Disease Burden
Information on the number of people infected is often difficult to obtain for tropical diseases. Many infected people may be without obvious symptoms, those with symptoms may not seek care at public health facilities, and those who do may not be reported. Routine health information systems provide little information on the number of people infected in the population. Surveys are more informative but are rarely done. A better picture emerges only when control programs need to map the distribution of the disease as a basis for targeting large-scale interventions. Hence, the apparent paradox is that intensification of disease control may result in a significant initial increase in estimates of the burden of disease through better epidemiological data.
Chagas Disease
Chagas disease is an important public health problem in 17 countries in Latin America. Estimates from the 1980s indicated that some 16 million to 18 million individuals were infected (WHO 1991), and in the 1990s, a series of multinational control initiatives was launched that was designed to interrupt transmission by eliminating domestic insect vectors and improving the serological screening of blood donors. As a result, estimates of the number of infected people were revised to 9.8 million in 2001 (Schmunis 2000). The estimated burden of disease in terms of disability-adjusted life years (DALYs) declined from 2.7 million in 1990 (World Bank 1993) to 586,000 in 2001 (Mathers and others 2006). Because of migration, T. cruzi-infected individuals can be found outside Latin America (for example, in Spain or the United States).
Estimates from the 1980s suggested that 5 million people in the Americas had symptoms of Chagas disease (WHO 1991). These estimates decreased to 1.2 million to 2.8 million in the 1990s. The World Health Organization (WHO) attributed 45,000 yearly deaths to Chagas disease (WHO 1991). WHO decreased its mortality estimates to 13,000 in 2001 (WHO 2002d).
In all affected countries, Chagas disease has been responsible for a high burden of disease and significant direct and indirect costs. Reports from Brazil in the late 1980s suggested that the aggregate costs for pacemakers and intestinal surgeries for Chagas disease were US$250 million per year, excluding the costs of consultations, care, and supportive treatment for chronic chagasic patients, which amounted to US$1,000 per year per patient, and disability awards, which in one state accounted for US$399,600 (Dias 1987; Schofield and Dias 1991). In Bolivia, in 1992, aggregate treatment costs were estimated at US$21 million. In Chile, in 1997, aggregate treatment costs for Chagas disease were estimated at US$14 million to US$19 million (Schenone 1998), and in Uruguay, in 1996, costs were estimated at US$15 million (Salvatella and Vignolo 1996).
Lymphatic Filariasis
LF is endemic in 83 countries, with 1.1 billion people living in known endemic areas. In 1992, the WHO Expert Committee estimated that 78 million people were infected (WHO 2002c). This estimate was later revised to 119 million, and current estimates indicate that LF is responsible for the loss of 4.6 million DALYs per year. Many endemic areas lack reliable data on the prevalence of LF, and estimates of the number infected may increase when more precise data become available from epidemiological mapping. Nationwide mapping in four neighboring countries in West Africa showed that LF was endemic in a much wider area than expected, and the findings resulted in a dramatic increase in the estimated number infected (Gyapong and others 2002).
Epidemiological trends have varied widely among different regions in recent decades. LF was controlled or eliminated from several islands in the Pacific, and China has seen a dramatic reduction in infection levels. Unfortunately, in India and Africa, the most endemic areas of the world, recent decades have witnessed little change (WHO 2002c).
The acute form of the disease is common and causes severe hardship in endemic communities. Infected individuals suffer from one to eight acute episodes per year, and during each episode, affected patients are bedridden for three to five days.
Morbidity caused by chronic LF is mostly lifelong, and the disease is considered the second leading cause of disability in the world (WHO 1995b). Patients affected by elephantiasis or hydrocele are often victims of societal discrimination, and the disease impairs their educational and employment opportunities, marriage prospects, and sexual life. Case-control studies in India revealed that affected individuals are 27 percent less productive than their uninfected counterparts (Ramu and others 1996). The patients work less and often switch to lighter jobs, leading to a loss of more than 1 billion person-days per year in India alone (Ramaiah and others 2000), which translates into an annual economic loss equivalent to 0.63 percent of gross national product.
Onchocerciasis
More than 99 percent of those infected with O. volvulus reside in 30 endemic countries in Africa, with the remainder living in the Republic of Yemen and six countries of the Americas. In 1995, the WHO Expert Committee on Onchocerciasis estimated that 17.7 million people were infected, of whom about 270,000 were blind and another 500,000 were severely visually impaired (WHO 1995a). However, more recent information from rapid epidemiological mapping of onchocerciasis (Noma and others 2002) by the African Programme for Onchocerciasis Control (APOC) indicates that the number of those infected is twice as high and that some 37 million people were infected in 1995. This revised estimate corresponds to an estimated 1.99 million DALYs lost because of onchocerciasis in 1995.
Using the most recent rapid epidemiological mapping data and the latest APOC data on treatment coverage and assuming that four rounds of ivermectin treatment will reduce the prevalence of troublesome itching by 85 percent and the burden of visual impairment and blindness by 35 percent give a DALY estimate of 1.49 million DALYS lost for 2003 (see table 22.1).
[Table .]
In addition to the burden of blindness and severe itching, onchocerciasis has important socioeconomic consequences. In the West African savanna, fear of blindness has resulted in the depopulation of fertile river valleys, severely affecting agricultural production. It was this socioeconomic impact, and not just the health impact, that led to the creation of the Onchocerciasis Control Program (OCP) in West Africa in 1975 (Remme 2004b).
Even though the importance of onchocercal blindness has long been recognized, only in 1995 did research demonstrate that the public health importance of onchocercal skin disease was even greater. Troublesome itching associated with dermal onchocerciasis makes working, studying, or interacting socially difficult (Murdoch and others 2002; Vlassoff and others 2000). Onchocercal itching now accounts for 60 percent of DALYs lost (Remme 2004a). Other skin manifestations, such as reactive skin lesions, are not included in the DALY estimates, even though they are highly prevalent and have major psychosocial and economic impacts. Onchocercal skin disease also diminishes people's income-generating capacity, and the school dropout rate is twice as high among children from households in which the head of household is affected by onchocercal skin disease (Benton 1998).
Leprosy
In May 2001, 10 years after the World Health Assembly had adopted a resolution to eliminate leprosy by the end of the millennium, the target—a prevalence rate of less than 1 per 10,000—had been achieved at the global level. The number of cases registered for treatment worldwide fell from 5.4 million in 1985 to 460,000 by the end of 2003 (WHO 2004a); however, this trend should not be taken at face value because the reduction is attributable mainly to such factors as the shortening of treatment duration for multibacillary patients and the cleaning up of patient registers.
Leprosy is reported from all regions of the world, but the burden of disease, which is estimated at 192,000 DALYs, is concentrated in a few countries. During 2003, 513,798 new cases were detected, of which more than 80 percent were in Brazil, India, Madagascar, Mozambique, Nepal, and Tanzania (WHO 2004a). India alone accounted for about 75 percent of the new cases. Case detection has remained remarkably stable over the past decade. Trends in case detection rates should be analyzed in conjunction with the proportion of new patients with grade 2 impairment (an indicator of the delay between onset of the disease and diagnosis) and the proportion of children among new cases (an indicator of recent transmission).
Virtually all published data on leprosy-related disability concern impairments. In 1997, WHO estimated the global prevalence of patients with visible impairments (disability grade 2) as 2 million. A similar number may have sensory impairment without deformity. Sensory and motor impairment that are already present at diagnosis are important risk factors for developing additional impairment and disability. Evidence indicates that sensory impairment itself causes significant functional disability.
The prevalence of activity limitations among people affected by leprosy is unknown. Van Brakel and Anderson's (1998) survey in Nepal finds that among those with any impairment, about 20 percent had limitations in relation to one or more indoor activities and up to 34 percent had significant limitations in relation to common outdoor activities. Even less is known about the prevalence of restrictions on social and economic participation. Surveys are urgently needed to assess the extent of patients with leprosy-related disabilities who require intervention.
Two difficulties affect the validity of DALY estimates for leprosy. The first is the lack of data, particularly on the burden of functional and psychosocial disability caused by leprosy. The second is that the effect of leprosy often goes well beyond the affected individual; the psychosocial consequences may affect the whole family. People without any visible signs of leprosy may be stigmatized simply because they are known to be a leprosy patient. Even after completing treatment, people may remain stigmatized.
Summary of DALY Estimates
Table 22.1 summarizes the DALY estimates for each of the four diseases by World Bank region. The high estimate for LF reflects not only its wider distribution and the larger number of people affected, but also the reduction in the burden for the other three diseases as a result of control efforts. For those diseases for which there has been significant progress toward elimination, public health officials should remain aware of the burden of disease that is currently averted but that might return if control were not to be sustained before transmission has been completely eliminated.
