18 INSECTS AFFECTING DOMESTIC ANIMALS. 
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dependent upon two hosts, between which they alternate for particular 
stages of existence. 
As regards the animals infested little need ie said. For the domes- 
ticated species of mammals, cattle evidently support the largest number 
of species of external parasites and the horse the greatest number of 
internal parasitic insects, while the hog has the least. 
Of the birds, honors are about even between the hen and the pigeon. 
The turkey is somewhat more exempt. 
EFFECTS OF PARASITES ON THE HOST. 
The effects of parasites, or the presence of noxious insects upon the 
animal economy, may be considered under three heads: A. As direct 
agents of loss to the animal economy. J. As earriers of contagious 
or infectious diseases. OC. As carriers of other forms of parasites. 
Under the first head we must consider (1) the effect produced by the 
extraction of certain portions of the nutritive fluids of the body, (2) 
the loss of vitality due to the production of sores, obstruction of vital 
functions by pressure or clogging and formation ot scales, ulcers, etc., 
by the presence of the parasites, (3) the loss of energy due to irritation, 
worrying, and fretting of the animal. 
For the first part it may be argued that the nutriment extracted is 
so infinitesimally small that it is not worthy of notice; but observe 
the amount one mosquito or louse is capable of holding, then multiply 
this by the thousands that may infest a single animal, then reflect that 
the substance they extract is already passed through the slow process 
of preparation for use by the tissues, and it will be seen that this drain 
is not insignificant. If left in the animal it would go to the formation 
of flesh, the laying on of fat, or the strengthening of nerve. 
This loss, however, may not be so important as the loss from degen- 
eration of tissue manifested in sores, ulcers, scabs, mange, etc., which 
make a heavy drain on the vitality of the animal and prevent the 
normal action of the skin, these in turn causing excessive irritation, 
uneasiness, and other derangements of the animal economy. Indeed, 
the occurrence of these conditions may be looked upon in many cases 
as the result of lack of nutrition of the affected parts due to the drain 
by the parasite. 
The irritation of the presence of insects or their attacks upon some 
sensitive part of the animal is often of the greatest injury. To anerv- 
ous horse the uneasiness caused by the buzzing of the mosquitoes or 
flies is worse than the actual bites or loss of blood, while the intense 
irritation of buffalo- gnats in the eyes and nostrils, aside from their 
bites, is productive of the greatest distress. Doubtless much of the 
injury caused by lice is due to the irritation of their presence rather 
than to the loss from their eating. Especially is this true of the biting 
and running lice which feed upon the epidermal scales, hairs, and 
feathers, of little or no value to the host animal. Indeed, Van Beneden 
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