ASCARIDINA 

 OXYUROIDEA 



1. Enterobius vermicularis 



111 spite of the fact that the oxyurid type of life cycle is 

 the simplest and probably the most primitive of any found 

 among nematodes parasitic in vertebrates, a search of the 

 literature has failed to reveal a single instance in Avhich a de- 

 tailed molt by molt account of the life cycle has been descrilied. 

 The life C3'cle of Entcrobiuf: vennicidaris, so far as it is knoivn, 

 will serve as an example of its type. 



The adult female worms, with the uteri filled with develop- 

 ing eggs, live in the lower part of the large intestine and par- 

 ticularlj' in the rectum. They do not ordinarily deposit their 

 eggs in the lumen of the intestine, but crawl out of the anus 

 and deposit them in the perianal region, leaving trails of eggs 

 as the}' creep about. Contact with air is apparently a stimulus 

 to oviposition (Philpot, 1!>24). Although they frequently re- 

 main outside the anus and release the eggs in showers when the 

 body ruptures, MacArthur (1930) and others state that they 

 commonly retreat into the rectum, to repeat their egg-laying 

 expeditions out of the anus over and over again, particularly 

 at night. 



The eggs when deposited by the females, or contained in the 

 uterus of females which have voluntarily migrated out of the 

 intestine, are fairly uniformly in the "tadpole" stage of de- 

 velopment, apparently being unable to progress beyond this 

 point without free oxygen. Within 6 hours after leaving the 

 body they develop a coiled larva (ring and-a-half embryo) 

 which is infective. According to Brumiit (1922) the larva 

 undergoes no molt before hatching nor, according to Philpot 

 (1924), as a free larva in water. However, Alicata (1934) sug- 

 gested that a molt within the egg shell might be general for 

 the Asearidina, and Entcrohin.i might well be reexamined. 

 Chitwood (personal communication) believes he has seen a 

 molt in the egg, and thinks there may be two. 



Development of the larva in the egg will occur in oxygenated 

 water, and in this medium the larvae commonly emerge in 

 from 9 to 24 hours at 37° C, but they only live for a few days, 

 so it is evident that water cannot be an important vehicle of 

 infection. Exposed to air a considerable proportion of the 

 eggs survive for at least 6 days at humidities above 62 percent 

 (Jones and Jacobs, 1939). 



When ingested the eggs hatch in the stomach or intestine, 

 and the worms live during the early part of their development 

 in the lower part of the small intestine, cecum and upper por- 

 tions of the colon, not infrequently invading the appendix. 

 Heller (1903) states that there definitely are two molts in the 

 small intestine, and probably three. Chitwood, (personal com 

 munication) reports having seen a molt in the epithelium of the 

 appendix. By analogy with other nematodes there is probably 

 a total of four molts. 



Although the worms have repeatedly lieen reported as bur- 

 rowing into the mucous membranes, especially of the appendix 

 (Penso, 1932), it seems probable that this is a habit only of the 

 fourth-stage larvae. Chitwood (personal communication) re- 

 ports having found the fourth-stage larvae in sections of the 

 appendix. He has observed a definite period 6 to 9 days after 

 infection when symptoms of invasion appeared, followed 4 to 7 

 days later by migration of the worms from the anus. Exposure 

 to air after operation would account for the deposits of eggs 

 which Penso reports and figures deep in the walls of the ap 

 pendix. 



There has been a large amount of discussion as to whether 

 internal autoinfeetion by the worms can occur. The fact that 

 infections persist even for many years in spite of the most care- 

 ful efforts to prevent reinfection from the anus via the hands 

 has lent support to this idea. However, the demonstration by 

 Lentze (193.5), and Nolan and Reardon (1939) of the ease 

 with which airborne infectious can occur seems sufficient to 

 account for the persistence of infections. On the other hand, 

 Zawadowsky and Schalimov (1929), Lentze (193.5) and others 

 have called attention to the failure of development and infec 

 tion of eggs or embryos left under conditions such as exist in 

 the lumen of the large intestine. It would be difficult to say 

 that internal auto-infection could never occur, but the evidence 

 is all in favor of the view that if it does occur it is an abnormal 

 and exceptional condition. 



Copulation of the young adult worms usually takes place in 

 the upper parts of the colon or in the cecum, where the males 

 live for some time. The females do not migrate to the rectum 

 until they contain developing eggs. Eipe females begin to ap- 

 pear about 15 days after infection. 



2. Other Oxvuroide.v 



The life cycles of other Oxyuridae are the same in essential 

 features, but differ in details. Oxyuris eqiii differs in that the 

 fourth-stage larva has a special structural development of the 

 anterior portion or "corpus" of the esophagus which 

 enables the larva to use it as a highly developed buccal 

 capsule for adhering to the mucosa (Wetzel, 1931). The ripe 

 females of this species creej) out of the anus as do those of 

 fJntcrobiua. but this is probably not true of forms parasitic in 

 rodents. The fourth stage larva of Dfnnatoxys vcUgera is also 

 provided with a special structure for adhering to the mucosa, 

 but in this case the end is accomplished by the development of 

 four conspicuous hooks on the head (Fig. 156, X, Y) (Dikmans, 

 1931), which is buried in the mucosa (Wetzel, 1932). These 

 specializations for maintaining a position in the colon are of 

 interest as indicative of a need for some sort of protection 

 against expulsion from the body before maturity is reached, a 

 need which may perhaps, as has already been suggested, have 

 led to a deeper burrowing into the mucosa and ultimately to a 

 parenteral migration. 



According to Philpot (1924), Aspiculuris ictraptcra has a 

 life cycle strikingly like that of Eiitrrobitix, differing only in 

 the earlier stage at which the eggs cease development before 

 expulsion, and their failure to hatch outside the body. Sypliacia 

 obvelata differs in that the eggs have developed embryos when 

 they leave the host. All stages of development from the young- 

 est larva to adult can be found in the cecum of naturally in- 

 fected mice, and are strikingly similar to those described and 

 figured for Aspiculuris. Tachygnnctria loju/icoUis and T. ilcn- 

 laia definitely undergo a molt before hatching from the egg. 

 PassaJunis cimbiniius, according to Penso (1932), is capable of 

 internal auto-infection; the gravid females burrow into the 

 mucosa to deposit their eggs, and the larvae subsequently 

 emerge to continue their development. Penso, however, postu- 

 lates a similar behavior on the part of Entcrobitis vermicularis, 

 and thinks that Wetzel's observations on Dcrmatoxys vcUycra 

 were in error, the larvae with buried heads being emerging 

 from, not entering, the mucosa, .\lthough Piissalurus ambigiins 

 may sometimes deposit its eggs in the mucosa, Penso 's observa- 

 tions need to be extended before this can be accepted as a 

 normal or usual procedure. 



Probstmayria rivipara (Atractidae) is, so far as known at 

 present, unique among nematodes that arc known to be obliga- 

 tory parasites of vertebrates in reproducing continuously gen- 

 eration after generation in a single host. It is among the. 

 nematodes what the Pupipara are among the Diptera, or Tiiiiga 

 among fleas. Its larvae hatch in the uterus and grow almost 

 to the size of the parents before being born (vide Ransom, 

 1907). They resemble the parents except for lack of develop- 

 ment of the genital organs. No stage of development is known 

 outside the body of the host. Transfer to new hosts is believed 

 by Jerke (1902) to be accomplished by contamination of food 

 or water by worms jiassed in the feces; such worms, he says, 

 remain alive in feces for several days. 



ASCARIDOIDEA 



In the Asearidoidca there are ahvays one or more molts be- 

 fore the embryos leave the eggs and, with few if any excep- 

 tions, there is a phase of burrowing into the mucosa, and in 

 many cases more extensive migration from the lumen of the in- 

 testine to the body cavity, liver, lungs or other tissues of the 

 definitive or of an alternating host. 



Heter.\kidae 



The members of this family bridge the gap between the typi- 

 cal oxyurid life cycle and that of the ascaridids. At least one 

 sjiecies, Subulura brumpti (see below), has become dependent 

 upon an intermediate host. 



The life cycle of Hetcrakis gallinac, according to Clapham 

 (1933), is of the typical oxyurid type except that the females 

 do not migrate out of the anus to deposit eggs, but oviposit in 

 the ceca. Earlier writers have reported burrowing and encyst- 

 nieiit in the cecal walls, or penetration into cecal glands, but 

 Clapham was unable to find any evidence of migration or bur- 

 rowing, the larvae passing directly to the ceca within 48 hours 

 and maturing in the lumen. The first molt occurs in the egg 

 (Alicata, 1934), the third not until 10 days after infection. 



Other species of Heterahis {isolonche, beramporia) burrow 

 into the intestinal mucosa at some time during development 

 and reach maturity in tumors which form around them. This 

 possibly is the first step in the direction of the Ascaris type of 

 life cycle. 



