Studies of Leptospirosis in Natural Host Populations I. Small Mammals of Waipio Valley, Island of Hawaii

Abstract

The small Indian mongoose, Herpestes auropunctalus (Carnivora: Viverridae), and the roof rat, Rattus rattus, and the Polynesian rat, Rattus exulans (both Rodentia: Muridae), are abundant in Waipio Valley, island of Hawaii. Two other murid rodents, the house· mouse, Mus musculus, and the Norway rat, Rattus norvegicus, are sporadic or rare in occurrence. As carriers of serotypes of the bacterium Leptospira interrogans (Spirochaetales: Treponemataceae), which is transmissible to humans, this assemblage of introduced mammals is of public health significance, for numerous cases of leptospirosis, or Weil's disease, have been traced to the valley. Population density of the mongoose was estimated at 2.3 per acre; for rats, it fluctuated seasonally from I to 11 per acre. The serotypes icterohemorrhagiae and sejroe were found in the mongoose in a 40:60 ratio by the kidney culture method. Combined kidney culture and serological tests on 180 mongooses showed a high of 34 percent overall infection in winter and a summer low of 9.4 percent. Of 33 house mice tested by culture only, ballum was isolated from 21 and icterohemorrhagiae from two. One isolation of icterohemorrhagiae was made from four Norway rats examined. For 126 roof rats tested by serology and kidney culture, 68 percent of adults and 26 percent of young were infected; and for 175 Polynesian rats, 34 percent of adults and 26 percent of young were infected. The Polynesian rat demonstrated a lesser persistence of the serum titer phase of the disease than did the roof rat. Icterohemorrhagiae made up 95 percent and ballum the remaining 5 percent of infections in the roof rat. For the Polynesian rat the ratio was 75: 25. Free-ranging rats under observation for as long as 8 months acquired or lost infections, as determined by repeated serological tests. The wet subtropical climate of Waipio Valley supports conditions for transmission of leptospirosis among small mammals, and possibly to humans, even in times of drought. No prominent differences were observed in the infection rates in the lower valley at 30 ft above sea level and 1.7 miles inland at 120 ft. In the forested watershed of the valley rim at 3000 ft, conditions of infection by species of host and by serotype of L. interrogans matched elosely those found on the valley floor. Tests of 152 water samples from streams, ponds, and taro patches resulted in isolations only of saprophytic leptospires, although temperatures, salinities, and pH concentrations appeared to be favorable for the support of pathogenic forms.