as giant lesions larger than 5 millimeters or eczematous lesions), and some of them may predispose the individual to secondary abscess development.
As molluscum lesions are usually characteristic and pathognomonic of the disease, clinical examination and evaluation are pivotal steps in diagnosing it. Sometimes they are very small, so using a magnification lens to observe their umbilicated nature can be of help in certain instances.
A more definite diagnosis can be established by performing a histological examination of biopsied or curetted lesions, especially in more complex cases or those that are not clinically straightforward. A demonstration of acidophilic, uniform bodies (also known as Henderson-Paterson bodies) is highly specific for molluscum contagiosum.
Nevertheless, biopsy is invasive, costly and more time-consuming than cytologic sampling (sometimes referred to as in-office squash preparation). The latter is hence often used as there are usually multiple lesions with a central pore from which keratinous material can be easily expressed.
Such expressions are then smeared on a slide, which is left unstained or stained with Giemsa, Wright, Gram, 10% potassium hydroxide or Papanicolaou stains to demonstrate large, brick-shaped inclusion bodies. In certain instances (mostly for experimental purposes) electron microscopy of fixed material is employed to visualize structures specific to poxviruses.
Immunohistochemical methods that use polyclonal antibodies allow recognition of molluscum contagiosum virus in fixed tissue, while in situ hybridization for the genetic material (DNA) of the virus has also been used. On the other hand, some classic virological approaches (such as complement fixation, neutralization, fluorescent antibodies and tissue cultures) are not routinely utilized for diagnosing this condition.
Skin conditions that need to be considered in differential diagnosis, or that may also coexist with molluscum contagiosum, are genital warts, basal cell carcinoma, keratoacanthoma, syringoma, sebaceous adenoma, lichen planus, varicella zoster virus, as well as histoid leprosy.
Also, it is important to exclude cutaneous cryptococcal infections in patients with acquired immunodeficiency syndrome (AIDS), as eruptions mimicking molluscum contagiosum have been described. Similarly, primary cutaneous histoplasmosis in HIV-positive patients can also be a confounding diagnosis.
Today, parents of affected children often use the Internet in order to self-assess and diagnose molluscum contagiosum in their children. But a recent study has shown that, albeit photographs alone may be an effective tool for correct diagnosis, in 35% of cases such an approach was insufficient for a definitive diagnosis.
Therefore the appropriate awareness of distinctive cytological features of molluscum contagiosum significantly aids in correctly diagnosing the condition – even in clinically unsuspected cases. Inability to identify the HP bodies may result in misinterpretation of molluscum lesions as inclusion epidermal cysts. If sexually acquired, molluscum contagiosum in adults should always raise the suspicion of potential co-infections, so screening panels for other sexually transmitted infections should be offered.