Chapter 15: Diagnosing Infections

Specimen Collection

Accurate diagnosis of infections begins with proper specimen collection, which is essential for reliable laboratory results.

• Aseptic technique: All specimens must be collected using aseptic methods to prevent contamination.

• "Clean catch" samples: Typically refers to urine samples collected midstream to minimize contamination.

• Sputum samples: Should be obtained by deep coughing into a sterile container; saliva is not suitable.

Antibody-Based Testing

Antibody-based tests are used to detect specific immune responses to pathogens.

• Antibody specificity: Antibodies bind to a single epitope, ensuring high specificity in tests.

• High specificity: Reduces false positives.

• Sensitivity: Ability to detect low levels of antibodies or antigens.

Testing Methods

Several laboratory methods are used to identify pathogens.

• Phenotypic: Based on observable characteristics (e.g., morphology, staining, biochemical reactions).

• Genotypic: Detects genetic material (DNA/RNA) of pathogens (e.g., PCR).

• Immunologic: Uses antigen-antibody reactions (e.g., ELISA, serotyping).

PCR vs. Antigen Tests

Both tests are used for pathogen detection, but differ in sensitivity and speed.

• RT-PCR: Detects viral RNA; highly sensitive, can identify low viral loads.

• Antigen tests: Rapid results but less sensitive; may miss early or low-level infections.

Coagulase Test

The coagulase test is a biochemical assay used to differentiate bacterial species.

• Purpose: Distinguishes Staphylococcus aureus (coagulase-positive) from other staphylococci.

• Classification: Biochemical test based on enzyme activity.

Antibody Titers

Antibody titers measure the concentration of antibodies in serum.

• Higher titer: Indicates greater antibody concentration and stronger immune response.

• Example: A titer of 1:256 means serum can be diluted 256-fold and still detect antibodies.

Matching Review: Key Terms

Chapter 16: Infectious Diseases Affecting the Skin and Eyes

Skin Lesions

Skin lesions are characteristic signs of many infectious diseases.

• Macules: Flat, discolored spots on the skin.

• Papules: Raised, solid lesions less than 1 cm in diameter.

• Pustules: Raised lesions containing pus.

• Vesicles: Small, fluid-filled blisters.

• Exanthem: Widespread rash, often associated with systemic infection.

Common Diseases

Several infectious diseases affect the skin and eyes.

• Rubella: Viral disease causing mild rash; dangerous in pregnancy.

• Ringworm: Fungal infection (dermatophyte); causes circular, itchy lesions.

• Impetigo: Bacterial infection (often Staphylococcus aureus or Streptococcus pyogenes); causes honey-colored crusts.

• Cellulitis: Bacterial infection of deeper skin layers; causes redness, swelling.

• Shingles: Reactivation of varicella-zoster virus; causes painful, unilateral rash.

• Leishmaniasis: Parasitic disease transmitted by sandflies; causes skin ulcers.

Smallpox

Smallpox is a highly contagious viral disease characterized by fever and pustular rash.

• Eradicated: Smallpox was eradicated globally in 1980; any outbreak is a medical emergency.

Clinical Scenarios

• Tree branch wound: Risk of tetanus or cellulitis.

• Unilateral facial rash: Suggests shingles (herpes zoster).

• "Slapped cheek" rash: Characteristic of parvovirus B19 infection (erythema infectiosum).

Chapter 17: Infectious Diseases Affecting the Nervous System

Meningitis

Meningitis is inflammation of the meninges, the protective membranes covering the brain and spinal cord.

• Symptoms: Fever, headache, neck stiffness, photophobia, altered mental status.

• Most serious acute bacterial: Neisseria meningitidis (meningococcus).

• Neonatal meningitis: Often caused by Group B Streptococcus, Escherichia coli, or Listeria monocytogenes.

• Diagnosis: Lumbar puncture to analyze cerebrospinal fluid (CSF).

Poliomyelitis

Poliomyelitis is a viral disease that can cause paralysis by infecting motor neurons.

• Prevention: Vaccination (Salk and Sabin vaccines).

Arboviruses

Arboviruses are viruses transmitted by arthropods (e.g., mosquitoes, ticks).

• Examples: West Nile Virus, Zika Virus, Eastern Equine Encephalitis.

Creutzfeldt-Jakob Disease

A rare, fatal neurodegenerative disorder caused by prions (misfolded proteins).

• Symptoms: Rapid dementia, motor dysfunction.

Chapter 18: Infectious Diseases Affecting the Cardiovascular & Lymphatic Systems

Ebola Virus Disease

Ebola is a severe, often fatal hemorrhagic fever transmitted by direct contact with bodily fluids.

• Symptoms: Fever, bleeding, organ failure.

Terminology

• Viremia: Presence of viruses in the blood.

• Septicemia: Bacterial toxins in the blood causing systemic illness.

• Bacteremia: Presence of bacteria in the blood.

Diseases & Transmission

• Tularemia: Caused by Francisella tularensis; transmitted by ticks, rabbits.

• Plague: Caused by Yersinia pestis; transmitted by fleas.

• Subacute endocarditis: Infection of heart valves, often by oral bacteria.

• Cat scratch disease: Caused by Bartonella henselae; transmitted by cat scratches.

Brucellosis

Brucellosis is a zoonotic infection caused by Brucella species.

• Causes: Contact with infected animals or animal products.

• Transmission: Ingestion, inhalation, or direct contact.

Spotted Fever Rickettsioses

Group of diseases caused by Rickettsia species, characterized by rash and fever.

• Rash: Begins on wrists and ankles, spreads to trunk.

• Cause: Rickettsia rickettsii (Rocky Mountain spotted fever).

HIV Case Study: Suzanne

• Transmission: Horizontal (sexual contact, blood), vertical (mother to child).

• Prevention: Safe sex practices, antiretroviral therapy during pregnancy.

Mixed Chapters: Matching Practice

Transmission and Disease Matching

Signs/Symptoms and Disease Matching

Written Response Topics

Smallpox Outbreak

A sudden outbreak of smallpox would be surprising and considered a medical emergency because the disease was eradicated globally in 1980. Any new cases would suggest bioterrorism or accidental release, requiring immediate public health response.

Salk vs. Sabin Polio Vaccine

The Salk vaccine is an inactivated (killed) polio vaccine administered by injection, while the Sabin vaccine is a live attenuated oral vaccine. The Salk vaccine is safer in immunocompromised individuals, whereas the Sabin vaccine provides better community immunity but carries a rare risk of vaccine-derived polio.