Disease Burden
All three diseases affect substantial populations. Globally, WHO estimates that 500,000 cases of DHF occur annually. Assuming that DHF cases constitute 6 percent of all clinical dengue cases (that is, all other cases are classical DF) implies a total of almost 8 million new infections per year. For leishmaniasis, current estimates suggest an overall prevalence of 12 million people infected in an at-risk population of 350 million, suggesting more than 2 million new infections each year. The prevalence of sleeping sickness is estimated at 300,000 people, with 60 million people considered to be at risk. Uncertainty about the true number of cases makes all these estimates approximate, particularly because incidence is increasing. In terms of disability-adjusted life years (DALYs) lost to these diseases (table 23.1), a dengue or trypanosomiasis death accounts for 27 to 28 DALYs lost. Leishmaniasis kills less often than trypanosomiasis, but each death is responsible for a loss of 34 DALYs.
[Table .]
Dengue
In hyperendemic areas of Southeast Asia, where multiple virus serotypes are circulating, DF and DHF are mainly childhood diseases and in some countries are leading causes of pediatric hospitalization and death, particularly in Cambodia, Myanmar, and Vietnam. Worldwide, 80 to 90 percent of deaths occur before age 15. In recent years in the Americas, DHF in adults in endemic form has been reported frequently (Diaz and others 1988; Guzman and others 1999; Harris and others 2000; Zagne and others 1994). In some countries, the disease is more frequent among females, and in Cuba, significantly more severe cases occur among Caucasians than among those of African descent (Kouri, Guzman, and Bravo 1987). Dengue causes relatively few deaths, estimated at 19,000 in 2001, corresponding to a case-fatality rate of 3.8 percent for DHF. Nonetheless, it can cause a substantial burden: in Puerto Rico during 1984-94, the DALY loss per million population was similar to that for the entire Latin America and Caribbean region from malaria, tuberculosis, intestinal helminths, and the childhood disease cluster (Meltzer and others 1998).
Economic Impact
Studies in Thailand estimated the economic impact of dengue as equivalent to US$12.6 million in 1994, of which patients and their families incurred 45 percent (Okanurak, Sornmani, and Indaratna 1997). Kouri and others (1989) estimate the cost of the 1981 DHF epidemic in Cuba at US$103 million, constituting US$41 million in medical care, US$5 million in social security disability payments, US$14 million in lost production, and US$43 million for vector control. For Southeast Asia, Shepard and others (2004) estimate individual treatment costs of US$139 for DHF and US$4.29 for DF (including health clinic visits, hospitalization, medications, travel expenses, and parents' time seeking treatment for their children). This estimate implies annual costs in the region of US$105 million—US$69.5 million for DHF and US$35.5 million for DF. Extrapolating from current trends (on the basis of cases reported to WHO since 1960), this figure will increase to an average of US$118 million each year through the first decade of the 21st century. Using the Thailand study to add the value of productive work lost doubles this figure to US$236 million. Thus, during the next decade, Southeast Asian economies could lose a total of US$2.36 billion because of DF and DHF. Additional economic losses are expected because of the impact of dengue outbreaks on tourism.
Social Impact
During the transmission season, parents familiar with DHF are anxious about their children's survival and the financial consequences of emergency medical care. In Cambodia, even relatively modest out-of-pocket health expenditures can lead to debt and poverty (Van Damme and others 2004). The psychological burden is poorly understood and warrants further study.
Even though dengue affects all strata of society, it may selectively affect the poorest. Most larval habitats in dengue-endemic communities are artificial: water storage containers, flower vases, discarded food containers, used tires, and habitats created by poor design of roof gutters and drains. Local vector ecology is largely determined by community social and cultural practices and infrastructure, and increasing urbanization typically attracts the poor to periurban settlements with deficient water supplies. Studies in the Republica Bolivariana de Venezuela (Barrera, Avila, and Gonzalez-Teller 1993; Barrera and others 1995) and in Thailand (Nagao and others 2003) have shown higher Ae. aegypti infestations where water distribution systems are deficient or unreliable. Along the border between Mexico and the United States, Reiter and others (2003) attribute the low dengue seroprevalence reported in Laredo, Texas, to factors such as air conditioning that limited human-vector contact, whereas in contiguous Nuevo Laredo on the Mexican side, where per capita income was one-seventh of that in Laredo, seroprevalence was much higher despite lower vector densities.
In countries with weak or unprepared health services, epidemics of dengue can be extremely disruptive and health services can be rapidly overwhelmed. Frequently, governments declare states of emergency to mobilize additional resources against dengue outbreaks, sometimes deploying the army to eliminate or apply larvicides to larval habitats. These responses are often launched at or after the peak of the epidemic, and the decline in transmission is unjustifiably attributed to the intervention rather than to the natural epidemic decline.
Leishmaniasis
In 2001, leishmaniasis killed an estimated 51,000 people, including 40,000 in South Asia and 8,000 in Sub-Saharan Africa, representing 0.3 percent and less than 0.1 percent, respectively, of all deaths (table 23.1). Nearly all deaths occurred at ages 5 to 29. Males were more affected than females, especially in Sub-Saharan Africa, where the ratio was three to one.
Economic Impact
Treatment for leishmaniasis is expensive, especially for VL. For many countries, the cost of treating all leishmaniasis patients would far exceed their total health budgets. For a WHO-recommended course of pentavalent antimonials, current drug costs per patient are US$150 for sodium stibogluconate, US$120 for meglumine antimoniate, and US$30 for generic sodium stibogluconate. Cases not responding to antimonials may require second-line, more toxic drugs, such as amphotericin B at a cost of US$60 or pentamidine at a cost of US$70. Less toxic amphotericin in liposomes is effective, but costs US$1,500 per patient. The first oral drug for VL, miltefosine, currently costs US$120 per patient.
In addition to drug costs, the additional costs of drug delivery can be high, especially for patients in remote areas. Patients often live far from a treatment center, and the expense of hospitalization may lead to interrupted treatment, facilitating resistance and requiring additional therapy. Without treatment, severe leishmaniasis can become chronic and debilitating, incapacitating patients and making them unable to work and vulnerable to poverty, malnutrition, and secondary infections.
Social Impact
Even self-limiting CL can leave disfiguring scars, which have associated stigma and may affect marriage prospects. CL can be disabling when lesions are numerous, and the most severe form, recidivans leishmaniasis, is difficult to treat, long-lasting, and disfiguring. In individuals with a defective cell-mediated immune response, the disseminated lesions of diffuse cutaneous leishmaniasis resemble those of leprosy. They do not heal spontaneously and frequently recur after treatment. Diffuse cutaneous leishmaniasis is recognized as a special public health problem, both clinically and because of its severe emotional consequences.
The lesions of mucocutaneous leishmaniasis can cause extensive destruction and distortion of oronasal and pharyngeal cavities, leading to mutilation of the face. Patients may be shunned and, in severe cases, even incarcerated. Although mainly associated with L. braziliensis and L. guyanensis in the Americas, mucocutaneous leishmaniasis has been reported in Africa, Asia, and Europe as a complication of L. donovani and L. major infections and, in immunosuppressed patients, of L. infantum.
Untreated VL is usually fatal. Even after recovery, patients may develop a chronic form of CL that usually requires prolonged and expensive therapy.
African Trypanosomiasis
WHO estimates the number of deaths caused by sleeping sickness as 48,000 in 2001 (table 23.1), although current estimates are in the range of 50,000 to 100,000 per year. Men are affected at nearly twice the rate of women. In relation to mortality, of all parasitic diseases in Sub-Saharan Africa, trypanosomiasis ranks behind only malaria. As concerns DALYs, the health burden is similar to that of schistosomiasis. In Sudan, 33 DALYs on average are lost because of each premature death from sleeping sickness (McFarland 2003). In Uganda, 23 DALYs were lost per death, but only 0.21 DALYs per successfully treated individual (Odiit and others 2004). Underreporting makes deriving estimates for the whole continent difficult: 100,000 deaths per year would imply more than 2 million DALYs lost, compared with the WHO estimate of 1.31 million (830,000 for males and 480,000 for females) (table 23.1).
Economic Impact
Information on age at death indicates that sleeping sickness mainly affects economically active adults. Data from Uganda show nearly 25 percent of cases occurring in those age 20 to 29 and more than 60 percent in those age 10 to 39 (Odiit 2003). Thus, when people become ill, their families not only become burdened with the care of seriously ill individuals but also often lose their breadwinners. Poor diagnostic support in many areas means that families often invest in a number of treatments that have no effect on the disease. In a T.b. rhodesiense area of Uganda, Odiit and others (2004) find that some patients made up to seven visits to health facilities before being correctly diagnosed, with just under three-quarters initially being diagnosed with malaria. For the 11 of 12 who were never diagnosed or were told that they had a different fatal disease, the costs to and burdens on their families can only be imagined.
In addition to the economic losses caused by interruption of their work, sleeping sickness patients face direct financial costs. Even though WHO now provides specific first-line drugs at no cost in excess of delivery and administration, hospitalization and treatment are expensive. In the DRC, Gouteux and others (1987) estimate that total costs are -equivalent to at least 25 percent of a year's income from agriculture.
Social Impact
The importance of sleeping sickness lies not only in the number of new cases reported, but also in its potential for epidemic outbreaks causing thousands of deaths: during recent epidemics in the DRC, in some villages up to 70 percent of the population became infected. Because of the severity of the disease, one case can affect all family members, placing a burden on the whole community, reducing the labor force, interrupting agricultural activities, and jeopardizing food security. Although untreated trypanosomiasis is lethal, treated patients often remain incapacitated, perpetuating the cycle of poverty, malnutrition, and disease. DALYs do not take into account the psychosocial impact and the "minor" disabilities. In adults, loss of memory and ability to concentrate is common. Such disabilities are often accompanied by reading and writing difficulties and occasionally by extreme incoherence. These disabilities greatly affect everyday life, particularly for those school-age children who, even after successful treatment, do not recover fully and cannot pursue their studies (Frezil 1983).
Burden of Animal Trypanosomiasis
Animal trypanosomiasis constrains agricultural production—in particular, the use of draft power. Cattle infected with T.b. brucei, T. vivax, or T. congolense quickly succumb to a wasting form of anemia. In many areas with a high tsetse challenge, such as Central Africa, cattle are few or not present at all. Elsewhere, countries invest an estimated total of US$30 million to US$50 million per year in some 35 million doses of veterinary trypanocides to prevent the disease in livestock (Geerts and Holmes 1998). About 60 percent of the cattle at risk are not treated, and the disease is thought to kill about 1 million a year. Current drugs are more than 40 years old, and drug resistance is increasing, as are problems of drug availability and accessibility, counterfeit drugs, and drug mismanagement (Geerts and Holmes 1998). Constraints on draft power mean that farmers can till only small plots, making subsistence farmers extremely vulnerable to food shortages. Milk yields are lower in infected cows, and animal trypanosomiasis lowers fertility and increases mortality, thereby constraining the overall growth rate of the number of livestock (Swallow 2000). Kristjanson and others (1999) estimate annual direct losses of US$1.3 billion per year as a result of lowered production of milk and meat, with aggregate agricultural losses attributable to trypanosomiasis estimated at US$4.5 billion per year.
