CHAPTER VI 



LIFE HISTORY (ZOOPARASITICA) 



II I'AKASI'I'KS OK VKirrKI'.KATKS 



ASA C. CHANDLER. Rico Institute, Houston, Texas; J. E. ALICATA, University of Hawaii, 

 Honolulu, T. H.; and M. B. CHITWOOD. Babylon, N. Y. 



In:roductIon 



The life cycles of tlie nematodes piiriisitic in veitelii:itis 

 differ in no essential from those of free livinj; nematodes, hnt 

 are subject to a numl)er of modifications which enable the 

 parasites to train access to new hosts with more facility and 

 greater certainty. With a few exceptions these nematodes 

 have tive stages of development separated by four moults as 

 do most free living nematodes, but in a few forms (e.g.. Con 

 tracacciim and Trivhiiullo) the number of molts is said to be 

 increased, and in some forms one or more of them is sujipressed 

 to the extent of being passed through rapidly in the egg, 

 or in hatched larvae with no intervening period of growth. 



The outstanding feature in the life cycle of parasitic nema 

 todes is a cessation of develoi)ment of the young worms aftei 

 reaching an infective stage, while they await an opjiortunity 

 to gain access to a new delinitive host. In most eases the or 

 ganisms pass through this jieriod of w.'iiting outside the body 

 of the original host, either (1) as embryos inside the egg 

 shells (oxyurjds, ascaridids, trichurids) ; (2) as free living but 

 nonfeeding third stage larvae, often enclosed in the .shed 

 cuticle of stage two (Stroii(iijlui<l< s, many strong.vlins) ; or 

 (3) as third stage larvae, usually encysted, in the bod.y of an 

 intermediate host, which in some cases is obligatory (e.g., 

 spiruroids, eamallanins, some metastrongylids) but in other 

 cases is optional (e.g., CapUlaria aiiiiiilain, Si/iinamiix Iraclira). 

 In many cases such larvae are capable of re-encystment, some- 

 times over and over again, in other hosts — transport hosts — 

 in which development to maturity does not occur. In a few 

 cases such secondary intermediate hosts have become neces- 

 sary parts of the life cycle (e.g., Gnalhostoma spinigeriim). 

 In the filariae and a few other nematodes (e.g., Hahronima) 

 the infective larvae do not become encysted, and habitually 

 emerge through a break in the labium of the vector as a re 

 suit of their own activities. A striking exception to the usual 

 waiting period outside the body of the host occurs in the case 

 of TrichiniUa spiralis, which passes its waiting period en 

 cysted in the flesh of the parental host. 



In considering the life cycles of parasitic nematodes from 

 an evolutionary standpoint it is necessary to consider possible 

 ways in which the nematodes may have developed into para 

 sites of vertebrates. One method was presumably the re 

 suit of ingestion by the host, followed by adaptation to the 

 environment encountered inside the alimentary canal. It 

 seems probable that the Oxyuridae, for instance, became para 

 sitic in this manner. Such nematodes might lie expected to 

 have the simplest possible type of life cycle, reproducing 

 generation after generation in the lumen of some part of the 

 alimentary canal, with enough eggs or larvae escaping witli 

 the feces to allow for spread to other hosts through the me 

 dium of contaminated food or water. It seems remarkable 

 that only a single instance (Probst mayria viripara) is known 

 of a parasite which has unequivocally adapted itself to this 

 type of life. The nearest approach, with the exception of 

 Probstmayria, is the facultative parasitism of a number of 

 species whose congeners are saprozoic free-living forms, e.g., 

 a species of Longibucca in the stomach of a snake (Chitwood, 

 1933), and another species of Longibucca in the stomach and 

 intestines of a bat (Mcintosh and Chitwood, 1934) ; Diplo 

 scapter coronata in ahydrochloric human stomachs (Chandler. 

 1!>38) ; and Ccphalobiis parasiticus in the stomachs and in 

 testines of monkeys (Sandgronnd, 1'.I39). In addition to these 

 cases, it is claimed by a number of writers (Koch, 1925; 

 Penso, 193:;) that Evlcrobius vrntiicufaris. Passalunis atn- 

 biguus and other oxyurids are callable of reproducing, gen 

 eration after generation, in the lumen aiul walls of the intcs 

 tine. This is denied by others (Zowadowsky and Schalimov, 

 1929; Lentze, 193.5) because of the demonstrated need of 

 oxygen by the embryos before they can complete their de 

 velopment. Even if the larvae can occasionally develop to 

 maturity in the gut walls, such an occurrence can certainly 

 be considered the exception rather than the rule. One other in 

 stance of repeated generations in a single host has been 

 claimed for Strongyluidcs stcrcoralis (Nishigori, 1928; Faust, 

 1931) but this is a case of short circuited rather than con 

 tinuous development, and occurs only under exceptional con 

 ditions. The offspring of wonns in the intestine do not 

 grow to maturity directly in the intestinal lumen, but migrate 



through the body as they wouhl if Ihcy h.-iii infected from 

 outside. 



With the few exceptions mentioned above, the simplest type 

 of life cycle in the case of obligatory parasites is that ex- 

 hibited by most species of oxyurids, in which the eggs fail 

 to develoi) beyond a certain point (morula stage in some, 

 "tadpole" in others) until exposure to oxygen outside the 

 body of the host, followed by reentrance of the embryonated 

 eggs or hatched larvae into the same or another host with 

 food or water contaminated by them. In many cases, jiossibly 

 in all. this simple cycle is modified further by a stage in 

 with the larvae attach themselves to the mucous membnuie, 

 liury their heads in it, or actually burrow into the walls of 

 the gut before they take up their residence in the lumen as 

 adults. 



Few parasites other than the Oxyuroidea have as simjile a 

 life cycle as that described in the last paragraph. Most of 

 them have an instinct for burrowing at some time during the 

 course of their development and exercise it either (1) by bur- 

 rowing through the skin and going on a tour of the body 

 via the circulatory system, lungs and throat before reaching 

 the intestine; (2) by burrowing into the mucous membranes 

 of the alimentary canal, either being content to live buried 

 in the gut wall for a few days, or entering the circulatory 

 system and going on a tour of the body similar to that of the 

 skin penetrators or, in some eases, burrowing directly through 

 into the body cavity or through mesenteries, parenteral tis 

 sues, etc.; or (3) by burrowing into the body cavity or tissues 

 of an intermediate host, either through the surface or through 

 the walls of the gut after being ingested. 



Two possible origins of this burrowing habit suggest them- 

 selves. One possible origin is as a useful instinct on the part 

 of gut parasites to serve either one or both of two purposes, 

 (1) to protect the young worms from being swept out of the 

 intestine with the feces, and (2) to provide a better type of 

 nourishment for the period of rapid growth and development. 

 There can be little doubt but that the burying of the head of 

 fourth-stage larvae of Dermatoxys vcligera (Wetzel, 1931) 

 and the use of the "corpus" of the esophagus of Oxyuris eqiii 

 as a mouth capsule (Wetzel, 1930) are steps in this direction. 

 One could then visualize as further developments complete 

 burrowing into the gut wall, penetration into the circulatory 

 system, and the circuit through the body that would neces- 

 sarily be entailed. 



Tlie alternative explanation is that the worms which mi- 

 grate through the body originally became vertebrate parasites 

 by burrowing through the skin. An initial step in this direc- 

 tion can be observed today in the occasional invasion of the 

 skin of dogs and sometimes of other animals by Ehabditis 

 strongyloides, the adults of which live in soiled straw bed 

 ding. Successful development of adult parasitism by this 

 method would necessitate an ultimate location in the body 

 whence the eggs or embryos could escape in order to reach 

 new hosts. This condition would be fulfilled in the case of 

 those parasites which, after penetration of the skin, reach 

 the circulatory system and eventually arrive in the lungs, 

 where, still imbued with an instinct for burrowing, they 

 would escape into the air spaces. Here they could successfully 

 reach maturity and reproduce (e.g., Metastrongylidae) or 

 they could be carried passively, via trachea and throat, to the 

 alimentary canal. Successful parasitism would, of course, be 

 dependent upon loss of the burrowing instinct after the third 

 moult, which actually occurs. 



The temporary burrowing into the intestinal mucosa of the 

 larvae of such worms as Ascaridia, Hacmonchus and Oesopha- 

 gostomum might, then, be construed either as a step in the 

 direction of a more extended migration, as practiced by re- 

 lated worms, or as a step in the direction of the simple 

 oxyurid type of life cycle, with abandonment of a primitive 

 but no longer necessary migration from skin to intestine. In 

 those species which usually perform the entire migration, the 

 failure of some individuals to do so (e.g., Ascaris. hookworms) 

 could be either atavistic or progressive. The fact that the 

 curtailed migration is more likely to occur in the normal than 

 in abnormal hosts is of little help in the matter, for it could 

 be argued either that the reason for the failure of migra- 

 tion in normal hosts is due to less restlessness in such hosts 

 and a consequent slipping back to ancestral ways, or that it is 



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