. "Appendix D-7: The Prospects for Immunizing Against Mycobacterium leprae." New Vaccine Development: Establishing Priorities: Volume II, Diseases of Importance in Developing Countries. Washington, DC: The National Academies Press, 1986.
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New Vaccine Development: Establishing Priorities, Volume II, Diseases of Importance in Developing Countries
this basis, the annual global incidence would be about 1.06 million cases. For the purposes of this assessment, all new cases are assumed to have some clinical symptoms that result in some permanent (but possibly mild) disability (category D)—such as minor neurological deficit. The proportion of cases in children (14 years of age and younger) is approximately 20 percent of detected cases (17.4 to 29 percent; Noordeen, personal communication, 1984) and is probably higher for new cases. Hence, it is assumed that 25 percent of new cases occur in the 5 to 14 years age group and 75 percent in the 15 to 59 years age group.
One-third of leprosy patients face the threat of progressive disease, which can result in severe physical disability and social stigmatization (SPRTTD, 1985). Patients who contract the disease early in life or who progress rapidly will spend part of their remaining lifetime with mild, moderate, and then severe chronic disability (categories D, E, and F). Patients who contract the disease later in life or who progress slowly may not experience the most severe consequences (some patients will not progress but will have already incurred mild chronic disability, category D). Most of the progression is thought to occur before the age of 60 (Bloom, personal communication, 1985). Hence, for ease of calculation, all transitions to moderate or severe categories are assumed to occur in the 15 to 59 years age group.
Leprosy is unique in exhibiting such progression and variability; it does not easily lend itself to description by the use of the morbidity category/age group matrix used in this exercise to display the incidence of disease consequences. To permit comparison of leprosy with other diseases by this method, some simplification of the epidemiological presentation of the disease is necessary.
One-third of all cases in any cohort are assumed to progress. This represents, on the average, 353,333 patients moving each year to the moderate or severe status. Half of these transitions are assumed to be to the moderate category and half to the severe category (176,667 to each). The annual number of new cases in category D in the 15 to 59 years age group (795,000) is reduced accordingly (by 353,333).
The life expectancy of leprosy patients is probably shortened by a few years, more so in the lepromatous forms (Sansarricq, 1981). Deaths are probably a consequence of other infections or intoxications (e.g., tetanus) resulting from physical injury or mutilation and/or depressed immune function. However, few data exist on which to base death estimates. It has therefore been assumed (arbitrarily) that leprosy patients have a 1 percent higher death rate than the general population. For a normal crude death rate of 10 per 1,000 for developing countries, this would result in about 1,060 premature deaths within a population of 10.6 million leprosy patients.
These deaths are assumed to occur predominantly before the age of 60 years (i.e., 90 percent in the 15 to 59 years age group and 10 percent in the 60 years and over age group).
The annual disease burden estimated to result from leprosy is shown in Table D-7.1.