Infectious Diseases Overview

Infectious Diseases Overview

• Infectious diseases are a broad category encompassing various illnesses caused by pathogenic agents.
• Learning objectives for this topic include understanding the pathophysiology, etiology, epidemiology, manifestations, complications, and management of key infectious diseases.
• Knowledge of clinical manifestations and investigations is vital for reasonable diagnosis and implementing an effective management plan.

Aetiological Classification

• Bacterial:
  • Gram-positive: streptococci, staphylococci
  • Gram-negative: Klebsiella, Pseudomonas, Haemophilus influenza
  • Anaerobic bacteria: Bacteroides
• Mycobacterial: Tuberculosis (TB), Leprosy
• Spirochetal: Syphilis, Leptospirosis
• Rickettsial: Typhus
• Chlamydia
• Viral: Dengue, influenza, Herpes Simplex Virus (HSV), Varicella-Zoster Virus (VZV), Cytomegalovirus (CMV), Epstein-Barr virus (EBV), Measles, Mumps, Rubella
• Protozoal: Malaria, Amebiasis, Bilharzia, Giardiasis
• Fungal: Candida albicans, Aspergillus niger

Fever and its Types

• Normal body core temperature ranges from 36.5°C to 37.2°C.
• Fever (pyrexia): an increase in body temperature above the normal upper limit.
• Types of fever:
  • Sustained fever: daily fluctuations of fever less than 1°C
  • Remittent fever: daily fluctuations of fever more than 1°C
  • Hectic fever: body temperature falls to normal once or more during the day.
  • Relapsing fever: days of fever interrupted by days of normal temperature
• Hyperpyrexia: body temperature > 40°C

Typhoid Fever

• Aetiology: Caused by Salmonella typhi and paratyphi A & B; transmitted through feco-oral route.
• Clinical manifestations:
  • Week 1: Step-ladder fever, headache, relative bradycardia, constipation, rash (rose spots).
  • Week 2: continuous fever, tachycardia, diarrhea, splenomegaly.
  • Week 3: convalescence begins
• Complications: Intestinal bleeding, peritonitis, meningitis, encephalitis, convulsions, pyelonephritis.
• Investigations: CBC (leucopenia with relative lymphocytosis), Widal test (+ve from 2nd week), blood, stool & urine cultures (+ve in week 1, 2, & 3, respectively – best diagnostic).
• Treatment: Ciprofloxacin, co-trimoxazole (septrin), ceftriaxone.

Brucellosis

• Etiology: Caused by Brucella (melitensis, abortus, or suis)
• Mode of infection: Contaminated milk or remnants of aborted animals
• Clinical picture: Bone & muscle pain, vomiting, constipation, relapsing fever (days of fever alternating with fever-free periods).
• Complications: Relapse, infective endocarditis, orchitis, paraplegia from transverse myelitis, abortion
• Investigations: CBC (lymphocytosis during fever spike), Blood Culture (+ve during fever), Serological tests (Brucella agglutination test - best diagnostic test)
• Treatment: Doxycycline or rifampicin for 6 weeks

Acquired Immunodeficiency Syndrome (AIDS)

• Aetiology: Infection with Human Immunodeficiency Virus (HIV).
• Modes of infection: Parenteral, sexual, vertical
• Clinical picture: Asymptomatic (lasting up to 10 years), Acute HIV syndrome (fever, rigors, arthralgia, rash for 2 weeks), symptomatic syndromes, and generalized lymphadenopathy.
• Opportunistic Infections: Cytomegalovirus (CMV), Herpes Simplex Virus (HSV), Tuberculosis (TB), Pneumocystis carinii Pneumonia.
• Secondary Neoplasms: Kaposi sarcoma, Non-Hodgkin's lymphoma.
• Investigations: HIV antibodies (screening), HIV-RNA by PCR (diagnostic).
• Treatment: Antiretroviral therapies (HAART), Zidovudine 600 mg/day orally.

Fever of Unknown Origin (FUO)

• Definition: Persistent elevation of body temperature over 38.5°C for 3 weeks, without a specific diagnosis after at least one week of inpatient investigations.
• Causes: Infections (bacterial, viral, protozoal), Malignancy (hematologic or non-hematologic), Collagen disease, other conditions.

Pulmonary Tuberculosis (TB)

• Definition: Chronic infection of the lung caused by Mycobacterium tuberculosis, aerobic, non-motile, acid-fast bacillus.
• Aetiology: M. Tuberculosis. Predisposing factors include children under 5, contacts with open TB patients, and immunocompromised individuals (e.g., AIDS, diabetes, leukemia, steroid use).
• Mode of transmission: Droplet infection.
• Symptoms: Toxic manifestations (night fever, sweating, weight loss), cough with sputum (mucoid, mucopurulent, sometimes blood-tinged), and chest signs.
• Clinical features: Consolidation to cavitation and fibrosis, possible CNS involvement.
• Investigations: Chest X-ray, ESR, C-reactive protein (CRP), sputum microscopy and culture, PCR, tuberculin skin test, and serological tests (Quantiferon Gold, anti-deaminase antibody (ADA)).
• Treatment: First-line drugs (isoniazid (INH), rifampicin, pyrazinamide, ethambutol, streptomycin) combined for a long period, standard regimen for 6 months.

Dengue Fever

• Definition: A viral disease caused by the dengue virus.
• Aetiology: RNA virus (Flavivirus, arbovirus).
• Vector: Mosquito Aedes aegypti
• Pathogenesis: Virus inoculated by mosquito bite, replication in reticuloendothelial system, leads to immune response, causing lymphocytic vasculitis and capillary leakage, resulting in possible complications.
• Clinical features: Incubation period (5-9 days), fever, rapid defervescence, headache, mouth/nose bleeding, muscle pain, vomiting, rash, photophobia, eye pain, and various others, depending on the stage and type of dengue infection.
• Complications: Dengue Shock Syndrome (DSS) and Dengue Hemorrhagic Fever (DHF).
• Investigations: Elevated hematocrit, leucopaenia, low platelets, elevated liver enzymes, and serological tests (dengue antibody IgM and IgG ELISA), and PCR.
• Management: Supportive, including intravenous crystalloids, paracetamol, platelet infusion and FFP.

Prevention

• Hand hygiene: 5 moments approach for hand hygiene, alcohol-based hand sanitizers, and use of appropriate personal protective equipment (PPE).
• Respiratory precautions: Report symptoms of acute respiratory illness, covering mouth, proper hand hygiene, and tissue disposal, and provision of protective measures in specific cases.

Other information

• The presentation includes references.