nivores mid pigs, tlirx ild not :i|>|U'iii' capaMc nt' Ihtoiiuiik in 

 t'cotivo. Tliosf wliii'h will lu'cnino iiitVctivc puss into tlio imis 

 I'K'S. Control lies ontirclv in :ippi'(>pi'i.'itt< fecdin);. I'lKS must 

 not lie pcrmitti'd to cat iincookiMl inont foods. 



KlPNKYWOKMS. Till' fggs of tlu' snino ki<lni'.v worm nri' 

 passoil ill the nriiio and cxposuro to drviiij; and 8iiidi);lit will 

 dostroy them. This can often l)C done liy the iirovision of a 

 l>are lot around the hoK h>t, kept free from grass or shade and 

 well drained. The ckks of the kidney worm of carnivores are 

 also passed in the urine hut m)thinf; is yet known of their 

 I'ionomics and so no control measures can he adopted. 



Mu'iiO>'il,.\Ri.v. Few lilaria worms are important in stock. 

 .Ml depend, however, on their removal from lilood or skin l)y 

 a hlood sucking insect. Control accordinijly deiieiids on insect 

 control, screening of houses and related measures. 



2. Methods of Destroying Infective Larvae 

 (free or enclosed in egg shells) 



(a> Disinfection 



(i.) Chemical — There is no good chemical disinfectant for 

 larvae enclosed in their vgg shells (e.g. Ascarids^ and disin 

 fectaitts for this ])uritose are jiractically useless; in f:ict they 

 may assist the larvae hy destroying fungi and bacteria which 

 are themselves harmful to the ]iarasites. The use of chcmi 

 cals against free lai*\"ae has lieeu discussi'd ahovc as it is not 

 po.ssihle to separate the actions of chemicals on the preinfectivc 

 l.-irvae from the action on infective larvae, although in general 

 the latter are more resistant. 



(ii) Urat is lethal for all forms of parasites and, in the 

 form of very hot water or live steam, is one of the most efti 

 sient disinfectants at our command. It is the only one of any 

 practical value against -X.scarids and its use is fundamental in 

 the control of these very serious parasites in awine and car 

 nivores. It can be used either as hot water (with lye or soap 

 to loosen the dirt) or steam. Its use is recommended in all 

 kinds of stables as it kills every kind of larva. While its value 

 has been recognized for many years, it is only reci'utly that 

 accurate knowledge has been obtained on the amount of heat 

 required and it was found that ascarid eggs could be killed in 

 1 second at 158° F., "J seconds at 140° F. and ." seconds at 

 140° F. 



(iii), Cleanliiics.i both of animals and of ijuarters, is of 

 great value in reducing numbers of parasites. Washing of 

 udders of sows with warm water and soap removed man.v infec- 

 tive ascarid eggs. Washing of stables (including window-sills 

 and other places where dust lies) mechanically removes selero 

 sfome larvae which have very great powers of resisting drying 

 and which otherwise would be blown on to the animals' feed 

 or water. 



(b) Pasture 



Little work has yet lieeu done on the control of parasites on 

 pasture and arable land although the sub.ject is of great impor 

 tance in all parts of the world. Pastures cannot l>e treated 

 daily as can a manure liejt]) and so it is uminly infective larvae 

 which have to be killed. They are then in position in which 

 destruction is difficult. Chemical treatment has many dis- 

 advantages — cost being one of the most obvious. Nevertheless 

 some chemicals have been tried; copper sulphate, bleaching 

 powder and lime have jiroved unsatisfactory in practice. The 

 mo.st promising appear to be those which could be applied as a 

 gas and retained on the ground by a mulch of paper, a tarpau- 

 lin trailed behind a tractor or some similar method. Such gases 

 as chloropicrin (a tear gas), calcium cyanide and others men- 

 tioned have at least possibilities in that direction. .Mternately 

 it may be practical to use a delii|uescent salt alone or mi.xed 

 with a very lethal chemical so that the larvae may be attracted 

 toward the moisture. 



These are suggestious for the future; meanwhile, the only 

 effective way is to collect the droppings daily before the larvae 

 can migrate to soil or grass. Obviously this method has very 

 considerable limitations, but it has been done on stud farms. 



.\s infective larvae of bursate nematodes do not feed but live 

 entirely on foodstuffs stored up in their bodies, their life can 

 be shortened by causing them to use up this source of eiu'rgy 

 more quickly than usual. Modeiate warmth and light are 

 natural stimulants and, as continual spreading and harrowing 

 e.xposes them to these physical ageuts, it is of assistance in 

 reducing the numbers but will not completelv eradicite them. 

 It is especially effective in dry, warm climates. 



The burning of grass — often a valuable agricnitur.il practice 

 - — theoretically must destroy some larval worms on the pasture 

 aud in the ground beneath; it cannot be relied uijon to destroy 

 them all aud it may also give the larvae access to the more 

 succulent grass beneath. It does, however, help to raise the 

 nutrition plane of the animals. 



.\ II worm lai'vac icqniii' ,-i degree of moisture for develop- 

 ment, allhongli only » few (e.g. Did jiiirniiliiK) .are capable of 

 de\cIopment in w:iter :ilone. Drainjige, an essential step in the 

 control of these luiigworms, is always of general value In damp 

 pastures, it impro\-es the (piality of the gr'ass ami so improves 

 the resistance of the host, but it is donbtfnl if it di'strovs many 

 larvae. 



Drying of larvae has very variable results. Oidin.ary drying 

 is quickly leth.il to uuiny of the lu'uiatode larvae of slieej) but 

 only slightly so to other larvae and forms enclosed in egg shells, 

 like .\scari<ls. Ten per cent of sclerostouie larvae can survive 

 4 nuinths air drying in an incubator at 7.")° to HU° F. and they 

 have been f(Uind alive in window dust in stables out of use for 

 several ye.ars. ()rdin;uy drying distroys only some larvae but 

 it may destr<iy sufficient to prevent disease. 



Urought has not nece.ss;irily the .same effect as draining of 

 |i;istures. In dry seasons grass is short and scarce, more must 

 be eaten, (especially by sheep) it is eaten "short" and a 

 greater area of the pasture is grazed daily. Larvae of some 

 worms tend to live near the roots of the grass and .so under 

 these conditions m;iny more reach the host: the i)arasite intake 

 varies directly with the time of grazing. The resistance of the 

 host, through poorrii'ss of the feed, is lowered and so serious 

 disease may result. 



.Moreover, embiyouated but uidiatched eggs of sheep gastro- 

 intestinal nematodes are often very resistant to drought aud so 

 these tend to accumulate. When the drought breaks they hatch 

 simultaneously, and if the moist weather continues for a week or 

 so, they may cause an explosive outbreak of disease. However, 

 short jieriods of drought interrupted by short showeis have an 

 ojiposite effect. 



Heavy rain is inimical to the development of sclerostome 

 larvae in their ])reinfective st;iges ; whether this is due to the 

 fact that subsecpient drying is easier, is not yet known. Heavy 

 rainfall in hill country often has the effect of mechanically 

 washing Larvae and faeces off' the hillsides and so reduces the 

 number of infective larvae; this is especially true in troi)ical 

 islands and uplands where rainfalls are often very heavy. How- 

 ever, it may concentrate infections in the valleys. In fiat country 

 the larvae are washed off the grass but quickly crawl back 

 again. (It should be noted that an excessive growth of grass 

 may encouiage sheep to feed between the tall grasses. It is in 

 this ])osition that most larvae are found and so heavy infections 

 ma.v result ) . 



Continued exposure to extreme cold undoubtedly has a serious 

 effect on the free stages of parasites but in many cases we 

 cannot rely on the natural cold of winter to act as an important 

 agent. Destruction dci)ends at least to some extent on snowfall. 

 Where the snow is adequate, the temperature of the grouiul be- 

 neath is almost independent of the air temperature, and even 

 when the air temperature falls to 0°F., the ground temperature 

 is still little below freezing. The type of winter most destruc- 

 tive is the siioiflixx winter and a comparatively mild winter 

 with little snow, is much more destructive than a severe one with 

 a heavy snowfall. 



Moreover, in countries with a normally cold winte~, animals 

 of all kinds are stabled during cold weather and the parasites 

 can be carried over as adults in the host or as eggs in the ma- 

 nure in the barn. 



In countries with a mild winter climate, frost may actually 

 increase the life span of the parasites, although repeated freez- 

 ing and thawing is much more lethal than continued freezing. 

 There is little accurate knowledge yet available on the lower 

 thermal death points of parasites. It is known, however, that 

 some forms, such as horse .sclerostome larvae, can survive very 

 low temiieratures (-3t)°F.) for long periods; others such as 

 sheep nodular worms, are easily killed by cold. Each species 

 has its own critical temperature or range of temperature and 

 this tends to control its distribution independently of man. 

 However, man may often jiermit a parasite to survive in an 

 otherwise unfavourable environment — by suitable methods of 

 animal husbandry. Eastern Canada has a hot summer and a 

 cold winter, but animals are housed for the coldest months of 

 the yea I'. This i)ermits of the existence of such parasites as 

 Orsopliai/fislfnninn c'lliunbinintni in eastern Canada, although it 

 is absent from British Columbia and Great Britain, both of 

 which are less extreme in temperature. 



Sunlight is harmful to most nematodes but whether because 

 of light or heat or both is not definitely known. Its value in 

 destloying larvae of pig kidney worms is well recognized. 



Dung feeding insects such as beetles, are known to destroy 

 many worm I'ggs but in some cases, the eggs appear to pass 

 through them and they may act as distributors rather than 

 destroyers. Such insects may act as true vectors of some 

 worms. 



Mixed glazing on pasture is usually of great value, as horses 

 will eat and digest the larvae of worms which mature in cattle 

 and so on. In this way, the number of infective larvae swal- 



S05 



