Host-parasite relations

Depending on their species, parasites may live in any organ or tissue of the host; they may live on its surface, or they may spend most of their time away from it. Special terms have been applied to these relationships. An endoparasite is a parasite that lives within the host’s body. An ectoparasite is one that lives on the outside of the body. An erratic (or aberrant) parasite is one that has wandered into an organ in which it does not ordinarily live. An incidental parasite is a parasite in a host in which it does not usually live. A facultative parasite is an organism that is capable of living either free or as a parasite. An obligatory parasite is an organism which must live a parasitic existence. A periodic parasite is one which makes short visits to its host to obtain nourishment or other benefits. A pseudoparasite is an object that is mistaken for a parasite. Parasites may themselves be parasitized by hyperparasites.

An organism which harbors a parasite is its host. There are several types of host. A definitive host is the host which harbors the adult stage of a parasite. An intermediate host is the host which harbors the larval stages of the parasite. A first intermediate host is the first host parasitized by the larval stages of the parasite. A second intermediate host is the host parasitized by the larval stages at a later period in the life cycle. A paratenic or transport host is a second (or third) intermediate host in which the parasite does not undergo any development but usually remains encysted until the definitive host eats the paratenic host.

The vector of a parasite or disease agent is an arthropod, mollusc or other agent which transmits the parasite from one vertebrate host to another. If the parasite develops or multiplies in the vector, it is called a biological vector. If the parasite does not develop or multiply in it, it is called a mechanical vector.

Intermediate hosts of helminths are biological vectors, but biological vectors are not necessarily intermediate hosts. Indeed, the latter term has no application to protozoa, bacteria, rickettsia or viruses, none of which have larvae. Mosquitoes are biological vectors of malaria and of yellow fever, and the tsetse fly is a biological vector of Trypanosoma brucei, for the parasites must develop in them to become infective for the next vertebrate host. However, tabanid flies are merely mechanical vectors of Trypanosoma evansi, since the parasites undergo no development in them.

The terms infection and infestation are used by different people in different ways. The former term originally referred to internal agents of disease, while the latter was used with reference to external harassing agents, including not only ectoparasites but also rodents, pirates and thieves. This usage was current during the latter part of the nineteenth century. Later on, it was felt desirable to distinguish between parasites which multiplied in their hosts and those which did not. "Infection" was then used for the former type of parasitism, and "infestation" for the latter. This usage was popular for a time, but it was never universally accepted. More recently there has been a trend toward the older usage. Most American parasitologists have accepted it, but most British ones prefer to speak of helminth infestations. In this book infection will be used to refer to parasitism by internal parasites, and infestation to parasitism by external parasites.

The term life cycle refers to the development of a parasite thru its various forms. It may be simple, as in an organism which multiplies only by binary fission, or it may be extremely complex, involving alternation of sexual and asexual generations or development thru a series of different larval forms. A monogenetic parasite is one in which there is no alternation of generations. Examples of this type are bacteria, flagellate protozoa such as Trichomonas, nematodes such as Ascaris and Ancylostoma, and the ectoparasitic fish trematodes of the order Monogenorida (= Monogenea). A heterogenetic parasite is one in which there is alternation of generations. Examples of this type are malarial parasites and coccidia, in which sexual and asexual generations alternate, the endoparasitic trematodes of higher vertebrates of the order Digenorida (= Digenea), in which there may be several larval multiplicative stages before the adult, and the nematode, Strongyloides, in which one generation is parasitic and parthenogenetic while another is free-living and sexual.

Depending on their type, parasites may live in only one or in a number of different types of hosts during the course of their normal life cycles. A monoxenous parasite has only one type of host - the definitive host. Examples are coccidia, amoebae, hookworms, fish trematodes, horse bots, streptococci and most pox viruses. A heteroxenous parasite has two or more types of host in its life cycle. Examples are the malarial parasites, most trypanosomes, trematodes of higher vertebrates, filariae, tapeworms, the rickettsiae, yellow fever virus and various encephalitis viruses.

These two pairs of terms are independent of each other. Parasitic amoebae and hookworms are monogenetic and monoxenous. Filariid and spirurid nematodes are monogenetic and heteroxenous. Strongyloides and most coccidia are heterogenetic and monoxenous. Malarial parasites and trematodes of birds and mammals are heterogenetic and heteroxenous.

Another group of terms deals with host range, i.e., the number of host species in which a particular parasite may occur. These parasites can be either monoxenous or heteroxenous, monogenetic or digenetic. Indeed, there may be a difference in host-restriction between the definitive and intermediate hosts of the same parasite. For example, the blood fluke, Schistosoma japonicum, can become adult in a rather wide range of mammals, but its larval stages will develop in only a few closely related species of snails.

The term, monoxenous parasite, is used by some authors for a parasite which is restricted to a single host species. Such parasites undoubtedly exist, but they are fewer than our present records indicate. The human malarial parasites were once thought to be monoxenous in this sense of the word, but they have more recently been found capable of infecting apes, and it is now known that chimpanzees in West Africa are naturally infected with P. malariae, the cause of quartan malaria in man (Garnham, 1958). Many species of coccidia are also known from but a single host, but for the most part closely related wild hosts have not been examined nor have cross transmission experiments been attempted with them. Because of this and because of the confusion arising between this usage of monoxenous and the one defined above, this usage should be avoided.

A stenoxenous parasite is one which has a narrow host range. Among the coccidia, members of the genus Eimeria are generally stenoxenous, as are the human malaria parasites and cyclophyllidorid tapeworms. Many nematodes such as the hookworms, nodular worms, filariids and spirurids tend to be stenoxenous. Both biting and sucking lice are stenoxenous, and many are even limited to specific areas on their host. Relatively few bacteria are stenoxenous, but Streptococcus agalactiae, Mycobacterium leprae, Vibrio, Mycoplasma, the spirochete, Treponema, the rickettsiae, Anaplasma, Eperythrozoon, Haemobartonella and Cowdria, and the viruses of hog cholera, duck hepatitis and yellow fever are stenoxenous.

An euryxenous parasite is one which has a broad host range. Among the coccidia, members of the genus Isospora are often euryxenous. So are most trypanosomes, most Plasmodium species (but not those affecting man), and many species of Trichomonas. Most trematodes are euryxenous, as are Trichinella, Dracunculus and Dioctophyma among the nematodes. Fleas, chiggers and many ticks are euryxenous. Most parasitic bacteria are euryxenous; examples are most species of Salmonella, Escherichia, Brucella, Erysipelothrix and Listeria. Among euryxenous rickettsiae are Rickettsia, Coxiella and Miyagawanella psittacii. Among euryxenous viruses are those of rabies and many encephalitides. Leptospira and Borrelia are euryxenous spirochetes.

The use of these two terms, however, may be deceptive. There exist in nature all inter grades between them, and all we have done has been to pick out the two extremes of a continuum and give them names.

Actually, the host range of most parasites is broader than generally supposed. The fact is that most animal species have not been examined for parasites. For example, the genus Eimeria is one of the commonest and best known among parasitic protozoa. Becker (1956) listed 403 species, of which 394 were from chordates and 202 from mammals. This is quite impressive, especially to someone who wishes to study their taxonomy. However, according to Muller and Campbell (1954), there are 33,640 known living species of chordates and 3552 of mammals. Some hosts have more than one species of Eimeria, but some coccidian species occur in more than one host. Assuming that these more or less cancel out, we can calculate that Eimeria has been described from only 1.17% of the world’s chordates and from 5.7% of its mammals. If all these possible hosts were to be examined, one might expect to find some 3500 species of Eimeria in mammals and 34,000 in chordates.

So far only the qualitative aspect of the host range has been discussed. However, altho a parasite may be capable of living in more than one host, it is much more common in some hosts than in others. The principal hosts of a parasite are those hosts in which it is most commonly found. The supplementary hosts are those of secondary importance, and the incidental hosts are those which are infected only occasionally under natural conditions. To these should be added experimental hosts, which do not normally become infected under natural conditions but which can be infected in the laboratory. This last category may include both incidental and supplementary hosts and also hosts never infected in nature.

In order to take into account this quantitative aspect of the host-parasite relationship, the terms quantitative host spectrum or quantitative host range are used. These give the amount of infection present in each infected species.

Several factors affect the quantitative host spectrum. One is geographic distribution. The natural quantitative spectrum may be quite different in one locality than in another. The species of animals present may be different, or the incidence of infection may be different. For example, a number of nematodes parasitize both domestic and wild ruminants. However, since the wild ruminants of North America and Africa are not the same, the quantitative host spectra of the same parasites on the two continents are different. The spectrum is still different in Australia, where there are no wild ruminants but where wild rabbits are susceptible to infection with a few ruminant nematodes.

A second factor is climate. Many of the same host species may be present in different areas but climatic conditions in one area may prevent or favor a parasite's transmission. For instance, the common dog hookworm in most parts of the United States is Ancylostoma caninum, but in Canada it is Uncinaria stenocephala. This is due to a difference in temperature tolerance of the free-living larval stages.

Local conditions such as ground cover are also important. If the vegetation is open so that the sunlight can get down to the surface of the soil where a parasite's eggs, cysts or free-living stages are found, survival will be much less, transmission will be reduced and the numbers of affected hosts will be fewer than if the vegetation is thick and protective. Or the kinds and numbers of parasites in a herd of animals confined to a low, moist pasture may be quite different from those in a herd kept on a hill pasture or on drylot.

A fourth factor is that of the distribution of acceptable intermediate hosts. Trypanosoma brucei occurs only in Africa because its tsetse fly intermediate hosts occur only there. The fringed tapeworm of sheep, Thysanosoma actinioides, is found in the western United States but not in the east despite the fact that infected sheep have repeatedly been introduced onto eastern pastures. A suitable intermediate host does not occur on these pastures, so the parasite cannot be transmitted.

A fifth factor is that of chronologic time. The quantitative host spectrum may be quite different in the same locality at different periods, particularly if an eradication campaign has been carried out in the interim. Echinococcosis is a case in point. At one time it was extremely common in the dogs, sheep and people in Iceland, but it has now been eradicated. Gapeworms were once common in poultry in the United States, but as the result of modern poultry management practices they are now exceedingly rare in chickens and turkeys, altho they are not uncommon in pheasants.

A sixth factor is that of the ethology or habits of the host. A species may be highly susceptible to infection with a particular parasite, yet natural infections may seldom or never occur. The habits of the host may be such that it rarely comes in contact with a source of infection even tho both exist in the same locality. For example, wild mink in the mid-western United States are not infrequently infected with the lung fluke, Paragonimus kellicotti. It is easy to infect dogs with this fluke experimentally, yet it is extremely rare in midwestern dogs. The reason is that dogs rarely eat the crayfish which are the fluke's intermediate host.

Because of these factors, we must speak of natural and potential host spectra. The latter term refers to the absolute infectability of potential hosts and not to the natural situation. The natural host spectrum is an expression of the actual situation at a particular time and place. The two spectra may be quite different, and of course the natural one will vary considerably, depending on the circumstances. The complete host spectrum has not been worked out for any parasite, and to do so would be a very time-consuming process. However, it will have to be done, at least for the more important parasites, before we can fully understand their ecology and the epidemiology of the diseases they cause.

Certain parasites and diseases occur in man alone, others in domestic animals alone, and others in wild animals alone. Still others, including some important ones, occur in both man and domestic animals, man and wild animals, domestic and wild animals, or in all three. A knowledge of their host relations is important in understanding their ecology and epidemiology.

A disease which is common to man and lower animals is known as a zoonosis. Zoonoses were redefined in 1958 by the Joint WHO/FAO Expert Committee on Zoonoses as "those diseases and infections which are naturally transmitted between vertebrate animals and man" (World Health Organization, 1959). Less than 20 years ago it was said that there were 50 zoonoses, but in the above report the World Health Organization listed more than 100, of which 23 were considered of major importance. Many more are certain to be revealed by future investigations.

Our thinking about parasites and diseases is ordinarily oriented toward either man or domestic animals. In this context, it is convenient to have a special term for hosts other than those with which we are primarily concerned. A reservoir host is a vertebrate host in which a parasite or disease occurs naturally and which is a source of infection for man or domestic animals, as the case may be. Wild animals are reservoirs of infection for man of relapsing fever, yellow fever and moist Oriental sore, while domestic animals are reservoirs for man of trichinosis and classical Oriental sore. Wild animals are reservoirs of infection for domestic animals of many trypanosomes, while man is a reservoir for domestic animals of Entamoeba histolytica.

Parasites and diseases may continue to exist indefinitely in their reservoir hosts, and man or domestic animals may become infected when they enter the locality where the parasites or diseases exist. Such a locality is known as a nidus (literally, "nest"). This term is used primarily in connection with vector-borne diseases, altho it need not be restricted to them.

Natural nidi may be elementary or diffuse (Palovsky, 1957). An elementary nidus is confined within narrow limits. A rodent burrow containing rodents, argasid ticks and relapsing fever spirochetes or a woodrat nest containing woodrats, kissing bugs and Trypanosoma cruzi is an elementary nidus. In a diffuse nidus the donors, vectors and recipients are distributed more widely over the landscape. A wooded region in which ticks circulate Rickettsia rickettsii among the rodents and lagomorphs is a diffuse nidus of Rocky Mountain spotted fever, as is an area where tsetse flies transmit trypanosomes among wild game. The nidality of a disease refers to the distribution and characteristics of its nidi.

The concept of the deme is useful in discussing host-parasite relationships, epidemiology, taxonomy, evolution, etc. (see Hoare, 1955). A deme is a natural population within a species. It lies more or less below the subspecies level, but it is not a formal taxon and is not given a Latin name. There are different types of deme. Nosodemes differ in their clinical manifestations. One example is Leishmania donovani, which has five nosodemes, Indian, Mediterranean, Sudanese, Chinese and South American, which produce different types of disease. Serodemes differ serologically. These are best known among the bacteria and viruses, but also occur among the animal parasites. Tritrichomonas foetus, for example, has several serological types or serodemes. Xenodemes differ in their hosts, and topodemes differ in geographic distribution.

There are also other types of demes. The population of a parasite species within a single host animal is a monodeme, and that in a single flock or herd is an ageledeme. Thus, a population of the stomach worm, Haemonchus contortus, in a single sheep is a monodeme, the population in all the sheep of a single flock is an ageledeme, that in all sheep is a xenodeme. The population in all cattle is another xenodeme and that in all goats is a third, the population of H. contortus in all hosts in North America is a topodeme, etc.

Each of these demes may differ morphologically and physiologically, and a large part of the taxonomist's work consists in determining the limits of their variation and deciding whether they are really demes or different species. Since the judgments of all taxonomists do not agree, there is some variation in the names which different parasitologists use. Demes are advance guards in the march of evolution, and no sharp line can be drawn beyond which they become subspecies or species. Taxonomists have been able to arrive at no better statement of how species are defined than to say that a species is what a specialist on its group says it is. And since some scientists are splitters and others are lumpers, their definitions vary with their temperaments. For most of us, the best rule is the pragmatic one of using those names which make for the greatest understanding of the organisms we study and of their relations with each other and with their hosts.

Parasite evolution

Parasites have evolved along with their hosts, and as a consequence the relationships between the parasites of different hosts often give valuable clues to the relationships of the hosts themselves. Certain major groups of parasites are confined to certain groups of hosts. Sucking lice are found only on mammals. Biting lice occur primarily on birds, but a few species are found on mammals. The monogenetic trematodes are found almost without exception on fish; some of the more highly evolved digenetic trematodes are found in fish, but more occur in higher vertebrates. There is a tendency, too, for the more advanced digenetic trematodes to occur in the higher host groups.

One would expect that, as evolution progressed in different host groups, there would develop in each one its own group of parasites. This has often occurred. Thus, of the 48 families of digenetic trematodes listed by Dawes (1956), 17 occur only in fish, 8 only in birds, 3 only in mammals, 2 in fish and amphibia, 3 in reptiles and birds, 6 in birds and mammals, 1 in fish, amphibia and reptiles, 2 in reptiles, birds and mammals, 1 in amphibia, reptiles and birds, 3 in all but fish, and 2 in all five classes of vertebrates. Of the 11 classes of tapeworms recognized by Wardle and McLeod (1952), 4 are found only in elasmobranch fish, 3 only in teleosts, 1 only in birds, 1 in teleosts, amphibia and reptiles, 1 in teleosts, birds and mammals, and 1 in amphibia, reptiles, birds and mammals.

This same tendency is apparent even in parasitic groups which are quite widely distributed. For example, many reptiles and mammals (but not birds) have pinworms of the family Oxyuridae, but each group has its own genera. Iguanas have Ozolaimus and Macracis, other reptiles have Thelondros, Pharyngodon and several other genera, rodents have Aspiculuris, Syphacia and Wellcomia, rabbits have Passalurus, equids have Oxyuris, ruminants have Skrjabinema, and man and other primates have Enterobius.

On the other hand, there are many exceptions to this general rule, and it cannot be used without corroboration as the sole criterion of host relationship. Many fish-eating birds and mammals have the same species of trematodes for which fish act as intermediate hosts. And the fact that the pig and man share a surprising number of parasites is no proof of their close relationship despite their similarity of character and personality; it simply reflects their omnivorous habits and close association.

Adaptation to parasitism

Adaptation to a parasitic existence has required many modifications, both morphological and physiological. Locomotion, at least of the parasitic stages, has often become restricted. Certain organs and organ systems may be lost. Tapeworms lack an intestine altho their ancestors presumably had one, and adult trematodes have no eye-spots altho their turbellarian ancestors and many of their larvae have them. Parasitic amoebae have no contractile vacuoles altho their free-living relatives do.

In contrast, many structures are modified or hypertrophied for the parasitic life. Many helminths have hooks and suckers to help them hold their position. The protozoon, Giardia, has turned most of its ventral surface into a sucking disc. The mouthparts of many insects and mites have become highly efficient instruments for tapping their hosts' blood supply. The chigger, which does not suck blood, has developed a method of liquefying its hosts' tissues. The food storage organs of many parasites have been enlarged. Many bloodsucking arthropods which are unable to obtain all the nutrients they need from blood, have established symbiotic relationships with various microorganisms and have formed special organs for them.

The reproductive system of many parasites has been hypertrophied to produce tremendous numbers of eggs. Other parasites, such as the trematodes, have developed life cycles in which the larvae also multiply.

In the parasites with high reproductive rates, infection is left largely to chance. Many other parasites, however, have developed life cycles in which chance is more or less eliminated. In these, the reproductive rate is low. The larva of the sheep ked, Melophagus minus, develops to maturity in the body of its mother and pupates immediately after emerging. The pupa remains in its host's wool. The female tsetse fly, too, produces fully developed larvae. The tropical American botfly, Dermatobia hominis, captures a mosquito and lays her eggs on it. These hatch when the mosquito lights to suck blood, and the larvae enter the host.

Morphological and developmental modifications are the most obvious ones, but biochemical ones are even more important. How do parasites survive in their hosts without destruction? What keeps those which live in the intestine from being digested along with the host's food? Why is it that morphologically similar species are restricted to different hosts which themselves may be morphologically quite similar?

The second question has been answered by saying that the same mechanism operates which prevents the hosts from digesting themselves, that the parasites protect themselves by producing mucus or that mucoproteins in their integument protect them, that they secrete antienzymes, or that the surface membrane of living organisms is impermeable to proteolytic enzymes. However, much more research must be done before a satisfactory answer can be given. Answers given to the first and third questions are vague. Compatibility of host and parasite protoplasm is invoked, but all this does is put a name to the beast. The question of how this compatibility is brought about remains unanswered, and a great deal of biochemical and immunochemical research must be done before it can be answered (see Becker, 1953; Read, 1950; von Brand, 1952).

Injurious effects of parasites on their hosts

Parasites may injure their hosts in several ways:
1. They may suck blood (mosquitoes, hookworms), lymph (midges) or exudates (lungworms).
2. They may feed on solid tissues, either directly (giant kidney worms, liver flukes) or after first liquefying them (chiggers).
3. They may compete with the host for the food it has ingested, either by ingesting the intestinal contents (ascarids) or by absorbing them thru the body wall (tapeworms). In some cases they may take up large amounts of certain vitamins selectively, as the broad fish tapeworm does with Vitamin B12.
4. They may cause mechanical obstruction of the intestine (ascarids), bile ducts (ascarids, fringed tapeworm), blood vessels (dog heartworm), lymphatics (filariids), bronchi (lungworms) or other body channels.
5. They may cause pressure atrophy (hydatid cysts).
6. They may destroy host cells by growing in them (coccidia, malaria parasites).
7. They may produce various toxic substances such as hemolysins, histolysins, anticoagulants, and toxic products of metabolism.
8. They may cause allergic reactions.
9. They may cause various host reactions such as inflammation, hypertrophy, hyperplasia, nodule formation, etc.
10. They may carry diseases and parasites, including malaria (mosquitoes), trypanosomosis (tsetse flies), swine influenza (lungworms), salmon poisoning of dogs (flukes), heartworms (mosquitoes) and onchocercosis (blackflies).
11. They may reduce their hosts' resistance to other diseases and parasites.

A great deal more could be said about this subject. Additional information is given in the symposium on mechanisms of microbial pathogenicity of the Society for General Microbiology (Howie and O'Hea, 1955).

Resistance and Immunity to Parasites

This is such a tremendous subject that its facets can only be hinted at. The general principles of immunology apply to animal parasites as much as they do to bacteria, viruses and other microorganisms. However, since the association of many of the larger parasites with their hosts is not as intimate as that of microorganisms, the hosts' immune responses may not be as great. This is especially true with regard to the formation of circulating antibodies.

Immunity or resistance may be either natural (innate) or acquired. Natural resistance is the basis of host-parasite specificity, but, as mentioned above, little is known of its mechanism. Acquired immunity may be either active or passive. Active immunity results from the body's own action. It follows exposure to living or dead disease agents, and can result from natural infection or artificial administration of virulent, attenuated or killed organisms.

One type of active immunity is premunition. This is immunity due to the continued presence of the disease agent. It occurs in such diseases as babesiosis and anaplasmosis.

Passive immunity results from the introduction of antibodies produced by some other animal. It may be acquired naturally, thru the colostrum or milk in mammals or thru the egg yolk in birds, or artificially by injection of antiserum. Passive immunity is seldom as long-lasting as active immunity.

Immunity against parasites and disease agents generally increases with age. There are exceptions, however. Young cattle, for instance, are more resistant to Babesia and Anaplasma than are adults. Age immunity may be either developed as the result of previous exposure or it may be natural. Not all the factors operating in the latter case are known. An important one is that very young animals cannot mobilize their body defenses against invasion as efficiently as adults. For instance, they do not produce antibodies at first, depending on those acquired from their mothers. Another factor, discovered by Ackert and his co-workers (cf. Ackert, Edgar and Frick, 1939) to explain the relative resistance of older chickens to Ascaridia galli, is that these birds have more intestinal goblet cells than do young birds. The goblet cells secrete mucus which inhibits the development of the worms. For further information on immunity in parasitic infections, see Taliaferro (1929), Culbertson (1941) and Soulsby (1960).

Genetic constitution is also important in determining resistance to parasites. For instance, Ackert et al. (1935) showed that Rhode Island Red and Plymouth Rock chickens are more resistant to Ascaridia galli than are Buff Orpingtons, Minorcas and White Leghorns. Cameron (1935) found that in a mixed flock of sheep, Cheviots were less heavily parasitized with gastrointestinal nematodes than Shetlands and Scottish Blackface, and that these in turn were less heavily parasitized than Border Leicesters. Stewart, Miller and Douglas (1937) found that Romney sheep were markedly resistant to infection with Ostertagia circumcincta, while Rambouillets were less so and Southdowns, Shropshires and Hampshires were least resistant. Certain individuals among the more susceptible breeds, however, were just as resistant as the Romneys. Whitlock (1958) has studied genetic resistance to trichostrongylidosis in sheep in some detail.

The nutritional status of the host may affect its resistance. Poorly nourished animals are usually more susceptible to infection and suffer more severely from its effects. Protein depletion or protein starvation is particularly important. Lack of specific vitamins and minerals generally decreases resistance, but there are cases in which lack of a certain vitamin which the parasite requires may affect the parasite adversely. Thus, Becker and Smith (1942) found that when calcium pantothenate was added to a ration containing restricted vitamins B1, B6 and pantothenate, the number of oocysts produced by Eimeria nieschulzi infections in the rat was increased.

Geographic Distribution

Some parasites, particularly those of man and his domestic animals, are worldwide in distribution, but others are much more restricted. But even a widely distributed species may be much more prevalent in one region than another. Many factors are responsible, some of which have already been discussed (pp. 7-8). A parasite which originated in a particular place in a particular host species may never have been introduced into some other locality or host where it could develop perfectly well. It may have been introduced but may have died out because a suitable vector was lacking or because the climate was not suitable. The ox warble has not been able to establish itself in the southern hemisphere because the reversal of seasons has prevented it from completing its life cycle.

Whenever domestic animals are introduced into a new region, there is a good possibility that they will pick up some of the parasites of their wild relatives there. The parasite spectrum of cattle in Africa differs from that in North America, both of these differ from the spectrum in Europe, and all three differ from the spectrum in Australia. Wild animals, too, may acquire parasites from domestic ones or from other wild species. Hence, the parasite spectrum of animals in zoos may be quite different from that in their normal habitat, and the success of an attempt to introduce a new game bird or mammal into a region may depend in part on the parasites and diseases that it encounters.

The importance of wildlife as a parasite reservoir for domestic animals is well illustrated by the report of Longhurst and Douglas (1953) on the interrelationships between the parasites of domestic sheep and Columbian black-tailed deer in the north coastal part of California, where the two live on the same range. They found in their survey of 63 sheep and 81 deer that 1 species of trematode, 5 of cestodes, and 13 out of 18 species of nematodes were common to both hosts.

Origin of Parasitism

Parasites originated from free-living ancestors. The process probably began soon after the first living forms appeared. The change from a free-living to a parasitic habitat has taken place many times in the course of evolution. It has occurred as new major groups appeared, it has taken place independently many times in each group, and it is undoubtedly still occurring. Once established, the parasites evolved along with their hosts.

In some cases, the parasites first invaded the host thru the integument, like Pelodera and related rhabditid nematodes. In other cases, the parasites were swallowed along with their host's food. Parasites with life cycles involving two or more hosts became established first in one host, and later on developed their more complicated life cycles. The trypanosomes, for instance, were originally gut parasites of insects and only later became blood parasites of vertebrates.

Preadaptation was necessary for parasites to become established. They must have had the ability to survive and reproduce in the host before they entered it. By far the great majority of free-living forms which entered the alimentary canal of some larger animal were killed and digested, but some of them were able to resist this process and a few were able to live there. Some of the factors involved have already been discussed (p. 10).

Economic Importance

Parasites are responsible for heavy economic losses to the livestock industry. These are due in part to death, but even more important are the losses due to illness, reduced growth rate, decreased meat, milk, egg and wool production and, in working animals, loss of working energy. It is impossible to quantitate these losses accurately, but rough estimates can be made. The U. S. Department of Agriculture (1954) made such an estimate for losses in agriculture during the ten-year period, 1942-1951. The figures on parasite losses in Table 1 are taken from this publication. Further details are given in the publication itself and by Schwartz et al. (1955).

Annual loses due to parasites of livestock in the U.S., 1942-1951

Parasites caused an estimated loss of $939,848,000 per year. All other diseases, both infectious and nutritional, were estimated to cause a total annual loss of $1,748,594,000, so parasites are considered to be responsible for about 35% of the losses in the American livestock industry. A billion dollars a year is a sizeable figure. We can hardly expect to eliminate this loss completely, but if every animal owner took advantage of our present knowledge, a half billion dollars a year, or even more, could be saved.

Scientific Names

There are several million species of animals in the world. Many of them are well enough known and easy enough to recognize to have received common names. However, these names vary from one language to another and from one locality to another among people who speak the same language. Furthermore, the same common name is often applied to different species in different regions.

In the United States, "cattle" refers to the ox, Box taurus, but in India it refers to the zebu, Bos indicus, and in England and some other countries (and in the Bible) to domestic livestock in general. "Fowl" has more than one meaning. It may refer to the chicken, Gallus domesticus, but it may refer to any bird raised for food, including the turkey, Meleagris gallopavo, and ducks. Most domestic ducks are Anas platyrhynchos, but the Muscovy duck is Cairina moschata. One of the worst offenders is "rabbit" which is applied indiscriminately to many quite different species. Rabbits are not rodents, but lagomorphs; they have four upper incisors, whereas rodents have only two. The domestic rabbit is the common wild rabbit of Europe, Oryctolagus cuniculus. The common wild rabbit of North America, however, is the cottontail, Sylvilagus, of which there are 13 species. In addition, there are several species of jack rabbits belonging to the genus Lepus.

In order to prevent the confusion which would be inevitable in dealing with these myriad species, a system of scientific names has been worked out. This system was first established by Linnaeus in the eighteenth century, and the starting point for the names of animals is the tenth edition of Linnaeus' Systerna Naturae, which was published in 1758. An International Code of Zoological Nomenclature was adopted in 1904; it was reviewed at a colloquium held in Copenhagen in 1953, and a new, revised code was adopted by another colloquium held in London in 1958. This code establishes rules for naming animal species and for indicating their relationships.

In the system of binomial nomenclature used for scientific names, each species is given two names. The first name, which is capitalized, is used for a group of closely related species; this group is called a and its name is the generic name. The second name, which is not capitalized, is used for a single species within the genus and is called the specific name. A particular generic name can be used for only a single group of species in the animal kingdom, but the same specific name can be applied to species in different genera. The generic and specific names are often derived from Latin or Greek, but they may also be based on the names of persons, geographic localities, etc. They must, however, have latinized endings. Both names are written in italics.

The name of the person who first named each species and the date when he did it are also part of its scientific name, altho these are often omitted in non-taxonomic writing. If the namer assigned the species to a different genus from the one which is accepted as correct, then his name and date are enclosed in parentheses and are followed outside the parentheses by the name of the person who assigned the species to its present genus with the date when he did it. If there has been no change in the genus designated by the original author, parentheses are not used. Thus, the common large roundworm of the dog, Toxo car a cams, was first described by Werner in 1782, but he assigned it to the same genus as the earthworm, Lumbricus. In 1905, Stiles established a new genus, Toxocara, for this species. The original name, then, was Lumbricus canis Werner, 1782, and the presently accepted name is Toxocara canis (Werner, 1782) Stiles, 1905. Similarly, in the early days of parasitology almost all tapeworms were assigned to a single genus, Taenia. As knowledge increased, more and more genera were split off from it. The common sheep tapeworm was called Taenia ex pans a by Rudolphi in 1805, but in 1891 Blanchard established a new genus, Moniezia, for it, so that its correct name is now Moniezia expansa (Rudolphi, 1805) Blanchard, 1891.

Genera are grouped together into families, families are grouped into orders, orders into classes, and classes into phyla. Each of these categories, and also each of the lower ones, is known as a taxon (pi., taxa). Subfamilies and superfamilies, suborders and superorders, etc. are often used, and in some cases so many relationship levels are recognized that it is necessary to introduce cohorts, tribes, etc.

Each family is based on one of its genera, known as the type genus, and the name of the family is obtained by attaching the ending, -idae, to the root of the name of the genus. Thus, Strongylus belongs to the family Strongylidae, and Trichomonas to the family Trichomonadidae. The subfamily ending is -inae.

While the botanists long ago adopted a system of uniform endings for the names of their higher taxa, the zoologists have never been able to agree on one. As a consequence, it is impossible to determine the ranks of the higher taxa with certainty from their names. In the present book, however, the system of uniform endings proposed by Levine (19 59) is used, so this problem does not arise. These are: Superclass, -asica; Class, -asida; Subclass, -asina; Superorder, -orica; Order, -orida; Suborder, -orina; Supercohort, -icohica; Cohort, -icohida; Subcohort, -icohina; Superfamily, -icae; Family, -idae; Subfamily, -inae; Supertribe, -ibica; Tribe, -ibida; Subtribe, -ibina.

Many scientific names appear quite formidable at first glance. They have definite meanings, however, and it helps in remembering them to know what these meanings are. Since most scientific names are based on Latin or Greek, a knowledge of some of the descriptive words from these languages is helpful. Much information can be obtained from a dictionary of derivations such as that of Jaeger (1955). The thorny-headed worm of swine is Macracanthorhynchus hirudinaceus. This name is derived from the Greek. The generic name means "large (macr-) thorny (acantho-) proboscis (-rhynchus)". The specific name is derived from the scientific name of the leech (Hirudo) and means "leechlike"; it was given because the worm is firmly attached to the intestinal wall and looks vaguely like a leech. The name of the whipworm, Trichuris, commemorates an error. This nematode looks a good deal like a buggy-whip, with a sturdy body and a long, whip-like anterior end about as thick as a hair. This is not permissible according to the rule of priority of the International Code of Zoological Nomenclature, so the error remains.

It is often discouraging to students and scientists alike to see the many changes in scientific names which continue to be made. These, however, appear to be inevitable. As new knowledge is gained, some species must be split up, others recombined and still others shifted from one genus to another. It is sometimes found that a name which has been long used and accepted must be dropped in favor of an unfamiliar one, either because it had been used first for some other species or because the less familiar name had been given earlier but overlooked.

Another reason for these changes lies in human nature itself. No satisfactory criteria have ever been established for the definition of species, and some taxonomists go into finer differences than others in separating them.

The taxonomists' difficulties arise because what they are dealing with are individual organisms, and all taxonomic schemes are the result of man's attempts to arrange these individuals in a system which shows their relationships. All taxa, whether species, subspecies, genera, families or whatnot, are products of this abstraction process and have no real existence outside the human mind. Many taxonomists, however, refuse to accept this idea, believing that species are real and external, and that their task is simply to discover and differentiate them. It is easy to understand why they do not like to believe that they are devoting their lives to figments of the imagination.

Without the labors of the systematists we should be in a state of hopeless confusion. Their scientific names and their taxonomic schemes are absolutely necessary if we are to carry out reproducible experimental work or understand practically all biological phenomena.