thoir (U'voli)|)niiMif laiiidly, :iii<l :i rliiiliilitifi)iiii liirvji iiiav 

 liatih witliin :;4 lunirs. This larva is aliout •_'."!() ^ loiiR, with nii 

 iMonuali'il liiiccal cavity ami a typical rhahilitiform csophaKiis 

 IKisscssiiiK csoiihaiioal valves. Tlu'so larvae were shown l>y Mc 

 Coy (IJIJill to (icvclop normally with oTily pnrc cultures of cer 

 tain species of living bacteria as food. Tniler favorable con 

 (litions the larvae underKo the first ecdysis or molt within 4S 

 li.niis atter liatchinj;, bnl second sta(;e larvae show very slight 

 niorpholoKical difTeronccs from first sta^e larvae, although they 

 arc about 400 to 4:U) A" long, .\ftor a niiniuiuni of about 2 more 

 d.-iys the larvae cease fecdiuK. undergo a second ecdysis, and 

 enter the third or infective staRO. The cuticle shod at this 

 molt is normally retaiiu'd as a protective sheath, though it may 

 occasionally be lost. The most important inori)holo^rical changes 

 in the infective larva are noted in the shape of the tail and the 

 structure of the esophaRUs. The tail is shorter and more stumi)y 

 than that of the prccediuK stages. The esojihaKus is "lilari 

 form," or preferably "strouByliform," i.e., it is more uni 

 form ill width with tapering anterior portion, and the esoplia 

 seal valves are lacking. The anterior portion of the lumen of 

 the stoma is closed and the reuuiining posterior portion remains 

 open in a characteristic shape. According to .\licata (l!>.'i.")) 

 the various third stage larvae of strongylid nematodes para 

 sitic m swine can be differentiated, among other ways, by the 

 form of the stoma; there is a jiossibility that this character 

 istic may hold true for other members of the Strongylina. 



The infective larvae climb up on ob.jects as high as a film of 

 moisture extends, and show positive thermotropism and thig- 

 motropism. They retire from excessive warmth in direct sun- 

 light. They migrate vertically if buried in soil, but migrate 

 laterally to a very slight extent (Chandler, 1925). 



.Mthough Leuckart (ISOO showed that .(. caiiiiiiim of dogs 

 could 1)0 transmitted per n.i, and Leicliteustern (ISS(i) proved 

 the same thing for .1. iltuHlciiali' of man, the usual mode of in- 

 fection is by iienetration of the skin; this method of infection 

 was first demonstrated by Looss (ISiiS). In subser|uent work 

 Looss (190.")) established the course which the larvae follow 

 in the body to reach the intestine. Skin penetration is accom 

 plishcd in a few minutes when the larvae are able to obtain 

 leverage, as in mud, to help them in their burrowing, but they 

 are nimble to penetrate when submerged iu water. Within 3.") 

 to 40 minutes the larvae, having left their sheaths behind them. 

 have reached the dermis and within a few hours are in the sub 

 cutaneous tissue. Many find their way into superficial lym 

 phatic capillaries, and a few directly enter blood vessels. Some 

 larvae are slow in entering the circulation, and may be en- 

 capsulated in the .skin, especially in hosts sensitized by previous 

 exposure. Certain "foreign" species of hookworms, e.g. 

 Aiicnlo.'ttoma braziUense and Uncinaria' stowccplmla in man, 

 commonly fail to enter the circulation at all but wander aim- 

 lessly in the skin, causing "creeping eruption." Although the 

 larvae may remain in the skin for considerable periods no de- 

 velopment takes place there (Fiilleborn, 1927). 



When larvae enter the lymphatics they are carried first to 

 the regional lymph glands, and then to the main lymph chan- 

 nels leading to the thoracic duct, through which they enter the 

 circulation. Such larvae, as well as those which entered the 

 blood system directly, eventually reach the right heart, whence 

 they are pumped out to the lungs. Here the ma.iority burrow 

 into the air spaces (Fiilleborn, 192.')), and are then mechani- 

 cally carried in mucus, helped by epithelial cilia, to the trachea 

 and throat. If .swallowed they now pass to the alimentary 

 canal, and grow to maturity in the intestine. 



Although .skin penetration is undoubtedly the usual mode of 

 infection, infection by mouth can also occur. There has been 

 considerable controversy as to whether swallowed larvae had 

 of necessity to penetrate the mucosa and migrate to the lungs 

 before growing to maturity, or whether they could develop to 

 maturity without such migration. Yokogawa (192(5) investi- 

 gated the matter and found that when A. canhnim larvae are 

 fed to puppies a few penetrate the walls of the alimentary 

 oanal and enter the circulation, but the great ma.iority of those 

 which develop at all do so directly, without migration. In ab- 

 normal hosts, however, such as rodents, most of them perform 

 the usual migration via the circulatory s.vstem, and a few mi- 

 grate through the tissues to the body cavity whence they enter 

 the liver, or go through the diaphragm to the ])leural cavity, 

 whence thev enter the lungs. This work was confirmed In- 

 Scott (1928). Fiilleborn (1926-1927) showed that the larvae 

 of Uncinaria strnociphala of dogs also develop directly after 

 oral infection, few migrating even in abnormal hosts. Several 

 Japanese workers, however, (Myiagawa, 191fi; Myiagawa and 

 Okada, liiSO, 19H1; Okada ig.'Sl) have persisted in the belief 

 that lung migration is a biological necessity for hookworms. 

 Foster and Cross (1934) carried through some further cxjjeri- 

 ments which conclusively confirm the earlier work, showing that 

 the lung .journey is not a biological necessity for these worms 

 (though it apparently is for Strongyloidex sfrrcoraUn.) Swal 

 lowed larvae rarely migrate in su.sceptible normal hosts, but 



coniinonly do so in abriormal hosts and in resistant normal ones. 

 I.oo.ss (1911) and S'okogawa (192(i) observed that sw;illowed 

 hookworm larvae remain in the stomach at least 2 days, and 

 I'illleborn (1927) found tin'y could remain there at least ") 

 (l.iys, partly in the lumen, jiartly deep in the mucous glaruls. 

 Ill' demonstrated that the larvae have an initial tendency to 

 burrow into the glands, later to return to the lumen, as is the 

 case with Asroriiiia. lie thinks that something in the secretion 

 of the mucous glands causes the larvae to lose their mobility; 

 possibly the .same mech.'inism is responsible for the loss of the 

 burrowing instinct in the l;irvae reaching the intestine from the 

 Inngs after skin penetration (see below). 



The minimum time rc(|uired for the larvae to reach the 

 trachea after skin penetration is usually about 3 days, but the 

 ma.iority re(|uire 4 or ."i days, and some still longer. By the 

 time the larvae appear in the bronchioles and tr;ichea they have 

 grown slightly in length, have developed a provisional mouth 

 capsule, and are ready for the third molt, although there is no 

 evidence tli:it the.v ever complete it before reaching the diges- 

 tive tract. The formation of the provisional, and subsefiuently 

 of the definitive, month capsules is accomplished by the devel- 

 opment of dorsal and ventral bladder like structures posterior 

 to the already existing mouth. These spread around tin' sides 

 and finally unite (Looss, 1905) (Figs. 181). 



Up to the time of the third molt the larvae grow very little 

 in length, but increase from about 20 m to 30 M in diameter. 

 The molt usually occurs very soon after the larvae reach the 

 intestine, and the larvae at this time lose their tendency to 

 burrow, so remain in the intestine. There is no evidence that 

 they temporarily burrow into the glands of the stomach as do 

 larvae that are directly swallowed. The young worms now 

 grow very rapidly. They may reach a length of 2.5 to 3 mm 

 within a few days. Sexual differentiation now begins, and in 

 from 4 to 6 days after the third molt the definitive mouth cap 

 sule is developed. By the time the worms have reached a 

 length of from 3 to 5 mm. the fourth molt takes place. There- 

 after the worms grow to maturity, copulate, and begin egg pro- 

 duction. In the case of Aitcylostoma dnodeiiale in man the 

 eggs first appear in the feces 5 to 6 weeks after infection, 

 whereas in A caiiiinim of dogs, eggs may appear as early as 15 

 d;iys (Herrick, 192S). 



2. IIaemonchus contortus 



The life cycle of this worm as worked out by Ran.soni (1906), 

 Veglia (1916) and others is essentially the same as that of the 

 ancylostomas in its free-living phase. The infective, ensheathed 

 third stage larvae, however, are not skin-penetrators, but have 

 a tendency to climb up on vegetation or other ob.iects where 

 they are in a favorable position to be ingested by their herbiv- 

 orous definitive hosts. Here they curl up, and are remarkably 

 resistant to cold and to moderate desiccation. Upon being in- 

 gested by the final host the larvae bury themselves in the mu- 

 cous glands and crypts of the abomasum, where they undergo 

 the third and fourth molts; the sdult stage is reached after 

 about the 9th to 11th days, and the worms emerge to live in the 

 lumen of the organ, beginning egg production about 3 weeks 

 after infection. Although there is no evidence that the worms 

 perform a parenteral migration in sheep, Ransom (1920) 

 showed that they do migrate to the lungs in guinea pigs. 



3. Syngamus trachea 



The life cycle of this worm was first experimentally worked 

 out by Ortlepp (1923). The eggs of the worm are laid in the 

 bronchi or trachea of the host in an advanced stage of seg- 

 mentation. Under favorable conditions the first-stage larva is 

 developed in about 3 days, but the egg does not become infec- 

 tive until after 1 to 2 weeks, whereupon they may or may not 

 hatch. Ortlepp observed only a single molt during the course 

 of development and interpreted the infective larva as a second- 

 stage larva but Wehr (1937) demonstrated that the develop- 

 ing larva undergoes two molts within the egg. Buckley (1934), 

 studying 5. ierci of cats, also observed the usual two molts. 

 Yokogawa (1922) also missed the first molt in the case of 

 Xippostrongylus muris, and in spite of the large amount of 

 experimental work done with that worm the missed molt was 

 not discovered until 1936, when Lucker demonstrated it. The 

 first cuticle in both these worms is extremely thin, and the 

 second ecdysis may be in progress before it is coinpletely shed. 



Infective larvae, whether hatched or .still in the eggs, are 

 infective when directly swallowed by susceptible hosts, but 

 very often they are swallowed by various invertebrates; when 

 this happens they penetrate the gut wall and become encap- 

 sulated in the body cavity. Walker (1.SS6) and Waite (1920) 

 both called attention, on epidemiological grounds, to the impor- 

 tance of earthworms in the dissemination of this parasite, but 

 Clapham (1934) first experimentally worked out the role played 

 by these annelids. Subsequently Taylor (1935) showed that 



273 



