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Lutte contre le trachome en Afrique subsaharienne

 | 
Anne-Marie Moulin
, 
Jeanne Orfila
, 
Doulaye Sacko
, 
et al.

Seconde partie. Chapitres analytiques

Chapitre 21. Which are the best surveillance methods for directing the fight against trachoma? What are the intervention and recovery (or elimination) thresholds? [Quelles sont les méthodes de surveillance les plus adéquates pour guider la lutte contre le rachome ? Quels sont les seuils d’intervention et les seuils de guérison (ou d’élimination)?]

Thomas Lietman

Texte intégral

1There are two issues here. What indicator(s) should be used, and how should sampling be performed.

Efficacy indicators and Process indicators

2There are no perfect indicators. Several groups have proposed standard indicators, often differing only in trivial details (e. g. 1-10 year olds or 1-9 year olds). There is some advantage to using the same indicators that other programs use, so results can be compared. Rather than developing a totally new set of indicators that differs only in minor details from existing proposals, it would be preferable to choose a subset of existing indicators that are most valuable to programs in West Africa.

3The WHO has recommended several efficacy and process indicators for surveillance of a trachoma program’s activities. Their definitions are still changing, particularly for the process indicators. Examples include.

Efficacy indicators from population-based surveys

4Clinical activity (WHO Simplified Grading, either TF or TI counting as clinically active) in children aged 1-9 years (after the 1st birthday and before the 10th birthday). These children are by far the most likely to display evidence of infection. Children under 9 months often do not display the characteristic signs of infection (follicles) even if they are infected, so children < 1 year are not included. Expressed as clinically active children/total children of this age group.

5Trichiasis. Uncorrected (unoperated or recurrent) trichiasis in adults (men and women) 40 years and older. Expressed as a fraction of discovered trichiasis cases over population in this age group.

6Scarring (TS) would be a useful intermediate indicator, but inexperienced graders find it the most difficult to grade consistently (Taylor et al., 1987). Corneal blindness (CO) is in a sense the most important outcome, as it is the direct cause of blindness, but it frequently has other causes: there is a low level of corneal opacity in communities without trachoma.

7Process indicators. Processes are more difficult to assess, the WHO definitions are still changing, as are changing the definitions of the various NGOs involved in trachoma control. These measures may be country specific.

8Surgery

  • Total number of surgeries performed during the previous year,

  • Trichiasis surgical coverage: number of TT operated divided by the number of TT operated + number of patients requiring surgery,

  • Recurrence rate as a proxy indicator for quality of surgery.

9Antibiotics

  • Total number of patients treated with antibiotics during the previous year,

10Antibiotic coverage: number treated/eligible and targeted for treatment.

11Facial cleanliness

  • Percent of children aged 1 to 9 years with clean faces.

12Environmental

  • Access to water: Percentage of population with access to water within 30 minutes travel time (or 1 kilometer),

  • Fly breeding sites: percentage of households using latrines

13National programs and individual organizations will develop their own subsets of indicators in order to fit their specific needs, particularly in terms of program’s management.

How should communities and individuals be sampled?

14The essence of determining prevalence is predicated on either examining all individuals (usually not realistic or necessary), or examining an unbiased, random sample of individuals in the population of interest. There are a number of sampling strategies, and the optimal strategy depends on the goals and resources of the program. Examples include:

  1. examine all individuals during a “trachoma day”, perhaps the same day that antibiotics are administered ;

  2. examine all individuals in a random sample of villages ;

  3. examine all individuals in a random sample of households ;

  4. examine children 1-9 and adults >40 years in a random sample of villages or households.

Comment on the WHO Trachoma Rapid Assessment protocol

15The WHOs Trachoma Rapid Assessment system (examine 50 children from poorest households, ask locals if there are any trichiasis cases, assess latrines and flies, etc.) may be useful for identifying trachoma-endemic areas, but it should be stressed that this methodology was not designed as a prevalence survey or for monitoring a program once it has started.

Intervention: Where should a trachoma program be started?

16It is necessary to estimate the prevalence of ocular chlamydial infection. Clinical activity in children appears to be a reasonable estimate of this in untreated communities, as mentioned above. The WHO has proposed the following guidelines for intervention:

17It should be noted that these guidelines are based on expert opinion more than data. From studies, we have learned the following:

  • While areas with >20 % clinical activity in children generally have correspondingly high ocular chlamydial infection by lab testing, areas with <5­10% clinical activity in children may have no ocular chlamydia by lab testing (Baral et al., 1999) ;

  • Mass treatment becomes less expensive than targeted treatment at higher levels of clinical activity (>20 or 25 %) (Holm et al., 2001);

  • In meso-endemic areas (5-20 %), treatment of all children or active children and their household appear to be equally effective (Holm et al., 2001).

18Elimination as a public health care concern: when can we stop a trachoma program? This is a difficult question. In particular, it depends on two things that we do not know:

  • Is there a magical threshold, from below which infection can never return to its original level? This is unknown, may be wishful thinking, that infection may return to previous level if nothing else is done (Gaynor et al., 2002).

  • Do hygiene and environmental programs have enough of an effect that infection will not return to previous levels even if antibiotics are stopped? This is not known too (Emerson et al., 2000). A well-designed controlled trial suggested that at 1 year hygiene could affect the severity of cases (less TI), but not the prevalence of clinical activity (TF and TI)(West et al., 1995). A small pilot study suggests that active trachoma was reduced more in two villages with an intensive (and perhaps unsustainable) fly control program than in two nearby villages without fly control (Emerson et al., 1999). A larger, well-designed fly control study involving multiple villages is now taking place in the Gambia to try and address this important issue (Emerson et al., 2002).

19Pending further studies, the WHO has recommended the following definitions of elimination:

20It should be recognized that guidelines of elimination often change during the course of a program (e. g. leprosy’s definition has changed several times). Also, it should be noted that these guidelines are, as above, more from expert opinion than from data. Studies include:

  • Antibiotic treatment is more effective in reducing infection (by lab testing) than in reducing clinical activity (by examination). For example, mass treatment with azithromycin may reduce infection in children from approximately 50% to <10 %, but clinical activity from approximately 50% to only 20-30 % (Schachter et al., 1999).

  • After multiple treatments, there may be, for example, 5 % clinical activity, but no detectable infection. It should be noted that it is possible that a small amount of chlamydia is present but undetectable by even the most sensitive lab techniques.

  • Ideally, as with sexually transmitted chlamydial infections, laboratory testing (PCR) would be performed to document elimination of infection. This could be made more affordable by sampling communities, sampling individuals within communities, and pooling several samples within the same laboratory test (Peeling et al., 1998; Diamont et al., 2001). Realistically, programs will rely on clinical examinations, but they should interpret their results according to what this implies about actual infection.

  • Remember, although elimination and eradication are synonymous in English. They can have different meanings in disease control (Dowdle, 1998). For example, elimination of leprosy as a public health concern is defined as a prevalence of <1:10000, and elimination of tuberculosis as a public health concern is defined as an incidence of <1:100000/year.

Bibliographie

References

Baral K. Osaki S., Shreshta B., Panta C.R., Boulter A., Pang F., Cevallos V., Schachter J., Lietman T., 1999 – Reliability of clinical diagnosis in identifying infectious trachoma in a low-prevalence area of Nepal. Bulletin of the World Health Organization, 77(6): 461-466.

Diamont J., Benis R., Schachter J., Moncada J., Pang F., Jha H.C., Bhatta R.C., Porco T., Lietman T., 2001 – Pooling of chlamydia laboratory tests to determine the prevalence of ocular chlamydia trachomatis infection. Ophthalmic Epidemiology, 8(2-3): 109-117.

Dowdle W.R., 1998 – The principles of disease elimination and eradication. Bulletin of the World Health Organization, 76 Suppl 2: 22-25.

Emerson P.M., Cairncross S., Bailey R.L., Mabey D.C., 2000 – Review of the evidence base for the 'F' and 'E' components of the SAFE strategy for trachoma control. Tropical medicine & international health, 5(8) : 515-527.

Emerson P.M., Lindsay S.W., Walraven G.E., Faal H., Bøgh C., Lowe K., Bailey R.L., 1999 – Effect of fly control on trachoma and diarrhoea. Lancet, 353(9162) : 1401-1403.

Emerson P.M., Lindsay S.W., Walraven G.E., Dibba S.M., Lowe K.O., BAILEY R.L., 2002 – The Flies and Eyes Project Design and methods of a cluster-randomised intervention study to confirm the importance of flies as trachoma vectors in the Gambia and to test a sustainable method of fly control using pit latrines. Ophthalmic Epidemiology, 9(2) : 105-117.

Gaynor B.D., Yi E., Lietman T., 2002 – Rationale for mass antibiotic distribution for trachoma elimination. International Ophthalmology Clinics, 42(1) : 85-92.

Holm S.O., Jha H.C., Bhatta R.C., Chaudhary J.S.P., Thapa B.B., Davis D., Pokhrel R.P., Yinghui M., Zegans M., Schachter J., Frick K.D., Tapert L., Lietman T.M., 2001 – Comparison of two azithromycin distribution strategies for controlling trachoma in Nepal. Bulletin of the World Health Organization, 79(3) : 194-200.

Peeling R.W., Toye B., Jessamine P., Gemmill I., 1998 – Pooling of urine specimens for PCR testing: a cost saving strategy for chlamydia trachomatis control programmes. Sexually Transmitted Infections, 74(1) : 66-70.

Schachter J., West S.K., Mabey D., Dawson C.R., Bobo L., Bailey R., Vitale S., Quinn T.C., Sheta A., Sallam S., Mkocha H., Mabey D., Faal H., 1999 – Azithromycin in control of trachoma. Lancet, 354(9179) : 630-635.

Taylor H.R., West S.K., Katala S., Foster A., 1987 – Trachoma : evaluation of a new grading scheme in the United Republic of Tanzania. Bulletin of the World Health Organization, 65(4): 485-488.

West S., Muñoz B., Lynch M., Kayongoya A., Chilangwa Z., Mmbaga B.B., Taylor H.R., 1995 – Impact of face washing on trachoma in Kongwa, Tanzania. Lancet, 345(8943): 155-158.

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