CHAPTER VII 



EPIDEMIOLOGY AND SANITARY MEASURES FOR THE CONTROL OF 

 NEMIC PARASITES OF DOMESTICATED ANIMALS 



T. W. M. CAMERON, Institute of Parasitology, Macdonald College, Quebec, Canada 



The parasitic existence of a nematode is dependent on its 

 finding a suitable environment in which it can mature and re- 

 produce and this involves four sets of factors: 



(a) SuccESSFUi, Admission to the Host. — The host must 

 traverse the ground where the free stages of the parasite are 

 found, it must eat suitable foodstuffs in or ou which tlie 

 larval stages occur; it must be exposed to the intermediate 

 host, and so on. Even slight differences in habits; e.g., such 

 as exist between a sheep and an ox in eating grass, may 

 make all the difference in the parasite gaining admission. 

 The anatomy of the host, including the thickness of the skin 

 is a factor to be considered under this head. 



(b) Suitable Environ ment.\l Conditions in the Host. — 

 Once inside the host, the parasite must find a suitable habitat 

 — type of mucosa in the intestine, length of intestine, pres- 

 ence of suitable food, and so on. 



(c) Possession of a Suit.able Protective Mechanism 

 Against the Normal. Metabolic Processes of the Host. — 

 When the parasite lies in the alimentary system, it must 

 possess some means of preventing itself being digested or 

 being passed out by peristalsis, etc. 



(d) Absence op a Host Reaction that Would Interfere 

 WITH THE Normal Metabolism of the Parasite. — This fac- 

 tor applies most obviously to parasites that leave the lumen 

 of the alimentary tract at some period of their life, but it may 

 apply to all. If there is any host reaction, the parasites must 

 be able to resist its effects. 



Under natural conditions, nematodes are more or less spe- 

 cific to a single species or a group of closely related species 

 of animals. In general, it may be taken that nematodes of 

 ruminants are not transmissible to horses, pigs, poultry, nor 

 those of the latter to each other. However, many parasites 

 of wild ruminants are transmissible to domesticated ruminants, 

 of wild carnivores to dogs, cats and foxes, of wild birds to 

 domestic ones, and so on. The important exceptions to this 

 generalization include the Trichina and some members of the 

 genus Tricltostrongyhis. 



Very few parasitic nematodes can complete their entire life 

 cycle within the same vertebrate animal, any more than they 

 can live a free independent existence. At one stage or an- 

 other they must leave the host to undergo some form of devel- 

 opment outside of it — either free, or in an alternate or inter- 

 mediate host. In effect, this means that a single young nema- 

 tode develops into a single adult only; there is no multiplica- 

 tion as in the case of bacteria. In both groups, disease de- 

 pends upon numbers, but whereas the entrance of a single 

 bacterium into the body may cause disease, the entrance of a 

 single larval nematode usually does not. Parasitic disease de- 

 jiends on actual numbers entering the body. 



The stages which leave the body, are never immediately 

 infective. Some essential development must take place before 

 they are ready to re-enter and this development takes a definite 

 period of time and requires a definite set of conditions — heat, 

 moisture, oxygen, presence of correct intermediate host and 

 so on — before they are able to infect. Once the infective 

 stage is reached, they are often able to live for a long period 

 before re-entry; while the minimum time necessary outside of 

 the body can be fairly accurately ascertained, the maximum 

 time is much more difficult to determine. 



In many cases, after entrance to the body, complicated 

 migrations through various organs are an essential part of 

 the life-cycle and the greatest damage to the host is often 

 caused at this period. We know of no means of preventing 

 these migrations and we know of no therapeutic agents which 

 can affect the nematodes during migration . Accordingly, 

 prevention of ingress is of the greatest importance. 



Scientific control consists in making development as difificult 

 as possible and so depends essentially on a knowledge of the 

 life history and bionomics of the parasite involved. This 

 necessitates the correct identification of the nematode con- 

 cerned. The need for correct identification is most important 

 as not only are no two parasites quite alike in their biology, 

 but treatment is often different. 



In determining control methods it is important to remem- 

 ber that there are economic aspects of the problem to be con- 

 sidered. The cost of control may be excessive and it must be 

 balanced against the loss to the stockowner — and loss should 

 include not only actual, but potential future loss. It is often 

 accordingly necessary to adopt several methods of control 



simultaneously rather than to employ a single method. Control 

 may be nationwide or it may be individual. Individual con- 

 trol is at best a palliative, and campaigns directed over a 

 wide area and infinitely more satisfactory. This not only in- 

 volves cooperation between veterinarians, agriculturists, ad- 

 ministrators and parasitologists but it involves careful co-ordi- 

 nation as well. A central authority and enabling legislation 

 are almost essential, but the legislation to be successful must 

 come as the result of a demand from the majority of the 

 farmers involved. A central laboratory with a good informa- 

 tion service is also desirable, with adequately staffed branch 

 or associate laboratories throughout the country. 



Control measures aim at breaking at some point the essen- 

 tial life-cycle of the parasite. If more than one point is at- 

 tacked, the chances of successful results are increased. These 

 measures will be discussed convenientl.v in several groups, al- 

 though it must be understood that such a hard and fast divi- 

 sion as is here adopted does not occur in nature and that 

 methods described under one may be equally applicable under 

 another. The parasite may be attacked at one or other of the 

 following points in its life cycle: — 



1. While in the ovum or as a young developing larva. 



2. During the developed infective stage, which does not 

 grow and ' ' rests ' ' until it enters the host. 



3. Before entrance of this larva into the host. 



4. Within the intermediate host or vector, within which 

 1 and 2 may be found and which may be the means of 

 entry to the host. 



-■). During the parasitic stages in the host. 



1. Methods of Destroying the Eggs of Pre-Infective 

 Larvae 



These are those stages passed in the faeces as well as the 

 subsequent stages which develop therein. (In a small minority 

 of cases the larvae are not passed in the faeces, but may 

 leave by the mouth or the urinary system or be abstracted 

 from the blood b.v blood-sucking animals, or may come to rest 

 in the host's muscles). As we know of no efficient method of 

 destroying the eggs in the host, this section is accordingly 

 mainly concerned with manure and its treatment. It is not a 

 new subject, having been advocated for years in connection 

 with human hookworm disease, but curiously enough, very lit- 

 tle indeed has been done about it in connection with nema- 

 todes of animals in which it is infinitely more important. 



There are two ways of treating manure. The first is to 

 disregard its parasite content and concentrate on its dispersal 

 in such a manner as to keep it out of harm's way. The sec- 

 ond is to treat it in such a way that its parasite content is 

 destroyed. Some twenty years ago, the late Dr. Maurice 

 C. Hall stated that this subject offered a field for a large 

 amount of investigation but this investigation has not been 

 done. 



"Broadly speaking," he said, "one would have to deter- 

 mine how long the larvae and eggs of the various species of 

 worms involved live in manure piles, in spread manure, in 

 closely packed manure; the effect of sunlight, of moisture, of 

 various chemicals, the chemicals in turn being of a nature 

 not to injure the fertilizing value of the manure. There are 

 practically no data on this and little could be surmised with- 

 out such data. ' ' 



Since that was written a considerable amount of investiga- 

 tion has been carried out on this subject and it may be con- 

 sidered under the following headings: 



1. Storage. The prompt daily collection of all manure in 

 the stables is an essential routine in farm practice and if 

 correctly carried out is a valuable preventive measure. If this 

 manure were stored in a proper container for a suflScient length 

 of time, without any other treatment all eggs and larvae would 

 be destroyed. Unfortunately, the time factor is too long to be 

 practical and resource to additional methods is necessary. 



'2. Heating. In piles of horse manure, all eggs and larvae 

 of the strougyle type are destroyed in 4 days by the natural 

 heat generated, with the exception of those in the outer 6 

 inches. As a temperature as high as 107 degrees F. is generated 

 in the central zone, all other parasites should be destroyed 

 also, although we are without definite knowledge of this. If 



302 



