Epidemiology and control of infectious diseases developing countries
Developing an insecticide treatment program as part of the Trypanosomiasis control strategy for the government of UgandaBackground
Trypanosomiasis is a vector-borne parasitic disease mainly affecting animals but can be transmitted to humans in places where there is considerable contact between them. The disease constitutes a significant public health burden in sub Saharan Africa where it occurs in about 37 countries covering an area approximately one-third of the continent's total land area (1). The economic implications of the disease on both human and animal populations are equally grim; an estimated 50 million cattle are at risk with direct costs to livestock producers and consumers put at about US$1.2 billion annually and a loss in GDP of about US$4.75 billion per year (2-3). The effect of the disease is so severe in some parts of the continent that it is the main consideration in the migration and settlement of populations. Trypanosomiasis and its associated human presentation (sleeping sickness) is mainly a disease of the poor; affecting mainly those living in very rural conditions with little resource base (livestock) and lacking access to health and support services.
The distribution of the disease typically follows that of the vector; the tsetse fly (Glossina spp). Two main disease forms of public health interest are caused by different strains of the parasite specie Trypanosoma brucei. These subspecies are morphologically similar; their classification being more due to their geographical distribution (4). T brucei gambiense, found in western and central Africa whereas and T brucei rhodensiense is a zoonosis seen in much of eastern and southern Africa. These disease forms also differ in terms of the degree of morbidity and the rate of disease progression. In recent times the disease has been characterised by a resurgence in the prevalence of cases in endemic areas, an increase in severity and duration of epidemics and the expansion of foci beyond previously determined boundaries (5).
The Ugandan context
Uganda is unique in the sense that it has endemic foci for both strains of the parasite; T brucei rhodensiense in the southeast and T brucei gambiense in the northwest (4, 6) with considerable overlap in distribution (7). In addition there have been reports of spread of the disease to areas previously unknown to have disease (8).
The major concerns are the risks to human populations through their contacts with both the reservoir and vector. The main reservoirs for T brucei rhodensiense are cattle and other domestic animals while humans are the main reservoirs for T brucei gambiense. Transmission is usually through the introduction of the parasite into a susceptible host or reservoir by an infected fly during a blood meal. Onset of symptoms is variable depending on the incubation of the different strains but range from days to weeks; T brucei gambiense infections are characterised by a long incubation period and a prolonged course of illness whereas T brucei rhodensiense infections have shorter incubation period, greater severity of disease. Both forms however can prove fatal if untreated.
Developing a control programme therefore needs to focus on aspects of the transmission pathway likely to give maximal benefits within the context of limits of resource available. The basic factors that contribute to the spread and persistence of the disease have been summarised to include the following often acting in concert (9):
Reservoir- fly contact
Introduction of infected reservoirs into tsetse fly infested areas
Displacement of uninfected susceptible hosts into areas endemic for disease
In the country, the above factors have been encouraged by the effects of displacement of large populations due to wars and drought and other natural disasters, resettlement activities and expansion of communities due to increased economic activities (8).
It is evident therefore that the central theme is the contact of the tsetse fly with the reservoir- in this case, cattle -and efforts targeting disruption of this pathway is critical to achieving control of the disease. The basic control tools that have targeted the parasite vector are; habitat modification, ground or aerial sprays and tsetse fly trapping. These tools have used in the past with reported success and therefore could form the starting blocks of the control programme.
Habitat modification: this is based on the based on the principle that understanding the natural habitat of the vector is important in designing any programme aimed at reducing the probability of contact between the reservoir and vector. The fly habitat is determined by the vegetation type and cover, temperature and humidity. Preferred breeding sites are the foliage along rivers and lakes, forest edges extending to areas of scrub savannah. These sites are an important point of contact as it is characterised by human activity; farming, animal rearing, hunting etc. With expanding populations, land development/ reclamation programmes are going on at an uncontrolled pace and direction encroaching into these areas. Control efforts by habitat modification requires that a â€œsafeâ€Â distance be kept between the fly and the reservoir but this is unlikely to be effective in the country because;
These expansion efforts are often uncoordinated being driven by local needs such as economic growth, displacement by war and drought.
There is either little or inaccurate data on the distribution of the fly habitat to help with planning of settlements
Such planned modifications are huge capital investments and are not very effective as stand-alone programmes in achieving control of the disease.
Vector control: methods directly targeting the tsetse fly vector include ground or aerial spraying and the use of insect traps and screens. Aerial spraying of insecticides against the tsetse fly is used to quickly reduce fly populations. It is often used in habitat modification programmes but it more often employed in epidemic situations. It is however a very expensive method therefore cannot be used on long term basis in endemic areas and, the effectiveness of the approach is often difficult to determine especially where the area to be covered in not well delineated. Ground spraying including spraying of cattle and other domestic animals have been recognised by farmers and local communities as a way to control the disease. The practice also has a number of advantages such as a reduction in irritation, local fly challenge and a reduced risk of re-infection (4, 12). The practice has also been noted to be prevent bites by other vectors (13). The use of screens is also of variable effectiveness and is likely to expensive depending on the material used as screens and the area to be covered.
Based on this background, a programme of insecticide treatment of the reservoir appears to a good option but successful implementation would require a number of measures resolved and questions answered.
Objectives: while the goal of the programme is to achieve control of the disease, it is important that specific targets be set and parameters for determining this is defined a priori. An option would be the burden of disease calculated by the disability-adjusted life year (DALY) estimate for the human host that can become infected from the reservoir. Objectives need to be defined within the context of current burden, resource and capacity levels to implement the programme. Many of such programmes in the past have started well but were unable to meet targets due to wrong judgements and budgeting. Resources are hardly ever enough and also tend to move in the direction of the fundersâ€™ preference or perceived urgency. The effect is that programmes with no clear goals and benefits become relegated. This is further compounded by the duplication of projects and an unstable political environment. Therefore the success of the project requires a good conceptualisation of its goals on the short and long run and adequate budgeting of needed resources to cover its various activities throughout its lifetime.
Targeting issues: insecticide treatment measures need to be implemented at points and periods of considerable exposure of the reservoir with the vector. How are these to be determined? This would require data on the local distribution of vectors and information on factors that determine the relationships between them and the reservoirs. Information on migration patterns of livestock across the country is necessary as it could predict the rate and direction of spread of disease. Some of this information may already be available from health inspection records and from the local and regional veterinary clinics, in which case the task would be to gather and analyze them. On the other hand they may either be unavailable or stored in a format that makes it unusable in which case they have to be gathered. Available methods used in generating information to enable targeting decisions in the past are based on and information derived from the review of existing literature (11). More recent innovation in data collection involve the analysis of land cover maps and entomological field studies (fly trapping studies) and predictions of tsetse fly distribution by remote sensing methods (using Geographic information systems technology) (10). Many of the interventions in current use are based on expert opinion (literature reviews) and it is difficult to validate such information and estimate their predictive ability in the face of changing climate seen in the country. The use of newer methods would generate more accurate data on the country and should therefore form an important research focus. These would be used to. This would serve a lot of functions; generate data to validate current data in use, build a spatial map displaying present situation, hotspots for immediate intervention and design models able to predict with greater precision, future trends that would be useful in planning and monitoring.
Treatment methodologies: the practice of insecticide treatment has previously been up to the farmers and owners of livestock and spraying methods and frequency are not standardized. The result is that spraying is probably determined by availability of funds or a perceived threat of disease. For effective use of this method, it is important to compare the effectiveness of treatment options; ground versus targeted spraying of the animals in reducing the carriage rates for the parasite. This coupled with a cost effectiveness component would help define best options in different settings where this would be applied. Other areas of interest would be the interval of spraying. This is defined by the concentration on the target surface and half life of the insecticide to be used. While the latter is known, the former is more difficult to determine. Where spraying is disproportionate, some areas have low concentrations and increase the risks of resistance to the drug building quickly. One option to determine this is by the use of filter paper which is placed on the target surface during spraying. After the exercise, it is removed and the concentration on the paper analyzed in the laboratory.
Insecticides: the success of the programme partly depends on the availability of an effective agent to which the fly is and remains sensitive to. It is important to have relevant information on all available agents; sensitivity, reactions, shelf life etc. These would be used to draft guidelines on first choice options and indications for choosing an alternative agent. This information needs to be regularly updated through surveys and resistance testing studies.
Surveillance: Even the best programmes are unlikely to succeed without a robust infrastructure supporting it. There must be a system of surveillance with well defined lines of communication and command from the livestock owner through the veterinary extension worker to the public health officers at the head of the team. It is important to emphasize that everyone at each level understand their roles and responsibilities. An integrated approach is best advised where all stakeholders are involved in the entire process. A good way to start is to identify the relevant stakeholders, their interest and level of influence in determining the direction of policy. The system should also provide a suitable platform for the research objectives to be met. Entomological studies on the tsetse fly; biology and drug metabolism pathways are necessary especially when considering options for drugs to be used as insecticides. It is also important to understand the efficacy of these drugs when used in different modes; aerial spraying, direct application or in screens, the duration of effect and any adverse effects on humans following inhalation or contact. For maximum benefits, these should be integrated into an already existing community based service provided by the health department. While active reporting may be considered as routine, efforts should be made to encourage reporting of cases by farmers, traders and health staff and such cases investigated. The practice of zero reporting should be encouraged. Surveillance sites should be spread across areas that are notably endemic for disease and from newly reported sites of disease.
Resource and staffing requirements: the control team should have members with both technical expertise and an understanding of the scientific issues. The team should also have access to resource persons with knowledge in specific areas that may be needed at some points. The range of skills needed include statistics, epidemiology, social science and a public health specialist with a good knowledge of the health system. The eventual size of the team would depend on the scale of the intervention intended and the resource constraints. However, these members must be able to commit a dedicated period of their time towards the programme and they would require training. The team must be able to develop a response protocol to reports from either the field or the laboratories with tasks clearly. This is important for the continuity of the program in the event that some members leave the team. It is also essential that the life span of members and the program be defined and considerations made for continuity, flexibility and the ability of the program to take on additional tasks provided it falls within the remit of the program. This is to guard against the usual adhoc operations seen during epidemic management programmes.
In conclusion, effective introduction of the insecticide treatment programme must be in synergy with other aspects of the national program against trypanosomiasis and sleeping sickness; chemoprophylaxis, active case detection and treatment of human disease. It must also integrate into the wider goals of food security and provision of essential services. The core components of the programme are summarized in the figure below (figure 1).
Figure 1. Chart indicating the interplay of the various components of the programme
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