Surveillance

The number of cases of most vector-borne diseases typically varies over both time and space. Information on the number of cases can be gathered from morbidity and mortality records maintained by state or national governmental agencies for the human population. Morbidity data are records of illness, whereas mortality data are records of the cause of death. These data vary greatly in their quality and timeliness, depending upon the accuracy of determining the cause of illness or death and the rapidity of reporting. In the United States, the occurrence of confirmed cases of many vector-borne diseases, including yellow fever, plague, malaria, and encephalitis, must by law be reported to municipal health authorities. However, infections with many arthropod-borne parasites, including Lyme disease and the mosquito-borne encephal-itides, frequently are asymptomatic or present variable clinical symptoms and therefore remain largely undiagnosed and underreported. The frequency of case detection and accuracy of reporting systems are dependent on the type of surveillance employed and the ability of the medical or veterinary community to recognize suggestive symptoms and request appropriate confirmatory laboratory tests. In addition, some laboratory tests vary in their specificity and sensitivity, thus complicating the interpretation of laboratory results. Cases may be classified as suspect or presumptive, based on the physician's clinical diagnosis, or confirmed, based on a diagnostic rise in specific antibodies or the direct observation (or isolation) of the parasite from the case. Surveillance for clinical cases may be active or passive.

Active surveillance involves active case detection in which health workers visit communities and seek out and test suspect cases. In malaria control programs, for example, a field worker visits every household biweekly or monthly and collects blood films from all persons with a current or recent fever. Fever patients are treated with antimalarial drugs presumptively, and these suspected cases are confirmed by detection of malaria parasites in a blood smear. Confirmed cases are revisited and additional medication administered, if necessary. This surveillance provides population infection rates regardless of case classification criteria.

Most surveillance programs rely on passive surveillance, which utilizes passive case detection to identify clinical human or veterinary cases. In this system, individuals seeking medical attention at primary health care organizations, such as physicians' offices, hospitals, and clinics, are diagnosed by an attending physician who requests appropriate confirmatory laboratory tests. However, because many arthropod-borne diseases present a variety of nonspecific symptoms (e.g., headache, fever, general malaise, arthralgia), cases frequently may be missed or not specifically diagnosed. In mosquito-borne viral infections the patient often spontaneously recovers, and cases frequently are listed under fevers of unknown origin or aseptic (or viral) meningitis without a specific diagnosis. In a passive case-detection system, it is the responsibility of the attending physician to request laboratory confirmation of suspect clinical cases and then to notify the regional public health epidemiologist that a case of a vector-borne disease has been documented.

The reporting system for clinical cases of vector-borne diseases must be evaluated carefully when interpreting surveillance data. This evaluation should take into account the disease, its frequency of producing clinically recognizable symptoms, the sensitivity and specificity of confirmatory laboratory tests, and the type and extent of the reporting system. Usually programs that focus on the surveillance of a specific disease and employ active case detection provide the most reliable epidemiological information. In contrast, broad-based community health care systems that rely on passive case detection typically produce the least reliable information, especially for relatively rare vector-borne diseases with nonspecific symptoms.

Diseases that are always present or reappear consistently at a similar level during a specific transmission season are classified as endemic. The number of cases in a population is expressed as incidence or prevalence. Population is defined as the number of individuals at risk from infection in a given geographical area at a given time. Incidence is the number of new cases per unit of population per unit of time. Incidence data are derived from two or more successive samples spaced over time. Prevalence is the frequency of both old and new infections among members of a population. Prevalence typically is determined by a single point in time estimate and frequently is expressed as the percentage of the population tested that was found to have been infected.

The level of parasite endemicity in a population may be graded as hypoendemic (low), mesoendemic (medium), or hyperendemic (high), depending upon the incidence of infection and/or the immune status of the population. In malaria surveys, for example, the percentage of children with palpable spleens and the annual parasite incidence are used to characterize the level of endemicity. In endemic disease, the percentage of individuals with sera positive for IgG-class antibodies typically increases as a linear function of age or residence history, whereas in hypoendemic disease with intermittent transmission, this function is disjunct, with certain age groups expressing elevated positivity rates. The occurrence of an extraordinarily large number of human infections or cases is termed an epidemic. Health agencies, such as the World Health Organization, typically monitor incidence data to establish criteria necessary to classify the level of endemicity and to decide when an epidemic is under way. A geographically widespread epidemic on a continental scale is called a pandemic.

Serological surveys (or serosurveys) are a useful epidemiological tool for determining the cumulative infection experience of a population with one or more parasite -and host-related factors affecting the efficiency or risk of transmission, and reinfection rates. When coupled with morbidity data, serosurveys provide information on the ratio of apparent to inapparent infections. Random sampling during serosurveys representatively collects data on the entire population and may provide ecological information retrospectively by analysis of data collected concurrently with each serum sample. This information may assign risk factors for infection, such as sex, occupation, and residence history, or it may help in ascertaining age-related differences in susceptibility to disease. Stratified sampling is not random and targets a specific cohort or subpopulation. Although stratified samples may have greater sensitivity in detecting rare or contiguously distributed parasites, the data are not readily extrapolated to infection or disease trends in the entire population. Repeated serological testing of the same individuals within a population can determine the time and place of infection by determining when individuals first become seropositive, i.e., serologically positive with circulating antibodies against a specific parasite. This change from seronegative to seropositive is called a seroconversion.

Forecasting the risk of human infection usually is accomplished by monitoring environmental factors, vector abundance, the level of transmission within the primary and/or amplification cycles, and the numbers of human or domestic animal cases. As a general rule, the accuracy of forecasting is related inversely to the time and distance of the predictive parameter from the detection of human cases. Surveillance activities typically include the time series monitoring of environmental conditions, vector abundance, enzootic transmission rates, and clinical cases.

Environmental conditions. Unusually wet or warm weather may indicate favorable conditions for vector activity or population increases, concurrently increasing the risk of parasite transmission. Parameters frequently monitored include temperature, rainfall, snow pack (predictive of vernal flooding), and agricultural irrigation schedules.

Vector abundance. Standardized sampling at fixed sites and time intervals can be used to compare temporal and spatial changes in vector abundance that are useful in detecting an increased risk of parasite transmission. Extraordinary increases in vector abundance and survival may forecast accurately increased enzootic transmission and, to a lesser extent, epidemics.

Enzootic transmission rates. Monitoring the level of parasite infection in vector or vertebrate populations provides direct evidence that the parasite is present and being actively transmitted (Fig. 2.4). The level of transmission usually is directly predictive of the risk of human or domestic animal involvement. Enzootic transmission activity may be monitored by vector infection rates, vertebrate-host infection rates, sentinel seroconversion rates, and clinical cases.

Vector infection rates. Sampling vectors and testing them for parasites determines the level of infection in the vector population (Fig. 2.4, C and D). When vectors are tested individually, prevalence data are expressed as percentages; e.g., 10 females infected per 50 tested is a 20% infection rate. When combined with abundance estimates, infection rates also may be expressed as infected vectors per sampling unit per time interval; 100 bites per human per night x 0.2 infection rate = 20 infective bites per human per night. These data provide an index of the transmission rate. When infection rates are low and vector populations large, vectors usually are tested in lots or pools. It is statistically advantageous to keep the pool size constant and thus keep the chance of detecting

FIGURE 2.4 Mosquito-borne encephalitis surveillance in southern California. (A) Coop with 10 sentinel chickens; (B) Taking blood sample from chicken; (C) Hanging mosquito trap on permanent standard (components from left to right are trap motor and fan assembly with collecting carton, dry-ice bait in a Styrofoam container, and battery); (D) Sorting mosquito collections by species to estimate relative abundance.

FIGURE 2.4 Mosquito-borne encephalitis surveillance in southern California. (A) Coop with 10 sentinel chickens; (B) Taking blood sample from chicken; (C) Hanging mosquito trap on permanent standard (components from left to right are trap motor and fan assembly with collecting carton, dry-ice bait in a Styrofoam container, and battery); (D) Sorting mosquito collections by species to estimate relative abundance.

infection the same. Because there may be more than one infected vector per pool, infection rates are expressed as a minimum infection rate = positive pools/total individuals tested x 100 or 1000.

Vertebrate-host infection rates. Introduced zoonoses, such as sylvatic plague in North American rodents, frequendy produce elevated mortality that may be used to monitor epizootics of these parasites over time and space. In contrast, endemic zoonoses rarely result in vertebrate host mortality. Testing reservoir or amplifying hosts for infection is necessary to monitor the level of enzootic parasite transmission. Stratified sampling for these parasites (directly by parasite isolation or indirectly by seroprevalence) usually focuses on the young of the year to determine ongoing transmission. For example, examining nestling birds for viremia can provide information on the level of enzootic encephalitis virus transmission.

Monitoring the incidence of newly infected individuals in a population over time is necessary to detect increased transmission activity. Because many parasites are difficult to detect or are present only for a limited time period, sampling frequently emphasizes the monitoring of seropositivity. Monitoring the IgM antibody, which rises rapidly after infection and decays relatively quickly, can indicate the level of recent infection, whereas monitoring the IgG antibody documents the population's historical experience with the parasite. Sampling, marking, releasing, recapturing, and resampling wild animals is most useful in providing information on the time and place of infection in free-roaming animal populations.

Sentinel seroconversion rates. Sentinels typically are animals that can be monitored over time to quantify the prevalence of a parasite. Trapping wild animals or birds is labor intensive, and determining seroprevalence may provide little information on the time and place of infection, especially if the host has a large home range. To circumvent this problem, caged or tethered natural hosts or suitable domestic animals of known infection history are placed in sensitive habitat and repeatedly bled to detect infection. A suitable sentinel should be fed upon frequently by the primary vector species, be easy to diagnose when infected, be unable to infect additional vectors (i.e., not serve as an amplifying host), not succumb to infection, and be inexpensive to maintain and easy to bleed or otherwise sample for infection. Chickens, for example, are useful sentinels in mosquito-borne encephalitis virus surveillance programs (Fig. 2.4, A and B). Flocks of seronegative chickens are placed at farmhouses and then bled weekly or biweekly to determine seroconversions to viruses such as WEE or SLE. Because the chickens are confined and the date of seroconversion known, the time and place of infection is determined, while the number seroconverting estimates the intensity of transmission.

Clinical cases. Detecting infection among domestic animals may be an important indication that an epizootic transmission is under way and that the risk of human infection has become elevated. Domestic animals often are more exposed to vectors than are humans and thus provide a more sensitive indication of parasite transmission. Clinical human cases in rural areas in close association with primary transmission cycles may be predictive of future epidemic transmission in urban settings.

Vector-borne diseases frequently affect only a small percentage of the human population, and therefore vector control remains the intervention method of choice. Control programs attempt to maintain vector abundance below thresholds necessary for the transmission of parasites to humans or domestic animals. When these programs fail, personal protection by repellents or insecticide-impregnated clothing, bed nets, or curtains is often the only recourse. Vaccination may be a viable alternative method of control for specific vector-borne diseases, if the vaccine imparts lasting immunity as in the case of yellow fever virus. However, many parasites, such as malaria, have evolved to the point where infection elicits a weak immune response that provides only short-term and marginal protection. The need for continued revaccination at short intervals severely limits their global usefulness, especially in developing countries. Although breakthroughs in chemotherapy have been useful in case management, it remains the mandate of the medical/veterinary entomologist to devise strategies which combine epidemiological and ecological information to effectively reduce or eliminate the risk of vector-borne diseases.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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