The protozoa occurring in the hind-gut of cockroaches. III. Factors affecting the dispersion of Nyctotherus ovalis

The only mode of transmission of Nyctotherus ovalis from one cockroach to another was shown experimentally to be by way of ingestion of the encysted stage of the ciliate. The cysts were remarkably resistant, being able to remain viable for 21 weeks at −18°C. if kept in dry faeces, or for 20 weeks in wet faeces at 4°–25°C. Their life was much shorter if kept at 37°, 25°, or 4°C. under dry conditions, or at −18°C. under wet. The cysts could withstand freeze-drying, or storage in pure oxygen, carbon dioxide or nitrogen, at least for short periods. Nyctotherus ovalis could excyst in any of the four species of cockroach which were tested (Periplaneta americana, Blatta orientalis, Blattella germanica, Blaberus giganteus), irrespective of the species of origin. The ciliate could establish an infection in a very young host; in a host which had ingested infected faeces once only; in hosts which were in a very dry or a very humid environment; in a host maintained at a temperature far below that required for in vitro excystation, subject only to the temperature being high enough for it to be possible for the cockroach to digest its meal; but Nyctotherus could not establish an infection in cockroaches which were about to moult, because there was no movement nor digestion of food in the gut. There was no evidence of an instinctive drive on the part of the cockroach to eat infected faeces. There was some evidence that the Blatta and Blattella types of trophozoite and cyst were associated with the presence or absence respectively of a type of flora typically associated with the hind-gut of Blatta. The process of excystation took as little as 3 hr. for completion in vivo, but this time was increased if faecal material was ingested along with cysts. The process was most rapid in the smallest cockroach (Blattella) and slowest in the largest (Blaberus). The process, once started, could be completed in vitro, but at a considerably slower pace. Digestion of the knob on the cyst by the cockroach's enzymes seemed to be necessary if the ciliate was to emerge. A small proportion of cysts hatched after passage through the gut of a locust, but no means were found of increasing this proportion, and an infection was never established. Excystation did not occur in any other arthropod, nor in the frog, which were tested. Cysts recovered from the mid-gut of a cockroach completed the process of excystation in vivo more rapidly if they were put in a buffer medium of pH approximating that of the mid-gut than if they were put in saline. Passage of the cysts through the foregut of the cockroach had no function in stimulating excystation: the agent seemed to be the trypsin secreted by the mid-gut caeca, or the products of its activity. The many failures in the series of experiments which are reported here can probably be attributed to contaminating bacteria. The complete process of excystation could occur in vitro if the cysts were incubated with faecal matter from the cockroach. The optimum temperature was 32°C. , and 2 days were needed for a significant number to hatch. No means were found of shortening this time, nor any agents which could be substituted for the faecal matter. The stimulus seemed to be provided by particular anaerobic bacteria of the faeces which were present in some cockroaches, but absent in others. Work by other authors on excystation in other Protozoa is reviewed. It was concluded that in vivo excystation of Nyctotherus ovalis occurs as a result of stimulation of the cyst by the digestive trypsin of the cockroach, and not by the products of the unidentified bacterium which is active in vitro. Cysts originating from Blaberus seemed to be less sensitive to this bacterial factor than some of those from Periplaneta or Blatta. This work was done during the tenure of a Research Studentship awarded by by the Agricultural Research Council. I am deeply indebted to Dr P. Tate for his encouragement and counsel at all stages of the work.