TICKS AND MITES
The other group or Phylum of animals with which we will be particularly
concerned is known as the Arthropoda, which means "jointed-feet" and
includes the crayfish, crabs, spiders, mites, ticks and insects. Of
these only the last three are of interest to us now. It is customary to
speak of spiders, mites and ticks as insects, but as they have four
pairs of legs, instead of three pairs, in the adult stage, and as their
bodies are not divided into three distinct regions as in the insects,
they are placed in a different class.
GENERAL CHARACTERS OF TICKS
The ticks are all comparatively large, that is, they are all large
enough to be seen with the unaided eye even in their younger stages and
some grow to be half an inch long. When filled with blood the tough
leathery skin becomes much distended often making the creature look more
like a large seed than anything else (Fig. 14). This resemblance is
responsible for some of the popular names, such as "castor-bean tick,"
etc.
The legs of most species are comparatively short, and the head is small
so that they are often hardly noticeable when the body is distended. The
sucking beak which is thrust into the host when the tick is feeding is
furnished with many strong recurved teeth which hold on so firmly that
when one attempts to pull the tick away the head is often torn from the
body and left in the skin. Unless care is taken to remove this, serious
sores often result.
Ticks are wholly parasitic in their habits. Some of them live on their
host practically all their lives, dropping to the ground to deposit
their eggs when fully mature. Others leave their host twice to molt in
or on the ground. The female lays her eggs, 1,000 to 10,000 of them, on
the ground or just beneath the surface. The young "seed-ticks" that
hatch from these in a few days soon crawl up on some near-by blade of
grass or on a bush or shrub and wait quietly and patiently until some
animal comes along. If the animal comes close enough they leave the
grass or other support and cling to their new-found host and are soon
taking their first meal. Of course thousands of them are disappointed
and starve before their host appears, but as they are able to live for a
remarkably long time without taking food their patience is often
rewarded and the long fast ended.
Those species which drop to the ground to molt must again climb to some
favorable point and wait for another host on which they may feed for a
while. Then they drop to the ground for a second molt and if they are
successful in gaining a new host for the third time they feed and
develop until fully mature and the female is ready to lay her eggs. The
Texas fever tick, and some others, as we shall see, do not drop to the
ground to molt but once having gained a host remain on it until ready to
deposit their eggs.
The young ticks have only six legs (Fig. 15) but after the first molt
they all have eight.
TICKS AND DISEASE
_Texas Fever._ Ever since stockmen began driving southern cattle into
states further north it has been noted that the roads over which they
were driven became a source of great danger to northern cattle. Often
80% to 90% of the native cattle died after a herd of southern cattle
passed through their region and the losses became so great that both
state and national laws were passed prohibiting the driving or shipping
of southern cattle into northern states.
[Illustration: FIG. 14--Castor Bean Tick (_Ixodes ricinus_) not fully
gorged.]
[Illustration: FIG. 15--Texas fever tick, just hatched; has only six
legs.]
[Illustration: FIG. 16--Texas fever tick (_Margaropus annulatus_) young
adult not fully gorged.]
[Illustration: FIG. 17--_Amblyomma variegatum_ several ticks belonging
to this genus transmit _Piroplasma_ which cause various diseases of
domestic animals.]
But for years the cause of this fever, which came to be known as the
Texas fever, was not known. The southern cattle themselves seemed
healthy enough and it was difficult to understand how they could give
the disease to the others. It was early noticed, too, that it was not
necessary for the northern cattle to come in direct contact with the
others in order to contract the disease. Indeed the disease was not
contracted in this way at all. All that was necessary for them was to
pass along the same roads or feed in the same pastures or ranges. Still
more puzzling was the fact that these places did not seem to become a
source of danger until some weeks after the southern cattle had passed
over them and then they might remain dangerous for months.
In 1886 Dr. Theobald Smith of the Bureau of Animal Industry, United
States Department of Agriculture, found that the fever was caused by the
presence in the infected cattle of a minute Sporozoan parasite
(_Piroplasma bigeminum_). Further investigations and experiments proved
conclusively that this parasite was transmitted from the infected to the
well animal only by the common cattle tick now known as the Texas fever
tick (Fig. 16).
The infection is not direct, that is, the tick does not feed on one
host then pass to another carrying the disease germs with it. Unlike
many other ticks the Texas fever tick does not leave its host until it
is fully developed. When the female is full grown and gorged she drops
to the ground and lays from 2,000 to 4,000 eggs which soon hatch into
the minute "seed-ticks" which make their way to the nearest blade of
grass or weed or shrub and patiently wait for the cattle to come along.
If the mother tick had been feeding on an animal that was infected with
the Texas fever parasite, her body was filled with the minute organisms
of which some found their way into the eggs so that the young ticks
hatching from them were already infected and ready to carry the
infection to the first animal they fed upon.
It took many years of hard patient work to learn all this, but the
knowledge thus obtained cleared up much of the mystery in connection
with the occurrence of the fever in the north and, as we shall see,
suggested the possibility of other diseases being communicated in the
same way.
It was found that the southern cattle in the region where the ticks
occur normally, usually have a mild attack of the disease when they are
young and although they may be infected with the parasite all the rest
of their lives it does not affect them seriously. These cattle are
almost always infected with ticks, and when taken north where the ticks
do not occur naturally and where the cattle are therefore non-immune,
some of the mature ticks drop to the ground and lay their eggs which in
a few weeks hatch out and are ready to infect any animal that passes by.
The northern cattle not being used to the disease soon sicken and die.
It is estimated that the annual loss due to this disease and the ravages
of the tick in the United States is over $100,000,000, so of course most
determined efforts are being made to stamp it out. Formerly various
devices for dipping the tick-infested cattle into some solution that
would kill the ticks were resorted to, but it was always expensive and
never very satisfactory. The immunizing of the cattle by inoculating
them when they were young with infected blood has been practised. Very
recent investigations have shown that it is possible and practicable to
rid pastures of ticks by a system of feed-lots and pasture rotation. The
aim is to have as many of the ticks as possible drop to the ground on
land where they may be destroyed and to so regulate the use of the
pasture that the ticks in all of them may eventually be left to starve.
Several similar diseases of cattle, many of them probably identical
with Texas fever, occur in other parts of the world where the losses are
sometimes appalling. Horses, sheep, dogs, and other animals are also
affected with diseases caused by the same group of Protozoan parasites.
Most of them have been shown to be transmitted by various species of
ticks (Fig. 17) so that from an economical standpoint these little pests
are becoming of prime importance. Not only do they transmit the disease
germs that infect domestic animals but they are known to be responsible
for at least two diseases of men, Rocky Mountain spotted fever and the
relapsing fevers.
_Spotted Fever._ The first of these is a disease that for some years has
been puzzling the physicians in Idaho and Montana and other mountainous
states. A few years ago certain observers recorded finding Protozoan
parasites in the blood of those suffering from the disease, and although
more recent investigations have failed to confirm these particular
observations it is now quite generally believed that the disease is
caused by some such parasite and that the organism is transferred from
one host to another by certain species of ticks that live on wild
mammals of the region where the disease exists. Dr. H.T. Ricketts, who
has made a special study of the disease, has shown:
"1. That the period of activity of the disease is limited to the
season during which the adult female and male ticks attack man.
"2. That in practically all cases of this disease it can be shown
that the patient has been bitten by a tick.
"3. That the period between the tick bite and the onset of the
disease in the many animals he has experimented with corresponds
very closely to this period as observed in man.
"4. That infected ticks are to be found in the locality where the
disease occurs.
"5. That the virus of spotted fever is very intimately associated
with the tissues of the tick's body as is shown by the fact that
the female passes the infection on to her young through her eggs,
and further, by the observation that in either of the two earlier
stages of the life cycle the disease may be contracted by biting a
sick animal and communicated to other animals after molting or even
after passing through an intermediate stage."
Professor R.A. Cooley of Montana, from whose report the above quotation
is taken, has also made studies of the habits of the tick and believes
there can be no doubt that it is the disseminator of the disease.
_Relapsing Fever._ The relapsing fever is an infectious disease or
possibly a group of closely related infectious diseases occurring in
various parts of the world. Occasionally it is introduced into America,
but it does not seem to spread here. It has been shown that the disease
is communicated from one person to another by means of blood-sucking
insects. In Central Africa where the disease is very prevalent a certain
common tick (_Ornithodoros moubata_) (Fig. 18) is known to transmit the
disease. This tick lives in the resting places and around the huts of
the natives and has habits very similar to the bedbug of other climes,
feeding at night and hiding during the day. It attacks both man and
beast and is one of the most dreaded of all the African pests.
Nathan Bank, our foremost authority on ticks, in summing up the evidence
against them says:
"It is therefore evident that all ticks are potentially dangerous.
Any tick now commonly infesting some wild animal, may, as its
natural host becomes more uncommon, attach itself to some domestic
animal. Since most of the hosts of ticks have some blood-parasites,
the ticks by changing the host may transplant the blood-parasites
into the new host producing, under suitable conditions, some
disease. Numerous investigators throughout the world are studying
this phase of tick-life, and many discoveries will doubtless
signalize the coming years."
MITES
The mites are closely related to the ticks, and although none of them
has yet been shown to be responsible for the spread of any disease their
habits are such that it would be entirely possible for some to transmit
certain diseases from one host to another, from animal to animal, from
animal to man, or from man to man. A number of these mites produce
certain serious diseases among various domestic animals and a few are
responsible for certain diseases of men.
_Face-mites._ Living in the sweat-glands at the roots of hairs and in
diseased follicles in the skin of man and some domestic animals are
curious little parasites that look as much like worms as mites (Fig.
19). Such diseased follicles become filled with fatty matter, the upper
end becomes hard and black and in man are known as blackheads. If one of
these blackheads is forced out and the fatty substance dissolved with
ether the mites may be found in all stages of development. The young
have six legs, the adult eight. The body is elongated and transversely
wrinkled. In man they are usually found about the nose and chin and neck
where they do no particular harm except to mar the appearance of the
host and to indicate that his skin has not had the care it should have.
Very recently certain investigators have found that the lepræ bacilli
are often closely associated with these face mites and believe that they
may possibly aid in the dissemination of leprosy. It is also thought
that they may sometimes be the cause of cancer, but as yet these
theories have not been proven by any conclusive experiment.
In dogs and cats these same or very similar parasites cause great
suffering. In bad cases the hair falls out and the skin becomes scabby.
Horses, cattle and sheep are also attacked. The disease caused by these
mites on domestic animals is not usually considered curable except in
its very early stages when salves or ointments may help some.
_Itch-mites._ "As slow as the seven-years' itch" is an expression, the
meaning of which many could appreciate from personal experience, for it
certainly seemed to take no end of time to get rid of the itch once it
was contracted. Just why seven years should have been set for the limit
of the disease is not clear, for if the little roundish mites that cause
the disease live for seven years on a host they are not going to move
out voluntarily even if their seven-year lease has expired.
[Illustration: FIG. 18--_Ornithodorus moubata_, the Tick that Transmits
Relapsing Fever. From Boyce's "Mosquito or Man."]
[Illustration: FIG. 19--The follicle mite (_Demodex folliculorum_).
(After Murray.)]
[Illustration: FIG. 20--Itch-mite (_Sarcoptes scabiei_). (After
Murray.)]
[Illustration: FIG. 21--Harvest-mites or "jiggers." (_Leptus irritaus_
and _L. americanus_.) (After Riley.)]
The minute whitish mites (Fig. 20) that cause this disgusting disease
are barely visible to the naked eye. They are usually very sluggish but
become more active when warmed. They live in burrows just beneath the
outer layer of skin, sometimes extending deeper and causing most intense
itching. As the female burrows, she lays her eggs from which come the
young mites that are to spread the infection. Various sulphur ointments
and washes are used as remedies. Cleanliness will prevent infection.
Closely related to the itch-mite of man (_Sarcoptes scabiei_) are
several kinds attacking domestic animals, causing mange, scab, etc. The
variety infesting horses burrows in the skin and produces sores and
scabs, and is a source of very great annoyance. These mites may also
migrate to man. Tobacco water and sulphur ointments are used as
remedies.
Horses and cattle are also infested by other mites (_Psoroptes
communis_) which cause the common mange. These do not burrow into the
skin but live outside in colonies, feeding on the skin and causing
crusts or scabs. The inflammation causes the animal to scratch and rub
constantly and often causes the loss of much of the hair.
_Harvest-mites._ A score or more of different varieties of mites cause
many other diseases of domestic animals, such as the scab of sheep and
hogs and chickens, various other manges of the horses and cattle and
dogs, etc. But we need to call attention to just one more example, that
of the harvest-mites or jiggers (Fig. 21). Professor Otto Lugger, from
whose report on the _Parasites of Man and Domestic Animals_ most of
these notes in regard to the mites are taken, thus feelingly refers to
this pest.
"About the very worst pests of man and domesticated animals are the
Harvest-bugs, Red-bugs or Jiggers.... Men and animals passing
through low herbage that harbors them are attacked by these pests,
which, whenever they succeed in finding a host, burrow in and under
the skin, causing intolerable itching and sores, the latter caused
by the feverish activity of the finger-nails of the host, if that
should be a man, whose energy in scratching, apparently, cannot be
controlled and who is bound forcibly to remove the intruders. The
writer has been there! Those who have ever passed through meadows
infested with red-bugs will remember the occasion."
Horses, cattle, dogs and cats and other animals suffer also. Again
sulphur ointments are the best remedies.
"The normal food of these mites must, apparently, consist of the
juices of plants, and the love of blood proves ruinous to those
individuals which get a chance to indulge it. For, unlike the true
chigoe, the female of which deposits eggs in the wound she makes,
these harvest-mites have no object of the kind, and when not killed
at the hands of those they torment they soon die victims to their
sanguinary appetite."