Some Possible Causative Factors.
Lack of Sunshine.
A possible physical factor is the prevalence of fogs, with the consequent exclusion of sunshine, affecting alike the riverside towns of Huntly and Kaitangata, and the high altitude townships of Denniston and Millerton. Through the courtesy of the Government Meteorological office, a table showing the hours of sunshine per annum for places throughout New Zealand was made available. While observations had not been made actually at these coal mining centres, statistics were available for four of the seven localities with the lowest mean caries indices, and for three of the seven with the highest. These figures showed the average hours of sunshine per annum over a period of five years, and the averages for the two sets of localities (low and high caries index) disclosed a difference of only 92.7 hours per annum, or about 15 minutes per day. This could scarcely be regarded as significant in a country with a temperate climate, where normally only the face and hands are exposed to the sun. Observations made at the actual coal mining centres would, however, be of interest.
Much has been written, particularly in recent years, in regrad to the influence of fluorine in the control of dental caries. It has been suggested that one part in a million in drinking water will confer increased immunity to caries. The limited number of analyses that are available show that the fluorine content of New Zealand waters is much lower than the optimum amount suggested, and this appears at first sight to support the hypothesis that lack of fluorine may be a factor in our high incidence of dental caries. We cannot overlook the fact, however, that the pre-European Maori had excellent teeth, and, according to Pickerill (1), was more immune to caries than any other race for which statistics are available. The Maoris drank the water from New Zealand streams and rivers, and gained their living off the soil, and there is no evidence to show that the fluorine content of New Zealand waters to-day, or the mineral content of the soil, is substantially different from what it was in pre-European days. Thus, while the low fluorine content of New Zealand waters may be a contributing factor, it is clearly not the only factor to account for the high incidence and degree of dental caries in this country.
The medical reports in this survey showed that an abnormally high proportion of the children at one of the coal mining centres exhibited gross manifestations of rickets. Now, M. Mellanby (2) and others have claimed that an association exists between the incidence of rickets and that of dental caries. Other workers refute this hypothesis, and say that there is no evidence of any correlation between the two conditions. Here, then, was an opportunity to test the conflicting theories under local conditions. Statistics were analysed in the case of 164 children who showed signs of rickets, and the result was that no correlation could be demonstrated between the incidence of rickets and that of dental caries, as far as the children who formed the subject of this study were concerned. More than half of the children with stigmata of rickets had teeth better than the average, and in several cases where the rachitic manifestations were described as slight, the caries indices were high. Similar results have been obtained in other countries, notably by Taylor and Day (3) in India.
The very nature of the coal mining industry tends to produce an environment in which many of the ordinary amenities of life may be absent, and which may tend to have an adverse affect upon the mode of life of those who have to live in coal mining areas.
Vernon (4) is of the opinion that the three factors of heredity, social environment and occupational environment are closely interlocked—heredity in that an individual who inherits a high standard of intelligence is better fitted to cope with risks to health; social environment (in which he includes nutrition) in that good home environment and wise selection of food play a vital part in the maintenance of health; occupational environment in that this influences to a considerable extent the mental outlook of the individual and determines his economic status. Thus the three factors are mutually dependent upon one another.
Is it possible, then, that these factors can have any influence on the incidence and degree of dental caries?
The data included the occupation of the fathers of most of the children examined, and on studying the list of occupations it was evident that they could conveniently be grouped under five headings—
(1) Manual occupations.
(3) Clerical work.
The occupations of the fathers were classified accordingly in 1,285 cases, and the mean caries index for each occupational category was calculated. This analysis disclosed a significant difference in the dental condition of the children in two categories in particular, namely, manual workers and professions. The children in the manual worker group had the highest mean caries index
(10.8), and those in the professional group the lowest (6.3, or excluding one isolated case, 4.6). Similar observations have been made by Wilkins (5) in England and by Greenwald (6) in the United States. Considered in relation to Vernon's hypothesis quoted above, the reason is perhaps to be found not in any one factor but in a combination of factors connected with environment.
Infant Feeding and Child Care.
In view of the almost universal adoption of the Plunket (or Truby King) system in New Zealand [78 per cent. of the babies born in New Zealand in 1942–43 came under the supervision of Plunket nurses (7)], the question of infant feeding and child care in relation to dental caries was also studied, but the number of children for whom we had the necessary data was somewhat limited—390 in all. The duration of breast-feeding and of Plunket supervision respectively were ascertained from the mothers in the course of the medical examination of the children, and the statistics were analysed. Of the 390 children seven had perfect teeth (cf. 31 with no caries in the whole group of 1,475 children), but it was observed that these had had the longest average period of Plunket supervision, and when the results were plotted in graph form, it was noted that the duration of Plunket supervision tended to fall as the caries index rose. The
Fig. 2.—Breast feeding and Plunket Supervision in relation to caries-index groups in 390 cases in age-group 5–6 years.
breast-feeding graph, on the other hand, although it showed minor variations, disclosed a considerable degree of uniformity
in regard to the duration of breast-feeding, irrespective of variations in the caries index, (see Fig. 2). This, however, should not be interpreted as indicating that breast-feeding does not confer increased immunity to dental caries (and the Plunket system stresses the prime importance of breast-feeding). In the age-group in question, ante-natal conditions will have had more influence than breast-feeding on the structure of the teeth. Nevertheless,' lack of breast-feeding may be a contributing factor in the onset of caries in later childhood through its influence on the structure of the permanent teeth, as well as in the resulting tendency to under-development of the jaws, with consequent irregularity of the teeth. The explanation of these observations would appear to be that the Plunket system has as its central feature a carefully arranged nutritional programme which must be followed with care, as it has to be modified from time to time according to age and progress. This programme, with all its associated features concerning the general welfare of the child, demands close attention, intelligence, self-discipline and patience on the part of the mother. Taking these facts into consideration, the evidence can be said to suggest that the application of sound dietary principles, involving the intelligent selection of foodstuffs, and carried out in accordance with a carefully prepared plan, is associated with a reduction in the incidence and degree of dental caries.
Relative Susceptibility of the Sexes.
A study of the dental condition of 747 males and 728 females in the age-group 5–6 years disclosed no significant difference in the average caries index as between males and females (10.3 for males and 9.9 for females).
The average number of teeth affected per head proved to be the same (10 8) for both males and females.
It can be assumed, therefore, that this is not a factor that has any bearing on the regional variations that have been demonstrated.