Infectious Diseases and Antibiotics

Fundamental Concepts of Infectious Disease

  • Definitions and Terminology:

    • Infectious Diseases: These are diseases caused by organisms known as pathogens. They are also known as communicable diseases because they can be passed from infected to uninfected individuals via disease transmission.
    • Disease: Often defined as an illness or disorder of the body or mind that results in poor health. It is characterized by a specific set of signs and symptoms.
    • Pathogen: An organism that causes disease, such as a protoctist, bacterium, or virus.
    • Infectious Cycle/Transmission Cycle: The process by which a pathogen passes from one host to another. Control methods aim to break these cycles by eliminating conditions that favor spread.
    • Direct Contact: Required for pathogens that cannot survive outside the human body.
    • Indirect Transmission: Occurs when pathogens survive in water, food, faeces, or animal vectors (including insects) before infecting a new host.
    • Disease Carriers (Symptomless Carriers): Individuals who spread a pathogen but do not show symptoms themselves, making them difficult to trace as sources of infection.
    • Endemic Disease: A disease that is always present in a population. Examples include Tuberculosis (TB) globally and Malaria in tropical/sub-tropical regions.
    • Disease Eradication: The complete breakage of a pathogen's transmission cycle so no cases occur worldwide. Successes include Smallpox (declared eradicated in 1980) and Rinderpest (livestock disease, 2011).

  • Quantitative Measures of Disease:

    • Incidence: The number of people diagnosed with a disease over a specific period (e.g., a week, month, or year).
    • Prevalence: The number of people who have a disease at any one point in time.
    • Epidemic: A sudden increase in the number of cases of a disease.
    • Pandemic: An increase in cases across a continent or throughout the entire world.
    • Mortality Rate: The number of deaths caused by a disease over a set length of time (usually a year).
    • Standardization: Data is typically expressed as a proportion (e.g., per 100,000100,000 people) to allow for valid comparisons between populations and across different time periods.

Major Infectious Diseases and Causative Agents

  • Cholera:

    • Causative Agent: Vibrio cholerae (Bacterium/Prokaryote).
    • Transmission: Food-borne and water-borne.
    • Global Distribution: Asia, Africa, Latin America.
    • Incubation Period: Two hours to five days.
    • Site of Action: Wall of the small intestine.
    • Clinical Features: Severe diarrhoea ('rice water'), loss of water and salts, dehydration, and weakness.
    • Diagnosis: Dipstick test of rectal swabs or identification of V. cholerae in faecal samples via microscopy.

  • Malaria:

    • Causative Agent: Four species of protoctist (Eukaryote) in the genus Plasmodium: P. falciparum, P. malariae, P. ovale, and P. vivax.
    • Transmission: Insect vector (female Anopheles mosquito), blood transfusion, re-use of unsterile needles, and across the placenta (mother to fetus).
    • Global Distribution: Throughout the tropics and sub-tropics (endemic in 106 countries).
    • Incubation Period: One week to one year.
    • Site of Action: Liver, red blood cells, and brain.
    • Clinical Features: Fever, anaemia, nausea, headaches, muscle pain, shivering, sweating, and enlarged spleen.
    • Diagnosis: Dipstick test for malaria antigens in blood or microscopical examination of blood.

  • HIV/AIDS:

    • Causative Agent: Human Immunodeficiency Virus (HIV) (Virus).
    • Transmission: Semen/vaginal fluids during sexual intercourse, infected blood products, contaminated hypodermic syringes, mother to fetus across placenta, at birth, or via breast milk.
    • Global Distribution: Worldwide, specifically sub-Saharan Africa and South East Asia.
    • Incubation Period: Initial weeks for flu-like symptoms, but years (up to 10 or more) before AIDS symptoms develop.
    • Site of Action: T-helper lymphocytes, macrophages, and brain cells.
    • Clinical Features: Opportunistic infections (pneumonia, TB, cancers), weight loss, diarrhoea, dementia.
    • Diagnosis: Testing blood, saliva, or urine for antibodies against HIV.

  • Tuberculosis (TB):

    • Causative Agent: Mycobacterium tuberculosis and Mycobacterium bovis (Bacteria/Prokaryotes).
    • Transmission: Airborne droplets (M. tuberculosis) and undercooked meat or unpasteurised milk (M. bovis).
    • Global Distribution: Worldwide.
    • Incubation Period: A few weeks to several years.
    • Site of Action: Primary infection in lungs; secondary in lymph nodes, bones, and gut.
    • Clinical Features: Racking cough, coughing blood, chest pain, shortness of breath, fever, sweating, and weight loss.
    • Diagnosis: Rapid molecular test for DNA, sputum microscopy, chest X-ray, or long-term culture (up to 12 weeks).

Cholera: Detailed Pathophysiology and Treatment

  • Infection Process: Bacteria must pass through the stomach. If stomach acidity is less than pH 4.54.5, bacteria are unlikely to survive. Once in the small intestine, they multiply and secrete the toxin choleragen.
  • Mechanism of Illness: Choleragen disrupts the epithelial lining of the intestine, causing salts and water to leave the blood and enter the intestinal lumen. This leads to severe diarhoea; fluid loss can be fatal if not treated within 2424 hours.
  • Oral Rehydration Therapy (ORT): A solution of salts and glucose. Glucose is crucial because its absorption is linked to the uptake of sodium and potassium ions (Na+Na^+ and K+K^+). Intravenous rehydration is used if the patient is too ill to drink.
  • Prevention: Sewage treatment and chlorinated piped water. In the USA (2016), an oral vaccine was approved for travelers. Mass vaccination has been used in Africa during epidemics.
  • Historical Context: Epidemics followed the 2010 Haiti earthquake and the 2016 Yemen civil war.

Malaria: Lifecycle and Control

  • Vector Role: Female Anopheles mosquitoes feed on blood for protein to develop eggs. They take up Plasmodium gametes with a blood meal. Gametes fuse in the mosquito gut and move to salivary glands. When the mosquito bites another human, it injects an anticoagulant and the infective stages of the parasite.
  • African Intensity: Approximately 90%90\% of cases are in Africa because the local Anopheles species have long lifespans and a strong preference for biting humans. P. falciparum is the most fatal species dominant there.
  • Anti-malarial Drugs:
    • Quinine and Chloroquine: Used for treatment; chloroquine is also prophylactic (inhibits protein synthesis).
    • Proguanil: Inhibits the sexual reproduction of the parasite in the mosquito.
    • Mefloquine: Used where resistance exists, though it has side effects (dizziness, disturbed sleep).
    • ACT (Artemisinin-based combination therapy): Current best treatment, using drugs from the Artimisia annua plant.
  • Control Methods:
    • Biological Control: Stocking water with fish that eat larvae; using Bacillus thuringiensis to kill larvae.
    • Environmental Control: Draining marshes and clearing vegetation; spreading oil on water surfaces to prevent larvae from breathing.
    • Personal Protection: Insecticide-treated nets (ITNs) and indoor residual spraying. ITNs should be replaced every 22 to 33 years.
  • Vaccination: The RTS,S/AS01 (Mosquirix) injectable vaccine provides partial protection and was piloted in Ghana, Kenya, and Malawi in 2019.

HIV/AIDS: Virology and Management

  • Retroviral Nature: HIV is a retrovirus; its genetic material is RNA. It uses the enzyme reverse transcriptase to convert RNA into DNA, which is then incorporated into human chromosomes.
  • Immune Impact: The virus destroys T-helper lymphocytes, which coordinate the immune response. When these cell counts drop, opportunistic infections occur.
  • AIDS Defined: Acquired Immunodeficiency Syndrome is a collection of opportunistic diseases (e.g., Candida albicans/oral thrush, Pneumocystis jiroveci pneumonia, Kaposi's sarcoma).
  • Economic Impact: In sub-Saharan Africa, AIDS has drained government funds and affected the most economically productive age groups (2020s and 3030s).
  • Drug Therapy: Zidovudine (similar to thymine nucleotide) binds to reverse transcriptase and blocks it. Combination therapy with multiple drugs prolongs life and reduces mother-to-child transmission.
  • Prevention Strategies: Education, condom use (femidoms, dental dams), needle-exchange schemes, contact tracing, and screening of donated blood (which is also heat-treated).

Tuberculosis (TB): Latency and Resistance

  • State of Infection: About 30%30\% of the world population has latent TB (inactive, non-infectious). It becomes active when the immune system is weakened (HIV, malnutrition, diabetes, smoking).
  • Co-infection: TB is the leading cause of death among people living with HIV.
  • Drug Resistance:
    • MDR-TB: Multiple-drug-resistant TB, resistant to first-line drugs isoniazid and rifampicin.
    • XDR-TB: Extensively drug-resistant TB, resistant to first-line and several second-line drugs.
  • Treatment: DOTS (Direct Observation Treatment, Short course) involves health workers watching patients take medication to ensure completion (six to nine months). Bedaquiline is a newer drug for MDR-TB.
  • Vaccination: The BCG vaccine (derived from M. bovis) protects children but its effectiveness decreases with age.

Antibiotics: Mechanisms and Resistance

  • Antibiotic Definition: A substance derived from a living organism that kills or inhibits microorganisms without harming host cells.
  • How They Work:
    • Penicillin: Inhibits enzymes that build peptidoglycan cross-links in bacterial cell walls. Bacteria secrete autolysins to create holes for growth; without cross-links, the wall weakens and the cell bursts due to osmotic turgor pressure.
    • Other Targets: Protein synthesis (ribosomes), DNA replication, cell membrane function, and enzyme action.
    • Selective Toxicity: Antibiotics do not affect viruses because viruses lack metabolic targets (like cell walls or ribosomes) and use host machinery. Eukaryotic cells are unaffected because their proteins differ from bacterial ones.
  • Antibiotic Resistance Mechanisms:
    • Enzymatic Inactivation: Beta-lactamases (like penicillinase) break down the antibiotic.
    • Permeability: Thicker cell walls or proteins that pump out (efflux) antibiotics.
    • Genetic Spread: Resistance genes often reside on plasmids and are transferred via conjugation.
  • Reducing Resistance Impact:
    1. Prescribing only when necessary (not for viral infections).
    2. Using narrow-spectrum instead of wide-spectrum antibiotics.
    3. Ensuring patients complete entire courses of medication.
    4. Regularly changing the types of antibiotics prescribed.
    5. Avoiding prophylactic use in farming.

Questions & Discussion

  • Question 1a: State one structural feature of Plasmodium cells that indicates it is eukaryotic.
  • Question 1b: Explain why viruses are not classified as prokaryotes.
  • Question 2: Explain why all viruses are parasites, but not all bacteria are parasitic.
  • Question 3: Describe how cholera is transmitted from person to person.
  • Question 4: One person can excrete 101310^{13} cholera bacteria a day. An infective dose is 10610^6. How many people could one person infect in one day? (Response calculation: 1013/106=10710^{13} / 10^6 = 10^7, or 1010 million people).
  • Question 5: Explain the high risk of cholera following natural disasters like earthquakes or floods.
  • Question 6: Describe precautions for visitors in cholera-endemic countries.
  • Question 7: Describe how malaria is transmitted.
  • Question 8: Explain how Plasmodium cells differ from host red blood cells based on micrographs.
  • Question 9: Data analysis of malaria cases and deaths in Zanzibar (1999-2008).
  • Question 10: List factors making malaria difficult to control.
  • Question 11: Describe personal precautions against malaria.
  • Question 12: Calculate percentage changes and explain the phrase "living with HIV" based on Table 10.5.
  • Question 13: Suggest advice for an HIV/AIDS education programme.
  • Question 14: Explain HIV risk for children receiving blood transfusions for sickle cell anaemia or malaria.
  • Question 15: Explain why early knowledge of HIV infection is important for transmission control.
  • Question 16: Process TB data from a South East Asian country (population 69 million, 106,000106,000 new cases, 11,50011,500 deaths) for valid comparisons.
  • Question 17: Explain the high TB death rate in high-prevalence HIV populations.
  • Question 18: Precautions for visitors in high-prevalence TB countries.
  • Question 19: Explain why antibiotics are not effective against viruses.
  • Question 20: Describe ways bacteria resist antibiotics.
  • Question 21: Suggest why multi-drug resistant organisms evolve in hospitals and prisons.
  • Question 22: Suggest how limiting use, changing types, and combining antibiotics reduces resistance development.
  • Question 23: Analyze an antibiotic sensitivity test (disc diffusion) for E. coli O157 and choose the appropriate treatment using zone diameters (A through F).