Infectious Disease Management, Pathogen Adaptations, and Pharmacology

Environmental Management and Mosquito Control Strategies

General Environmental Management: Efforts to control vector-borne diseases often focus on minimizing the presence of still water where mosquitoes breed. * Examples of practical management include cleaning out roof gutters and fixing potholes in the ground to prevent stagnant water accumulation.
Personal Protection Measures: In many South Asian and Asian regions, such as Sri Lanka, mosquito nets are draped over beds to protect individuals while they sleep. Additionally, insect repellent is used to prevent physical contact with vectors.
Genetic Engineering via CRISPR: * Gut Modification: One method involves modifying the gut of the mosquito to ensure it cannot properly host the Plasmodium parasite. * Offspring Modification: A second example involves editing female mosquitoes so they do not survive into adulthood. These genetically modified genes are released into the wild population. * Mechanism of Spread: Through natural selection, the modified genes are passed on by males to the wild population. Since the female offspring die before breeding, the overall population decreases. * Ethical Implications: Altering a food web can cause a "catapult chain reaction" or a cascade effect. There are also inherent ethical concerns regarding the intentional eradication of a species, despite mosquitoes often being viewed as annoying or dangerous.

Prevention vs. Treatment of Infectious Diseases

Focus on Prevention: For diseases like Malaria or Dengue, it is more effective in the long term to focus on controlling the cause rather than just treating cases.
Economic and Practical Rationale: Investing in prevention is more sustainable; otherwise, treatment must be provided indefinitely.
Epidemiological Terms: * Prevalence: The total number of cases that exist in a population at a given time. For Malaria, prevalence and infection rates are exceptionally high.
The Transmission Link: The spread of disease is directly related to the movement and density of the vector. Therefore, controlling the vector is the most effective way to halt the spread.

Comparative Adaptations of Viruses and Bacteria

Stages of Infection: Pathogens generally progress through three stages to cause disease: * Transmission: How the pathogen moves from one host to another. * Adhesion: How the pathogen stays attached to the host. * Invasion: How the pathogen enters the cells and causes disease.
Viral Adaptations: * Transmission: Direct contact or indirect transmission (e.g., coughing or sneezing). Viruses can stay suspended in the air for long periods and resist desiccation (drying out). Some viruses specifically stimulate sneezing and coughing in the host to facilitate their own spread. * Adhesion: Viruses bind to specific host cell receptors. For example, HIV (Human Immunodeficiency Virus) adheres to $CD4$ receptors on T helper cells and macrophages. * Invasion: Viruses are intracellular pathogens. They enter cells via receptor-mediated endocytosis, where the cell membrane essentially "hugs" the virus and takes it inside. * Enveloped Viruses: Are enclosed in an endosome (a lipid vesicle). * Non-enveloped Viruses: Often form a pore in the host cell membrane to deliver the viral genome directly. * Replication and Release: Viruses hijack the host's DNA/RNA machinery to replicate their genome and proteins. They eventually trigger apoptosis (programmed cell death), causing the cell to explode and release new viruses to infect more cells.
Bacterial Adaptations: * Transmission: Bacteria often reside in reservoirs like food, water, or air (indirect transmission). They may induce vomiting or diarrhea in the host to facilitate spread. * Adhesion: Bacteria use structures such as pili and fimbriae (hair-like projections) or adhesion proteins to latch onto mucus or membranes (e.g., in the bladder or stomach). They also form a biofilm, an extra protective layer that helps them stick to surfaces. * Invasion: * Capsule: A hard outer layer that resists phagocytosis (the process where host cells engulf and destroy pathogens). * Granulomas: Bacteria can hide inside granulomas, which are clusters of white blood cells, effectively shielding them from immune detection. * Enzymes: Some bacteria produce enzymes like IgA protease, which specifically destroys $IgA$ antibodies, a primary human defense.

Pathogenesis of Malaria (Plasmodium falciparum)

Pathogen Classification: The malaria parasite, Plasmodium falciparum, is a protozoan.
Transmission Mechanism: Transmitted via the saliva of the female Anopheles mosquito. When the mosquito bites, it injects saliva containing the pathogen into the human bloodstream.
Invasion of the Liver: * The parasite travels from the skin to the blood circulation and then to the liver. * It infects hepatocytes (liver cells), where it utilizes the liver's high nutrient content for asexual reproduction. * Staying inside cells allows it to avoid immune detection (B cells and T cells do not recognize the infected cell as foreign from the outside).
Invasion of Red Blood Cells: * After the liver stage, the parasite triggers apoptosis, enters the blood, and infects red blood cells. * Red blood cells carry oxygen via hemoglobin ( $HB$ ). Infected cells lose their discoid shape, becoming shriveled. * Symptoms: This leads to anemia (lack of hemoglobin/oxygen), causing fatigue. The activation of the immune system causes fever.
Cerebral Malaria: * While the blood-brain barrier usually prevents pathogens from entering the brain, infected red blood cells can carry the parasite there. * Cerebral malaria is often fatal.
Cycle Completion: A new mosquito bites the infected person, takes up the infected blood into its saliva, and the process repeats.

History and Classification of Antibiotics

Discovery: * The first antibiotic was Arsphenamine (used for syphilis), but it was toxic due to arsenic content. * Penicillin: Discovered by accident by Alexander Fleming from the fungus Penicillium notatum. It was effective in very low concentrations.
The Golden Age: Between the $1950s$ and $1960s$ , most modern antibiotics were discovered, greatly increasing human life expectancy.
Classification by Method: * Bactericidal: Suicidal/kill. These kill the bacteria by attacking the cell wall causing lysis. This is irreversible. These do not require the immune system to finish the job, making them ideal for immunocompromised patients (elderly, organ transplant recipients, or those with autoimmune diseases). Example: Penicillin. * Bacteriostatic: Static/stop. These "pause" the replication (DNA and proteins) of the bacteria. This is reversible. They work in conjunction with the immune system to clear the infection. Example: Tetracycline (or Doxycycline, used for pneumonia).
Classification by Target: * Broad Spectrum: Attacks many types of bacteria (both Gram-positive and Gram-negative). Used when the specific cause of a severe infection is unknown and treatment cannot wait. These disrupt the host's natural microflora (biological barriers) and are a major driver of antibiotic resistance. * Narrow Spectrum: Targets specific strains of bacteria. These are used when the pathogen is identified via a swab or test. They are less likely to cause widespread resistance or disrupt the healthy microbiome.

Antibiotic Resistance (AR)

Process of Selection: * Large Population: Bacteria exist in millions. * Reproduction: They reproduce asexually via binary fission every $4$ to $20$ minutes. * Variation/Mutation: Rapid reproduction leads to high mutation rates because bacteria lack sophisticated error-checking mechanisms. * Selective Pressure: The antibiotic acts as the pressure. Those with a mutation for resistance have a selective advantage. * Horizontal Gene Transfer: Bacteria can pass resistance genes to others in the same generation via circular DNA called plasmids.
Examples of Resistant Strains: * MRSA: Methicillin-resistant Staphylococcus aureus (Golden Staph). * VRE, MDR TB, CRE: Other common resistant strains frequently found in hospitals.
Prevention of Resistance: * No Antibiotics for Viruses: Using them for viral infections unnecessarily exposes gut bacteria and pathogens to the drug. * Finishing the Full Course: Even if a patient feels better, some pathogens remain. Stopping early allows the most resistant survivors to multiply. * Hygiene: Prevents the spread of resistant alleles between populations.

Antiviral Medications

Challenges: Because viruses live inside host cells, killing the virus often risks killing the host cells. Antivirals generally do not "cure" the infection but focus on preventing reproduction.
Mechanisms of Action: * Inactivating proteins on host cells so the virus cannot attach/adhere. * Inhibiting host cell transcription and translation so viral proteins aren't made. * Inhibiting the release of new viruses from the cell (stopping apoptosis).
Timing: Antivirals are most effective when given before mass reproduction. Once symptoms (fever, pain, swelling) appear, the virus has usually already replicated extensively.
Case Studies: * HIV/AIDS: Antivirals must be taken daily to slow viral spread. While they extend life, they do not cure the disease; the immune system eventual weakens, leading to secondary infections. * COVID-19 (Paxlovid): Consists of two parts: the $N$ part (inhibits the virus) and the $R$ part (stops the liver from breaking down the $N$ part too quickly).

Questions & Discussion

Q: Why focus on controlling the cause (prevention) for Malaria rather than just treatment?
A: Prevalence and infection rates are too high for treatment to be the sole strategy. It is more cost-effective and safer to stop the vector (mosquito) because the spread of the disease is directly linked to the vector.
Q: Which modes of transmission do viruses use?
A: Primarily direct and indirect transmission.
Q: What is the definition of validity in a bacterial experiment (e.g., using a blank agar plate)?
A: Validity ( $V$ for variables) refers to how well the experiment addresses the aim by controlling variables. A blank agar plate acts as a control, which confirms the validity of the results.
Q: When should antivirals be administered?
A: Ideally after exposure but before mass reproduction. However, they can be given after symptoms in serious cases to minimize further development.
Q: Why is overprescription of antibiotics bad?
A: It creates unnecessary exposure to selective pressure, kills healthy gut bacteria (increasing infection risk), and allows pathogenic bacteria to develop resistance without cause.