Vector-Borne Diseases and Their Transmission

Transmission Objectives

• Discuss various routes of transmission, especially concerning vector-borne diseases.

Reading for Today:

• "The Unified Theory of Perspiration" – Desowitz.
• "Resurgent Vector-Borne Diseases" - Gubler.

Reading for Thursday:

• "Common Strategies for Antigenic Variation" Deitsch et al.

Transmission Overview

• Microbes must leave the body and be transmitted to fresh hosts to thrive.
• Three major factors in transmission:
  1. The quantity of microorganisms shed: More pathogens released increases the likelihood of finding a new host.
  2. Microorganism stability in the environment: Some organisms resist drying, heat, and chemical insults via spores or cysts.
  3. The quantity of microorganisms needed to infect a fresh host: Different organisms have varying virulence thresholds; for example, only 10 Shigella bacteria are needed for infection while 10,000,000 Salmonella bacteria are required.

Factors Affecting Transmission

• Genetic Susceptibility: Individual differences in immune response.
• Upper Respiratory Issues: Coughing and sneezing promote pathogen spread.
• Intestinal Activity: Diarrhea serves as a mechanism to shed pathogens.

Types of Human Transmission:

• Transmission occurs through various bodily fluids (blood, mucous, urine, seminal fluid, milk, saliva).
• Skin-to-skin contact.
• Vertical Transmission: From parents to offspring.
• Invertebrates (e.g., insects, ticks, mites) as carriers.
• Vertebrates (e.g., mammals, birds) representing zoonoses.

Transmission Risk Reduction

• Proper mask use dramatically reduces transmission risks:
  • 90% Risk
  • 30% Risk
  • 5% Risk
  • 1.5% Risk
  • 0% Risk
• 6 FT social distancing can help minimize risk.

Horizontal vs. Vertical Transmission

• Horizontal Transmission: Common infections like polio and influenza spread among individuals via air, food, contact, or vectors.
• Vertical Transmission: Involves transmission through reproductive means including ovum/sperm to offspring (Examples: HTLV, HIV, CMV, rubella).

Types of Transmission Overview

1. Between People: Directly between individuals.
2. Soilborne Pathogens: Includes fungal pathogens, mycoses, tetanus, and various spore-forming bacteria (e.g., Clostridium, Bacillus).
3. From Vectors/Animals to People: Encompasses arthropod-borne infections and zoonoses:

List of Vector-Borne Diseases:

• Examples of Arthropod-Borne Pathogens:
  • Malaria
  • Sandfly Fever
  • Typhus
  • Brucellosis
  • Rabies
  • Yellow Fever

Zoonotic Diseases

• 60% of existing human infectious diseases are zoonotic.
• 75% of emerging human infectious diseases have an animal origin, including significant diseases like Ebola and HIV.

Major Arthropod-Borne Pathogens

Pathogen Types and Associated Diseases:

• Viruses: Flaviviruses, Bunyaviruses, examples include yellow fever, dengue.
• Bacteria: Yersinia (plague), Rickettsias (spotted fever).
• Protozoa: Trypanosomes leading to diseases such as Chagas' disease.

Lyme Disease and Other Tick-Borne Illnesses

• Lyme Disease: Caused by Borrelia burgdorferi; noted for its geographical distribution in the US.
• Tick-borne Encephalitis: Affects the nervous system; involves multiple species of flaviviruses.
• Major Tick-Borne Diseases: A comprehensive understanding does include statistics on annual cases and affected geographical areas.

Treatment and Prevention Techniques

• Ivermectin and Diethylcarbamazine are used to treat various parasitic infections, targeting different mechanisms of action.
• Integrated Control Strategies: Contextual disease management approaches combining vector control, human treatments, sanitation, and monitoring.

R0 and Infection Dynamics

• Basic Reproductive Ratio (): Fundamental in understanding disease dynamics; determines if a disease will spread (R0 > 1), maintain itself (R0 = 1), or dwindle (R0 < 1).
• R0 Influences: Contact rate, infectious period, disease transmission modes.

Environmental Influences on Transmission

• Impact of climate factors on vector prevalence.
• Factors like sanitation, behavior, and environmental modifications influence disease spread.

Future Considerations

• Potentiality of "Altruistic Vaccines" that could prevent disease transmission without benefiting the individual directly.
• The relationship between vector dynamics and pathogen transmission patterns in response to climate change.

Closing Note

• Future studies and discussions will focus on understanding the complexities of vector-borne diseases and their impacts on public health, as well as exploring solutions through education about these topics in focused courses like Medical Entomology.