Intrinsic Incubation

FIGURE 2.1 Components of the transmission cycle of an anthro-ponosis such as malaria or louse-borne typhus. (Original by Margo Duncan)

mechanisms for tolerating high constant body temperatures and evading the complex immune systems of the vertebrate hosts as well as for tolerating variable body temperatures and avoiding the very different defensive mechanisms of the arthropod vectors. Asexual parasites, such as viruses and bacteria, employ essentially the same life form to infect both vertebrate and arthropod hosts, whereas more highly evolved heterosexual parasites, such as protozoa and helminths, have different life stages in their vertebrate and arthropod hosts. Some asexual parasites, such as the plague bacillus, intermittently may bypass the arthropod host and be transmitted directly from one vertebrate host to another.

Among sexually reproducing parasites, the host in which gametocyte union occurs is called the definitive host, whereas the host in which asexual reproduction occurs is called the intermediate host. Vertebrates or arthropods can serve as either definitive or intermediate hosts, depending upon the life cycle of the parasite. For example, humans are the definitive host for the filarial worm, Wuchereria ba-nerofti, because adult male and female worms mate within the human lymphatic system, whereas the mosquito vector, Culex quinquefasciatus, is the intermediate host where development occurs without reproduction. In contrast, humans are the intermediate host of the Plasmodium protozoan that causes malaria, because only asexual reproduction occurs in the human host; gametocytes produced in the human host unite only in the gut of the definitive mosquito host.

A disease is the response of the host to infection with the parasite and can occur in either vertebrate or arthropod hosts. Immunity includes all properties of the host that confer resistance to infection and play an important role in determining host suitability and the extent of disease or illness. Some species or individuals within species populations have natural immunity and are refractory to infection. Natural immunity does not require that the host have previous contact with the parasite, but it may be age dependent. For example, humans do not become infected with avian malaria parasites, even though infective Culex mosquito vectors feed frequently on humans. Conversely, mosquitoes do not become infected with the measles or poliomyelitis viruses that infect humans, even though these viruses undoubtedly are ingested by mosquitoes blood feeding on viremic hosts.

Individuals within populations become infected with parasites, recover, and in the process actively acquire immunity. This acquired immunity to the parasite ranges from transient to lifelong and may provide partial to complete permanent protection. A partial immune response may permit continued infection but may reduce the severity of disease, whereas complete protection results in a cure and usually prevents immediate reinfection.

Acquired immunity may be humeral and result in the rapid formation of antibodies, or it may be cellular and result in the activation of T cells and macrophages. Antibodies consist of five classes of proteins called immunoglobulins that have specific functions in host immunity. Immunoglobulin G (IgG) is most common, comprising over 85% of the immunoglobulins present in the sera of normal individuals. The IgGs are relatively small proteins and typically develop to high concentrations several weeks after infection; they may persist at detectable and protective levels for years. In contrast, IgMs are large macroglobulins that appear shortly after infection but decay rapidly. For the laboratory diagnosis of many diseases, serum samples typically are tested during periods of acute illness and convalescence, 2 to 4 weeks later. A fourfold increase in parasite-specific IgG antibody concentration in these paired sera provides diagnostic serological evidence of infection. The presence of elevated concentrations of IgM presumptively implies current or recent infection. T cells znd macrophages axe several classes of cells that are responsible for the recognition and elimination of parasites. In long-lived vertebrate hosts, acquired immunity may decline over time, eventually allowing reinfection.

Clinically, the host response to infection ranges from inapparent or asymptomatic to mildly symptomatic to acute. Generally it is beneficial for the parasite if the host tolerates infection and permits parasite reproduction and/or development without becoming severely ill and dying before infecting additional vectors.

One or more primary vertebrate hosts are essential for the maintenance of parasite transmission, whereas secondary or incidental hosts are not essential to maintain transmission but may contribute to parasite amplification. Amplification refers to the general increase in the number of parasites present in a given area. An amplifying host increases the number of parasites and therefore the number of infected vectors. Amplifying hosts typically do not remain infected for long periods of time and may develop disease. A reservoir host supports parasite development, remains infected for long periods, and serves as a source of vector infection, but it usually does not develop acute disease.

Attributes of a primary vertebrate host include accessibility, susceptibility, and transmissibility.

Accessibility. The vertebrate host must be abundant and fed upon frequently by vectors. Host seasonality, diel activity, and habitat selection determine availability in time and space to host-seeking vectors. For example,

The Vertebrate Host the avian hosts of eastern equine encephalomyelitis (EEE) virus generally begin nesting in swamps coincidentally with the emergence of the first spring generation of the mosquito vector, Culiseta melanura, thereby bringing EEE virus, susceptible avian hosts, and mosquitoes together in time and space. Diel activity patterns also may be critical. For example, larvae (microfilariae) of W. bancrofti move to the peripheral circulatory system of the human host during specific hours of the night that coincide with the biting rhythm of the mosquito vector, Cx. quinque-fasciatus. Historically, epidemics of vector-borne diseases have been associated with increases in human accessibility to vectors during wars, natural disasters, environmental changes, or human migrations.

Susceptibility. Once exposed, a primary host must be susceptible to infection and permit the development and reproduction of the parasite. Dead-end hosts either do not support a level of infection sufficient to infect vectors or become extremely ill and die before the parasite can complete development, enter the peripheral circulatory system or other tissues, and infect additional vectors. Ideal reservoir hosts permit parasites to survive in the peripheral circulatory system (or other suitable tissues) in sufficient numbers for sufficiently long time periods to be an effective source for vector infection. Asexual parasites, such as viruses and bacteria, typically produce intensive infections that produce large numbers of infectious organisms for relatively short periods during which the host either succumbs to infection or develops protective immunity. In the case of EEE virus, for example, 1 ml of blood from an infected bird may contain as many as 1010 virus particles during both day and night for a 2- to 5-day period; birds that survive such infections typically develop long-lasting, protective immunity. In contrast, highly evolved parasites produce comparatively few individuals during a longer period. W. bancrofti, for example, maintains comparatively few microfilaria in the bloodstream (usually <10 microfilaria per cubic millimeter of blood), which circulate most abundantly in the peripheral blood during periods of the day when the mosquito vectors blood feed. However, because both the worms and the human host are long-lived, transmission is enhanced by repeated exposure rather than by an intense parasite presentation over a period of a few days. Infection with >100 microfilaria per female mosquito may prove fatal for the vector; therefore, in this case, limiting the number of parasites that infect the vector may increase the probability of transmission.

Transmissibility. Suitable numbers of susceptible vertebrate hosts must be available to become infected and thereby maintain the parasite. Transmission rates typically decrease concurrently with a reduction in the number of susceptible (i.e., nonimmune) individuals remaining in the host population. The epidemic threshold refers to the number of susceptible individuals required for epidemic transmission to occur, whereas the endemic threshold refers to the number of susceptible individuals required for parasite persistence. These numerical thresholds vary depending on the immunology and dynamics of infection in the host population. Therefore, suitable hosts must be abundant and either not develop lasting immunity or have a relatively rapid reproductive rate, ensuring the rapid recruitment of susceptibles into the population. In the case of malaria, for example, the parasite elicits an immune response that rarely is completely protective, and the host remains susceptible to reinfection. In contrast, encephalitis virus infections of passerine birds typically produce lifelong protection, but bird life expectancy is short and the population replacement rate is rapid, ensuring the constant renewal of susceptible hosts.

Literally, a vector is a "carrier" of a parasite from one host to another. An effective vector generally exhibits characteristics that complement those listed above for the vertebrate hosts and include host selection, infection, and transmission.

Host selection. A suitable vector must be abundant and feed frequently upon infective vertebrate hosts during periods when stages of the parasite are circulating in the peripheral blood or other tissues accessible to the vector. Host-seeking or biting activity during the wrong time or at the wrong place on the wrong host will reduce contact with infective hosts and reduce the efficiency of transmission. Patterns of host selection determine the types of parasites to which vectors are exposed. Anthropophagie vectors feed selectively on humans and are important in the transmission of human parasites. Anthropophagie vectors which readily enter houses to feed on humans or to rest on the interior surfaces are termed en-dophilic (literally, "inside loving"). Vectors which rarely enter houses are termed exophilic (i.e., "outside loving"). Zoophagic vectors feed primarily on vertebrates other than humans. Mammalophagic vectors blood feed primarily on mammals and are important in the maintenance of mammalian parasites. In contrast, ornithophagic vectors feed primarily on avian hosts and are important in the maintenance of avian parasites. There is a distinction between vectors attracted to a host and those which successfully blood feed on the host. Mammalophagic vectors therefore represent a subset of those mammalophilic vectors that are attracted to mammalian hosts.

Infection. The vector must be susceptible to infection and survive long enough for the parasite to complete multiplication and/or development. Not all arthropods

The Arthropod Vector that ingest parasites support parasite maturation, dissemination, and transmission. For example, the mosquito Cx. quinquefasciatus occasionally becomes infected with western equine encephalomyelitis (WEE) virus; however, because this virus rarely escapes the midgut, this species rarely transmits WEE virus. Some arthropods are susceptible to infection under laboratory conditions, but in nature they seldom feed on infected vertebrate hosts and/or survive long enough to allow parasite development. The transmission rate is the number of new infections per unit of time and is dependent upon the rate of parasite development to the infective stage and the frequency of blood feeding by the vector. Because many arthropod vectors are poikilothermic and contact their homeothermic vertebrate hosts intermittently, parasite transmission rates frequently are dependent upon ambient temperature. Therefore, transmission rates for many parasites are more rapid at tropical than at temperate latitudes, and at temperate latitudes they progress most rapidly during summer. The frequency of host contact and, therefore, the transmission rate also depend upon the life history of the vector. For example, epidemics of malaria in the tropics transmitted by a mosquito that feeds at 2-day intervals progress faster than epidemics of Lyme disease at temperate latitudes, where the spirochetes are transmitted to humans principally by the nymphal stage of a hard tick vector that may have one generation and one blood meal per life stage per year.

Transmission. Once infected, the vector must exhibit a high probability of refeeding on one or more susceptible hosts to ensure the transmission of the parasite. Diversion of vectors to nonsusceptible or dead-end hosts dampens transmission effectiveness. The term zooprophy-laxis (literally, "animal protection") arose to describe the diversion of Anopheles infected with human malaria parasites from humans to cattle, a dead-end host for the parasites. With zooprophylaxis the dead-end host typically exhibits natural immunity, in which host tissues are unacceptable to parasites and do not permit growth or reproduction. Alternatively, transmission to a dead-end host may result in serious illness, because the host-parasite relationship has not coevolved to the point of tolerance by the dead-end host. WEE virus, for example, can cause serious illness in humans, which are considered to be a dead-end host because they rarely produce a viremia sufficient to infect mosquitoes.

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