Interseasonal Maintenance

that do not play a substantial role in either enzootic maintenance or epidemic transmission. Vector incrimination combines field and laboratory data that measure field infection rates, vector competence, and vectorial capacity.

Infection rates. The collection of infected arthropods in nature is an important first step in identifying potential vectors, because it indicates that the candidate species feeds on vertebrate hosts carrying the parasite. Infection data may be expressed as a percentage at one point in time or an infection prevalence (i.e., number of vectors infected/number examined x 100). The more commonly employed infection rate refers to infection incidence and includes change over a specified time period. When the infection prevalence is low and arthropods are tested in groups or pools, data are referred to as a minimum infection rate (number of pools of vectors positive/total specimens tested/unit of time x 100 or 1000). Minimum infection rates are relative values with ranges delineated by pool size. For example, minimum infection rates of vectors tested in pools consisting of 50 individuals each must range from 0 to 20 per 1000 females tested.

It is important to distinguish between infected hosts harboring a parasite and infective hosts capable of transmission. In developmental and cyclodevelopmental vectors, the infective stages may be distinguished by location in the vector, morphology, or biochemical properties. Distinguishing infective from noninfective vectors is difficult, if not impossible, with viral or bacterial infections, because the parasite form does not change. The ability to transmit may be implied by testing selected body parts, such as the cephalothorax, salivary glands, or head. With some tick pathogens, however, parasite movement to the mouthparts does not occur until several hours after attachment. As mentioned previously, the transmission rate is the number of new infections per time period. When standardized per unit of population size, the transmission rate may be expressed as an incidence. The annual parasite incidence is the number of new infections per year per 1000 population.

The entomological inoculation rate is the number of potentially infective bites per unit of time. This frequently is determined from the human or host biting rate and the proportion of vectors that are infective and is calculated as bites per human per time period x infectivity prevalence.

Vector competence is defined as the susceptibility of an arthropod species to infection with a parasite and its ability to transmit this acquired infection. Vector competence is determined quantitatively by feeding the candidate arthropod vector on a vertebrate host circulating the infective stage of the parasite, incubating the blood-fed arthropod under suitable ambient conditions, refeed-ing the arthropod on a noninfected susceptible vertebrate host, and then examining this host to determine if it became infected. Because it often is difficult to maintain natural vertebrate hosts in the laboratory and control the concentration of parasites in the peripheral circulatory system, laboratory hosts or artificial feeding systems frequently are used to expose the vector to the parasites. Susceptibility to infection may be expressed as the percentage of arthropods that became infected among those blood feeding. When the arthropod is fed on a range of parasite concentrations, susceptibility may be expressed as the median infectious dose required to infect 50% of blood-fed arthropods. The ability to transmit may be expressed either as the percentage of feeding females that transmitted or the percentage of hosts that became infected.

Failure of a blood-fed arthropod to become infected with or transmit a parasite may be attributed to the presence of one or more barriers to infection. For parasites transmitted by bite, the arthropod midgut provides the most important barrier. Often parasites will grow in a nonvector species if they are inoculated into the hemo-coel, thereby by-passing this gut barrier. After penetrating and escaping from the midgut, the parasite then must multiply and/or mature and be disseminated to the salivary glands or mouthparts. Arthropod cellular or humeral immunity may clear the infection at this point, creating a dissemination barrier. Even after dissemination to the salivary glands, the parasite may not be able to infect or be transmitted from the salivary glands due to the presence of salivary gland infection or salivary gland escape barriers, respectively.

For parasites transmitted at the posterior station, vector competence may be expressed as the percentage of infected vectors passing infective stages of the parasite in their feces.

The concept of vectorial capacity summarizes quantitatively the basic ecological attributes of the vector relative to parasite transmission. Although developed for mosquito vectors of malaria parasites and most easily applied to anthroponoses, the model provides a framework to conceptualize how the ecological components of the transmission cycle of many vector-borne parasites interact.

Vectorial capacity is expressed by the formula:

C= ma2(Pn)/(-\nP), where C is the vectorial capacity as new infections per infection per day, ma is the bites per human per day, a is the human biting habit, P is the probability of daily survival, and n is the extrinsic incubation period (in days).

The biting rate (ma) frequently is estimated by collecting vectors as they attempt to blood feed and is expressed as bites per human per day or night (e.g., 10 mosquitoes per human per night). The human biting habit (a) combines vector feeding frequency and host selection. The feeding frequency is the length of time between blood meals and frequently is expressed as the inverse of the length of the gonotrophic cycle. Host selection patterns are determined by testing blood-fed vectors to determine what percentage fed on humans or the primary reservoir. Therefore, if the blood feeding frequency is 2 days and if 50% of host-seeking vectors feed on humans, a = (1/2 days) x (0.5) = 0.25. In this example, ma2 = 10 bites/human/night x 0.25 = 2.5; a is repeated because infected vectors must refeed to transmit.

The probability of the vector surviving through the extrinsic incubation period of the parasite, P", requires information on the probability of vector survival (P) and the duration of the extrinsic incubation period (»). P is estimated either vertically, by age-grading the vector population, or horizontally, by marking cohorts and monitoring their death rate over time. In Diptera, P may be estimated vertically from the parity rate (proportion of parous females per number examined). In practice, P = (parity rate)1^, where ß is the length of the gonotrophic cycle. The extrinsic incubation period may be estimated from ambient temperature from data gathered during vector competence experiments by testing the time from infection to transmission for infected vectors incubated at different temperatures. Continuing our example, if P = 0.8 and » = 10 days, then the duration of infective life is Pn/(-lnP) = 0.810/(-lnx 0.8) = 0.48. Therefore C = 2.5 x 0.48, or 1.2 parasite transmissions per infective host per day

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