siiiiils iiiul sliiRS wuiild also si'ivo as transport liosts, ami later 

 found that tlio pneystod larvae would remain viable in these 

 niolUises for several years. More reeeiitly Clnpham (l!>Sila, 

 l!).!i>h"l showed that niaRBots. erane tiy larvae, spring tails ami 

 eentipedes would serve in a similar eapaeity, and that the 

 worms were alile to survive metamorphosis in the tissues of 

 tlies. 



When iiiKesteil liy these hosts the infeetive larvae lialih fnini 

 the eRKS if they have luit already done so, penetrate the gut 

 wall, and enter the body eavity, where they are eventually en 

 eapsulated by the host tissues, t'lapliani has shown that the life 

 eycle is eompleted somewhat nuire readily with the aid of a 

 vector than without, and was able to infect chickens readily 

 with a starlint; strain when an earthworm vector was used, 

 whereas Taylor ( 1!>11S^ had had difficulty in doinjr so by direct 

 infection. I'lapham calls attention to the fact that Siiii'li'miix 

 trachea is evidently nndergoinK evolution in its life cycle; at 

 present it can still develop without an intermediate host, and 

 iuis not as yet adapted its requirements to any imiliciilar inter 

 mediate host, Init can use almost any that happens to swallow 

 it. She makes the reasonable suggestion, however, that in time 

 difTerent strains may adapt themselves to different intermedi 

 ate hosts, as determined by the food habits of the final hosts. 

 and thus i>erhaps give rise to new species. At present, however, 

 the effect of living in a transport host seems to be to rliiiiiiiair 

 physiological differences; for example, in the case of starling 

 strains developing in chickens. Tt is ]>ossible that some species 

 of StjiiDamiis may already have reached the stage of reciiiirixn 

 an intermediate host, since Buckley (1S134) was unable to ii\ 

 feet cats with eggs containing third stage larvae of S. irrci. 



After infection by swallowing eggs, free larvae, or larvae 

 contained in invertebrate transport hosts, .S". trachra appar 

 ently reaches the lungs via the circulatory .system. Orflepp 

 (1923) found the larvae in the lung tissues within 24 hours 

 and Wehr as early as 17 hours after infection. Welir found 

 fourth stage larvae after 3 days and immature adults after 7 

 days; some of the latter were already in cDpiila even before 

 entering the trachea. 



Variatioks in the Life Cycle in Other Stroncyi-oide.v 



AND TrICHOSTRONGYLOIDEA 



The preparasitic stages of nearly all the members of the 

 Strongyloidea and Trichostrongyloidea, except the Syngamidae, 

 are remarkably similar, involving two free living rhabditiform 

 stages separated liy a molt, and a strongyliform third stage, in 

 which the shed cuticle is usually retained as a sheath. The 

 time intervals between the molts and the total time required to 

 reach maturity vary considerably; in some species, e.g. Ornitlio- 

 strongi/lus quaflrirndiatiis, the infective third stage may lie 

 reached within 3 days. The infective larvae are distinguish 

 able by characters of the mouth, buccal cavity, esophagus, 

 shape of tail, length of sheath, etc., and also, as Lucker has 

 shown in a series of papers (e.g., I^ucker, 1938) by the num 

 ber and arrangement of cells in the intestine. 



The only important variation from this formula is the molt- 

 ing of some species within the egg, thus eliminating a period 

 of feeding and growth outside the host ; this, as already noted, 

 occurs in Synfiamim and it also occurs in XcmatoiUnis spp. 

 (Ransom, 1911; Maupas and Seurat, 1913) and in OsuaUlo 

 cruzia fijiformis (Slrongyhis auriciilari.t, Zeder) (Maupas and 

 Seurat, 1913 >. According to the latter authors, Ostcrtagio 

 marshalli hatches as a second stage larva and undergoes its 

 second molt 2 or 3 days later without feeding. This is not 

 true, however, of 0. circumcincia. When botli molts occur 

 inside the egg the infective embryos may or may not hatch 

 prior to being swallowed by a host, eggs containing infeetive 

 third-stage larvae being infective as well as the free larvae. 



Strong ylacantha glycirrhi^a, according to Seurat (1920b), 

 hatches at the end of 48 hours but the larvae fail to feed, and 

 at the end of a month hare molted twice and are ensheathed 

 in both shed cuticles, .just as in the case of Dictyncaiiliis (see 

 below). 



A striking exception to the usual course of events occurs in 

 the case of Olliilaniis iricuspis, according to Cameron (1927). 

 This parasite of the stomach of eats is viviparous. The eggs 

 hatch in the uterus of the mother, and the larva undergoes its 

 first molt before it is born, acquiring the typical tri cuspid 

 tail. Third stage larvae are found free in the stomach of the 

 cat, but it is not certain whether the second molt occurs before 

 or after birth. This form is believed by Cameron to leave the 

 stomach with the vomitus of the eat. When eaten by another 

 cat with the vomitus the larvae change to fourth-stage larvae 

 and finally adults. Some part of this development is believed 

 to take place in the depths of the mucous membrane. No other 

 method of exit from the cat has yet been found ; no larvae were 

 ever seen in the intestine, nor were mice infected when fed on 

 cat stomach or infected vomitus. Continuous auto infection is 

 believed possitde but improbable; Cameron suggests the pos- 



sible production of a substance inhibiting complete larval de- 

 velojiment, as postulate<i by Fiilleborn in tin' case of UhitlxUas 

 hiifonis in the lungs of frogs. 



The mode of access of the infective l:nvae to the final host 

 varies in different species, even, sonietinu's, within the same 

 genus. There arc three possibilities: (1) pcTwtration of the 

 skin; (2) ingestion with food or water; (3; ingestion with a 

 transport host. Skin penetration is characteristic of most of 

 the hookworms (l*'amily Aucylostomatidae) — Avrylontoma, 

 Xecalor, i'ncinaria iind (laigerin — but Iliiiioslomiiiii seems to 

 be an exception in that, althinigh the larvae, at least of B. 

 Irigonncrplialiim, seem to be capable of penetrating under cer- 

 tain conditions (Ortlepp, 1937, p. 2(17), they do not clo so as 

 re.idily as other hookworm larvae (Cameron, 1923; Schwartz, 

 192.'>), and normally infect by mouth. Although most of the 

 hookworms ;ire able to infect the host by mouth as well as 

 through the skin, and may even be able to dispense with the 

 parenteral migration (see above), Ortlepp (1937) was unable 

 to cause infection in sheep by the oral route with larvae of 

 Caigeria pacliy.scrli.i. Most other members of the Strongy- 

 loidea and Trichostrongyloidea fail to i)enetrate the skin al- 

 though a few {Slcphaiiiiriis deniattis, Nippostrongylux miirix, 

 Langixlriata mii.seiili, Trichostrongyliin caJcaralus) are able to 

 do so. Other species of Trirhostrongylux apparently do not 

 penetrate the skin. Xippo.ilroiigyhix miirix is almost wholly 

 dependent upon skin penetration ( Vokogawa, 1922), whereas 

 for Langiatriaia miisciili oral infection is probably more impor- 

 tant in nature (Schwartz and Alicata, 1936). 



The great majority of the worms belonging to the groups we 

 are considering normally enter the host by mouth, with con- 

 taminated water or food. In most cases the larvae climb up on 

 living vegetation and are more or less resistant to desiccation. 

 This is true of all the Strongylidae so far as known (except 

 Stephaniinix), and all of the Trichostrongyloidea with the ex- 

 ception of the few mentioned in the preceding paragraph, and 

 OUiiIanus. 



The development within the host involves varying degrees 

 and types of migiation. Skin-penetrating larvae usually follow 

 the route described above for ancylostoraes, but Schwartz and 

 Alicata (1936) showed that the larvae of Longislriata muscnU 

 do not normally do so; they appear in the stomach within a 

 few hours after skin penetration, and in the intestine soon after 

 that, but they were not found in the liver, lungs or stomach 

 walls. Their actual route was not determined. In the case of 

 this worm, whether infection is by skin or mouth, the entire 

 development takes |)lace in the intestine, contrary to what 

 happens in other skin-penetrating £orms, even in the nearly 

 related Xippo.itrongyliis. 



Nematodes infecting by mouth may or may not migrate via 

 the blood stream. Most of the Trichostrongyloidea (e.g. 

 Cooperia, OrnithoxtroiigyJiis, Ostcrtagia, Obeliscoidcs, Graphid- 

 iiiin. Eacmonchu.^, Hyoxtrougyliis, most species of Trichostrong- 

 his, Xematodinis) perform no migration at all beyond a more 

 or less temporary invasion of the glands or crypts of the 

 stomach or duodenum. Some forms, e.g., Ornilhostrongybis 

 qiiadriradiatiia, may reach the adult stage of development as 

 carlv as the third" or fourth day after infection (Curillier, 

 1937). 



The Strongylidae show various gradations from invasion 

 of the circulatory system and transportation with the blood, to 

 mere temporary invasion of the glands. Of the three common 

 species of Strongylus in horses each shows characteristic fea- 

 tures in its migration, the larvae of S. vulgaris being found in 

 aneurisms in the anterior mesenteric vein, those of S. ideniatus 

 under the peritoneal walls of the abdominal cavity, and those 

 of S. equinus in liver and pancreas. According to the usually 

 accepted view (see, for example, Xeveu-Lemaire, 19.36) .S. vtil 

 garis penetrates the walls of the jntestine and migrates through 

 the body via the circulatory system, passing through the capil- 

 laries of both liver and lungs to be distributed all over the body 

 by the systemic arterial circulation. Ninety percent stop in the 

 anterior' mesenteric artery, to the walls of which they adhere 

 by using the mouth as a sucker. The resulting irritation leads 

 to the formation of an aneurysm and thromboses. Here they 

 remain for .") months, meanwhile growing and passing through 

 two molts; one at a length of 3 to 4 mm, the other at a length 

 of 7 to 10 mm. Having jiassed the final molt they release their 

 holds and are carried by the blood stream to the walls of the 

 cecum or colon. They remain imbedded in the walls in little 

 nodules under the mucosa for about a month, and finally make 

 their exit into the lumen. Olt (1932) thinks that the normal 

 migration is via the lungs and trachea as in the case of hook- 

 worms, but that some larvae burrow through the intestinal 

 walls and l)etween the laminae of the mesenteries until they 

 reach a large bloodvessel. If this is a large, heavy-walled 

 vessel the slow jiassage through it leads to inflammation and 

 the characteristic aneurysms. Wetzel and Enigk (1938a), on 

 the other hand, believe they have convincing evidence that no 



