|Year : 1964 | Volume
| Issue : 2 | Page : 50-58
Trachoma - a bio-meteorological approach
|Date of Web Publication||14-Feb-2008|
S N Cooper
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Cooper S N. Trachoma - a bio-meteorological approach. Indian J Ophthalmol 1964;12:50-8
In a previous study by Ursekar (1955) in our department at King Edward Memorial Hospital,- Parel, attention was first drawn by him to the unequal distribution of trachoma (Map 1]-[Figure - 1] and [Map 2)-[Figure - 2]. We arrived at our figures by a statistical analysis in which the incidence of trachoma per cent of eye diseases was ascertained from a questionnaire to heads of ophthalmic hospitals and through the Director General of Health Services. In a consideration of the geological and meteorological factors to explain such an uneven distribution we had implicated the southwest premonsoon and monsoon winds to explain this peculiar endemicity of trachoma.
Since then the Trachoma Pilot Project (Indian Council of Medical Research) has carried out an exhaustive survey of the incidence of trachoma in the villages in the 15 states all over the country Gupta and Preobraganski (1964). Strangely the map of incidence of trachoma of this Project, except in the case of Mysore, is very similar, almost within 10%, to our findings, (cf. map on p. 40). This establishes our first observation that the incidence of trachoma has a peculiar pattern distribution, being highest in Rajasthan and Punjab from which the incidence drops towards the East and South, the least-affected areas being Bengal, in the East, and the Southern states. When the drop is from 80% to 1% or less it needs no statistician to prove that the drop is significant.
The Trachoma Pilot Project has made other observations of great significance. In Utter Pradesh where these observations were made in the district of Alighar-Gupta and Preobraganski, (1964), it was found that ocular bacterial infections occur in the form of two peaks, March-May and August-October which correspond with the premonsoon and post monsoon periods. It was also noted that during these peak periods comparatively more new cases of trachoma were discovered and mild cases passed on to a florid form.
A third observation, which requires consideration, is that the above periods (March-May and August-October) also correspond with the peak periods of the fly index, which, according to Gupta and Preobraganski may be responsible for the peak periods of conjunctivitis and exacerbations of trachoma in Uttar Pradesh.
Such observations have made us reconsider the factors prevailing during monsoon to answer the following two posers.
1) Why is the incidence of trachoma so peculiarly distributed in India?
2) How can one explain the seasonal peaks of bacterial infections and new trachoma cases?
The bio-meteorological factors which prevail during the Indian monsoon are (a) flies and insects, (b) rainfall, (c) moisture, (d) prevailing winds, (e) storms.
(a) Flies and insects: The part played by flies, although a tempting consideration in view of the coinciding peaks of the fly-index and secondary bacterial infections, does not merit special attention from the fact that flies in Bengal are also plentiful during the monsoon but the incidence of trachoma is lowest here.
It is also known that during the monsoon, not only flies, but a variety of beetles and insects invade the country. During this season, swarms of the Indian locust (Schistocerca Gregaria) which are to be found in areas where desert edges are bordered by grassland, migrate towards Rajasthan and the Punjab. This is the so-called desert locust, which inhabits North and East Africa, Arabia, Israel, Syria, Iraq, Jordan, Persia and India.
Therefore, out of all the varieties of insects, many of which could be vectors for viruses and viral infections, as far as Indian trachoma is concerned, the one that deserves more attention than others is the locust. Flies and beetles are all over India, but the habitat of the Indian locust is where grassland areas adjoin deserts, that is adjoining the deserts of Cutch and Thar. Such areas are breeding grounds for these locusts, particularly grounds the months be fore the monsoon. It will be thus appreciated that the locust-infested areas of India are also trachomatous areas of high endemicity.
(b) The rainfall map although approaching nearest in similarity inversely to that of the incidence of trachoma, rainfall by itself cannot be considered as a likely cause for this uneven distribution. A glance at map 3-[Figure - 3] can show convincingly how Rajasthan and the southern plateau of Deccan, although having similar rainfalls compare so differently in the incidence of trachoma. (Map 2)-[Figure - 2].
(c) Moisture: Similarly [Table - 1] supplied by the meterological section, Poona observatory clearly shows that atmospherical humidity does not seem to play any convincing part in the unequal distribution of trachoma. It may he noted however, that in the high endemic regions (Chandigarh, New Delhi, Bikaner, Jodhpur, Ajmer) the humidity is lowest in the months of April, May, June when it is also the peak period for bacterial infections in Uttar Pradesh, whereas it is relatively high during these months in the low incidence areas, Bengal, Bombay, Madras.
(d) Winds: For our consideration here, we must confine ourselves to the surface winds upto a height of 0.5 Km (1.600 feet). Referring to the Climatological Atlas for Airmen we find that during the months of May to September two winds prevail over India: (1) the Arabian Winds and (2) the Maritime Winds.
In the previous paper by Ursekar (1955) we had implicated the South-West monsoon winds as responsible for the trachoma infection. We had come to this conclusion by studying the pattern of the winds prevailing only over India as taken from the Oxford Atlas. We explained the unequal distribution by considering that the mountain ranges of the Vindhyas and the Aravali Hills divert these winds to Rajasthan and Punjab. There was however a great weakness in this explanation, as it was not possible to explain by the same implication why the South of India and Bengal which receive the same winds during the same months are so immune.
This concept can now be clarified by studying the prevailing winds over India in conjunction with those over the Middle East from the Airmens Atlas, which we present in a simplified form in Maps 4-[Figure - 4] and Map 5-[Figure - 5]. By referring to map 4-[Figure - 4] it can be seen that during the months from May to September there are two winds that prevail over India, (a) the Arabian winds and (b) the Maritime winds.
(a) The Arabian Winds: They take their origin as the Shamal winds from the cool northern end of Zagros mountains bordering Iran on the West. They blow down south-eastwards from Iraq along the Persian gulf, the right edge of which sweeps across the Syrian and Arabian deserts. Once over the Arabian Sea they blow eastwards towards India. Reaching the west coast of India, these winds are obstructed by the mountain ranges of the Western Ghats and the Vindhyas and are directed northwards where they enter India at the Cutch peninsula.
In May, June and July the center for the lowest isobar of atmospheric pressure is located just above the Thar desert, (L in Map 4)-[Figure - 4] and consequently the Arabian winds are also drawn towards this depression during these months.
In this change of direction northwards they are also helped by the Maritime Winds blowing from the south-west (see below). From the map it will be easy to trace the further course of these winds. The Aravali Hills prevent them from spreading eastwards and so they sweep across the deserts of Cutch and Thar, over Rajasthan and Punjab. These winds are heavily laden with fine dust, evidence of which can be seen in the brown fog which hangs around in this region before the onset of monsoon. Coming vis a vis the Himalayan ranges the course is diverted east-south-east along Uttar Pradesh towards Bengal. Here they meet the Maritime Winds from the Bay of Bengal. (see below)
It is to be noted that these Arabian winds sweep across four deserts-Syrian, Arabian, Cutch and Thar.
It is also to be noted that the Maritime Winds push the Arabian Winds northwards, but if the former are delayed, the Arabian Winds can reach further South where their influence can be felt when the monsoon is delayed.
(b) The Maritime Winds blow from the South-West. They reach the West coast of India further South than the Arabian Winds. They are obstructed by the Western Ghats, but the winds further South reach the Bay of Bengal and proceed towards Burma. Here again, they are diverted northwards by the high Arakan mountains in Burma, and enter the Eastern part of India through Bengal. A second deviation is caused by the Himalayan range, and now the Maritime Winds blow westwards to meet the Arabian Winds, where they neutralise each other.
We shall presently see how these winds can be responsible for the unequal distribution of trachoma.
The North-East Trades : In the months of September to February, the winds take a definite change, the month of August being a transitional one where the south-westerly Maritime winds and the Arabian Winds change to the North-Easterly Trade winds originating from the Himalayas (Map 5)-[Figure - 5]. It will be seen from this map, that with the cooler season setting in the northern hemisphere and summer in the southern, winds originate from the cold Himalayan ranges and blow towards the hotter potentials in the South. The winds thus become now, north-easterly over the whole of India, in a direction reverse to the summer and monsoon winds. These are clean winds which sort of sweep the country of aerial infection, with a consequent drop of ocular infections from November onwards till March and April, which again are transitional months making way for the commencement of the summer winds, to complete the cycle.
(5) Storms : Map 6-[Figure - 6] depicting storms clearly indicate the prevalence of dust storms in regions of high trachoma endemicity and thunder-storms in regions of low endemicity.
| Discussion|| |
It is a well-known fact about India that viral and bacterial infections are much more common in pre-monsoon and early monsoon months. Herpes, Beale's conjunctivitis, superficial kerato-conjunctivities are all too common in the months of April to July in Bombay and the West-coast and so is trachoma in the north-west sector of India.
From the above study it can be seen that the north-west of India starting from Cutch, going upwards through the deserts of Cutch and Thar, onto Rajasthan and Punjab is subjected to four biometeorological factors, (1) the Arabian winds, (2) the dust-storms, (3) the locust population and scourge, (4) trachoma.
The origin of the trachoma virus is unknown, but the fact that trachoma blazes along the trail of the Arabian winds suggests that the peculiar entry of these Arabian winds through the portal at Cutch is an extremely tempting theory to investigate as regards the influence of these winds on the apportment of new trachoma infection into India.
Similarly, the theory about the locust as a vector for the trachoma virus, again seems a very tempting one to investigate, regarding the part played by these insects in Indian Trachoma.
These Arabian winds kicking up a lot of dust in their passage across the deserts and over locust-infested areas, are perhaps the ill winds that bring trachoma to the North-West. On the other hand, the Maritime winds are devoid of this infection and counter the effect of the Arabian Winds as they meet somewhere in east Uttar Pradesh. The extra immunity of Bengal against this infection (it is the lowest endemic area in India) is perhaps due to the fact that it is favoured by thunderstorms which are protective against the trachoma-bearing Arabian winds. The dust-storms in Rajasthan and Punjab are contributory to the trachoma infection.
It is also to be noted that the Arabian winds which blow over other parts of the Middle-East and Arabia, also sweep over deserts, the edges of which are also locust-infested and these lands are notoriously trachoma afflicted.
These considerations alone, seem to explain the peculiar distribution of Trachoma in India, highest in the north-west, lowest in the South and East.
| Summary|| |
The peculiarly irregular distribution of trachoma in India, highest incidence in the north-west (80%) lowest in the South (5 %) and - Bengal (0.5%) is sought to be explained by bio-meteorological considerations. (1) The Arabian winds coming in from the deserts of Syria, Arabia, Cutch and Thar, proceed towards the areas of highest endemicity in India. (2) The locust population is prevalent in the same areas in the same months (3) Trachoma incidence is highest along with secondary infections of the conjunctiva in the same months and same regions. These facts, since they coincide require a more careful study of the influence of the first two over the third.
The maritime winds with their thunderstorms in Bengal seem to have a protecting influence on trachoma infection in India.
| Acknowledgements|| |
We are most thankful to Dr. Pisharoti and Dr. Nedungadi of the Meteorological Department, Poona and Bombay for supplying the various meteorological data and for permission to use the material from the Climatological Atlas for Airmen.
We have to thank Dr. F. Fernandes, microbiologist Ciba Research Laboratories, Goregaon, for his information on the locust population in India.
| References|| |
Gupta U. C. and Preobraganski V. V. (1964) J. All-India Ophthal. Soc. 12, 1.
Ursekar (1955) J. All-India Ophthal. Soc. 2, 94.
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6]
[Table - 1]