Chapter 4

Question 1

Define Epidemiology

Answer 1

the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems

Question 2

Define vector-borne disease

Answer 2

a illness caused by a pathogen that is transmitted by an arthropod

Question 3

Facultative parasites & Obligate parasites

Answer 3

Facultative parasites have both free-living and parasitic forms

Obligate parasites are totally dependent on their host(s) to sustain them

Question 4

Ectoparasites and Enddoparasites

Answer 4

Ectoparasites
* live on or outside the host
* produce an infestation that typically remains topical or peripheral

Endoparasites
* live inside the host
* Ectoparasites produce an infestation that typically remains topical or peripheral, whereas endoparasites produce an infection on invasion of host tissues or cells

Question 5

The components of a transmission cycle of an arthropod-borne disease are:

Answer 5

A. A parasite that can develop and/or multiply within both vertebrate host and vector tissues

B. A vertebrate host (or hosts) that develops a level of infection with the parasite that is infectious to a vector

C. A vector that acquires the parasite from the infectious vertebrate host and is capable of transmission

Question 6

Host Immunity

Answer 6

Some species or individuals within a population have natural (or innate) immunity and are refractory to infection. Natural immunity does not require that the host have previous contact with the parasite but may be age dependent. Individuals may acquire immunity after becoming infected with parasites. This acquired immunity to the parasite ranges from transient to life-long and may provide partial to complete protection against future infections with the same or related parasites

Question 7

Host Immunity (IgG and IgM)

Answer 7

The IgGs are relatively small proteins, typically reaching high concentration several weeks after infection, and may persist at detectable and protective levels for years. Therefore, parasites such as many arboviruses that induce long- lasting immunity are good candidates for vaccine development

In contrast, immunoglobulin M (IgM) is a large macroglobulin that appears shortly after infection but decays rapidly relative to IgG

For the laboratory diagnosis of many diseases, serum samples typically are tested during periods of acute illness and again during convalescence 2 to 4 weeks later. A 4-fold increase in parasite-specific IgG antibody concentration in these paired sera provides diagnostic evidence of infection. The presence of elevated concentrations of IgM presumptively implies current or recent infection

Question 8

What cells are responsible for recognition and elimination of parasites?

Answer 8

T cells and macrophages

Question 9

What is a dead-end hosts

Answer 9

Dead-end hosts do not 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 the parasite can develop or reproduce to infect additional vectors

Question 10

primary host

Answer 10

A primary host must be susceptible to infection with a parasite and permissive to its development and reproduction such that the host eventually becomes infectious to vectors

Question 11

What are Anthropophagic vectors

Answer 11

feed selectively on humans and are important in the transmission of parasites that use humans as their primary host

Question 12

What are Zoophagic vectors, Mammalophagic Vectors, ornithophagic

Answer 12

Zoophagic vectors feed primarily on nonhuman vertebrates.

Mammalophagic vectors blood-feeding on mammals

Ornithophagic vectors feed primarily on avian hosts

Question 13

Vector competence

Answer 13

• Susceptible to infection and capable of becoming
  infectious to vertebrate hosts
• Survive long enough for the parasite to complete reproduction and/or development within the vector

Question 14

Explain the mode of transmission

Answer 14

• Vertical transmission = the passage of parasites directly to subsequent life stages or generations within vector populations
• Horizontal Transmission = the passage of parasites between vectors, either by transmission to and from vertebrate hosts or directly between vectors

Question 15

2 Types of Vertical transmission

Answer 15

Transstadial transmission = the sequential passage of parasites acquired during one life stage, through the molt to the next stage(s)

Transgenerational transmission = the vertical passage of parasites by an infected vector to its offspring.

Question 16

What are the 2 ways of Transgenerational transmission?

Answer 16

Transovarial transmission (more efficient) = occurs through the ovaries after the parasites infect the ovarian germinal tissue and then are transmitted transstadially to the next reproductive or blood-feeding stage

Transovum transmission (less efficient) = parasite remains on the surface of the egg after being laid by an infected vector and eventually infects the offspring on eclosion.

Question 17

2 Types of Horizontal transmission

Answer 17

• Anterior-station transmission occurs when parasites are liberated from the mouth parts or salivary glands during blooding feeding (e.g. malaria parasites, encephalitis viruses, filarial worms)
• Posterior-station (or stercorarian) transmission occurs when parasites remain within the gut and are transmitted via contaminated feces.

Question 18

5 Types of Horizontal transmission

Answer 18

• Mechanical
  Occurs when the parasite is transmitted among vertebrate hosts without amplification or development within the vector, usually by contaminated mouthparts
• Propagative
  The parasite multiples asexually within the vector and is transmitted only after a suitable incubation period is completed
• Venereal Transmission
  The passage of parasites between male and female vectors during mating and is relatively rare
• Cyclodevelopmental
  The parasite develops, but does not multiply, within the vector. E.g. Filariasis
• Cyclopropagative
  The parasite develops and reproduces asexually within the arthropod vector.

Question 19

Different between Anterior-station transmission and Posterior-station (or stercorarian) transmission

Answer 19

Anterior-station transmission occurs when parasites are liberated from the mouth parts or salivary glands during blooding feeding

Posterior-station (or stercorarian) transmission occurs when parasites remain within the gut and are transmitted via contaminated feces.

Question 20

Transmission Cycles (anthroponosis and zoonoses)

Answer 20

A vector-borne anthroponosis = a disease resulting from a parasite that normally infects only humans and one or more anthropophagic vectors.

Vector-borne zoonoses = diseases caused by parasites of animals that occasionally infect humans

Question 21

Transmission cycle ( Venezuelan Equine Encephalitis Virus (VEEV) )

Answer 21

Enzootic transmission cycle = maintained among rodents and other vertebrates as reservoirs and mosquitoes as primary vectors

Epizootic VEEV strains = transmitted by several mosquito vectors (e.g. Aedes and Psorophora spp.) to susceptible amplification hosts, horses.