HOST SPECIFICITY, PATHOGENICITY, AND POTENTIAL USE AS A BIOCONTROL AGENT OF WILD RATS

Host specificity and pathogenicity of Sarcocystis singaporensis were investigated as a prerequisite to a subsequent application of the parasite as a biocontrol agent of wild rats in Egypt. After inoculation of 7 snake species comprising the families Elapidae, Viperidae, Colubridae, and Boidae with sarcocysts, sporocyst development was only observed in a reticulated python. Among amphibians, reptiles, and rodents that orally received various sporocyst doses in the laboratory, 2 x 104 sporocysts or more were lethal to roof rats Rattus rattusfrugivorous, brown rats Rattus norvegicus, and bandicoot rats Nesokia indica. Sarcocysts developed in Rattus spp. and Nile grass rats Arvicanthis niloticus. Subsequently, the pathogenicity of S. singaporensis was tested under natural control situations offering bait pellets containing high amounts of sporocysts to a free-living population of roof rats, which was monitored by indirect census baiting commonly used in rodenticide evaluation. Ten days after consumption of the bait pellets, the infected population collapsed, leading to a control success of 73%. A negative control population, which received a placebo, remained stable. These data demonstrate for the first time that S. singaporensis can be used as a biocontrol agent of wild rats. However, an immunization experiment with roof rats in the laboratory showed that these are capable of mounting a rapid specific immune response resulting in survival of acute sarcocystosis. Sarcocystis singaporensis is a cyst-forming coccidian with an obligate heteroxenous life cycle between snakes as definitive, i.e., Python reticulatus, and rats of the genera Rattus and Ban- dicota as intermediate hosts (Zaman and Colley, 1975; Brehm and Frank, 1980; Beaver and Maleckar, 1981; Hiifner and Frank, 1984). It is endemic in the southeast Asia region (Kan and Dissanaike, 1977; Lai, 1977; Kan, 1979; O'Donoghue et al., 1987). Sarcocystis singaporensis is highly pathogenic and 300 spo- rocysts can be lethal to Wistar rats within 2 wk after infection due to extensive development of schizonts in endothelial cells (Brehm and Frank, 1980). Wood (1985) was the first to suggest S. singaporensis as a possible biocontrol agent of wild rats, as he observed malaysian Rattus tiomanicus, which died in his laboratory after infection with S. singaporensis. We have ex- tended this concept to field experiments under natural condi- tions. As we intended to conduct a field experiment with S. sin- gaporensis and roof rats in Egypt, a zoogeographically distinct area, and to estimate the risk of a release of this parasite in a foreign environment, our initial investigations concentrated on host specificity. Additionally, we wanted to know if high spo- rocyst doses are harmful to animals other than the known in- termediate hosts. Then, we focussed on the impact of acute sarcocystosis on the mortality of wild rodents, especially roof rats. The aims of our study were: (1) to demonstrate the patho- logical effects in wild rats; (2) to investigate how immunity to infection limits the pathogenic potential of the parasite; and (3) to provide experimental support for the assumption that the parasite can also be used to control a free-living rat population.