Leprosy Mailing List – December 21 , 2020
Ref.: (LML)
Epidemiology of HD
From:
Joel Almeida, London and Mumbai
Dear Pieter and colleagues,
Understanding
unexpected successes and unexpected failures is very helpful in achieving major
epidemiological impact. Some field programmes achieved 16% to 90% real annual
decline in new cases, while others showed a disappointing impact (0% to 3% real
annual decline). The underlying epidemiology can be summarised as below. This
might encourage great colleagues in endemic countries to implement increasingly
effective interventions.
Fig. 1
The neglected multitude of previously treated anergic patients maintains
transmission despite all other efforts. 16% to 20% / year real decline in new
cases is available from repeated skin camps with competent clinicians to
diagnose subtle LL signs + MDT (prolonged for high BI patients to prevent
reinfection/recurrence).(1-3) 90% / year real decline in new cases
is available by adding repeated mass multi-drug administration to suppress
bacilli in sub-clinical disease or missed cases, alongside skin camps and
MDT.(4) By contrast, fixed duration MDT for LL patients failed to sustain a
rapid decline,(5) with patients who had a high initial BI showing a
significant risk of recurrent disease after even 24 months of MDT.(6)
Fig. 2 Only a very small fraction of the population has genetically linked(7-10) anergy to HD bacilli, with consequently low ID50s. ID50 is the dose of bacilli sufficient to infect 50% of individuals of a given description.
Fig. 3 Nearly all HD patients are non-infectious, for practical purposes. Transmission is almost exclusively by unrecognised LL or previously treated LL patients with recurrent disease, whose nasal discharge can contain up to tens of millions of viable bacilli per day.(11)
Reliable
epidemiology is like a reliable map. It is better than driving blindfolded.
Further, our understanding keeps improving as clues from the front-lines
are unearthed by great colleagues. Endemic countries need the elbow room to
sustain and spread the successes they have demonstrated in exemplary
programmes.
Joel Almeida
References
1. Tonglet R, Pattyn SR, Nsansi BN et al. The reduction of the leprosy endemicity in northeastern Zaire 1975/1989 J.Eur J Epidemiol. 1990 Dec;6(4):404-6 reviewed in: 23a. Almeida J. Reducing transmission in poor hyperendemic areas - evidence from Uele (DRC). LML 29 Nov 2019
2.
Norman G, Bhushanam JDRS, Samuel P. Trends in leprosy over 50 years
in Gudiyatham Taluk, Vellore, Tamil Nadu. Ind J Lepr 2006. 78(2):
167-185. reviewed and analysed further in: 20a Almeida J. Karigiri, India:
How transmission rapidly was reduced in a low-income population LML 29 October 2020
3. Li HY, Weng XM,
Li T et al. Long-Term Effect of Leprosy Control in Two Prefectures of China,
1955-1993. Int J Lepr Other Mycobact Dis. 1995 Jun;63(2):213-221. reviewed
& analysed further in: 22a. Almeida J. What really happened in Shandong? LML 16 Nov 2019
4. WORKSHOP ON THE
PREVENTION OF LEPROSY, POHNPEI, FEDERATED STATES OF MICRONESIA. 25-27 MAY 1999
sponsored by the Sasakawa Memorial Health Foundation Tokyo, Japan and the
Western Pacific Regional Office of the World Health Organization. Int J Lepr,
67 (4) (SUPPLEMENT)
5.
Scheelbeek PFD, Balagon MVF, Orcullo FM et al. A retrospective study of
the epidemiology of leprosy in Cebu: an eleven-year profile. PLoS Negl Trop
Dis. 2013 Sep 19;7(9):e2444. doi: 10.1371/journal.pntd.0002444. eCollection
2013.
6.
Balagon MF, Cellona RV, dela Cruz E et al. Long-Term Relapse Risk of
Multibacillary Leprosy after Completion of 2 Years of Multiple Drug Therapy
(WHO-MDT) in Cebu, Philippines. American Journal of Tropical Medicine and
Hygiene, 2009; 81, 5: 895-9. reviewed and analysed further in 19a. Almeida
J Recurrence rate among MB patients following RFT. LML 2 June 2019.
7. Gaschignard J,
Grant AV, Thuc NV, Orlova M, Cobat A, Huong NT, et al. Pauci- and
Multibacillary Leprosy: Two Distinct, Genetically Neglected Diseases. PLoS Negl
Trop Dis 2016; 10(5): e0004345. https://doi.org/10.1371/journal.pntd.0004345
8. Chakravarti MR,
Vogel F. A twin study on leprosy Georg Thieme Publishers, Stuttgart, Germany;
1973.
9. Cambri G, Mira MT. Genetic Susceptibility
to Leprosy—From Classic Immune-Related Candidate Genes to Hypothesis-Free,
Whole Genome Approaches. Front. Immunol., 20 July 2018 | https://doi.org/10.3389/fimmu.2018.01674
10. Sartori PVU, Penna GO, Bührer-Sékula S et
al. Human Genetic Susceptibility of Leprosy Recurrence. Scientific Reports
2020; 10: Article number: 1284
11. Davey TF, Rees RJ. The nasal dicharge in leprosy: clinical and bacteriological aspects. Lepr Rev. 1974 Jun;45(2):121-34.
PS. In medical
exams someone from our college was asked, "What is the function of the
muscle spindle?" The classmate replied "Sir, that is not well
understood."
The examiner shot back, "By whom, Sir? By the world at large or just by
you?"
In the epidemiology
of HD, despite whatever might remain to be discovered, it seems wise to
recognise and apply the knowledge already available.
LML - S Deepak,
B Naafs, S Noto and P Schreuder
LML
blog link: http://leprosymailinglist.blogspot.it/
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