Infectious Diseases Overview

Infectious Diseases

• Infectious diseases are a broad category encompassing various illnesses caused by pathogens.
• Important infectious diseases include typhoid fever, brucellosis, and acquired immunodeficiency syndrome (AIDS).
• Normal body temperature ranges from 36.5°C to 37.2°C.
• Fever is defined as having a body temperature above the normal upper limit.
• Hyperpyrexia is a fever exceeding 40°C.
• Types of fever include sustained, remittent, hectic, and relapsing fever.

Aetiological Classification

• Bacterial:
  • Gram-positive: streptococci, staphylococci
  • Gram-negative: Klebsiella, Pseudomonas, Haemophilus influenza
  • Anaerobic bacteria: bacteroides
• Mycobacterial: Tuberculosis (TB), Leprosy
• Spirochetal: Syphilis, Leptospirosis
• Rickettsia: 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, Bilharziasis, Giardiasis
• Fungal: Candida albicans, Aspergillus niger

Typhoid Fever

• Etiology: Salmonella typhi and paratyphi A and B
• Mode of Infection: Feco-oral transmission
• Clinical Manifestations:
  • First week: Fever (stepladder pattern), headache, relative bradycardia, constipation, rose spots
  • Second week: Continuous fever, tachycardia, diarrhea, splenomegaly
  • Third week: convalescence begins
• Complications: Intestinal bleeding, peritonitis, meningitis, encephalitis, convulsions, pyelonephritis
• Investigations: CBC (leucopenia with relative lymphocytosis), Widal test (positive from the second week – prognostic rather than diagnostic), cultures (blood, stool, urine, in weeks 1-3 respectively).
• Treatment: Ciprofloxacin, Co-trimoxazole, ceftriaxone

Brucellosis

• Etiology: Brucella (melitensis, abortus, or suis)
• Mode of Infection: Contaminated milk, or remnants from aborted animals.
• Clinical Picture: Bone and muscle pain, vomiting, constipation, relapsing fever. Fever for 10 days followed by 10 days of apyrexia- cycling fever. Hepatosplenomegaly and lymph node enlargement.
• Complications: Relapse, infective endocarditis, orchitis, paraplegia (transverse myelitis), abortion.
• Investigations: CBC (lymphocytosis positive during fever spike), Blood culture, Serological tests (Brucella agglutination test - the best diagnostic test).
• Treatment: Doxycycline or rifampicin for 6 weeks.

Acquired Immunodeficiency Syndrome (AIDS)

• Etiology: Human Immunodeficiency Virus (HIV)
• Modes of Infection: Parenteral, sexual, vertical
• Pathogenesis: HIV infects T-lymphocytes, suppressing cellular and humoral immunity. A fall below 200 T-cells/dL leads to opportunistic illnesses and malignancies.
• Clinical Picture: Asymptomatic phase (up to 10 years), acute HIV syndrome (fever, rigors, arthralgia, rash), symptomatic syndromes (generalized LN enlargement, Constitutional symptoms, Neurological disease, Opportunistic infections, Secondary neoplasms).
• Investigations: HIV antibodies (screening, appear within 2 weeks of infection), HIV-RNA by PCR (best diagnostic test).
• Treatment: Anti-retroviral (HAART), Zidovudine 600 mg/day orally (note that HAART involves multiple drugs in practice).

Fever of Unknown Origin (FUO)

• Definition: Persistent elevation of body temperature above 38.5°C for 3 weeks without a diagnosed cause. Requires 1 week of inpatient investigations.
• Causes: Bacterial (TB, infective endocarditis, typhoid fever, brucellosis, lung abscess, pyelonephritis), Viral (EBV, CMV, HIV, hepatitis), Protozoal (malaria, amebiasis), Malignancy (hematologic and non-hematologic), Collagen diseases (rheumatic fever, PAN, SLE), Others (inflammatory bowel disease, hemolysis, cerebral hemorrhage).
• Undiagnosed Often the cause is unidentified even after comprehensive investigation.

Other Topics/Notes

• Preventive measures (hand hygiene, respiratory precautions)
• Investigations (laboratory, imaging, endoscopy, biopsies, etc.)
• Treatment regimens (for TB, malaria, etc.). Details on the use of different medicines are not included in this summary for safety and efficacy reasons.

Pulmonary Tuberculosis (TB)

• Definition: Chronic lung infection caused by Mycobacterium tuberculosis.
• Etiology: Mycobacterium tuberculosis (acid-fast, intracellular, aerobic, non-motile bacillus)
• Risk Factors: Children under 5, close contacts with active TB patients, immunocompromised individuals (AIDS, diabetes, leukemia, steroid users)
• Mode of Transmission: Droplet infection.
• Clinical Picture: Symptoms (night fever, sweating, weight loss, appetite loss), cough and expectoration (thick mucoid), hemoptysis (blood-tinged sputum), sings (inspection, percussion, auscultation), complicated TB with massive systemic spread in immunocompromised patients, tubercles in CNS, lungs, pericarditis, Pott's disease of spine, generalized lymphoma.
• Investigations: Chest X-ray, ESR and CRP (elevated), sputum examination (acid-fast bacilli staining), PCR (Mycobacterial DNA detection), Seriological tests (Quantiferon Gold, ADA), Tuberculin skin testing.
• Treatment: First-line drugs (isoniazid, rifampicin, pyrazinamide, ethambutol, and streptomycin), combination therapy for long periods, standardized 6-month regimen. Indications for corticosteroids vary.

Dengue Fever

• Definition: Viral disease caused by the dengue virus.
• Etiology: Dengue virus (RNA virus, Flavivirus, arbovirus).
• Vector: Aedes aegypti mosquito.
• Pathogenesis: Virus inoculated by the bite of an infected mosquito, replicates in the reticuloendothelial system. Secondary infections may cause antibody response and complications.
• Clinical Features: Incubation (5-9 days), social history (residence in or travel to endemic region). Initial fever (2-4 days), rapid defervescence, afebrile period (1 day), second fever period (2-3 days). Generalised lymphadenopathy, skin rash (itchy maculopapular), GIT issues, CNS and CVS symptoms,
• Complications: Dengue shock syndrome (DSS), Dengue haemorrhagic fever (DHF).
• Investigations: Haematology (Elevated Hct, leucopenia, low platelets), Biochemistry (Elevated liver enzymes), Immunology (Dengue Antibody - IgM and IgG ELISA), Microrbiology (PCR possible).
• Management: Support (intravenous fluids, colloids, paracetamol). Possible platelet transfusion.
• Prevention: Vector control, biological agents
• Prognosis: Mortality 10-30% in dengue haemorrhagic fever.