Leprosy Mailing List – January 26th, 2007
Ref.:
30% annual reduction of leprosy case detection in India.
From:
Richardus J. H., Fischer E., Rotterdam, The Netherlands
Rotterdam,
25th January 2006.
Dear Dr. Noto,
We would like to reply to the messages put in the
LML by Dr. Reddy from Kathmandu regarding the reduction in leprosy
case detection in India over the past four years. It was suggested previously in
LML that such strong reduction is biologically not possible for a
disease that has a 2 to 11 year gestation period. Dr. Reddy forwards the
hypothesis that this is biologically not impossible and requests
arguments one way or the other.
It is not helpful scientifically to approach this
issue in absolute terms such as impossible. Let us consider the
observation of the strong decrease in leprosy case detection and look
for information and evidence that could clarify matters.
It has been
documented that India experienced a drastic reduction in annual
detected cases from 473,658 in 2002 to 161,457 in 2005 (1). The annual
reduction rate of leprosy case detection in this period in India was
thus over 30% per year.
Meima et al. (2) published a paper in 2004 in which
the expected trends in incidence rates of leprosy were calculated for
the period 2000-2020. In
this paper, the annual reduction during this period was calculated to
be between 2 and 12 %, based on observed data in the period 1985-1998. In other words, if there are
no drastic changes in the epidemiological leprosy situation as
observed in the period before the year 2000 (including factors such
BCG vaccination coverage), the annual reduction afterwards cannot be
expected to be more than 12% in the most favourable circumstances. Hence, something additional
has to have happened to explain the observed 30% annual reduction of
leprosy case detection in India.
Leprosy, both paucibacillary and multibacillary,
has a long and variable incubation period, with estimated averages of
3.5 and 8 years. The
reduction of new case detection in India started in 2003. This would imply an intensive
decrease in transmission of M. leprae that started between 1995
and 1999. It is indeed
true that leprosy case detection, including active case finding, has
been stepped up in this period. Finding
cases early and putting them on treatment takes out sources of
infection, but it is questionable whether this has such a strong
impact on the ongoing transmission of M. leprae in the whole
population to explain the recent annual reduction of new cases of 30%. MDT does not prevent
sub-clinical cases developing disease and spreading M. leprae
during heir incubation period. Also,
multibacillary leprosy is hard to detect in its early stages and is
often not recognized during active case finding campaigns. To our knowledge there have
been no successful systematic interventions in India (or
elsewhere in the world) focussing on the primary prevention of leprosy
(vaccination) or on the prevention of disease in pre-clinical cases
(chemoprophylaxis).
We conclude that part of the reduction in leprosy case detection in
India may well be explained by a longer existing natural decline of
the transmission of M. leprae in the population, possibly
accelerated by the intensified control activities of the past 10
years. But it is not
plausible to expect a relatively sudden annual reduction of more than
12% in the absence of any drastic nation-wide intervention with the
effect of interrupting transmission of M. leprae such as
vaccination or chemoprophylaxis and which would have started in the
late 1990's. If such
intervention existed, it would be important to know!
So while it may not be biologically impossible to
achieve such drastic reduction in case detection, it is not very
likely that the observed reduction is explained completely on
biological or medical grounds. Other
factors have probably contributed as well, including operational
factors such as case finding activities and administrative factors
such as registration procedures.
We hope that our comments contribute to further
discussion on the interpretation of the global leprosy data.
With kind regards,
Egil Fischer, MSc
Jan Hendrik Richardus, MD PhD
Department of Public Health
Erasmus MC, University Medical Center, Rotterdam
The Netherlands
(1) Anonymous. Global leprosy situation 2006. Weekly Epidemiological
Record 2006; 81(32):309-16.
(2) Meima et al. The future incidence of leprosy: a scenario analysis.
Bull World Health Organ. 2004; 82(5):373-80.
