Ticks: Tick Surveillance
Authors : Prof Maxime Madder, Prof Ivan Horak, Dr Hein Stoltsz
Licensed under a Creative Commons Attribution license .
The surveillance of vectors and vector-borne diseases is essential for their control. As the livestock populations grow and with increased trade worldwide, there is the increasing likelihood of vector-borne disease outbreaks and many are expanding their range into new areas. After the recent import of Brazilian cattle in Benin and Ivory Coast early in the 21 st century, the cattle tick Rhipicephalus (Boophilus) microplus was found to have been introduced on the animals (Madder et al., 2007, 2011 and 2012). An adequate surveillance system could have identified the introduction in time allowing successful eradication in an early stage.
Vector surveillance can be defined as the monitoring of arthropod populations responsible for the transmission of pathogens. Vector surveillance can be used to:
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Better understand vector ecology, for example: o Vector population distribution or density o Vector species diversity o Seasonal variation and population dynamics. This could be important to understand the transmission dynamics of a pathogen and the resulting epidemiological situation of the disease.
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Detect the presence/absence of a vector population, for example: o Detection of an “exotic” vector species in a region not known to be colonized o Evaluation of vector control programmes o Surveillance of the presence of insecticide resistance genes in a vector population;
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Assess the risk of vector-borne pathogen transmission, for example: o An early-alert system based on routine pathogen detection in vector populations o The evaluation of vector abundance. This information can be used in pathogen transmission models to estimate the abundance threshold (ratio between vector and host numbers) above which an epidemic may occur.
Routine sampling of vector populations is critical in order to understand the estimated levels of both infected and uninfected arthropods. Arthropods are typically collected, sent to an appropriate laboratory alive, or preserved in ethanol (70%), and assayed for identification and infection. The methods of collection (e.g. dragging the ground with a flannel cloth) will vary with the vector as well as the handling and packaging methods and according to the pathogen and vector involved. For surveillance purposes, arthropods are
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Ticks: Tick surveillance trapped, identified, sorted by sex, age, physiological type etc., counted and stored for later assays (Armed
Forces Pest Management Board, 1998).
Arthropod sampling data in surveillance involves an estimation of vector density. Vector density in a region is important to understand because high vector densities have been shown to be associated with (high risk) outbreaks of certain vector-borne diseases. Many sampling tools are available and the choice of a particular tool depends on the species and the surveillance question. Because many hard ticks “quest for a host” on vegetation, they are collected by dragging a large square piece of cloth over the ground. If the vegetation is too thick, then a square cloth can be made into a flag that can be waved across the vegetation. In addition, for ticks that “hunt” for their hosts, CO
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traps are placed on the ground to attract ticks to a “sticky” surface where they can be collected or they are trapped within a receptacle. Ticks can also be collected from hosts, but this method may select for tick species that remain on the host for long periods of time or certain stages such as adult males, and is further complicated by the movement of the host; all of these factors make sampling design key in determining relative abundance in an area.
In a vector surveillance program it is essential to collect vectors systematically in time and space and to determine the species either morphologically or molecularly. In addition, vector surveillance programmes should include systematic detection and identification of pathogens from a sample of vectors to monitor the introduction of pathogens transmitted by local tick vectors or newly introduced tick vectors. If the objective is to isolate the pathogen for identification, then ticks should be collected alive and stored properly for testing.
Alcohol (70%) is used routinely for this purpose. Accurate tick identification is very important especially because most tick species transmit specific pathogens. After collecting, sorting, identification, labelling, and placement in a suitable container, the ticks are delivered to an appropriate reference laboratory where they can be assayed for a pathogen.
The likelihood of the spread of vector-borne diseases with climatic changes and globalization will lead to greater use of vector surveillance systems. These systems will benefit from improvements in diagnosis, knowledge of vector-borne disease ecological systems and reporting (Hitchcock et al., 2007). With the development and improvement in geological information systems (GIS) that display and analyze epidemiological data, we have seen an improvement in accuracy, usefulness and timeliness of information being processed. We can track seasonal and year-to-year trends in animal disease incidence, and by overlaying climate, vegetation, and other factors, make valuable predictions about potential outbreaks of vector-borne diseases. There are a variety of satellite derived environmental variables such as temperature, humidity, and land cover type with vector density that are used to identify and characterize vector habitats.
Remote sensing techniques have been used to map several vector-borne diseases due to mosquitoes, ticks, black flies, tsetse flies, and sand flies (Kalluri et al ., 2007).
Climate changes influence the epidemiology of vector-borne disease and these changes can influence both vector and pathogen distributions, how pathogens are transmitted, and interactions between vectors and hosts (Tabachnick, 2009). The challenges today are the development of vector surveillance systems that continue to collect epidemiological information on vectors, storage of that data, processing of the data, and
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Ticks: Tick surveillance analyses that will allow for monitoring of current changes in vector populations and prediction of future populations changes with our changing global environment. Geographical and seasonal distributions of vectors are influenced by climatic and land-use changes and thus climate-related environmental factors can be used as predictive indicators in association with on-going vector surveillance activities. Satellite measurements and remote sensing techniques cannot identify the vectors themselves, but they can identify and characterize suitable vector habitats. Remote sensing techniques can aid in the development of distribution maps and disease risk on a seasonal basis and monitor changes in distributions and disease risk over time. Maps showing seasonal risks of vector-borne diseases will be critical in monitoring the impacts of global climate changes on vectors. Remote sensing can be used to determine the influence of environmental factors on the spread of vectors or possible increases in distributional boundaries. Remote sensing and other geospatial technologies are integral to any vector surveillance programme and remains an important tool in predictive veterinary epidemiology (Martin et al ., 2007).
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