Study Notes on Vector-Borne Infections

Geographical Restriction of Vector-Borne Illnesses
- Discuss the geographical regions where these illnesses are prevalent, focusing on factors like climate, ecology, and evolution.
Definitions
- Intrinsic Incubation: The period from when a host is infected until the pathogen can be transmitted to another host (within the host).
- Extrinsic Incubation: The time it takes for the pathogen to develop and be transmitted via a vector after being acquired from a vertebrate host.
Plasmodium spp. Life Cycle
- Outline the phases of the Plasmodium life cycle, including its transmission among mosquitoes and humans.
Clinical Presentation of Malaria
- Symptoms include fever, chills, headache, and potential complications leading to severe illness.
Clinical Presentation of Rocky Mountain Spotted Fever (RMSF)
- Discuss common symptoms: chills, confusion, headache, muscle pain, and potential rash.
RMSF Transmission & Characteristics
- Explain how RMSF is spread via tick bites and describe the causative agent, Rickettsia spp.
Diagnostic Challenges of RMSF
- Highlight the difficulties in diagnosing RMSF due to overlapping symptoms with other diseases.
Treatment of RMSF
- Review preferred therapeutic options, emphasizing prompt antibiotic therapy.

Annually, vector-borne illnesses result in over 700,000 deaths worldwide, constituting 17% of total infectious diseases.
The highest disease burden is noted in tropical and subtropical regions.

Vector: An arthropod responsible for the transmission of pathogens.
Vector-Borne Illness: A disease caused by a pathogen transmitted through an arthropod or invertebrate.
Zoonosis: Infections transmitted from animals to humans; human-to-human transmission is rare (exceptions include bubonic plague and Ebola virus).

Requirements for Transmission Cycles:
1. The parasite must develop and reproduce within both the vertebrate host and vector tissues.
2. The vertebrate host must reach a level of infection that is transmissible to the vector.
3. The vector must acquire the parasite from the infected vertebrate host and transmit it to another host.
Important Note: Not all parasites carried by a vector can infect a host, and not all parasites in a host can infect a vector.

Incidental/Accidental Hosts:
- Humans, while often not essential, can still contract infections (e.g., West Nile Virus).
Definitive and Intermediate Hosts:
- Definitive Hosts: Species where sexual reproduction occurs (e.g., in Plasmodium).
- Intermediate Hosts: Species where asexual reproduction occurs; can be either vertebrates or vectors.

Contact Degree: Different vectors have different feeding behaviors affecting transmission:
- Intermittent (e.g., mosquitoes) vs. Continuous feeders (e.g., lice).
Infection Requirements:
- A vector usually needs to take multiple blood meals to successfully transmit parasites.
Factors Affecting Transmission:
- Body Temperature: The ability of the parasite to withstand varying temperatures.
- Immune Evasion: The parasite's mechanisms to evade both the host's and vector's immune systems.
- Life Form: Asexual replication vs. sexual reproduction affecting life stages in hosts and vectors.

Fleas: Plague (Bacteria), Tungiasis (Ectoparasite).
Lice: Typhus (Bacteria), Louse-borne relapsing fever (Bacteria).
Sandflies: Leishmaniasis (Parasite), Sandfly fever (Virus).
Ticks: Crimean-Congo hemorrhagic fever (Virus), Lyme disease (Bacteria), RMSF (Bacteria).
Mosquitoes (Aedes): Chikungunya (Virus), Dengue (Virus), Zika (Virus).
Aquatic Snails: Schistosomiasis (Parasite).

Entry: Plasmodium spp. sporozoites are injected into the vertebrate host by Anopheles mosquitoes.
Liver Stage: Sporozoites quickly reach the liver, interact with host cells, and replicate to form schizonts.
Red Blood Cell Stage: Merozoites released from hepatocytes infect red blood cells, leading to further replication and differentiation into gametocytes.
Pathogenesis: Hemolysis of red blood cells, obstruction of capillaries, and triggering of immune responses resulting in symptoms like fever.

The genus Rickettsia is divided into different groups that include notable pathogens.
Rickettsia rickettsii:
- Obligate intracellular bacterium unable to grow on artificial media, replicating in eukaryotic cell cytoplasm.

Causative Agent: Rickettsia rickettsii.
Transmission: Spread via tick bites; incubation period averages 1 week.
Symptoms: Chills, confusion, headache, muscle pain, and rash (not always present); high mortality risk without antibiotic treatment.
Diagnosis: Often difficult due to lack of specific early symptoms; confirmed during convalescence with titer increases.
Preferred Treatment: Doxycycline is the treatment of choice, critical to initiate without waiting for lab results.

Possible nerve damage, hearing loss, incontinence, paralysis, and gangrene of extremities.
Leakage of blood vessels leads to encephalitis, pneumonitis, and arrhythmia.

Vector-borne illnesses are transmitted by invertebrate vectors and primarily affect tropical and subtropical regions: climate change may influence their distribution.
Control of vector populations is crucial due to the lack of vaccines for many vector-borne diseases.
Understanding the biology of the vector, host, and parasite is essential for developing effective interventions against these diseases.