Dermatology - Cellulitis, Erysipelas, Lymphangitis, Bites, and Tick-Borne Diseases (Study Notes)

Cellulitis

  • Definition: An acute bacterial infection of the dermis and subcutaneous tissue; in severe cases, infection can spread to other parts of the body.
  • Classic signs: redness and warmth of the skin; edges may be marked by a clinician to monitor improvement or progression.
  • Etiology:
    • Bacterial penetration through a compromised epidermal barrier into deep dermis and subcutaneous tissues.
    • Common pathogens: B-hemolytic streptococci (groups A, B, C, G, F), Staphylococcus aureus including MRSA, Gram-negative aerobic bacilli, Pseudomonas aeruginosa.
  • Risk factors:
    • Disruption of skin barrier from trauma, infection, insect bites, injection drug use, body piercing, maceration, ulceration, chronic wounds.
    • Patient factors: elderly, diabetes, hypertension, cancer, obesity.
  • Clinical presentation:
    • Erythema, pain, swelling, warmth, local tenderness, regional lymphadenopathy.
    • Borders: non-elevated, poorly demarcated.
  • Diagnosis:
    • If signs of systemic infection or doubt persists, obtain:
    • CBC
    • Blood cultures
  • Differential diagnosis:
    • Toxic Shock Syndrome (TSS)
    • Gout
    • Venous stasis dermatitis
    • Deep vein thrombosis (DVT)
    • Thrombophlebitis
    • Drug reaction
  • Management/Treatment:
    • Non-purulent cellulitis (outpatient):
    • Keflex (cephalexin) — first line
    • Penicillin VK
    • Clindamycin
    • Non-purulent cellulitis (inpatient):
    • Cefazolin — first line
    • If penicillin-allergic: vancomycin
    • Purulent drainage (outpatient):
    • Bactrim (trimethoprim-sulfamethoxazole)
    • Clindamycin
    • Doxycycline
    • Purulent drainage (inpatient):
    • Vancomycin
  • Monitoring/notes:
    • Track edge of erythema to assess response to therapy.

Erysipelas

  • Background:
    • Distinct form of cellulitis; caused by Streptococcus pyogenes (S. pyogenes).
    • Affects superficial skin layer; differs from cellulitis which involves deeper layers.
  • Clinical presentation:
    • Well demarcated, raised borders; non-purulent erythema; warm
    • Usually involves the face or lower extremities
  • Treatment:
    • Non-purulent drainage (outpatient):
    • Keflex (cephalexin) — first line
    • Penicillin VK
    • Clindamycin
    • Non-purulent drainage (inpatient):
    • Cefazolin — first line
    • If penicillin allergy: vancomycin
    • Purulent drainage (outpatient):
    • Bactrim
    • Clindamycin
    • Doxycycline
    • Purulent drainage (inpatient):
    • Vancomycin
  • Notes:
    • In severe cases, management is similar to cellulitis and may require vancomycin.

Lymphangitis

  • Clinical presentation:
    • Acute or chronic inflammation of lymphatic vessels.
    • Red, tender streaks extending proximally to regional lymph nodes.
    • Most commonly affects the extremities.
    • Common causative organism: Streptococcus pyogenes (S. pyogenes).
  • Differential diagnosis:
    • Thrombophlebitis
    • Contact dermatitis
    • Allergic reaction
  • First-line treatment:
    • Oral amoxicillin
  • Lymphangitis: management
    • (Note: The slide lists management generally but specific outpatient/inpatient steps aren’t fully detailed beyond the first-line antibiotic.)

Bites (Part II)

Spider Bites

  • Brown Recluse Spiders

    • Geography: Mostly southwestern and midwestern USA
    • Appearance: brown color; six eyes; may have a violin pattern on the anterior cephalothorax
    • Clinical presentation:
    • Local effects initially painless blisters 2–8 hours after bite; then burning, itching, swelling, erythema
    • Progressive stages: 1) bright red papule within hours; 2) firm purple necrotic lesion within a few hours; 3) black eschar (necrotic scar) visible 7–14 days post-bite; healing may take weeks to months with deep scarring
    • Systemic symptoms (less common): malaise; nausea/vomiting; fever; rhabdomyolysis (myalgias with dark urine); acute hemolytic anemia (pallor, jaundice, icterus, painless dark urine)
    • Differential diagnosis:
    • Infections; solitary ulcerated lesion; vascular disease; Pyoderma gangrenosum; vasculitis; other arthropod bites
    • Differential diagnostic features for recluse bites:
    • Typically a single focal lesion; often indoors (attic/garage/closet)
    • Timing: lesions from November–March less likely
    • Red center with pale center; lesions may be flat or sunken; chronic lesions longer than weeks unlikely
    • Treatment/management:
    • Local wound care: clean with soap and water; cold packs; elevate limb if possible
    • Pain control: NSAIDs; opioids if severe; tetanus prophylaxis if needed
    • Wound progression: debridement only if improves healing; historical use of dapsone; antibiotics only if secondary infection

  • Black Widow Spiders

    • Geography: Found in every US state except Alaska
    • Appearance: shiny black; red hourglass or anvil-shaped mark on ventral abdomen
    • Clinical presentation:
    • Latrodectism: local bite pain; systemic symptoms begin 30 minutes to 2 hours after bite
    • Muscular pain, spasms, and rigidity; distal to proximal spread (extremities, back, abdomen)
    • Neurotoxin effects: nausea, vomiting, headache, fever, syncope, paresthesias, convulsions (toxic reaction)
    • Usually self-limited, resolving in 1–3 days
    • Differential diagnosis:
    • Acute abdomen; renal colic; opioid withdrawal; tetanus
    • Treatment/management:
    • Local wound care; apply ice to constrict vessels
    • Pain control: NSAIDs; opioids if severe; muscle relaxers
    • Tetanus prophylaxis if needed
    • Severe cases: antivenom

  • Active Learning tip (exam focus):

    • Brown Recluse bites: necrotic wounds with pain, erythema, ecchymosis, bleb formation; possible surrounding ulceration/necrosis
    • Black Widow bites: neurologic symptoms (muscle pain/spasms) may occur with minimal local bite findings

Human Bites

  • Etiology (pathogens):
    • Human oral flora: Eikenella corrodens; Peptostreptococcus
    • Human skin flora: Staphylococci and streptococci
    • Bloodborne pathogens: HBV, HCV, HIV (less likely but possible)
  • Clinical presentation:
    • Delayed presentation common due to injury circumstances
    • Intentional bite: semicircular/oval erythema and bruising with/without skin break
    • Clenched-fist injury: wounds over metacarpophalangeal joints from striking against teeth
  • Workup:
    • Laboratory: CBC; wound cultures if infected
    • Hepatitis B/C and HIV testing if warranted and via shared decision making
    • Imaging: plain radiographs for clenched-fist injuries to assess fracture, joint disruption, or retained foreign body
  • Treatment/management:
    • Local wound care: control bleeding; clean with soap & water, iodine, or antiseptic; copious irrigation with sterile saline
    • Antibiotics (outpatient): Augmentin (amoxicillin-clavulanate) first line
    • Antibiotics (alternative if PCN allergy):
    • Adults: doxycycline ± clindamycin
    • Pediatrics: Bactrim + clindamycin
    • Tetanus prophylaxis
  • Mandatory reporting (nonspecific guidelines; varies by jurisdiction):
    • Generally no universal reporting requirements for human bites
    • If a crime is suspected: ensure patient safety, offer to contact police
    • Specific concern: human bite marks on a child with intercanine distance > 3 cm likely from an adult
    • Practical steps: assess safety, provide resources, and report as required by local policy

Animal Bites

  • Dog bites
    • Account for ~90% of all animal bites
    • Pathogens: Pasteurella multocida (most common), anaerobes, Streptococcus, Staphylococcus
  • Cat bites
    • 60–80% become infected
    • Pathogen: Pasteurella multocida
  • History taking (risk assessment):
    • Complications based on symptoms; higher risk with immunocompromise, liver disease, foreign implants
    • Immunization history: tetanus status; rabies exposures; geographic location
    • Animal information: provoked vs unprovoked; wild vs domestic vs stray; current location of animal; rabies vaccination status; health status of animal
  • Physical examination:
    • Wound preparation: control bleeding; clean; irrigate
    • Assess proximity to joints, tendons, bones; evidence of neurovascular compromise; wound depth
  • Workup and investigations:
    • Laboratory: CBC; wound cultures if infected
    • Imaging: plain radiographs to assess for embedded tooth or fracture
  • Cat Scratch Disease (complication):
    • Etiology: Bartonella henselae from scratch or bite of domestic cat
    • Clinical: regional lymphadenopathy 7–10 days after exposure; commonly in upper extremity, neck, head, groin; fever, malaise, headache; may last up to four months
  • Treatment/management:
    • Vaccines: Rabies and Tetanus as indicated
    • Antibiotics (for bites): Augmentin first line for both cats and dogs
    • If PCN allergy and dog bite: clindamycin + fluoroquinolone
    • If PCN allergy and cat bite: doxycycline or cefuroxime (ceftin)
    • Reporting: typically, all animal bites are reported; Maryland-specific: report to the county Animal Control where the bite occurred
  • Administrative/illustrative material:
    • An example reporting form is provided in the transcript (not required for clinical notes but indicates mandatory reporting processes)

Tick Bite Diseases

Lyme Disease

  • Etiology and transmission:
    • Caused by Borrelia burgdorferi (spirochete)
    • Transmitted by Ixodes species ticks (deer ticks)
  • Geographic distribution:
    • Northeastern coastal region; Minnesota; Wisconsin; California; Oregon; Utah; Nevada
  • Seasonality:
    • Most transmission occurs between May–September
  • Clinical presentation:
    • Early localized disease:
    • Erythema migrans: expanding, warm, annular erythematous rash with central clearing or bull’s eye
    • Systemic symptoms: fever, arthralgias, regional lymphadenopathy
    • Early disseminated disease (days to months after infection):
    • Multiple erythema migrans lesions
    • Neurologic manifestations: facial nerve palsy (Bell’s palsy); headache and neck stiffness (meningitis)
    • Cardiac manifestations: irregular pulse, syncope, dizziness
    • Late disease (months after exposure):
    • Recurrent synovitis; recurrent tendonitis and bursitis
    • Encephalopathic symptoms: severe headaches, neck stiffness; confusion; facial palsy (one or both sides)
    • Peripheral nerve involvement: radiculoneuropathy; numbness, tingling, shooting pain, or weakness
  • Differential diagnosis:
    • Other tick-borne illnesses: RMSF, ehrlichiosis, babesiosis
    • Autoimmune processes: juvenile RA, SLE
  • Diagnosis/testing (CDC two-tier testing):
    • Tier 1: EIA/ELISA for IgM and IgG
    • Tier 2: Immunoblot (Western blot)
    • Rules:
    • If the immunoblot is negative, no further testing is needed
    • If the immunoblot is equivocal or positive, reflex testing is performed
    • Symptoms duration considerations:
      • ≤ 30 days: perform both IgM and IgG immunoblots
      • > 30 days: perform only IgG immunoblot
  • Treatment/management:
    • Tick removal as soon as possible
    • Antibiotics (active disease):
    • Doxycycline 100 mg, orally, twice daily (BID) for 21–28 days
    • Amoxicillin 500 mg, three times daily (TID) for 21–28 days
    • Cefuroxime 500 mg, twice daily (BID) for 14–21 days
    • Prophylaxis (post-bite): Doxycycline 200 mg within 72 hours of tick bite

Rocky Mountain Spotted Fever (RMSF)

  • Etiology and transmission:
    • Caused by Rickettsia rickettsii
    • Transmitted by Dermacentor ticks (American dog tick)
  • Geographic distribution and seasonality:
    • Occurs in all states; most common in Arkansas, Missouri, North Carolina, Tennessee, Virginia, Oklahoma
    • Transmission most common May–August
  • Clinical presentation:
    • Early: flu-like symptoms (fever, headache, malaise, myalgia, arthralgia, nausea) one week after bite
    • Rash: appears 2–4 days after fever; starts as small blanching pink macules on wrists and ankles and spreads to trunk; involves palms and soles by day 5–6
    • Endothelial infection leads to: decreased platelets (thrombocytopenia), edema, hypotension/hypovolemia
    • Severe manifestations: pulmonary edema, acute renal failure, meningoencephalitis, severe coagulopathies
    • Physical findings: fever > 102°F; rash may progress to necrotic or gangrenous lesions; AMS; lymphadenopathy; hepatosplenomegaly; edema of dorsum and feet
  • Differential diagnosis:
    • Viral exanthemas (hand-foot-mouth, measles, rubella, roseola)
    • Viral gastroenteritis; mononucleosis; pharyngitis
    • URI, UTI; TTP/ITP; other vasculitides; toxic shock syndrome
    • Meningoencephalitis; meningococcemia; other tick-borne infections (typhus, ehrlichiosis, babesiosis, leptospirosis)
  • Diagnosis/testing:
    • Laboratory: IgM or IgG antibody testing; CBC: leukocytes normal or low; thrombocytopenia; CMP: hyponatremia; increased BUN
  • Complications (long-term):
    • Paraparesis; hearing loss; peripheral neuropathy; bowel/bladder incontinence; cerebellar/vestibular/motor dysfunction
  • Treatment/management:
    • Remove the tick
    • Antibiotics:
    • First line: doxycycline
    • Second line: chloramphenicol

References

  • The content above is derived from the lecture slides: Dermatology: Infectious Disease — Ty ler McCauley, MS, PA-C; PAS506: Clinical Medicine I (Fall 2025). The notes summarize the differential diagnosis, diagnostic approaches, and treatment guidelines as presented in the slides.