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In leprosy both the reference points for measuring the incubation period and the times of infection and onset of disease are difficult to define; the former because of the lack of adequate immunological tools and the latter because of the insidious nature of the onset of leprosy. Even so, several investigators have attempted to measure the incubation period for leprosy. The minimum incubation period reported is as short as a few weeks and this is based on the very occasional occurrence of leprosy among young infants. [1] The maximum incubation period reported is as long as 30 years, or over, as observed among war veterans known to have been exposed for short periods in endemic areas but otherwise living in non-endemic areas. It is generally agreed that the average incubation period is between 3 to 5 years.
(shown on the right).
Measuring the disease burden: incidence, prevalence and new case detection
Although annual incidence – the number of new cases occurring each year - is important as a measure of transmission, it is difficult to measure in leprosy due to its long incubation period, delays in diagnosis after onset of the disease and the lack of laboratory tools to detect leprosy in its very early stages.
Instead, the registered prevalence is used. Registered prevalence is a useful proxy indicator of the disease burden as it reflects the number of active leprosy cases diagnosed with the disease and retrieving treatment with MDT at a given point in time. The prevalence rate is defined as the number of cases registered for MDT treatment among the population in which the cases have occurred, again at a given point in time [2].
New case detection is another indicator of the disease burden and usually reported by countries on an annual basis. It includes cases diagnosed with onset of disease in the year in question (true incidence) and a large proportion of cases with onset in previous years (termed a backlog prevalence of undetected cases). The new case detection rate (NCDR) is defined by the number of newly detected cases, previously untreated, during a year divided by the population in which the cases have occurred.
Endemic countries also report the number of new cases with established disabilities at the time of detection, as an indicator of backlog prevalence. However, determination of the time of onset of the disease is generally unreliable, is very labour-intensive and is seldom done in recording these statistics.
| Country | Registered Prevalence (rate/10,000 pop.) | New Case Detection (rate/100,000 pop.) | ||||
|---|---|---|---|---|---|---|
| Start of 2004 | Start of 2005 | Start of 2006 | During 2003 | During 2004 | During 2005 | |
| Brazil | 79,908 (4.6) | 30,693 (1.7) | 27,313 (1.5) | 49,206 (28.6) | 49,384 (26.9) | 38,410 (20.6) |
| Dem. Rep. of Congo | 6,891 (1.3) | 10,530 (1.9) | 9,785 (1.7) | 7,165 (13.5) | 11,781 (21,1) | 10,737 (18.7) |
| Mozambique | 6,810 (3.4) | 4,692 (2.4) | 4,889 (2.5) | 5,907 (29.4) | 4,266 (22.0) | 5,371 (27.1) |
| Nepal | 7,549 (3.1) | 4,699 (1.8) | 4,921 (1.8) | 8,046 (32.9) | 6,958 (26.2) | 6,150 (22.7) |
| Tanzania | 5,420 (1.6) | 4,777 (1.3) | 4,190 (1.1) | 5,279 (15.4) | 5,190 (13.8) | 4,237 (11.1) |
| Totals | 106,578 | 55,391 | 51,098 | 75,603 | 77,579 | 64,905 |
Notes: 1) Madagascar is not included as it reached elimination at the national level in September 2006 2) Nepal detection reported from mid-November 2004 to mid-November 2005
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