In previous papers it has been demonstrated that ground-dwelling rats are the principal reservoir of leptospirosis in Malaya. The present paper considers the distribution of infection by sex and weight in the ten principal rat species. There appears to be a general tendency for females to be infected more frequently than males, but significant differences were demonstrated only in R. sabanus (more than three times as many females as males infected) and R. whiteheadi. In Malaya, where seasonal changes are minimal, weights can be used as a good index of age in rats. In rat species with a low incidence of infection the incidence appeared to rise steadily with age. In species with a medium incidence the infection rate rose at first with age, fell in the 6-8-month age-group, and then rose again. In high-incidence species the rate rose rapidly from the second month.There appear to be three types of enzootic infection; (1) intensive transmission of a single serogroup in a crowded population of rats of a single species (transmission probably being through urinary contamination of damp soil); (2) low-intensity transmission of several serogroups among ground-rats frequenting wet places (probably with urinary transmission); and (3) low-intensity transmission of several serogroups among ground-rats in dry places (the transmission may be venereal).
In recent years leptospirosis has been shown to be an important cause of human febrile illness in Malaya. Studies were therefore undertaken to determine its animal reservoirs and the factors influencing spread of infection from them to man and domestic animals. This paper presents the board picture obtained. A wide range of animal species were trapped in forest localities, ricefield areas, areas of scrub and cultivation and in several towns and villages. The maintenance hosts of leptospirosis in Malaya appear to be mainly or entirely rats, although evidence of infection has been found throughout the animal kingdom. Some rat species have characteristics which suggest that they are better maintenance hosts than others. Evidence was found of practically every serogroup of leptospires infecting animals in Malaya. Altogether 104 strains were isolated and identified, and 155 animals were found to have serological evidence of infection. Of 1763 rodents examined, 194 had evidence of infection, and 41 of 1083 other animals. A serum survey of domestic animals showed the highest incidence of antibodies to be in goats and the lowest in oxen.
One of the factors on which the incidence of leptospirosis is dependent is the survival time of shed leptospires in surface water or soil water, and this time is in turn affected by the acidity or alkalinity of the water. The authors have therefore studied the survival of four leptospiral serotypes in buffered distilled water at pH's ranging from 5.3 to 8.0. All survived longer in alkaline than in acid water, and significant differences between the serotypes were found in response to pH. Survival at pH's under 7.0 ranged from 10 to 117 days and at pH's over 7.0 from 21 to 152 days. Survival was also studied in aqueous extracts of soil samples from different areas in Malaya; no correlation was found between pH and survival time.It was also noted that in a group of Malayan ricefields a low incidence of leptospirosis in man was accompanied by a high infection rate among rodents, and when it was found that this phenomenon could not be explained by pH or salinity, attention was turned to the soil. Bentonite clay, similar to the montmorrillonite clay of the ricefields, was found to adsorb about half the leptospires in suspension. The authors recommend that field study of this laboratory observation be undertaken.
In a previous paper the authors reported on the methods and zoogeographical background of a survey of animal leptospirosis in Malaya, giving a broad analysis of results. In the present paper the localities studied in towns and villages, in ricefields, in secondary forest and scrub and in primary forest are compared in detail. In towns and villages infection rates in rats were low, except in a seaport town where the invading R. norvegicus was heavily infected. In ricefields infection is maintained in R. argentiventer, alone or in association with R. exulans. In secondary forest and scrub there is overlap with forest species and the main hosts of leptospires appear to be R. exulans and R. jalorensis. In primary forest giant rats and, to a lesser degree, spiny rats are the main hosts.Ground-living rats appear to be better maintenance hosts than those scrambling on vegetation or arboreal rats. With some exceptions the incidence of infection of a rat species in an area was found to be in direct relation to the proportion that species formed of the total rat population. The critical number of rats for maintenance of leptospirosis in an area is estimated to be about two rats of the maintenance species per hectare.