Rita Martinez
Humbio 153
3/9/12
Trachoma: Epidemiology, Vector Biology, and Environmental Risk Factors
Learning Objectives:
-Understand the basic biology of trachoma
-Know the worldwide distribution of trachoma and its contribution to the global disease burden
-Understand the different modes of transmission and relevant vector ecology
-Understand the major environmental risk factors and identify risk populations
-Understand the main prevention and intervention strategies used to control trachoma transmission
-Understand how the context of a humanitarian crisis exacerbates the transmission of trachoma and long-term societal effects
Background
Chlamydia trachomatisis the bacterial pathogen that leads to the development of trachoma, which is characterized by three main phases. The incubation period is about 5-12 days, and the initial stage of infection (conjunctivitis)is characterized by inflammation, itchiness, and ocular and nasal secretions that resemble “pink eye”(Burton, 2009). Later stages of trachoma result in the scarring of the inner eyelid due to repeated untreated infections over time. After a period of about 10-20 years of active trachoma, the eyelashes begin to turn inwards and start scratching the cornea, which is called trichiasis (WHO factsheet). This stage of trachoma requires surgery; otherwise the continuous scratching of the cornea will lead to permanent blindness.
While economic development and improved hygiene have been effective at eliminating the disease in North America and Europe, it continues to afflict regions with hot, dry climates, where access to clean water, sanitation, and health care is compromised (Hotez, 2008). Poverty and sanitary conditions are strongly linked to the presence of trachoma as they foster the conditions necessary for the transmission of the bacteria. Children are the most significant reservoirs for the bacteria, which are transmitted both directly through contact with ocular and nasal secretions as well as indirectly through flies as mechanical vectors.
In the early 1900’s, the influx of new immigrants in the U.S. and the consequent overcrowding of towns led to a surge in trachoma cases. An initiative was enacted to screen all new immigrants-- this focus on diagnosis and treatment through antibiotics and surgery, alongside improved standards of living under the Wilson administration enabled the eradication of trachoma in the U.S. and is reflective of similar efforts in Europe (Hotez, 2008).
Epidemiology
Trachoma represents the leading infectious cause of blindness in the world, with an estimated 84 million people infected in 55 countries (Hotez et al, 2007).There are 259 million people worldwide suffering from visual impairment, and trachoma accounts for about 15% of these cases (Hotez, 2008). Trachoma is most predominant in Sub-Saharan Africa, the Middle East, and Northern Africa, and is also present in South America, Southeast Asia, India, and aboriginal communities in Australia (Hotez et al, 2007). Additionally, trachoma represents the sixth most prevalent neglected tropical disease, with 84 million infected and a total of 590 million people at risk (Hotez et al, 2007). One factor that accounts for the neglect of this disease is a result of the commonness of the condition in places made inaccessible due to conflict.
By and large, trachoma is found in regions afflicted by poverty and poor sanitation. Locally, trachoma tends to be observed in resource poor settings coupled with very dry climates, which further limits sanitation efforts due to a limited access to clean water and lack of latrines. In highly endemic areas, research indicates that the prevalence among young children (reservoirs) can be as high as 60-90%, with women being three times as likely to be infected than men due to their closer interaction with children (Hotez, 2008). This can have resounding effects for women’s lives, since when mothers are no longer able to care for the household the responsibility is usually placed on the daughter, who would have to abandon educational opportunities. It is estimated that an annual $5.3billion are lost due to the global trachoma disease burden (Hotez, 2008).
Morocco represents a contemporary success story of an effective trachoma control efforts. Their government launched the National Blindness Control initiative in 1991 and implemented the SAFE strategy (Surgery, Antibiotics, Face washing, and Environmental improvement) from 1997-1999 to effectively reduce the presence of active trachoma in children under ten by 99 percent (Center for Global Development, 2012). The strong political commitment from the government as well as public-private partnerships enabled Morocco to reach WHO standards of less than 5% prevalence rate among children under ten in a rapid time. The combination of all four strategies proved effective in this country in order to significantly reduce infection rates, and they are currently in a stage of surveillance to keep these numbers down.
The effects of trachoma extend beyond health and the resulting visual impairment, but also translate into sustained social equality and a cycle of poverty. Trachoma not only affects the ability for infected persons to work and lead healthy lives, but particularly impacts women and children which leads to broader detrimental societal effects. Because women are most likely to be in contact with children, who are the most significant reservoir of trachoma, they are three times more likely than men to become infected. While trachoma does not have major immediate health impacts, over time, the societal effects of the disproportionate number of women that are blinded by trachoma should not be understated. What is more, the resulting disability incurred by the infection hinders their ability to pursue both economic and educational opportunities, and thus perpetuating a cycle of poverty.
Vector Biology
The most significant vector for the mechanical transmission of trachoma is the bazaar fly, muscasorbens. In this case, Chlamydia trachomatis is transmitted by flies who feed off of the ocular and nasal secretions of infected persons and land on other people’s faces, effectively spreading the infection (Medical Ecology, 2004). Because the fly is not required for the life cycle of the bacteria, it is only a mechanical and not biological vector of trachoma.
The house fly, muscadomestica can also transmit trachoma as it can harbor the bacteria in its alimentary tract for up to six hours, but is generally a less effective vector than muscasorbens. A third species, muscavetustissima is a significant vector, but only in Australia (Emerson et al, 2001). While all three species exhibit eye-seeking behavior that enable them to transmit infectious bacteria from person to person, muscasorbens the most well-adapted vector, as it is the most commonly observed species on the faces of children in endemic areas.
The life cycle of both muscasorbens and domestica are very similar, and can be summarized as follows:
-the laying of the egg and its maturation (one day)
-three larval feeding stages (ranges from 4-10 days)
-pupa stage (ranges from 3-6 days)
-adult fly emerges (lifespan ranges from 15-30 days)
Poor sanitation and crowding provide great breeding grounds for muscasorbens and muscadomestica. While animal feces provides viable breeding ground for the larval stages of growth, some studies have indicated that isolated human feces results in more numerous flies emerging (Emerson et al, 2001). This further highlights the importance of latrines in terms of vector population control, as the liquefied excrement forms do not allow for viable vector breeding ground for these flies.
Another important ecological consideration in addition to the presence of latrines is the presence of livestock. Human populations living in close proximity to livestock are vulnerable to trachoma infection due to the cultivation of the vector population in the case of inadequate waste disposal.
In order to better understand the importance of muscasorbens as a vector, Emerson et al conducted a study that used insecticide to decrease the vector population resulted in a 75% decrease in new infection rates for that year (Emerson et al, 1999).
Environmental Risk Factors
The major environmental risk factors for trachoma can be summarized by the six D’s and five F’s: Dirt, Dust, Dung, Dry, Discharge, Density and Flies, Feces, Faces, Fingers, and Fomites. The majority of these risk factors center on the issue of sanitation and access to clean water, and secondly, the presence of flies to serve as mechanical vectors.
Since trachoma mainly plagued very poor regions of the world, especially those with dry climates, it is important to conceptualize its transmission as focused on how these realities limit broader sanitation efforts and consequently compromise personal hygiene. A commonly used phrase to depict a major risk factor for trachoma is the “dirty face” example. Since children are the biggest reservoirs for the bacteria because of their limited face and hand washing, they can spread the infection to many others through direct contact with their ocular and nasal secretions. In areas where it is particularly dry, it is not surprisingly to see children with “dirty faces,” since there is a limited ability to access clean water for face and hand washing (Hotez, 2008). This means that infected children do not adequately wash away the ocular and nasal secretions or discharge that is known to contain the infectious bacteria that leads to trachoma. Direct contact with this discharge will result in trachoma infection, but it is important to note that sneezing and coughing from an infected person can transmit the infection to others. Subsequently, “dirty faces” coupled with lack of access to clean water and lack of latrines facilitates both direct and mechanical transmission of trachoma.
Regions with limited access to clean water may also not have many latrines to adequately dispose of human waste, and this in turn fosters an effective breeding ground for the fly population. The close proximity with human populations and their eye-seeking behavior make flies excellent mechanical vectors of trachoma. The aridity of the region contributes to the flies seeking out moisture in children’s faces, and so access to water could divert some attention towards these water sources as opposed to faces. Additionally, the presence of livestock and overcrowding, as is experienced in refugee camps also fosters a high fly population. In refugee camps specifically, sanitation efforts can be extremely compromised, and the high number of people confined in such proximity makes them especially vulnerable to infection due to their increased contact with infected persons, limited access to clean water for hygienic purposes, and access to latrines, which in turn contribute to a larger fly population that can further facilitate trachoma transmission.
Trachoma is easily spread within households and among school age children who inadvertently touch their faces and later contaminate other people or objects that can then spread the bacteria indirectly. Fomitesare inanimate objects, like dirty face cloths, pillowcases, etc. that can become contaminated with bacteria and are another important mode of indirect trachoma transmission (Hotez, 2008).
Children under ten are at heightened risk for trachoma due to their relatively poorer hygiene but also due to their increased interactionswith children who are infected. Women also represent a significant risk group as they are also the most common caretakers and will usually become infected while caring for other children with active trachoma (Ngondi et al, 2009).
Current Challenges
Trachoma is completely preventable and treatable. Addressing trachoma is a feasible, and the main limitations with control strategies center around issues of access to essential medicines like azythromicin and feasibility of public health interventions (Hotez et al, 2007). This situation is further exacerbated within the context of a humanitarian crisis where resources are extremely limited and reserved for life-threatening illnesses or other immediate needs. Additionally, many highly endemic areas remain plagued with trachoma because they have been made inaccessible due to conflict or political instability.
A concerted, cross-sector effort is necessary to effectively tackle endemic areas worldwide, and a consensus is missing on what should be prioritized and in what situations. Some central questions to be addressed in terms of priorities include the following:
-Vector control versus mass drug administration of azythromicin
-Targeting specific risk populations versus broader community
-Diagnosis versus treatment
-Mortality versus morbidity
-Latrines/vector control versus access to clean water/sanitation efforts
Specifically, within the context of a humanitarian crisis, the biggest question becomes how to efficiently allocate very limited resources.
-Diagnosis versus treatment
-Mortality versus morbidity
-Endemic areas made inaccessible through conflict
-Lack of political will or political instability
The answer usually lies in how one can save the most lives or alleviate the most suffering. Under this framework, the reasoning behind treating trachoma in this context may seem less compelling but must not be understated. It should be stressed that under these conditions where sanitation is severely compromised and overcrowding is a problem, the possibilities for trachoma outbreaks are very likely, even though the immediate effects might not be life threatening.
In these cases, diagnosis through visual inspection of the inner eyelid may not be the priority, especially if at least 10% of the children under ten appear to be infected with active trachoma. It would be more efficient to advocate for the inclusion of azythromicin into the regular cocktail of drugs that are usually provided through relief organizations to these refugee camps (Gebre et al, 2011). In the case of limited resources, it would also be more efficient to target the drug administration to children under ten and women, as this can result in an overall decrease in new infections by reducing the size of the trachoma reservoir.
Where endemic areas are made inaccessible due to conflict, it is incumbent upon international organizations to endeavor to reach these communities either through their governments or by supporting their health-based grassroots efforts. Providing this support through drugs and other forms of advocacy could be beneficial when governments are unresponsive to community needs due to political or economical instability.
While the current strategy to address trachoma accurately identifies the best strategies to control transmission, this SAFE strategy needs to be customized using country-specific considerations in order to be most effective. For example, if one country has very limited resources, and an NGO only has funding to administer azythromicin to a portion of the population, it is relevant for them to attempt to identify the biggest risk populations and treat them in hopes that this will reduce the overall prevalence rate by virtue of them tackling the most significant reservoirs. Research highlights the effectiveness of targeting children under ten in order to significantly reduce infection rates in a severely endemic area (Gebre et al, 2011).
References (in order of appearance)
1. Burton MJ, Mabey DCW (2009) The Global Burden of Trachoma: A Review. PLoSNeglTropDis 3(10): e460. doi:10.1371/journal.pntd.0000460
2. WHO. Prevention of Blindness and Visual Impairment 2012. Accessed March 2, 2012.
3. Hotez, Peter J. Forgotten People, Forgotten Diseases: the Neglected Tropical Disease and their Impact on Global Health and Development. Washington, D.C: ASM Press; 2008
4. Hotez, Peter J, et al. Control of Neglected Tropical Diseases. New England Journal of Medicine 2007;357:1018-22007
5. Center for Global Development. Case 10: Controlling Trachoma in Morocco 2011. Accessed February 28, 2012.
6. Emerson, P. M., Bailey, R. L., Walraven, G. E. L., et al (2001), Human and other faeces as breeding media of the trachoma vectorMuscasorbens. Medical and Veterinary Entomology, 15:314–320.doi:10.1046/j.0269-283x.2001.00318.x
7. Emerson, P.M.,Lindsay, S.W., et al (1999)Effect of fly control on trachoma and diarrhoea.Lancet,353,1401–1403.
8. Jeremiah Ngondi, Mark H. Reacher, et al. Risk factors for trachomatoustrichiasis in children: cross-sectional household surveys in Southern Sudan, Transactions of the Royal Society of Tropical Medicine and Hygiene, Volume 103, Issue 3, March 2009, Pages 305-314, ISSN 0035-9203, 10.1016/j.trstmh.2008.08.024. (
9. TeshomeGebre, BerhanAyele, et al (2011), Comparison of annual versus twice annual mass azythromicin treatment for hypoendemic trachoma in Ethiopia. The Lancet, DOI:10.1016/S0140-6736(11)61515-8
Sources:
10. Medical Ecology: Trachoma 2004. Accessed on February 16, 2012.
11. Bio Ventures for Global Health. Trachoma Health Primer, 2011. Accessed February 27, 2012.
*Interview with Expert: Jeremy Keenan, Assistant Professor of Ophthalmology at UCSF. (Referred to article, reference number 9)
1