Chapter 21: Skin and Eye Infections - Vocabulary Flashcards

Bacterial infections of the skin: Staphylococcus aureus

Gram status and morphology: Staphylococcus aureus is a gram-positive cocci, commonly in clusters; historically familiar from lab work.
Key pathogenic features: exotoxin production and the enzyme penicillinase contribute to high virulence and antibiotic resistance.
Antibiotic resistance: S. aureus rapidly develops resistance to various antibiotics; MRSA = methicillin-resistant Staphylococcus aureus (the most well-known resistant strain).
Trends in MRSA: data show increasing MRSA prevalence across patient groups (ICU to outpatients) over time; graph shown to illustrate the rise, but not something to be tested on directly.

Localized infections caused by Staphylococcus aureus

Boils (furnuncles): generally benign but rupture creates portals for secondary bacterial infections from organisms beneath the nails or skin, risking bacteremia and systemic infection.
Impetigo: superficial infection with blisters, often on the face of children; staph aureus is a common cause but not the only one. Adults can get impetigo too. Scratching facilitates transmission to other children. Variability in severity shown in images.
Systemic infections related to S. aureus exotoxins: systemic infections affect multiple sites; boils and impetigo are localized, whereas scalded skin syndrome is a systemic/exotoxin-mediated presentation.
Scalded skin syndrome (exfoliative toxin-mediated): widespread exfoliation and peeling of skin; looks like severe burn or scald; can be gruesome but is preventable with good hygiene and treated as a systemic infection.
Visuals illustrate the face and large areas of skin peeling in severe cases.

Bacterial infections of the skin: Streptococcus

Morphology and gram status: Streptococcus species (gram-positive cocci).
Hemolysis on blood agar: hemolytic enzymes range from beta (β) to alpha (α) to gamma (γ); degree of hemolysis correlates with pathogenicity.
Antibiotic resistance context: rising resistance in nosocomial pathogens includes vancomycin-resistant Enterococci and penicillin-resistant Streptococcus pneumoniae; trends show increasing penicillin-resistant S. pneumoniae in various settings.

Localized infections caused by Streptococcus

Strep throat: caused by Streptococcus pyogenes; 5–15% of individuals are carriers, though exam questions about exact percentages are unlikely. Many sore throats are of viral origin; testing is needed to confirm strep as the cause.
Classic strep throat symptoms: red and inflamed epiglottis, throat swelling, general sore throat. Antibiotics can treat strep throat if caused by S. pyogenes.
Persistent and severe systemic infection: necrotizing fasciitis (NF) or flesh-eating bacteria.
- NF can progress rapidly, destroying tissue and potentially causing death within 12–24 hours; not every case is this extreme, but rapid progression is possible.
- Risk factors: immunocompromised individuals; recent chickenpox increases risk due to potential secondary infection (strep entering via skin lesions or scratching). Any itchy rash could be a portal of entry and bloodstream invasion.
- Clinical presentation: early mild symptoms can obscure the source; NF is highly destructive and results in significant tissue necrosis if untreated.
- Prevention and treatment: standard infection control and antibiotics; management can be difficult due to rapid progression.

Pseudomonas infections

Microbiology: Pseudomonas is a gram-negative bacillus; highly motile with multiple flagella; forms a protective capsule (glycocalyx) and produces both endotoxins and exotoxins.
Ecological niche and epidemiology: ubiquitous in soil and water; primarily affects immunocompromised hosts; a common nosocomial pathogen; prevention in hospital settings is critical (e.g., restrictions on entering ICU with food, plants, and water sources to limit spread).
Clinical impact: can be localized infections in immunocompetent individuals but is dangerous and can be fatal in immunocompromised patients; untreated infections in select cases carry about a 50% fatality rate in compromised hosts.
Distinctive clinical features: hot tub folliculitis (rash after exposure to warm water), especially in warm environments; characteristic grape-like odor; blue-green pus produced in infections; can cause pneumonia with blue-green sputum; bacteremia if systemic.
Prevention and treatment: control of environmental exposure and appropriate antibiotic therapy; emphasis on nosocomial risk.

Viral infections affecting the skin and eyes (overview)

Human papillomavirus (HPV): causes warts on skin (non-genital and genital locations). Warts are common and spread easily because the virus infects the top layer of the epidermis; thus, regrowth is not immediately recognized by the immune system, preventing prompt regression. Transmission can be direct (skin-to-skin) or indirect (via surfaces like door handles). Treatments vary; duct tape therapy is mentioned as a non-traditional method that works by creating a low-oxygen, warm environment under the tape, killing affected epidermal cells rather than directly targeting the virus. Warts come in various shapes (classic wart vs. cauliflower-type wart).
Herpes simplex virus (HSV): two types, HSV-1 and HSV-2, with a common misconception that HSV-1 causes only oral herpes and HSV-2 only genital herpes. Either type can infect either site. HSV is enveloped, enabling longer survival on fomites; infection persists for life with periodic outbreaks. Outbreaks may be triggered by UV light, hormonal changes, and stress. Healing typically spans $2 ext{ to } 4 ext{ weeks}$ , though some people experience no symptoms or intermittent outbreaks. Self-inoculation can spread lesions if fingers contact another site. Routes of transmission: direct contact with vesicle fluid or through mucosal surfaces; crucial for pregnancy due to neonatal herpes risk.
Varicella-zoster virus (VZV): causes chickenpox and later can reactivate as shingles. Chickenpox is spread primarily by respiratory transmission; the virus remains latent and may reactivate years later as shingles. A vaccine was licensed in 1995; long-term (lifelong) immunity after vaccination is still under study. Chickenpox can have internal complications (e.g., encephalitis). Shingles tends to be more common after age ~ $50$ due to waning cell-mediated immunity; shingles is extremely painful and can have lasting postherpetic neuralgia. If chickenpox occurs naturally, shingles is not contagious; vaccination status complicates this transmission possibility. Antiviral therapy (e.g., acyclovir) is used for VZV infections.

Eye infections from viruses

Conjunctivitis (pink eye): can be bacterial or viral, and sometimes an allergen or irritant; infectious potential varies depending on cause.
Herpes conjunctivitis: caused by HSV, though less common, but significantly more serious due to potential spread along the optic nerve to the CNS.

Fungal infections of the skin

Ringworm (dermatophytosis): caused by dermatophyte fungi; extremely contagious and itchy; the name is historical (not caused by a real worm). The infection forms a circular, raised, red-edged lesion; spread can be indirect (from animals, especially cats) or direct. Hair and nails can be affected.
Clinical features and treatment: skin infections are generally treatable with topical antifungals; hair or nail involvement may require oral antifungal pills; secondary bacterial infections can occur due to itching and skin breaks. Prevention includes hygiene and limiting contact with infected animals.

Eye infections caused by helminths (parasitic worms)

River blindness (Onchocerca volvulus): vector-borne infection transmitted by black flies in tropical regions (Africa is most affected; ~90 ext{%} of cases occur in Africa). Key features include intense itching and eye lesions leading to blindness with repeated exposure near river banks where black flies breed. Adult female worms and larval production lead to a heavy worm burden: roughly $1{,}300$ to $1{,}900$ worms per day deposited in tissues, with the reproductive life of $9 ext{ to } 11 ext{ years}$ . Management relies on ivermectin, though the disease is challenging to eradicate in remote areas due to logistics and vector exposure.
Loa loa (loiasis): transmitted by deer flies; larvae enter via bite and mature into adult worms that migrate through skin. Incubation can be months to years; larvae are active in the blood/lymph during the day and migrate to the lungs at night. Severe disease can lead to death or serious complications such as encephalitis. Ivermectin is used for treatment but can provoke toxicity due to parasite death and immune activation. The worm can sometimes be observed migrating under the eye, which aids diagnosis.
General note on helminths: these infections are multicellular eukaryotes; treatments (like ivermectin) can be toxic because drugs target eukaryotic organisms similar to human cells; drug-induced immune responses can cause additional tissue damage.

Practical and public-health implications

Hygiene and infection control are repeatedly highlighted as preventive measures (e.g., for scalded skin syndrome, halting spread of impetigo, and reducing nosocomial Pseudomonas infections).
The content emphasizes the ecological niches and vectors (black flies for river blindness; deer flies for Loa loa; hot tubs for Pseudomonas) as crucial factors in transmission and epidemiology.
The discussion integrates resistance trends (MRSA, penicillin-resistant streptococci, vancomycin-resistant Enterococci) to illustrate the importance of antibiotic stewardship and infection control in clinical settings.
The material connects skin infections to potential systemic complications (e.g., NF, bacteremia, conjunctivitis with CNS involvement, and parasites causing ocular damage) to underline the importance of early recognition and treatment.
After Chapter 21, the course transitions to pathogens affecting the nervous system (Chapter 22), signaling a broader framework that links local infections to systemic and neurotropic consequences.

Quick reference: key numbers and identifiers (LaTeX-formatted)

Staph aureus MRSA trend: increases over time; no fixed percentage given.
Scalded skin syndrome: systemic exotoxin effect with widespread skin peeling.
NF (necrotizing fasciitis): can destroy tissue and cause death in $12 ext{ to } 24 ext{ hours}$ ; linked to cut injuries and potential secondary infection.
Strep throat: carrier rate around 5 ext{-}15 ext{%} (range not fixed; exam not focused on exact value).
NF risk factors: immunocompromised status; chickenpox as a risk factor due to secondary infection potential.
Pseudomonas: nosocomial bias; localized infections in immunocompetent adults; 50 ext{%} fatality in immunocompromised without treatment.
Pseudomonas characteristics: gram-negative bacillus; motile with multiple flagella; capsule (glycocalyx); produces endo- and exotoxins; grape-like odor; blue-green pus.
HPV warts: external, non-invasive to immune detection; transmission via direct/indirect contact; variable regression.
HSV (type I and II): two types; both may cause oral or genital lesions; enveloped virion; lifelong persistence; healing in $2 ext{ to }4 ext{ weeks}$ ; triggers include UV, hormones, stress; fetal risk if transmitted during delivery.
Varicella-zoster virus: chickenpox; respiratory transmission; latency with potential reactivation as shingles; vaccine licensed in 1995; lifelong immunity still under study; shingles more common after $ext{age} \ge 50$ ; antiviral therapy (e.g., acyclovir).
Ringworm (dermatophytes): contagious and itchy; circular rash with raised red edge; hair/nail involvement requires oral antifungals; contact with infected animals (especially cats) is a key risk.
Styes: typically caused by Staphylococcus aureus; multiple styes may require antibiotics.
Conjunctivitis: bacterial, viral, or allergic causes; herpes conjunctivitis is particularly serious due to CNS spread via the optic nerve.
River blindness (Onchocerca volvulus): vector = black fly; incubation ~ $1 ext{ year}$ ; adult worms and larval burden high; $1{,}300 ext{ to }1{,}900$ worms per day; reproductive life $9 ext{ to }11 ext{ years}$ ; ext{about }50 ext{%} of men over $40$ years blinded; Africa accounts for ~90 ext{%} of cases; treatment = $ext{ivermectin}$ .
Loa loa: vector = deer fly; diurnal/nocturnal larval migration (blood/lymph during day; lungs at night); incubation months to years; encephalitis risk; treatment similar to river blindness; toxicity considerations.

Connections to foundational principles and real-world relevance

Microbial pathogenesis concepts: exotoxins, endotoxins, cell wall components, and virulence factors (e.g., penicillinase) underpin disease manifestations and antibiotic resistance patterns discussed.
Host-pathogen interactions: how immune status (immunocompromised vs immunocompetent) shapes disease severity and outcome (e.g., Pseudomonas infections, NF progression, helminthic drug toxicity).
Epidemiology and public health: nosocomial infections, environmental reservoirs, vectors, and barriers to transmission control illustrate the importance of infection control and vaccination programs (e.g., HPV, varicella, and herpes management).
Clinical reasoning: distinguishing viral vs bacterial vs fungal etiologies (e.g., conjunctivitis causes; viral vs bacterial styes) and recognizing when systemic involvement warrants escalation.
Ethical/practical implications: the use of vaccines (varicella) and antibiotic stewardship to reduce resistance, plus considerations for treatment in resource-limited settings (river blindness/loiasis) where logistics limit access to care.

Note on transitions

Chapter 21 culminates with helminthic eye infections and a reminder of the complexity of parasites as multicellular eukaryotes, highlighting treatment toxicity and host responses.
The content sets the stage for Chapter 22, which will focus on nervous system pathogens.