Modes Of Transmission

The transmission of parasites by vectors may be vertical or horizontal. Vertical transmission is the passage of parasites direcdy to subsequent life stages or generations within vector populations. Horizontal transmission describes the passage of parasites between vector and vertebrate hosts.

Vertical Transmission

Three types of vertical transmission are possible within vector populations: transstadial, transgenerational, and venereal transmission.

Transstadial transmission is the sequential passage of parasites acquired during one life stage or stadium through the molt to the next stage(s) or stadium. Transstadial transmission is essential for the survival of parasites transmitted by mites and hard ticks that blood feed once during each life stage and die after oviposi-tion. Lyme disease spirochetes, for example, that are acquired by larval ticks must be passed transstadially to the nymphal stage before transmission to vertebrates.

Transgenerational transmission is defined as the vertical passage of parasites by an infected parent to its offspring. Some parasites may be maintained transgenerationally for multiple generations, whereas others require horizontal transmission for amplification. Transgenerational transmission normally occurs transovarially (through the ovary) after the parasites infect the ovarian germinal tissue. In this situation most of the progeny are infected. Other parasites do not actually infect the ovary and, although they are passed on to their progeny, transmission is not truly transovarial. This situation is usually less efficient and only a small percentage of the progeny are infected. Transgenerational transmission in vectors such as mosquitoes also must include transstadial transmission, because the immature life stages do not blood feed.

Venereal transmission is the passage of parasites between male and female vectors and is relatively rare. Venereal transmission usually is limited to transovarially infected males who infect females during insemination, which, in turn, infect their progeny during fertilization.

La Crosse virus (Fig. 2.2) is an example of a vertically maintained parasite where the arthropod host serves as the reservoir. This virus is maintained by transgenerational transmission within clones of infected Aedes trise-riatus mosquitoes and is amplified by horizontal transmission among squirrels and chipmunks. Because this temperate mosquito rarely has more than two generations per year, La Crosse virus spends long periods in infected vectors and relatively short periods in infected vertebrate hosts. Females infected vertically or horizontally transmit their infection transovarially to first-instar larvae. These larvae transmit the virus transstadially through the four larval stadia and the pupal stage to the adults. These transgenerationally infected females then take a blood meal and oviposit infected eggs, often in the same tree hole from which they emerged. Some blood meal

Transmission Type

Vertical Horizontal

Transovarial

Transstadial

Infected Males

Infected Females

Eggs

Larvae pl

Pupae

Vertical Trans

Reservoir generational Cycle

Uninfected Females -

Infected, - Females

Horizontal Amplification V

Cycle Susceptible '■■■■■■ Vertebrate Hosts

FIGURE 2.2 Modes of transmission of a vertically maintained parasite, La Crosse encephalitis virus.

hosts become viremic and amplify the number of infected Ae. triseriatus females by horizontal transmission. Venereal transmission of the virus from transgenerationally infected males to uninfected females has been demonstrated in the laboratory and may serve to establish new clones of infected females in nature.

Horizontal Transmission

Horizontal transmission is essential for the maintenance of almost all vector-borne parasites and is accomplished by either anterior (biting) or posterior (defecation) routes. Anterior-station transmission occurs when parasites are liberated from the mouthparts or salivary glands during blood 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. The trypanosome that causes Chagas disease, for example, develops to the infective stage within the hindgut and is discharged onto the host skin when the triatomid vector defecates during feeding. Irritation resulting from salivary proteins introduced into the host during feeding causes the host to scratch the bite and rub the parasite into the wound. Louse-borne relapsing fever and typhus fever rickettsia also employ posterior-station modes of transmission.

There are four types of horizontal transmission, depending upon the role of the arthropod in the life cycle of the parasite: mechanical, multiplicative, developmental, and cyclodevelopmental.

Mechanical transmission occurs when the parasite is transmitted among vertebrate hosts without amplification or development within the vector, usually by contaminated mouthparts. Arthropods that are associated intimately with their vertebrate hosts and feed at frequent intervals have a greater probability of transmitting parasites mechanically. The role of the arthropod is essentially an extension of contact transmission between vertebrate hosts. Eye gnats, for example, have rasping, sponging mouthparts and feed repeatedly at the mucous membranes of a variety of vertebrate hosts, making them an effective mechanical vector of the bacteria which cause conjunctivitis or "pink eye." Mechanical transmission also may be accomplished by contaminated mouthparts if the vector is interrupted while blood feeding and then immediately refeeds on a second host in an attempt to complete the blood meal.

Multiplicative (or propagative) transmission occurs when the parasite multiplies asexually within the vector and is transmitted only after a suitable incubation period is completed. In this case, the parasite does not undergo metamorphosis and the form transmitted is indistinguishable from the form ingested with the blood meal. St. Louis encephalitis (SLE) virus, for example, is not transmitted until the virus replicates within and passes through the midgut, is disseminated throughout the hemocoel, and enters and replicates within the salivary glands. However, the form of the virus does not change throughout this process.

Developmental transmission occurs when the parasite develops and metamorphoses, but does not multiply, within the vector. Microfilariae of W. bancrofti, for example, are ingested with the blood meal, penetrate the mosquito gut, move to the flight muscles, where they molt twice, and then move to the mouthparts, where they remain until they are deposited during blood feeding. These filarial worms do not reproduce asexually within the mosquito vector; i.e., the number of worms available for transmission is always equal to or less than the number ingested.

Cyclodevelopmental transmission occurs when the parasite metamorphoses and reproduces asexually within the arthropod vector. In the life cycle of the malaria parasite, for example, gametocytes that are ingested with the blood meal unite within the mosquito gut and then change to an invasive form that penetrates the gut and forms an asexually reproducing stage on the outside of the gut wall. Following asexual reproduction, this stage ruptures and liberates infective forms that move to the salivary glands, from where they are transmitted during the next blood meal.

The extrinsic incubation period is the time interval between vector infection and parasite transmission when the parasite is away from the vertebrate host. The intrinsic incubation period is the time from infection to the onset of symptoms in the vertebrate host. Repeated lag periods of consistent duration between clusters of new cases at the onset of epidemics were first noticed by early epidemiologists who coined the term extrinsic incubation. These intervals actually represent the combined duration of extrinsic and intrinsic incubation periods.

The duration of the extrinsic incubation period is typically temperature dependent. The rate of parasite development normally increases as a linear degree—day function of ambient temperature between upper and lower thresholds. After being ingested by the mosquito vector, WEE virus, for example, must enter and multiply in cells of the midgut, escape the gut, be disseminated throughout the hemocoel, and then infect the salivary glands, after which the virus may be transmitted by bite. Under hot summer conditions, this process may be completed within 4 days, and the vector mosquito, Cx. tarsalis, is capable of transmitting the virus during the next blood meal. In contrast, under cooler spring conditions transmission may be delayed until the third blood meal. Some parasites may increase the frequency of vector blood feeding and thereby enhance transmission. The plague bacillus, for example, remains within and eventually blocks the gut of the most efficient flea vector, Xenopsylla cheopis. Regurgitation occurs during blood feeding, causing vector starvation and, therefore, transmission at progressively more closely spaced intervals before the vector succumbs to starvation.

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Responses

  • Linda
    What is transgenerational transmission of parasite?
    2 years ago
  • florian
    What are the mode of transmission of blackflies?
    2 years ago
  • James Quattlebaum
    What are the mode of transmission of black flies?
    1 year ago

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