How it attains the human intestine is not known, but as it is a common parasite of the pig and apes, in which it occasions a fatal form of dysentery, it is likely that one or other of these animals is a usual source of infection. As attempts to infect animals experimentally have failed, it is reasonable to conclude that the infusorian, on occasion, assumes some resistant form which enables it to withstand the gastric and intestinal juices on the way to its habitat, the end of the ileum and the large intestine. Although it may live for a considerable time in water or fæces (one hour to three days), it has not been grown on culture media. In liquid stools the balantidium exhibits great activity, indulging in locomotive as well as in rotary movements.
The symptoms of balantidium dysentery are, in the present state of knowledge, indistinguishable from those of other forms of dysentery. The disease is chronic in type, its special nature being discoverable only on microscopical examination of the stools. Generally only one or two balantidia are found, but as many as twenty may be seen at a time in every field of the microscope.
Fig. 82.—Balantidium coli.
In the large number of cases collected from different authors by Strong, together with those observed by himself, the mortality amounted to about 30 per cent. In many cases there were no symptoms save occasional diarrhœa. How much of this was due to the parasite and how much to concurrent and independent disease it is difficult to state.
In 30 cases in which autopsies have been made a variety of dysenteric lesions, from catarrhal congestion and diphtheritic patches to extensive ulceration, have been found. On section Strong demonstrated the balantidium not only in exudate on the surface of the bowel, but congregated in large numbers in. the follicles, and embedded in the tissues forming the base of the ulcerations, including submucosa and muscular coat, and even in the lumen of blood-vessels and lymphatics. Bowman mentions that the colon is affected in its
