Thk Life-Histoky of the Wire-Worm of Sheep 



eggs are oval in shape and very small; only about one iour-huudredth 

 part of an inch in length. The nnmher which can be passed out by 

 a ])adly infected sheep is enormous, and even an infected sheep whicli 

 is still inifirardlii liealthii in appearauce may pass as many as three 

 million per day. The eggs then hatch out in the droppings on t]je 

 veld, provided conditions of temperature and moisture are suitable. 

 The warm wet weather of summer is most favourable, and hence 

 .spread of wii'eAvorm infection is worst at this season. Hatching is 

 ver3- rapid and may begin at once, since development of the eggs 

 occurs even in passing through the intestines. Under favourable 

 conditions, eggs may hatcli 19 hours after being laid. 



First Larval Staijc. — When hatching occurs, a small thread-like 

 "larva" (immature developmental stage), about one-eightietli part 

 of an inch in length, crawls out. Tliis larva feeds on the manure in 

 which it hatched, grows a little, forms a new skin, then moults or 

 ■casts the old skin, and emerges as the second larval stage The first 

 larval stage is passed through in about one day. 



Second Larval Statje. — At this stage, the worm is about one- 

 fiftieth of an inch in length, is very lively and continues to feed on 

 the droppings of the shee]). It then grows another skin, partially 

 •detaches the old skin while developing towards the third larval stage, 

 and in this *' ensheathed form " is called the " mature larva." The 

 whole process takes less than two days, and the mature larval stage 

 may therefore be reached about three days after hatching, provided 

 conditions of warmth and moisture are favourable. It is in this 

 stage that the worm is infective. Neither the egg' nor tiie first Jarva 

 cvAX infect an animal, but at the mature larval stage the tiny worm 

 proceeds to find a " host," i.e. a sheep or other ruminant, in whicJj 

 to complete its development. At this stage it is about one-thirtieth 

 of an inch in length, can live for a long time, and is migratory in 

 habit. It leaves the medium (dung of the sheep) in wliich it devel- 

 oped, and travels up the damp blades of grass. In eating the graf-:'S 

 the sheep takes the matured larvae into the stomach, and in this way 

 becomes infected. 



Tliird Larval ,S7(/<y6.— The worm now commences the parasitic 

 part of its life (first parasitic stage), casts its detached skin (remaining 

 from the second stage), continues its evolution, forms a third skin, 

 jnoults again, and emerges as the fourth larval stage. 



Fourth Larval Stage. — At this ''fourth larval" or "second 

 parasitic " stage, the worm commences to bore into the surface of the 

 st(nnach, causing a slight effusion of blood. Within the blood clot so 

 formed, the larval worm is found. Growth continues, and a ler.gth 

 of about one-fifth of an inch is reached in rather over a week, \\hile at 

 the same time differentiation of the sexes occurs. Male and female 

 worms can then he distinguished. A fourth skin [or3ns. is duly cast, 

 and the young adult worms appear : the whole process being completed 

 about a fortnight from the time the mature infectiAc larvae entered 

 ihe stomach. 



Adult Stage. — This third parasitic or last stage of growth is 

 -completed in about another fortnight, during which time the worms 

 grow from about a quarter of an inch in leng'ti) to full size of one 

 incli. and become sexually mature. Fertilization then takes place. 



