6.2 Clinically Relevant Viruses & Parasitic Infections

6.2 Clinically Relevant Viruses & Parasitic Infections

Independent study guide by OpenExamPrep.

Core Examination Pearl: Board examiners heavily test the clinical and histopathologic differentiation between Verruca plantaris (plantar wart) and Heloma durum (hard corn), the molecular mechanisms of HPV oncoproteins (E6/E7), the serologic interpretation of Hepatitis B panels (vaccine vs. natural immunity vs. chronic carrier), and the diagnosis of lower extremity parasitic infestations such as cutaneous larva migrans and scabies.

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1. Human Papillomavirus (HPV): Plantar Warts & Oncogenesis

Human Papillomavirus (HPV) is a non-enveloped, double-stranded circular DNA (dsDNA) virus belonging to the family Papillomaviridae. It exhibits strict tropism for stratified squamous keratinocytes, infecting basal epithelial stem cells exposed via microscopic abrasions.

Clinical Podiatric Manifestations & HPV Genotypes

• Verruca Plantaris (Plantar Wart): Caused predominantly by HPV-1 (deep, painful, hyperkeratotic myrmecia warts), as well as HPV-2, HPV-4, and HPV-63.
• Verruca Vulgaris (Common Wart): Caused by HPV-2 and HPV-4; exophytic hyperkeratotic papules on digits and dorsal feet.
• Mosaic Warts: Caused primarily by HPV-2; multiple coalescent, superficial, plaque-like clusters of plantar warts covering large areas of the heel or metatarsal surface, notoriously resistant to destructive therapy.
• Condylomata Acuminata (Anogenital Warts): Benign genital warts caused by low-risk mucosal types HPV-6 and HPV-11.

Clinical Differentiation: Verruca Plantaris vs. Heloma Durum (Corn)

The differential diagnosis between a plantar wart and a mechanical keratoma (heloma durum / hard corn / callus) is one of the most frequently tested concepts in podiatric board examinations:

+-----------------------------------------------------------------------------------------+
|                       VERRUCA PLANTARIS vs. HELOMA DURUM / CALLUS                       |
+----------------------------+-----------------------------+------------------------------+
| Clinical Feature           | Verruca Plantaris (Wart)    | Heloma Durum / Callus (Corn) |
+----------------------------+-----------------------------+------------------------------+
| Primary Etiology           | Viral infection (HPV 1, 2)  | Repetitive friction & shear  |
| Dermatoglyphics            | **Interrupted / Diverge**   | **Intact & Continuous**      |
| (Skin Lines)               | around the lesion           | passing through the lesion   |
| Scalpel Paring             | **Pinpoint bleeding dots**  | **Hard translucent core**    |
| (Debridement)              | (thrombosed capillaries)    | (central enucleation; dry)   |
| Provocative Pain Vector    | **Lateral Squeezing**       | **Direct Perpendicular**     |
|                            | (side-to-side pinching)     | (downward vertical pressure) |
| Anatomic Predilection      | Any plantar site / non-WT   | Over osseous prominences     |
| Histopathology             | **Koilocytes**, papilloma   | Compact orthokeratosis       |
+----------------------------+-----------------------------+------------------------------+

Histopathology of Verruca Plantaris

• Koilocytes (Pathognomonic): Cytopathic hallmark consisting of enlarged keratinocytes residing in the upper stratum spinosum and granulosum displaying pyknotic, hyperchromatic eccentric nuclei surrounded by a clear, prominent perinuclear cytoplasmic halo.
• Additional Histologic Features: Marked hyperkeratosis, papillomatosis (finger-like projection of dermal papillae), parakeratosis (persistence of nuclei in the stratum corneum), elongated and tortuous dermal capillaries directed toward the surface, and coarse, irregular aggregates of keratohyalin granules.

Molecular Oncogenesis of High-Risk HPV (Types 16 and 18)

High-risk mucosal types (HPV-16, HPV-18, 31, 33) drive cervical, anal, and penile squamous cell carcinoma through the expression of two potent viral oncoproteins that integrate into host genomic DNA:

• E6 Oncoprotein: Binds to the cellular tumor suppressor protein p53 and recruits an E3 ubiquitin ligase (E6-AP), targeting p53 for degradation via the 26S proteasome. Loss of p53 eliminates the cell cycle checkpoint for G1/S arrest and impairs apoptosis following DNA damage.
• E7 Oncoprotein: Binds to and phosphorylates the hypophosphorylated Retinoblastoma protein (pRb), inducing its dissociation from the E2F transcription factor. Free E2F translocates to the nucleus and drives transcription of genes required for entry into the S phase of the cell cycle.

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2. Herpesviridae in Lower Extremity & Systemic Pathology

Members of the family Herpesviridae are large, enveloped, double-stranded linear DNA (dsDNA) viruses possessing an icosahedral capsid surrounded by a proteinaceous tegument. Their defining biological characteristic is the ability to establish lifelong latent infection within sensory or lymphoid cells, with periodic reactivation:

1. Herpes Simplex Virus Types 1 and 2 (HSV-1 & HSV-2)

• Latency: HSV-1 establishes latency in the trigeminal ganglion (predominantly oral-labial lesions). HSV-2 establishes latency in the sacral sensory ganglia (S2–S4) (predominantly genital herpes).
• Herpetic Whitlow: A painful, grouped, non-purulent vesicular eruption on an erythematous base occurring on the distal phalanx of a digit (toes or fingers). Occurs in healthcare personnel (dentists, surgical podiatrists) following direct percutaneous exposure to infected oral secretions, or via autoinoculation from genital lesions.
• Diagnostic Testing:
  • Tzanck Smear: Scraping of the base of an unroofed vesicle stained with Giemsa or Wright stain. Demonstrates multinucleated giant cells with nuclear molding and Cowdry A inclusion bodies (large, eosinophilic intranuclear inclusions surrounded by a clear halo). (Note: Tzanck smear cannot distinguish HSV-1, HSV-2, or VZV).
  • PCR (Polymerase Chain Reaction): The clinical diagnostic gold standard with superior sensitivity and type-specificity.

2. Varicella-Zoster Virus (VZV / HHV-3)

• Primary Infection (Varicella / Chickenpox): Asynchronous, pruritic rash ("dewdrops on a rose petal" evolving from macules to papules to vesicles and crusts). Establishes lifelong latency in the dorsal root ganglia or cranial nerve sensory ganglia.
• Reactivation (Herpes Zoster / Shingles): Triggered by cellular immune decline or physical stress. Presents as a severe, stabbing, dermatomal neuralgic pain followed by a unilateral, closely grouped vesicular eruption restricted strictly to a single dermatome that does not cross the anatomical midline.
  • Lower Extremity Zoster: Frequently involves the L4, L5, or S1 dermatomes, presenting with excruciating radicular shooting pain, dysesthesia, and vesicular eruptions across the anterolateral leg, dorsum of the foot, or plantar heel.
  • Post-Herpetic Neuralgia (PHN): The most frequent debilitating complication, defined as severe neuropathic pain persisting for more than 90 days following resolution of the acute cutaneous rash.

3. Cytomegalovirus (CMV / HHV-5)

• Establishes latency in mononuclear leukocytes (monocytes, dendritic cells). Causes severe opportunistic retinitis, colitis, and painful lower extremity polyradiculopathy / peripheral neuropathy in severely immunosuppressed patients (AIDS with CD4 <50 cells/mm$^3$).
• Histopathology: Characteristic "owl's eye" inclusion bodies (massive basophilic intranuclear inclusion surrounded by a clear halo within markedly enlarged cytomegalic endothelial cells).

4. Epstein-Barr Virus (EBV / HHV-4)

• Binds the CD21 receptor on naive B lymphocytes. Causes infectious mononucleosis (triad of fever, exudative pharyngitis, and posterior cervical lymphadenopathy). Diagnostic markers: atypical reactive CD8+ T cells (Downey cells) on peripheral smear, and positive Monospot test (detects heterophile antibodies agglutinating sheep or horse red blood cells).

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3. Poxviridae: Molluscum Contagiosum

Molluscum Contagiosum Virus (MCV) belongs to the family Poxviridae. Poxviruses are exceptional among DNA viruses because they are large, brick-shaped, enveloped dsDNA viruses that replicate entirely within the host cytoplasm (encoding their own DNA-dependent RNA polymerase).

• Clinical Presentation: Firm, dome-shaped, smooth, pearly or flesh-colored papules (2 to 5 mm) with a pathognomonic central umbilication from which a curd-like waxy core can be expressed. Most common on the trunk and extremities in children, or genital/lower extremity regions in sexually active adults.
• Immunocompromised Host: In patients with advanced HIV/AIDS, molluscum lesions become giant (>1 cm), widely disseminated across the extremities, and highly recalcitrant to therapy.
• Histopathology: Large, eosinophilic, smooth intracytoplasmic inclusion bodies within keratinocytes that displace the host nucleus, termed Henderson-Patterson bodies (molluscum bodies).

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4. Viral Hepatitis: Occupational Exposure & Serology in Surgical Podiatry

Podiatric surgeons face continuous occupational exposure to bloodborne viral pathogens through sharps injuries, power-saw aerosols, and bone fragments during foot and ankle reconstructive surgery.

+-----------------------------------------------------------------------------------------+
|                        BLOODBORNE OCCUPATIONAL TRANSMISSION RISKS                       |
+-----------------------+-----------------------------+-----------------------------------+
| Viral Pathogen        | Viral Nucleic Acid & Family | Transmission Risk via Needlestick |
+-----------------------+-----------------------------+-----------------------------------+
| Hepatitis B (HBV)     | Partially dsDNA / Hepadna   | **~30%** (highest risk; 6–30%)    |
| Hepatitis C (HCV)     | (+) ssRNA / Flaviviridae    | **~3%** (intermediate; 1.8–3%)    |
| HIV-1                 | Diploid (+) ssRNA / Retro   | **~0.3%** (lowest risk; 0.2–0.3%) |
+-----------------------+-----------------------------+-----------------------------------+

1. Hepatitis A Virus (HAV)

• Virology: Picornaviridae; positive-sense single-stranded RNA (+ssRNA), non-enveloped. Transmitted via the fecal-oral route (contaminated water, shellfish).
• Clinical Course: Acute, self-limited hepatitis with jaundice; never establishes a chronic carrier state.
• Serology: Anti-HAV IgM signifies acute active infection; Anti-HAV IgG denotes prior exposure or vaccination (protective long-term immunity).

2. Hepatitis B Virus (HBV)

• Virology: Hepadnaviridae; partially double-stranded circular DNA with an envelope. Possesses an internal reverse transcriptase that synthesizes genomic DNA from an intermediate pregenomic RNA template.
• Transmission: Percutaneous, sexual, or perinatal bloodborne transmission.
• Serologic Profiling & Board Interpretation:
  • HBsAg (Surface Antigen): The first serologic marker to appear; signifies active infection (either acute or chronic if persisting >6 months).
  • Anti-HBs (Surface Antibody): Indicates clinical recovery and protective immunity (produced following successful vaccination OR resolved natural infection).
  • HBcAg (Core Antigen): Internal nucleocapsid antigen; sequestered within hepatocytes and not detectable in circulating serum.
  • Anti-HBc IgM: The first antibody to arise; definitive marker of acute infection. Crucially, it serves as the sole positive serologic marker during the "window period" (the interval where HBsAg has been cleared by host defenses but Anti-HBs has not yet reached detectable serum titers).
  • Anti-HBc IgG: Marker of past natural infection (persists for life in resolved natural infection or chronic carrier states). Anti-HBc IgG is strictly ABSENT following recombinant HBsAg vaccination!
  • HBeAg (Envelope Antigen): Soluble nucleocapsid protein secreted into serum; indicates active viral replication, high infectivity, and extreme transmission risk.
  • Anti-HBe: Signifies cessation of active replication and reduced transmission risk.

Table 1: Interpretation of Hepatitis B Serologic Panels

3. Hepatitis C Virus (HCV)

• Virology: Flaviviridae; enveloped, positive-sense single-stranded RNA (+ssRNA). Exhibits marked genetic hypervariability due to an RNA-dependent RNA polymerase lacking 3'-to-5' proofreading activity (antigenic drift of envelope glycoproteins).
• Natural History: Acute infection is asymptomatic in ~80% of patients, but chronic carrier state develops in 75% to 85% of infected individuals, leading to cirrhosis and hepatocellular carcinoma.
• Podiatric Systemic Manifestation (Mixed Cryoglobulinemia): HCV is the leading cause of Type II and Type III mixed cryoglobulinemia. Cryoglobulins (immunoglobulins that precipitate reversibly at temperatures <37°C) deposit in the microvasculature of the lower extremities, presenting with palpable purpura on the legs and feet, digital ischemia, painful peripheral sensory neuropathy, arthralgias, and membranoproliferative glomerulonephritis.

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5. Retroviruses: HIV & Lower Extremity Clinical Manifestations

Human Immunodeficiency Virus (HIV) belongs to the genus Lentivirus in the family Retroviridae. It is an enveloped, diploid positive-sense RNA virus containing two identical single-stranded RNA strands.

Viral Architecture & Structural Genes

• Gag Gene: Encodes core structural proteins: p24 (capsid protein, targeted in initial antigen/antibody screening tests) and p17 (matrix protein).
• Pol Gene: Encodes essential viral enzymes: Reverse Transcriptase (synthesizes complementary DNA from viral RNA template), Integrase (integrates proviral cDNA into the host cell chromosome), and Protease (cleaves polyprotein precursors into functional mature virions).
• Env Gene: Encodes the envelope precursor glycoprotein gp160, cleaved by host enzymes into:
  • gp120: Surface glycoprotein that binds to the primary host CD4 receptor on helper T cells and co-receptors (CCR5 on macrophages for R5 strains; CXCR4 on T cells for X4 strains).
  • gp41: Transmembrane stalk glycoprotein mediating viral fusion and penetration into the host target membrane.

CD4 Count Thresholds & Podiatric Opportunistic Pathology

Monitoring absolute CD4+ T-cell counts guides the staging of HIV progression and predicts clinical vulnerability to lower extremity manifestations:

+-----------------------------------------------------------------------------------------+
|                         HIV CD4 COUNT & PODIATRIC MILESTONES                            |
+-----------------------+-----------------------------------------------------------------+
| CD4+ T-Cell Count     | Associated Lower Extremity & Systemic Pathologies               |
+-----------------------+-----------------------------------------------------------------+
| >500 cells/mm³        | Normal immune surveillance; minor increase in tinea pedis       |
+-----------------------+-----------------------------------------------------------------+
| 200–500 cells/mm³     | • Recalcitrant, extensive verruca plantaris (resistant to acid) |
|                       | • Severe, recurrent interdigital tinea pedis                    |
|                       | • Oral candidiasis (thrush); multidermatomal herpes zoster      |
+-----------------------+-----------------------------------------------------------------+
| <200 cells/mm³        | • AIDS Definition established                                  |
|                       | • Pneumocystis jirovecii pneumonia (PCP) prophylaxis mandatory  |
|                       | • Kaposi Sarcoma (HHV-8) - violaceous plaques on feet/soles     |
+-----------------------+-----------------------------------------------------------------+
| <100 cells/mm³        | • Cryptococcus neoformans (meningitis, cutaneous papules)       |
|                       | • Toxoplasma gondii encephalitis; Bartonella henselae           |
+-----------------------+-----------------------------------------------------------------+
| <50 cells/mm³         | • Cytomegalovirus (CMV) retinitis & lower extremity neuropathy  |
|                       | • Mycobacterium avium complex (MAC) bacteremia                  |
+-----------------------+-----------------------------------------------------------------+

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6. Parasitology of the Lower Extremity

1. Sarcoptes scabiei (Scabies)

• Etiology: Sarcoptes scabiei var. hominis, an obligate human parasitic mite (arachnid).
• Pathogenesis: The fertilized adult female mite burrows into the stratum corneum of the epidermis, traveling up to 2–3 mm per day while laying 2 to 3 eggs daily and depositing fecal pellets (scybala). The intense pruritus represents a Type IV delayed hypersensitivity reaction to mite antigens, ova, and feces, typically developing 3 to 6 weeks after initial primary exposure.
• Clinical Presentation: Intensely pruritic, small, erythematous papules and thread-like, thin, serpiginous burrows (often capped by a tiny vesicle). A hallmark feature is severe nocturnal exacerbation of pruritus. Predilection sites include the interdigital web spaces of the toes and fingers, flexor wrists, ankles, dorsum of feet, axillae, and genitalia.
• Crusted (Norwegian) Scabies: Occurs in severely immunocompromised patients (e.g., advanced AIDS, organ transplant recipients, lepromatous leprosy). Characterized by thick, hyperkeratotic, psoriasiform, fissured crusts teeming with millions of live mites. Pruritus is notably mild or absent due to deficient cellular immunity; highly contagious.
• Diagnosis & Treatment: Microscopic identification of adult mites, oval ova, or scybala on mineral oil skin scrapings of unexcoriated burrows. Treatment of choice is topical 5% permethrin cream applied from neck to toes (washed off after 8–14 hours) or oral ivermectin.

2. Ancylostoma braziliense (Cutaneous Larva Migrans / "Creeping Eruption")

• Etiology: Zoonotic hookworms of domestic dogs and cats—principally Ancylostoma braziliense and Ancylostoma caninum.
• Transmission: Adult hookworms shed eggs in canine and feline feces, which hatch into rhabditiform and then infective filariform larvae in warm, moist, sandy soil or beaches. Humans contract infection by walking barefoot on contaminated sand or soil.
• Pathophysiology: Filariform larvae penetrate the intact human epidermis. Because humans are accidental hosts, the larvae lack the specialized collagenases and hyaluronidases necessary to penetrate the dermoepidermal basement membrane into the dermis and systemic venous circulation. Consequently, the larvae remain trapped within the stratum germinativum of the epidermis, wandering aimlessly at a rate of several millimeters to a few centimeters per day.
• Clinical Presentation: An intensely pruritic, raised, erythematous, serpiginous (snake-like), tortuous creeping eruption advancing across the dorsum of the foot, plantar sole, or interdigital spaces. Excoriation frequently leads to secondary bacterial staphylococcal infection.
• Treatment: Oral albendazole or ivermectin, or topical albendazole.

3. Leishmania (Cutaneous Leishmaniasis)

• Etiology: Leishmania braziliensis, L. tropica, or L. mexicana; flagellated protozoa.
• Transmission: Transmitted through the bite of the female phlebotomine sandfly (Phlebotomus in the Old World; Lutzomyia in the New World).
• Life Cycle & Pathology: The sandfly injects flagellated promastigotes into human skin during a blood meal. Promastigotes are phagocytosed by dermal macrophages, where they transform into non-flagellated amastigotes that multiply intracellularly and lyse host macrophages.
• Clinical Presentation: Begins as an asymptomatic erythematous papule on exposed lower extremities, which enlarges over weeks to form a chronic, indurated, painless or tender crateriform ulcer with raised, rolled, indurated violaceous borders ("oriental sore").
• Diagnosis: Giemsa-stained smear of ulcer scrapings or punch biopsy demonstrating tiny oval amastigotes with a visible nucleus and rod-shaped kinetoplast inside dermal macrophages.

Systemic Protozoa and Helminths

The parasitology heading also covers organisms that are not specific to the skin. High-yield examples with lower-extremity relevance:

Eosinophilia with skin entry through bare feet should prompt questions about travel and walking barefoot.