Theme 12

Acquired Immunodeficiency Syndrome (AIDS)

Overview and Etiology

AIDS is a disease caused by infection with the Human Immunodeficiency Virus (HIV), which targets the immune system, specifically CD4+ T cells. The progressive loss of these cells leads to immunodeficiency, making individuals susceptible to opportunistic infections and certain cancers.

• HIV Structure: Enveloped virus with two copies of single-stranded RNA, reverse transcriptase, and surface glycoproteins (gp120, gp41).

• Transmission: Primarily through sexual contact, blood, perinatal exposure, and contaminated needles.

• Pathogenesis: HIV binds to CD4 receptors and co-receptors (CCR5 or CXCR4) on host cells, enters the cell, and integrates its genome into host DNA.

HIV Infection and Progression

The progression of HIV infection is divided into three phases, each characterized by changes in CD4+ T cell counts and clinical symptoms.

• Phase 1: Acute infection with high viral load and mild or no symptoms. CD4+ T cell count remains relatively normal.

• Phase 2: Clinical latency with slow decline in CD4+ T cells. Opportunistic infections may begin to appear.

• Phase 3: AIDS, marked by severe immunodeficiency (CD4+ T cell count < 200 cells/μL) and increased susceptibility to opportunistic diseases.

Table: Selected Diseases Commonly Associated with AIDS

Pathogen or Disease	Disease Description
Pneumocystis jirovecii	Pneumonia
Mycobacterium tuberculosis	Tuberculosis
Candida albicans	Oral and esophageal candidiasis
Toxoplasma gondii	Encephalitis
Herpes simplex virus	Chronic ulcers
Kaposi's sarcoma	Vascular cancer

HIV Transmission and Epidemiology

HIV is transmitted through direct contact with infected body fluids. Epidemiological studies show varying prevalence worldwide, with sub-Saharan Africa having the highest rates.

• Prevention: Safe sex practices, screening blood products, and needle exchange programs.

• Diagnosis: Serological tests (ELISA, Western blot), PCR for viral RNA.

Antiretroviral Therapy (ART)

ART involves the use of multiple drugs to inhibit different stages of the HIV life cycle, improving patient outcomes and reducing transmission.

• Drug Classes: Reverse transcriptase inhibitors, protease inhibitors, integrase inhibitors, fusion inhibitors.

• Mechanism: ART suppresses viral replication, increases CD4+ T cell counts, and delays progression to AIDS.

Table: Drugs that Inhibit the HIV Life Cycle

Drug Class	Target
Reverse transcriptase inhibitors	Viral RNA to DNA conversion
Protease inhibitors	Viral protein processing
Integrase inhibitors	Integration of viral DNA into host genome
Fusion inhibitors	Viral entry into host cell

Challenges in HIV Vaccine Development

Developing an effective HIV vaccine is complicated by the virus's high mutation rate, genetic diversity, and ability to evade immune responses.

• Antigenic variation: HIV rapidly changes its surface proteins.

• Latency: HIV can remain dormant in host cells, escaping immune detection.

Rabies

Etiology and Pathogenesis

Rabies is a viral disease caused by the Rabies virus (genus Lyssavirus), typically transmitted through the bite of infected animals. The virus infects peripheral nerves and travels to the central nervous system, causing fatal encephalitis if untreated.

• Transmission: Animal bites (dogs, bats, raccoons, skunks).

• Symptoms: Fever, headache, muscle spasms, hydrophobia, confusion, paralysis.

• Diagnosis: Detection of viral antigens in tissue samples, PCR, and serology.

• Treatment: Post-exposure prophylaxis (PEP) with rabies vaccine and immunoglobulin.

Epidemiology and Prevention

Rabies is rare in developed countries due to vaccination programs for pets and wildlife control. Human cases are more common in regions with limited access to vaccines.

• Prevention: Vaccination of domestic animals, wildlife management, and prompt medical care after exposure.

Microbial Diseases of the Respiratory System

Upper Respiratory Tract Infections

The upper respiratory tract is susceptible to various microbial diseases, including bacterial and viral infections.

• Bacterial Diseases: Streptococcal pharyngitis, scarlet fever, diphtheria, otitis media.

• Viral Diseases: Common cold (caused by rhinoviruses, coronaviruses).

Table: Microbial Diseases of the Upper Respiratory System

Disease	Pathogen	Symptoms	Treatment
Streptococcal pharyngitis	Streptococcus pyogenes	Sore throat, fever	Antibiotics
Scarlet fever	S. pyogenes	Red rash, fever	Antibiotics
Diphtheria	Corynebacterium diphtheriae	Throat membrane, fever	Antitoxin, antibiotics
Otitis media	Various bacteria	Ear pain, fever	Antibiotics
Common cold	Rhinoviruses, coronaviruses	Sneezing, runny nose	Supportive care

Lower Respiratory Tract Infections

Lower respiratory tract infections are often more severe and include bacterial, viral, and fungal diseases.

• Bacterial Diseases: Pertussis (whooping cough), pneumonia (various pathogens).

• Viral Diseases: Influenza, respiratory syncytial virus (RSV), viral pneumonia.

• Fungal Diseases: Histoplasmosis, coccidioidomycosis.

Influenza (Flu)

Influenza is a highly contagious viral disease caused by Influenza virus types A, B, and C. The virus undergoes frequent genetic changes, leading to seasonal epidemics and occasional pandemics.

• Structure: Enveloped virus with segmented RNA genome and surface proteins hemagglutinin (HA) and neuraminidase (NA).

• Symptoms: Fever, cough, sore throat, muscle aches.

• Diagnosis: Rapid antigen tests, PCR.

• Treatment: Antiviral drugs (oseltamivir, zanamivir).

• Prevention: Annual vaccination.

Fungal Diseases of the Respiratory System

Fungal infections such as histoplasmosis are acquired by inhaling spores from the environment. These diseases are more common in immunocompromised individuals.

• Histoplasmosis: Caused by Histoplasma capsulatum, leading to lung infection.

• Symptoms: Fever, cough, chest pain.

• Diagnosis: Culture, serology, imaging.

• Treatment: Antifungal drugs (itraconazole, amphotericin B).

Key Concepts and Equations

• CD4+ T cell count: Used to monitor HIV progression. AIDS is defined as CD4+ T cell count < 200 cells/μL.

• Viral load: Measurement of HIV RNA in blood, used to assess treatment efficacy.

Equation:

$\text{AIDS diagnosis:} \quad \text{CD4}^+ \text{T cell count} < 200 \, \text{cells}/\mu L$

Example: A patient with a CD4+ T cell count of 150 cells/μL and recurrent pneumonia is diagnosed with AIDS.

Additional info: Some content was inferred and expanded for completeness, including details on ART drug classes, rabies epidemiology, and fungal diseases of the respiratory system.