1-27 Med/Vet Ent.

Transmission of Vector-Borne Diseases

Introduction to Vector-Borne Diseases

  • Focus on definitions relevant to vector-borne diseases.

  • Reference to a variety of diseases transmitted by arthropod vectors in the United States.

    • Examples of local diseases to be discussed (e.g., West Nile virus).

    • Mention of globally significant diseases like malaria.

Increase in Vector-Borne Diseases

  • Despite effective public health measures in the U.S., cases of vector-borne diseases are increasing.

  • Contributing factors:

    • New diseases emerging

    • Spread of vectors into new regions due to climate change

    • Civil unrest leading to outbreaks

  • Data showing trend of increasing cases of vector-borne diseases from 2004 to February 2018.

Case Study: West Nile Virus

  • First discovered in New York City in 1999.

  • Originally from Africa; outbreak linked to bird mortality.

  • Spread timeline:

    • 1999: First human case in New York.

    • 2000: Spread throughout the Northeastern U.S.

    • 2001: Found in multiple Eastern states.

    • 2002: Coast-to-coast spread.

    • 2005: Present in all 48 contiguous states.

  • Transmission and symptoms:

    • Transmitted primarily by mosquitoes of the genus Culex (notably Culex tarsalis).

    • Symptoms include mild flu-like symptoms (fever, headache).

    • Severe cases can result in central nervous system infections such as encephalitis or meningitis, with about a 10% mortality rate.

Case Study: Chikungunya

  • Identified in Eastern Africa in 1952; with limited spread until the early 2000s.

  • Spread timeline:

    • 1958: Strains appear in Asia.

    • 2007-2010: Re-emergence in Europe.

    • 2014: Spread to South America.

    • 2015-2016: Emergence in the United States.

  • Vectors: Transmitted by Aedes mosquitoes (Aedes aegypti and Aedes albopictus).

  • Symptoms include flu-like symptoms with severe rash; low mortality rate but high CNS involvement severity in certain cases.

  • No known treatment or vaccine available.

Case Study: Zika Virus

  • Initially unknown in the U.S.; first cases identified post-travel in South America (2015).

  • 2016: Major outbreak with over 4900 cases in the U.S., primarily travel-related.

  • Transmitted by Aedes mosquitoes (Aedes aegypti and Aedes albopictus).

  • Symptoms are flu-like, but notable severe effects include:

    • Guillain-Barré syndrome (autoimmune disorder causing paralysis).

    • Microcephaly in fetuses when pregnant women are infected, resulting in permanent disability.

  • Currently no vaccine or effective treatment; prevention focused on avoiding mosquito bites.

Definitions and Concepts in Vector-Borne Disease Cycles

  • Disease Cycle Components:

    • Pathogen: Can be viruses, bacteria, protozoa, or helminths (nematodes).

    • Vector: Arthropods (e.g., mosquitoes, ticks) involved in disease transmission.

    • Host: Organisms that carry the disease; can be anthroponotic (human-only) or zoonotic (animal and human).

    • Anthroponotic Diseases: Exclusively affect humans (e.g., malaria).

    • Zoonotic Diseases: Present in animals and can affect humans (e.g., West Nile, plague).

Key Definitions in Pathogen Categories

  • Obligate Pathogens: Only survive in a host; dependent on host for existence.

  • Facultative Pathogens: Can live both in and outside a host; free living in the environment.

  • Reproduction in Pathogens:

    • Sexual Reproduction: Genetic mixing occurs; gametes fuse to form new zygotes.

    • Asexual Reproduction: Cloning process; identical offspring to the parent.

  • Definitive Host: Host where sexual reproduction of the pathogen occurs.

  • Intermediate Host: Asexual reproduction or no reproduction occurs; involved in the pathogen’s lifecycle.

Vectors and Their Role in Disease Transmission

  • Definition of vectors: Organisms that actively transmit pathogens from one host to another.

  • Types of vectors discussed:

    • Arachnids (ticks, mites)

    • Insects (fleas, flies, mosquitoes)

  • Vector Competence: The ability of a vector to transmit a pathogen; varies among species within a genus.

Extrinsic and Intrinsic Incubation Times

  • Extrinsic Incubation Time: The time a pathogen remains within a vector before it can be transmitted to a new host.

  • Intrinsic Incubation Time: The period before symptoms appear in a host after infection.

  • Dead-end Hosts: Species that become infected but do not transmit the disease further.

Epidemiological Terms Related to Disease Spread

  • Endemic: Disease consistently found within a population or region.

  • Epidemic: Sudden increase in disease incidence that is not usually present.

  • Pandemic: An epidemic that spreads across regions or worldwide.

Data and Monitoring Strategies in Epidemiology

  • Monitoring vector populations through traps and sentinel animals (e.g., chickens).

  • Studying animal hosts for infection rates.

  • Conducting public health outreach for reporting dead birds or other hosts.

Evolution and Adaptation in Vector Species

  • Discussion of the evolutionary history of parasitism in diverse arthropods (ticks, fleas, lice).

  • Importance of studying fossil records to understand the evolution of parasitic vectors.

  • Relationship between parasitism and the development of host interactions over time.