3.1 Common Infections in Primary Care

Common Infections in Primary Care

Infectious diseases are among the most frequent reasons for visits to primary care clinics. A structured, evidence-based approach to diagnosis and risk stratification is essential to guide clinical decision-making, optimize empiric antibiotic therapy, and prevent antibiotic resistance. This section reviews three common primary care infections: community-acquired pneumonia (CAP), uncomplicated cystitis, and acute gastroenteritis (AGE).

Community-Acquired Pneumonia (CAP)

Community-acquired pneumonia is defined as an acute infection of the lung parenchyma in a patient who has not been hospitalized or resided in a long-term care facility for 14 days or more before the onset of symptoms.

Pathophysiology and Pathogens

The most common pathogen in CAP across all age groups is Streptococcus pneumoniae (pneumococcus). Other typical pathogens include Haemophilus influenzae and Moraxella catarrhalis (frequently seen in patients with underlying COPD). "Atypical" pathogens include Mycoplasma pneumoniae (common in school-aged children and young adults), Chlamydia pneumoniae, and Legionella pneumophila (associated with contaminated water sources and severe disease). Viral pathogens, including influenza, respiratory syncytial virus (RSV), and SARS-CoV-2, are also significant etiologies.

Clinical Presentation and Diagnosis

Patients characteristically present with acute onset of fever, chills, cough (often productive of purulent or rust-colored sputum), dyspnea, and pleuritic chest pain. Elderly patients may present atypically, often displaying confusion or a decline in baseline functional status without fever. Physical examination findings include tachypnea, tachycardia, and localized chest findings such as crackles, bronchial breath sounds, increased tactile fremitus, and egophony (E-to-A changes) over the area of consolidation.

The gold standard for diagnosing CAP is the presence of a new pulmonary infiltrate or consolidation on a chest radiograph (X-ray) in a patient with compatible clinical features.

Risk Stratification: The CURB-65 Score

Determining whether a patient can be safely treated as an outpatient or requires hospitalization is a critical safety decision. The CURB-65 scoring system is a validated tool to guide this decision:

Management Based on CURB-65 Score:

• Score 0–1: Low risk. Safe for outpatient management.
• Score 2: Moderate risk. Consider short-stay inpatient admission or close, supervised outpatient management.
• Score 3–5: High risk. Mandates inpatient admission. Scores of 4 or 5 should prompt assessment for intensive care unit (ICU) admission.

Empiric Antibiotic Regimens for CAP

Empiric therapy must target the most likely typical and atypical pathogens.

1. Outpatient Management (No Comorbidities)

For patients who are healthy, have no chronic comorbidities (e.g., heart, lung, liver, or renal disease, diabetes, alcoholism), and have no risk factors for MRSA or Pseudomonas aeruginosa:

• Amoxicillin 1 g orally three times daily (first-line for typical coverage), OR
• Doxycycline 100 mg orally twice daily (provides atypical coverage and typical coverage), OR
• Macrolide (e.g., Azithromycin 500 mg on day 1, then 250 mg daily) only if local pneumococcal resistance to macrolides is < 25%.

2. Outpatient Management (With Comorbidities)

For patients with comorbidities (COPD, diabetes, renal failure, congestive heart failure, liver disease, active malignancy, or alcoholism):

• Combination therapy: A beta-lactam (e.g., Amoxicillin/Clavulanate 875/125 mg twice daily or Cefuroxime 500 mg twice daily) PLUS a macrolide (e.g., Azithromycin) OR Doxycycline 100 mg twice daily.
• Monotherapy: A respiratory fluoroquinolone (e.g., Levofloxacin 750 mg daily or Moxifloxacin 400 mg daily).

3. Inpatient Management (Non-Severe)

• Combination therapy: An intravenous beta-lactam (e.g., Ceftriaxone 1–2 g daily or Ampicillin/Sulbactam 1.5–3 g every 6 hours) PLUS an oral or IV macrolide (Azithromycin 500 mg daily) or Doxycycline.
• Monotherapy: IV or oral respiratory fluoroquinolone (Levofloxacin or Moxifloxacin).

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Uncomplicated Cystitis

Acute uncomplicated cystitis is a superficial bacterial infection of the bladder mucosa in non-pregnant, premenopausal, immunocompetent adult women who have no structural or functional abnormalities of the urinary tract.

Pathogens and Clinical Features

The primary pathogen in over 80% of cases is Escherichia coli. Other common organisms include Staphylococcus saprophyticus (associated with young, sexually active females), Proteus mirabilis, and Klebsiella pneumoniae.

Typical symptoms include dysuria, urinary frequency, urgency, and suprapubic pain. Gross or microscopic hematuria may also be present. Crucially, systemic symptoms such as fever, chills, rigors, or flank pain are absent; their presence suggests upper urinary tract involvement (pyelonephritis) or systemic infection.

Diagnosis

In a patient presenting with classic symptoms and no vaginal symptoms (such as pruritus or discharge), the probability of acute cystitis exceeds 90%. A urinalysis (either dipstick or microscopy) is useful to support the diagnosis.

• Leukocyte Esterase: Highly sensitive marker for pyuria (white blood cells in the urine).
• Nitrites: Highly specific marker indicating the presence of enterobacteria (like E. coli), which reduce dietary nitrates to nitrites.
• Urine Culture: Not routinely indicated for initial episodes of uncomplicated cystitis. It should be obtained in patients with atypical symptoms, suspected pyelonephritis, recurrent infections, or if symptoms fail to resolve after initial empiric therapy.

Empiric Treatment Algorithms

First-line antimicrobial options for uncomplicated cystitis must offer high efficacy, low rates of resistance, and minimal systemic side effects:

1. Nitrofurantoin Monohydrate/Macrocrystals: 100 mg orally twice daily for 5 days. Note: Avoid in patients with a creatinine clearance < 30 mL/min, as the drug will not reach therapeutic concentrations in the urine.
2. Trimethoprim-Sulfamethoxazole (TMP-SMX) Double Strength (DS): 160/800 mg orally twice daily for 3 days. Note: Avoid if local resistance patterns for E. coli exceed 20% or if the patient has used this agent for a UTI within the past 3 months.
3. Fosfomycin Tromethamine: 3 g orally as a single dose.

Second-line options include oral beta-lactams (e.g., Amoxicillin/Clavulanate or Cefpodoxime) for 5–7 days. Fluoroquinolones (e.g., Ciprofloxacin) are highly effective but should be reserved for more complicated infections due to the risk of disabling adverse events (tendonitis, aortic aneurysm rupture, QTc prolongation).

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Acute Gastroenteritis (AGE)

Acute gastroenteritis is characterized by a rapid onset of diarrhea (three or more loose stools per day), with or without vomiting, nausea, fever, or abdominal pain, typically lasting less than 14 days.

Pathogens

• Viral: The most common cause of AGE. Norovirus is the leading cause across all age groups and is associated with outbreaks in closed communities (hospitals, schools, cruise ships). Rotavirus remains a significant cause in unvaccinated infants and young children.
• Bacterial: Often associated with contaminated food or water ("food poisoning"). Pathogens include Campylobacter jejuni (often from undercooked poultry), non-typhoidal Salmonella, Shigella, and various strains of Escherichia coli. Enterotoxigenic E. coli (ETEC) is the primary cause of traveler's diarrhea. Enterohemorrhagic E. coli (EHEC), particularly strain O157:H7, produces Shiga toxin and causes bloody diarrhea.
• Parasitic: Typically presents with prolonged or chronic diarrhea (> 14 days). Pathogens include Giardia duodenalis (associated with wilderness water sources) and Cryptosporidium.

Clinical Evaluation and Management

The primary clinical goal in AGE is assessing the patient's hydration status. Signs of moderate-to-severe dehydration include dry mucous membranes, delayed capillary refill, decreased skin turgor, tachycardia, hypotension, and oliguria.

1. Rehydration Therapy

• Oral Rehydration Therapy (ORT): The cornerstone of management. Oral rehydration salts (ORS) containing glucose and electrolytes utilize sodium-glucose cotransporters in the small intestine to maximize water absorption. ORT is preferred over intravenous fluids for mild-to-moderate dehydration.
• Intravenous Rehydration: Indicated for patients with severe dehydration, altered mental status, or intractable vomiting preventing oral intake.

2. Symptomatic Therapy

• Antidiarrheal Agents: Loperamide (an antimotility agent) can be used to decrease stool frequency in mild-to-moderate watery diarrhea. However, it is strictly contraindicated in patients with high fever or bloody diarrhea (dysentery), as delaying gut transit can worsen bacterial invasion, increase toxin absorption, and precipitate toxic megacolon.
• Antiemetic Agents: Oral Ondansetron can be used to facilitate oral rehydration in patients with persistent nausea or vomiting.

3. Empiric Antibiotic Therapy

Most cases of viral and mild bacterial AGE are self-limiting and resolve with supportive care. Antibiotics are generally withheld to prevent disruption of the gut microbiome and avoid antibiotic resistance.

Indications for empiric antibiotic therapy include:

• Severe diarrhea (more than 8 stools per day, severe dehydration, or hospitalization).
• Signs of systemic involvement (high fever, severe abdominal pain, sepsis).
• Immunocompromised host status or advanced age.
• Severe traveler's diarrhea.

Empiric Antibiotic Regimens:

• First-line: Azithromycin 500 mg orally once daily for 3 days, OR Ciprofloxacin 500 mg orally twice daily for 3 days.

Important Warning: Do not use antibiotics in cases of suspected EHEC (characterized by acute onset of bloody diarrhea without high fever) as antibiotics can trigger increased Shiga toxin release, significantly increasing the risk of Hemolytic Uremic Syndrome (HUS).