76 



HISTOMONAS 



intracytoplasmic movement. The flagella 

 produce a characteristic, jerky, oscillat- 

 ing movement resembling that of tricho- 

 monads, but Hislomonas can be differen- 

 tiated from them because it lacks an 

 undulating membrane and cixostyle. 

 Wenrich (1943) found peculiar, cylindrical 

 feeding tubes in about 15"( of the individuals 

 from one of the 2 pheasants he examined 

 and also in some individuals from a chicken. 

 These sometimes extended out as much as 

 the body diameter and often had internal 

 extensions as long or longer. 



This form is sometimes present in 

 large numbers in the lumen of the ceca, 

 but it is ordinarily absent or very difficult 

 to find. 



Life Cycle : Reproduction is by binary 

 fission, and there is no evidence of a sex- 

 ual cycle. Wenrich (1943) considered the 

 larger, 4-flagellate forms in the ceca to 

 be adult. There are no cysts. 



The naked trophozoites are delicate 

 and do not survive more than a few hours 

 when passed in the feces. Turkeys can be 

 infected by ingesting trophozoites, and this 

 mode of infection plays a part in transmit- 

 ting the parasites once disease has ap- 

 peared in a flock (Tyzzer and Collier, 

 1925). However, large numbers of the 

 parasites must be ingested. Tyzzer (1934) 

 pointed out that oral infection with infected 

 liver tissue or cecal discharges is some- 

 what unreliable because of the death of the 

 protozoa during their passage thru the al- 

 imentary tract. Lund (1956) found that 

 oral administration of 10,000 to 100,000 

 protozoa in saline caused infections in 

 about 40% of 6- to 9-week-old poults, and 

 illness in about 20%. However, when di- 

 gestible materials were added to the inoc- 

 ulum, the infection and morbidity rates 

 fell sharply. The protozoa remained in 

 the gizzard and upper intestine longer in 

 the presence of food, and were destroyed 

 before they reached the cecum. Horton- 

 Smith and Long (1956a) found that infections 

 with trophozoite suspensions could be pro- 

 duced only in starved chickens or, in 

 chickens that were feeding, by giving them 

 an alkaline mixture just before dosing 

 them. They believed that successful in- 



fection depends on the pH of the gizzard 



and possibly upper intestine. The pH of 

 the starved gizzard is 6. 3 to 7. 0, that of 

 chickens on feed is 2. 9 to 3. 3, and that of 

 chickens on feed which have received 

 alkali is 6. 2 to 6. 5. 



By far the most important mode of 

 transmission is in the eggs of the cecal 

 worm, Heterakis galli)iariim. Its discov- 

 ery by Smith and Graybill (1920) was a 

 milestone in the history of parasitology. 

 This mode of transmission has been amply 

 confirmed by many workers, and is the 

 preferred method of producing experimental 

 infections (Tyzzer and Fabyan, 1922; Tyzzer, 

 1926; Swales, 1948; McKay and Morehouse, 

 1948; Lund and Burtner, 1958). The para- 

 sites are carried inside the Heterakis eggs; 

 eggs treated with disinfectants or other 

 chemicals which do not kill them are still 

 infective. 



Infection of Heterakis eggs is so wide- 

 spread that Histouionas infections can be 

 produced with batches of eggs taken from a 

 very high percentage of turkeys or chickens 

 even if the hosts do not appear sick. Not 

 every egg is infected, however. Lund and 

 Burtner (1957) found that less than 0. 5% of 

 the embryonated eggs from experimentally 

 infected chickens contained the protozoa, 

 that less than half of the cecal worms they 

 examined from these birds contained Hist- 

 o)iiuiias -iniected eggs, and that positive 

 worms contained an average of only 2 in- 

 fected eggs each. 



The Heterakis eggs must hatch and 

 liberate larvae in order to transmit the 

 protozoa. Histomonas has never been seen 

 in the infective eggs, its presence being 

 inferred from the experimental results. 

 However, Tyzzer (1926) found the protozoa 

 in half-grown Heterakis from birds with 

 histomonosis, and (1934) in the cells of the 

 intestinal wall of 10-, 12-, and 21 -day old 

 worms from experimentally infected birds, 

 and Kendall (1959) found them in a 4-day - 

 old H. galluiarum larva. 



The possibility that arthropods may 

 transmit histomonosis has been considered 

 by a number of authors. Mechanical trans- 

 mission by flies and even grasshoppers is 



