AUREL O. FOSTER.
PARASITES and parasitic diseases are of a nature that sets them apart and is the basis of special principles that determine and explain the measures used to control them.
Animal parasites are a universal hazard to livestock production, although individual species are limited in distribution and intensity by a sensitive adjustment to climate (and other factors) and by the kinds of animals in which they develop.
They are abundant in numbers and kinds and are chiefly injurious because of the inapparent, unrecognized loss from subclinical parasitism. They are, then, especially dangerous because of their insidious and unspectacular effects despite the fact that they are generally better known for their ability to cause disease and death in livestock.
They are also commonly injurious to their hosts in a quantitative way. In most instances, as in the case of almost all the parasitic worms, or helminths, they do not multiply in or on affected animals. Even in protozoan and arthropod parasites (the major groups other than helminths), the individual cycles of development within the host animals are so limited, fixed, and finite that, despite multiplication in and on their hosts, the damaging effects generally are directly correlated with degree of exposure and magnitude of infection which cannot be said exactly of expo- sure or infection involving bacteria, viruses, and other disease-causing organisms.
Parasitism is essentially a herd or flock disease rather than one of individual animals. Measures to control parasites are effective therefore only it applied to a whole herd or flock as though it were but a single animal.
That is clearly true of sanitation a broad term that includes about every means of minimizing exposure short of chemicals that have specific and selective action. But it is also true of preventive medication, because the kind and extent of such medication depend on the hazards of specific parasitism. It also is true (with minor exceptions) of the use of chemotherapeutic, or curative, agents. Those agents often are unreliable in heavily parasitized animals and should therefore be used when parasitism in a flock is most responsive to control by them, but the urgency of treating all animals of a flock or herd is dictated by the fact that parasitism ordinarily is recognized first in one or a few animals when it is incubating in all the others.
Every parasite, moreover, has a relatively fixed cycle and rate of development, despite the abundance of different species, each of which has its own forms, habits, modes of life, and potentialities for causing disease and injury.
Each species therefore must fight its own battle for survival. The strongest attack must be made on the parasite's stage of development which is most vulnerable.
Immunological techniques cannot be used as a major aid to systematic control. That circumstance, more than any other, forces us to rely mainly on chemical measures of prevention and eradication.
PARASITE CONTROL is the judicious use of feasible, profitable measures to minimize the losses and the hazards of parasitism.
The term "parasite control" connotes something quite beyond "keeping parasites in check," "holding the line" against them, or "maintaining the status quo." By modern standards, eradication is the only rational goal, however remote that prospect or possibility may be.
Eradication of most parasitic infections is unfeasible in practice if not impossible. Measures are available, nevertheless, to effect attrition, prevention, or containment of most of the injurious species. The measures are based largely on sanitation and medication, the keystones of parasite control, which, if applied strongly enough, must ultimately spell eradication. There is therefore no logical basis for compromise or adjustment in the control of parasitic diseases.
THE GOAL has been achieved in a few instances. Tick fever a disease that caused an estimated loss of 73 million dollars in 1906 was eradicated from the United States after an unrelenting campaign against two ticks, Boophilus annulatus and B. microplus, transmitters of the microparasite of the blood, Babesia bigemina.
Dourine and surra of horses, trypanosome infections that occur all over the world, were eradicated by slaughtering affected horses. One can resort to that costly procedure only when the hazard is great and the number of affected animals is small. It is the one sure method of controlling diseases of whatever cause that have not yet secured a foothold in our animals. Its usefulness depends on recognition of diseases at the very beginning and on vigilance against their importation into this country.
But those cases are exceptions. Economic and other basic influences lead to compromise and indifference in many quarters where weapons of a sort are at hand for more vigorous onslaught.
We could feasibly eradicate scabies from cattle, sheep, and swine. We could eradicate lice, warbles, screw-worms, sheep nose bots, horse bots, and sheep keds. Almost within reach is the control of anaplasmosis and bovine venereal trichomoniasis. It is likely that by an all-out attack we could eradicate lancet flukes and common stomach worms of sheep and cattle, stomach hairworms of cattle,and nodular worms of sheep. We could also eradicate trichinosis and hydatid disease, both a constant risk to man. That we have not ended them is a measure of the job ahead in research and education.
THE DEVELOPMENT of control measures requires a full knowledge of injuriousness, parasite-host relationships, life cycles, sources of infection and means of transmission, epizootology, bionomics, immunity, factors that augment host resistance, and geographical distribution.
To eradicate tick fever it was necessary to have many basic facts: That the microparasites in the blood of sick animals were the sole cause of the disease; these organisms were tick-borne and were transmitted by only one kind of tick; this tick was a one-host species; there were no uncontrollable reservoirs of infection; the disease was transmitted by successive generations of ticks rather than by successive stages of the same tick; the disease could not exist if the ticks were eradicated; the developmental cycle of the tick was such that successive, definitely timed treatments would destroy all ticks before they could produce a new generation; and that efficient chemical measures could be devised for destroying the ticks. All those facts and more were used in devising the simple scheme of dipping of cattle south of the quarantine zone twice a month in a prescribed arsenical solution.
That is the "life cycle approach." A concerted attack on all fronts achieves best results, but "breaking the chain at its weakest link" is the essence of every effort directed against parasites.
Nearly every species has an environmental phase outside its host, which alternates with the parasite's stages in or on the animal. The former may involve an intermediate host for example, the cattle tick in the instance of the microparasite of cattle fever. There are one or more intermediate, reservoir, or vector hosts of many other parasites the trypanosomes, theilerias, anaplasms, histomonads (blackhead organisms of turkeys and chickens), leishmanias, liver flukes and other trematodes, tapeworms, many stomach worms, filarial worms, thorn-headed worms, and numerous others, including even an arthropod species, the tropical warblefly. These, like the parasite of tick fever, almost always are more subject to indirect attack by destruction of their intermediate hosts than by direct attack on the organism.
Often there is no intermediate host. In the direct cycle, the environmental phase may consist of free-living, pre-infective, and infective stages (as in gastrointestinal nematodes) or it may be the adult stage, as in flies that produce myiasis (screwworms, blowflies, cattle grubs, horse bots, the sheep nose bots, and others).
In any, there are successive host stages preadults, adults, eggs, larvae and embryos, in the case of parasitic worms that alternate with extra-host stages, such as the free-living pre-infective and infective larvae.
A parasite usually is more vulnerable to attack at one stage of this cycle than at another. Anthelmintics, for example, are commonly employed to destroy the adult stage of worm parasites in their hosts. Free-choice and low-level administrations of phenothiazine, however, act primarily upon the eggs and preinfective larvae of parasites outside the host, although there are also other significant actions, such as depression of the reproductive potential of female worms in their hosts.
The radiobiological attack on screw-worms is an attack on the egg stage of flies on potential hosts and also on the reproductive potential of adult flies.
Infestation by the most damaging gastrointestinal roundworm parasites is transmitted by means of the feces. Measures to prevent and destroy contamination of pastures, such as light stocking, resting and rotating pastures, stock rotation, chemical disinfestation, and general sanitation, consequently are the basis of many recommendations for control.
The time and environmental conditions that a parasite needs to develop are significant. Some parasites, such as cattle grubs and swine kidney worms, take so long to complete their life cycles that marketing the host animals often interrupts the cycle. Marketing practices, then, can influence parasitism, but time factors have greater significance in defining when, how often, and at what intervals medication can best destroy the parasite. Cattle thus are dipped or sprayed twice at an interval of 10 days for scabies, and swine are treated twice at an interval of 10 weeks to combat large roundworms.
Temperature, precipitation, and the other factors of climate determine the distribution, seasonal occurrence, and abundance of many parasites. Warmth and moisture generally favor their development. Many internal worm parasites must overwinter in breeder animals because the free-living infective stages cannot survive the cold of winter on pastures. They can be destroyed by anthelmintics between grazing seasons; so it is possible to put clean animals on clean pastures in the spring.
External parasites often exhibit just the opposite pattern. Lice, for example, survive in small numbers in the summer but become abundant in the winter. It is quite easy, therefore, to eradicate them by efficient treatment of animals in early fall. These off-season treatments are the basis of many measures of systematic, preventive medication. Generally it is advantageous to drain, fill, or fence swampy areas of pastures because they propagate parasites. Irrigation of and lands favors parasitism and thus creates special problems of control. Always, however, good control measures take advantage of environmental influences.