11.2 Urinary Tract, Skin/Soft Tissue & Sexually Transmitted Infections

Urinary Tract, Skin/Soft Tissue & Sexually Transmitted Infections

Urinary tract infections (UTIs), skin and soft tissue infections (SSTIs), and sexually transmitted infections (STIs) constitute high-volume clinical encounters across community pharmacies, outpatient clinics, and emergency departments. Effective pharmacotherapy requires precise differentiation of localized superficial infection from deep tissue invasion, recognition of evolving resistance patterns (such as community-associated MRSA and fluoroquinolone-resistant uropathogens), and rigorous adherence to public health contact tracing protocols.

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Urinary Tract Infections: Uncomplicated Cystitis vs. Pyelonephritis

Urinary tract infections are classified by anatomical location and host complexity:

• Uncomplicated Cystitis: Localized bladder infection occurring in otherwise healthy, non-pregnant, premenopausal adult females with structurally and functionally normal genitourinary tracts.
• Complicated UTI: Infection occurring in males, pregnant individuals, patients with indwelling catheters, anatomical abnormalities, nephrolithiasis, immunosuppression, or renal impairment.
• Acute Pyelonephritis: Invasive infection of the renal parenchyma and pelvicalyceal system presenting with systemic signs.

Microbiology of Uncomplicated UTIs

• Uropathogenic Escherichia coli (UPEC): 75%–90% of isolates.
• Staphylococcus saprophyticus: 5%–15% (predominantly in sexually active young women).
• Klebsiella pneumoniae, Proteus mirabilis, Enterococcus faecalis: 5%–10%.

First-Line Pharmacotherapy for Uncomplicated Cystitis

Clinical guidelines emphasize narrow-spectrum agents that concentrate heavily in urine while causing minimal disruption to normal bowel microbiota:

Acute Pyelonephritis: Clinical Management

Acute pyelonephritis presents with classic upper tract findings: fever (> 38°C), rigors, costovertebral angle (CVA) tenderness, flank pain, nausea, and vomiting, often alongside lower urinary symptoms.

• Outpatient Management (Mild-to-Moderate, Hemodynamically Stable, Tolerating Oral Intake):
  • Oral Ciprofloxacin: 500 mg PO BID for 7 days (or ciprofloxacin extended-release 1,000 mg PO daily for 7 days).
  • Oral Levofloxacin: 750 mg PO daily for 5 days.
  • Initial IV Dose Rationale: If local uropathogen fluoroquinolone resistance exceeds 10%, administer an initial loading dose of Ceftriaxone 1 g IV in the clinic before commencing oral fluoroquinolone therapy.
  • Alternative if fluoroquinolones contraindicated: TMP-SMX DS 1 tab PO BID for 14 days (if susceptibility confirmed) or oral beta-lactam (e.g., Cefixime 400 mg PO daily for 10–14 days, preceded by IV ceftriaxone).
• Hospital Admission Red Flags: Sepsis, hemodynamic instability, intractable nausea/vomiting, pregnancy, male sex, urinary tract obstruction / nephrolithiasis, solitary kidney, immunocompromise, or failure of outpatient oral therapy after 48 hours.

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Asymptomatic Bacteriuria (ASB): Strict Indications & Overtreatment Harms

Asymptomatic bacteriuria (ASB) is defined as the isolation of a specified quantitative count of bacteria (≥ 10^5 CFU/mL [≥ 10^8 CFU/L] in a clean-catch voided specimen) in a patient without any signs or symptoms referable to urinary tract infection (no dysuria, frequency, urgency, suprapubic pain, flank tenderness, or systemic fever).

                     [ Positive Urine Culture (>= 10^5 CFU/mL) ]
                                         │
                  Does the patient have signs or symptoms of UTI?
               (Dysuria, frequency, urgency, fever, flank pain, CVA tenderness)
                                         │
                        ┌────────────────┴────────────────┐
                       YES                                NO
                        │                                 │
                 [ Symptomatic UTI ]              [ Asymptomatic Bacteriuria ]
                        │                                 │
                 Treat according to           Is patient PREGNANT or undergoing
                 Cystitis/Pyelonephritis      invasive urologic surgery with
                 guidelines                   anticipated mucosal bleeding?
                                                          │
                                         ┌────────────────┴────────────────┐
                                        YES                                NO
                                         │                                 │
                                 [ TREAT with Targeted ]          [ DO NOT TREAT ]
                                 [ Antimicrobial Therapy]         [ Withhold Antibiotics ]

The Two Validated Indications for Screening and Treating ASB

Canadian and international guidelines universally endorse antimicrobial treatment of ASB in only two clinical scenarios:

1. Pregnant Individuals: Screen at 12 to 16 weeks gestation with a quantitative urine culture. Untreated ASB in pregnancy carries a 20%–40% risk of progressing to acute pyelonephritis, precipitating preterm labor, low birth weight, and perinatal mortality. Treat with a 5- to 7-day course of cephalexin, amoxicillin-clavulanate, or nitrofurantoin (avoid nitrofurantoin near term, 36–40 weeks, due to theoretical risk of neonatal hemolytic anemia).
2. Patients Undergoing Invasive Urologic Procedures: Specifically procedures where mucosal bleeding is anticipated (e.g., transurethral resection of the prostate [TURP]). Administer targeted antimicrobial therapy 30–60 minutes pre-operatively to prevent severe bacteremia and urosepsis.

Populations Where ASB Must NEVER Be Treated

Treating ASB provides zero clinical benefit, does not prevent symptomatic UTI, and causes substantial patient harm (antimicrobial resistance, adverse drug reactions, and C. diff infection). Do NOT screen or treat ASB in:

• Elderly individuals residing in community or long-term care facilities.
• Patients with indwelling Foley catheters or chronic suprapubic tubes.
• Patients with diabetes mellitus.
• Patients with spinal cord injuries.
• Non-pregnant healthy pre- or post-menopausal women.

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Skin & Soft Tissue Infections (SSTIs): Non-Purulent vs. Purulent

Clinical management of SSTIs hinges on a fundamental diagnostic fork: distinguishing non-purulent infection (spreading erythema, warmth, edema without drainage) from purulent infection (abscess, furuncle, carbuncle with fluctuance or purulent exudate).

                   [ Acute Skin & Soft Tissue Infection ]
                                     │
                       Is purulence/fluctuance present?
                                     │
                     ┌───────────────┴───────────────┐
                    NO                              YES
                     │                               │
          [ Non-Purulent Cellulitis ]      [ Purulent Skin Infection ]
          (Erysipelas, Cellulitis)         (Abscess, Furuncle, Carbuncle)
                     │                               │
          Pathogens: Group A Strep,        Pathogen: S. aureus (MSSA & MRSA)
                     MSSA                            │
                     │                     PRIMARY THERAPY:
          ANTIBIOTICS PRIMARY:             Incision and Drainage (I&D)
          Cephalexin 500 mg PO QID                   │
          or Cloxacillin 500 mg PO QID     Are systemic signs present?
          (MRSA coverage NOT routine)      (Fever, tachycardia, extensive redness)
                                                     │
                                     ┌───────────────┴───────────────┐
                                    NO                              YES
                                     │                               │
                              No Antibiotics                  Add Oral MRSA Agent:
                              Needed (I&D alone)              TMP-SMX DS 1-2 tabs BID
                                                              or Doxycycline 100 mg BID

1. Non-Purulent Cellulitis and Erysipelas

• Microbiology: Overwhelmingly caused by Beta-hemolytic Streptococci (Streptococcus pyogenes [Group A Strep], Group C and G Strep) and Methicillin-Susceptible Staphylococcus aureus (MSSA). MRSA is exceptionally rare in pure, non-purulent cellulitis.
• Empiric Oral Therapy:
  • Cephalexin: 500 mg PO QID for 5–6 days.
  • Cloxacillin: 500 mg PO QID on an empty stomach for 5–6 days.
  • Cefazolin: 1 g IV Q8H (if inpatient parenteral therapy required).
  • Severe Penicillin Allergy: Clindamycin 300 to 450 mg PO TID for 5–6 days.
• Duration: 5 to 6 days is clinically equivalent to 10 days, provided erythema has begun to recede and systemic signs have resolved.

2. Purulent Skin Infections (Abscesses, Furuncles, Carbuncles)

• Microbiology: Staphylococcus aureus, with Community-Associated MRSA (CA-MRSA) accounting for 40%–70% of isolates in many Canadian urban centers.
• Drainage plus MRSA coverage: A drainable abscess needs incision and drainage (I&D). Randomized trials (2016–2017) showed that adding TMP-SMX or clindamycin after drainage improves cure, even for small abscesses. PEBC's published sample items reflect this: a drainable abscess in localized purulent cellulitis is the feature that calls for an MRSA-active antibiotic, and TMP-SMX is the preferred oral choice for purulent cellulitis with fever.
• Features that strengthen the case for systemic antibiotics (and closer follow-up or IV therapy if severe):
  • Systemic inflammatory signs (fever > 38°C, tachycardia > 90 bpm, tachypnea, leukocytosis).
  • Severe, extensive, or rapidly progressive cellulitis surrounding the abscess.
  • Multiple sites of infection or location in difficult-to-drain areas (face, hands, genitalia).
  • Immunocompromised host, advanced age, or failure of prior I&D alone.
• Oral MRSA-Active Antimicrobial Options:
  • Sulfamethoxazole-Trimethoprim (TMP-SMX): 1 to 2 DS tablets PO BID for 5–7 days.
  • Doxycycline: 100 mg PO BID for 5–7 days.
  • Clindamycin: 300 to 450 mg PO TID for 5–7 days (only if local MRSA clindamycin resistance is < 10%–15% and the D-test for inducible clindamycin resistance is negative).

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Sexually Transmitted Infections: Public Health Agency of Canada Protocols

Sexually transmitted infections require rapid, accurate empirical management to eradicate mucosal infection, prevent pelvic inflammatory disease (PID), tubal scarring, and infertility, and interrupt ongoing community transmission.

1. Chlamydia trachomatis

• Clinical Shift: In updated Public Health Agency of Canada (PHAC) and international guidelines, oral Doxycycline is now preferred over single-dose Azithromycin for uncomplicated urogenital, anorectal, and pharyngeal chlamydia.
• Preferred Regimen: Doxycycline 100 mg PO BID for 7 days.
• Pharmacological Rationale: Randomized trials demonstrate that single-dose azithromycin (1 g) yields unacceptably high microbiological failure rates (15%–20%) in rectal chlamydia, which often co-occurs silently with urogenital infection.
• Alternative Regimen (Adherence Concerns or Pregnancy): Azithromycin 1 g PO single dose (pregnancy: amoxicillin 500 mg PO TID for 7 days is also acceptable).

2. Neisseria gonorrhoeae

• Resistance Landscape: N. gonorrhoeae has developed chromosomal and plasmid-mediated resistance to penicillins, tetracyclines, and fluoroquinolones. High-level resistance to oral cephalosporins (cefixime) has prompted elevated parenteral dosing.
• Preferred Regimen: Ceftriaxone 500 mg IM as a single dose (increased from historical 250 mg dose; use 1,000 mg IM for patients weighing ≥ 150 kg).
• Co-Treatment Rule: If Chlamydia trachomatis infection has not been ruled out by nucleic acid amplification testing (NAAT), add Doxycycline 100 mg PO BID for 7 days.
• Severe Cephalosporin Allergy: Gentamicin 240 mg IM single dose PLUS Azithromycin 2 g PO single dose.

3. Syphilis (Treponema pallidum)

• Staging and Pharmacotherapy:

4. Trichomoniasis (Trichomonas vaginalis)

• Preferred Regimen: Metronidazole 500 mg PO BID for 7 days (superior to single-dose 2 g therapy in preventing microbiological failure in females).
• Patient Counseling: Complete avoidance of alcohol during metronidazole therapy and for at least 24 to 48 hours following the final dose to avoid severe disulfiram-like reactions (flushing, throbbing headache, palpitations, nausea, vomiting).

Partner Management & Public Health Mandates

• Contact Tracing / Partner Notification: All sexual partners within the preceding 60 days must be clinically evaluated, tested, and empirically treated regardless of current symptoms.
• Sexual Abstinence: Patients and their partners must abstain from all sexual intercourse until 7 full days after completion of therapy (or 7 days following single-dose treatment) AND until all symptoms have completely resolved and partners have been treated.
• Test of Cure and Rescreening (PHAC): For gonorrhea, do a test of cure at every positive site: culture 3 to 7 days after treatment, or NAAT 2 to 3 weeks after. For chlamydia, test of cure is needed in pregnancy, when adherence is doubtful, or with an alternative regimen. Because reinfection is common, rescreen people treated for gonorrhea or chlamydia about 6 months after treatment.