EPIDEMIOLOGY OF VECTOR-BORNE DISEASES

Vector -borne Parasites

collective term for the increased outbreaks of many pathogens

Anthropogenic changes, Parasite ooportunities to expand to their distribution and parasites have evolved into more virulent or drug-resistant forms

Vector-borne parasites happen via

1. Discovery of Parasites as Causative Agents

2. Identify mode of transmission

3. Monitoring and Forecasting

4. Control

Methods of Investigation

Discovery of Parasites as Causative Agents

standard clinical case definitions are established, enabling the tracking of human or animal disease

Identifying mode of transmission

- evaluates vector abundance, host selection patterns, field infection rates, and vector competence through ecological investigation and laboratory experimentation

Monitoring and Forecasting

surveillance, epidemic transmission patterns have been described

Control

management of health outcomes/cases

Epidemiology

deals with the natural history and spread of diseases within human and animal populations

Vector-borne diseases

arthropod vector, a vertebrate host, and parasite

Vector

arthropod responsible for transmission of parasites (not diseases) among vertebrate hosts

Disease

response of the host to infestation or infection with a parasite outside or inside

Parasite

any organism (e.g. viruses, bacteria, protozoa, helminths, or arthropods) that is dependent on the host for its survival

Disease Agent/Pathogen

when a parasite injures its host and causes disease

Facultative Parasites

both free-living and parasitic forms

Obligate Parasites

totally dependent on their host(s) to sustain them

Ectoparasites

live on or outside the host; produce an infestation that remains topical or peripheral

Endoparasites

live inside the host; infection on invasion of host tissues or cells

Infected

host carrying a parasite

Infectious

infected host capable of transmitting a parasite

Carrier

host capable of parasite maintenance, particularly in an infectious form, without clinical symptoms

Gonotrophic Cycle

• reproductive cycle of arthropod

• includes sequence of searching for a host, blood feeding, blood meal digestion, egg maturation, and oviposition

Parous female

have a greater probability of being infected with parasites than nulliparous females that have not reproduced.

Parasite

develops and/or multiply within both vertebrate host and vectors

Vertebrate Host

develops infection with the parasite that is infectious to a vector

Vector

acquires the parasite from the infectious vertebrate host and is capable of transmission

8-12 days

Days for Extrinsic Incubation

4-7 Days

Days for Intrinsic Incubation

Extrinsic Incubation

From vector ingestion to potential parasite transmission

Intrinsic Incubation

how long the affected becomes infectious/show symptoms

1. Natural Immunity

2. Acquired Immunity

2 Types of Host Immunity

Natural (innate) immunity

doesn’t require host to have previous contact with the parasite

Acquired Immunity

after becoming infected with parasites, ranges from transient to life-long

partial immunity

allow some parasite development or reproduction within the host and may reduce the severity of disease

Complete Protection

clearance of the initial infection and usually prevents immediate reinfection

Antibodies

immunoglobulins that have specific functions in host immunity

Immunoglobulin G, Immunoglobulin M, T cells and macrophages

Types of Antibodies

Immunoglobulin G (IgG)

most abundant; more than 75% of the immunoglobulins present in the sera of normal individuals, small proteins, reaching high concentration several weeks after infection

Immunoglobulin M (IgM)

large macroglobulin that appears shortly after infection but decays rapidly relative to IgG, implies current or recent infection

T cells and Macrophages

responsible for the recognition and elimination of parasites

Primary/definitive vertebrate hosts

essential (accessible & susceptible) for the maintenance of parasite transmission

Secondary/incidental hosts

not essential to maintain transmission and may or may not contribute to parasite amplification

Amplification

increase in the number of parasites or prevalence of parasite infection in a given area

Amplifying Host

permissive to infection which increases number of parasites sufficient to infect susceptible vectors, don’t remain infected for long periods of time and exhibit disease

Reservoir host

supports parasite development and remains infected and potentially infectious to vectors for long periods but usually does not develop acute disease

Dead-end hosts

don’t contribute to transmission, either because they do not support a level of infection sufficient to infect vectors or because they become extremely ill and die before parasite can develop or reproduce to infect additional vectors

Vector

- “carrier” of a parasite from one host to another

Abundance, Vector Competence, Host Fidelity

Characteristics of an Effective Vector:

Abundance

ensure frequent feeding on hosts

Vector Competence

susceptible to infection and capable of becoming infectious to vertebrate hosts and must survive long enough for the parasite to complete reproduction and/or development within the vector

Host Fidelity

feeds on infective vertebrate hosts during periods when stages of the parasite are circulating in the blood or other tissues accessible to the vector

Anthropophagic vectors

·        feed selectively on humans

Zoophagic vectors

feed on nonhuman vertebrates

Mammalophagic vectors

·        feed primarily on mammals

Ornithopagic vector

feed primarily on avian host

Horizontal Transmission and Vertical Transmission

Modes of Transmission

Horizontal Transmission

between vectors, either by transmission to and from vertebrate hosts or directly between vectors

Vertical Transmission

directly to subsequent life stages or generations within vector populations

Transstadial Transmission and Transgenerational Transmission

2 Kinds of Vertical Transmission

Transstadial Transmission

• passage of parasites acquired during one life stage

• E.g., mites and hard ticks that blood feed once during each life stage and die after oviposition

Transgenerational Transmission

• passage of parasites by an infected vector to its offspring

• E.g., Infected Aedes triseriatus female mosquitoes with La Crosse virus (LACV) take a blood meal and lay infected egg.

Horizontal Transmission

maintenance of almost all vector-borne parasites accomplished by either anterior (biting) or posterior (defecation) route

Anterior-station transmission

parasites are liberated from the mouth parts or salivary glands during blood feeding (e.g., malaria parasites, encephalitis viruses, filarial worms)

Posterior-station (or sterocorarian) transmission

parasites remain within the gut and are transmitted via contaminated feces (e.g., Chagas disease, louse-borne relapsing fever, typhus fever rickettsia)

Mechanical Transmission, Multiplicative Transmission, Cyclodevelopmental Transmission and Cyclopropagative Transmission

Kinds of Horizontal Transmission

Mechanical Transmission

• parasite is transmitted among vertebrate hosts without amplification or development within the vector, usually by contaminated mouthparts

• E.g., Eye gnats (Hippelates sp.) have sponging mouthparts and feed repeatedly on secretions at the mucous membranes of vertebrate hosts, making them vector of the bacteria and viruses that cause conjunctivitis, or “pink eye”

Multiplicative (propagative) transmission

• parasite multiplies asexually within the vector and IS transmitted only after suitable incubation period is completed

• E.g., Arboviruses are not transmitted until the virus infects and escapes the midgut of the mosquito vector, and enters and replicates within the salivary glands

Cyclodevelopmental transmission

• parasite develops, but does not multiply, within the vector.

• E.g., Microfilariae of Wuchereria bancrofti are ingested with the blood meal, penetrate the mosquito gut, move to the flight muscles where they molt twice, and then migrate to the mouthparts where they remain until they exit during blood feeding

Cyclopropagative Transmission

• when the parasite develops and reproduces asexually within the arthropod vector

• E.g., Gametocytes of malaria parasite are ingested with the blood meal unite within the mosquito gut and then change to an invasive form (ookinete) that penetrates the gut and forms an asexually reproducing stage (oocyst) on the outside of the gut wall