this disease which has greatly reduced the mortality in human subjects. Its
application to animals, however, is difficult and requires considerable
time and expense. A method of vaccination applicable to animals, consisting
of a single injection of a suspension of "fixed" rabies virus, is now being
quite extensively employed by veterinarians. Sanitary regulations which
seek to control effectively the disease by exterminating it among dogs are
most likely to prove successful. The measures which are adopted to this end
can not be discussed in this place, but it is a striking fact that where
the muzzling of all dogs has been rigidly enforced, as in England and in
certain German districts, the disease has been practically stamped out.
TUBERCULOSIS.
[Pls. XXXIV-XXXVIII.]
Tuberculosis is an infectious and communicable disease characterized in its
early stages by the formation, in various organs of the body, of minute
nodules or tubercles, which contain _Mycobacterium tuberculosis_, the cause
of the disease.
The disease, in its various manifestations, has been known for many
centuries, and legislative enactments having reference to the destruction
of affected animals and forbidding the use of the flesh date far back into
the Middle Ages. The opinions entertained regarding the nature and the
cause of the malady varied much in different periods and very markedly
influenced the laws and regulations in vogue. Thus, in the sixteenth
century, the disease was considered identical with syphilis in man. In
consequence of this belief very stringent laws were enacted, which made the
destruction of tuberculous cattle compulsory. In the eighteenth century
this erroneous conception of the nature of the disease was abandoned and
all restrictions against the use of meat were removed. Since that time,
however, its communicable nature has been established by many
investigators, and the tide of opinion has again turned in favor of
repressing the disease and prohibiting the sale of contaminated products.
_Occurrence._--The statistics concerning tuberculosis show that it is a
disease prevalent in all civilized countries. In some countries, such as
the northern part of Norway and Sweden, on the steppes of eastern Europe
and Russia, in Sicily and Iceland, and in Algiers, it is said to be quite
rare.
The returns from testing British cattle with tuberculin, supplied by the
Royal Veterinary College, as stated in March, 1900, showed that among
15,392 animals tested 4,105, or 26 per cent, reacted.
During the slaughter of cattle for pleuropneumonia careful examinations of
the carcasses were made for tuberculosis. Of 300 head killed near Edinburgh
120, or 40 per cent, were tuberculous. Of 4,160 killed in England 20 per
cent were tuberculous. Of one of these lots of cattle (451 animals) the
president of the Lancashire Farmers' Association testified that they were
fairly representative cattle--cows, heifers, and growing stock--a
thoroughly mixed lot; 20 per cent of them had tuberculosis.
Of 398 bovine animals taken haphazard in the city of Manchester, 120, or 30
per cent, were tuberculous. Among them were 168 cows, 69, or 41 per cent,
being tuberculous, and 2 having diseased udders.
The result of testing the Queen's herd at Windsor was that 36 out of 40, or
90 per cent, were found tuberculous.
The investigations made by the British Dairy Farmers' Association deserve
particular attention, coming as they do directly from a cattle owners'
organization. The council of this association "resolved to submit the
general consideration of the question to a committee, with a view to some
more definite understanding as to the possible extent to which tuberculosis
exists in dairy cattle." The secretary was instructed to write to a number
of dairy farmers being members of the association, asking their cooperation
and the use of their herds for the application of the tests. Of the herds
offered, 9 were selected, containing 461 cows and 12 bulls, and 188 of
these animals reacted, being 40.8 per cent. There were among these cattle
335 Shorthorns, of which 119, or 35 per cent, reacted; 67 crossbreds, of
which 28, or 42 per cent, reacted; 47 Ayrshires, of which 37, or 80 per
cent, reacted.
Another experiment of much interest is that of the Cheshire County Council.
The technical instruction committee set aside £250 to be used by a joint
committee from the agricultural and horticultural schools and Worleson
Dairy Institute for applying the tuberculin test to their herds. The tests
were made February 15, 1899. The results were: Worleson herd of 54 animals,
16 diseased, or 29.6 per cent; agricultural school herd of 17 animals, 4
diseased, or 23.5 per cent. The Worleson herd consisted of Shorthorn cows.
In each herd the purebred Shorthorn bull was tuberculous. The results of
the tuberculin test were confirmed by the slaughter of the animals and
examination of the carcasses.
Sir T. D. G. Carmichael, member of Parliament for Midlothian, gave evidence
before the royal commission that his Polled Angus herd was tested in the
spring of 1895. "The results of the test were fearfully unexpected and
alarming." Of 30 tested 13 showed decided reaction--43 per cent. Again, he
speaks of having 41 animals tested the same spring and 16 reacted--39.5 per
cent.
Of 80 Shorthorn cattle intended for export which were tested 34 reacted, or
42 per cent.
Of a herd of 25 British Shorthorns recently tested in quarantine 40 per
cent were found tuberculous.
The addition of these animals above referred to gives 20,930 head examined
and 5,441, or 26 per cent, pronounced tuberculous. And these herds were not
selected because they were supposed to be tuberculous, but represent the
general cattle stock of the country. These animals included at least 470
head of Shorthorns, of which 170, or 34 per cent, were tuberculous.
To these facts may be added the evidence of Prof. Bang that in the first
half of the nineteenth century tuberculosis was brought to Denmark by
cattle from Switzerland, Schleswig, and England, and that the same thing is
now going on in Sweden and Norway, particularly through English cattle.
Also the evidence of M. Sivori, chief of section at the ministry of
agriculture, Argentina, who has investigated tuberculosis in that country
and who says that "30 or 40 years ago tuberculosis was unknown in Argentine
cattle, and it is still unknown among the native (criollo) cattle. Its
appearance dates from the introduction of pure breeding animals. Statistics
prove that tuberculosis is observed among the grades--above all among those
of the Durham and less among the Hereford."
Moreover, the reports of the royal commission of Victoria, Australia, and
of the New Zealand department of agriculture show a large proportion of
tuberculous cattle in those colonies, where the disease was almost
certainly carried by British cattle.
In the same manner that tuberculosis has been carried from Great Britain to
Denmark, Sweden, Norway, Argentina, and Australia, it has also been taken
to Canada. In one herd of imported cattle slaughtered in the Canadian
quarantine station, 13 of the 14 animals were found tuberculous. One of the
largest Shorthorn herds in Canada was some time ago tested because an
animal from it was condemned when offered for shipment to the United
States. This herd was found to be very badly affected, and an effort is
being made to eradicate the disease by the Bang method. A Canadian official
publication says of another Shorthorn herd, which at one time had a very
high reputation, that when an investigation in regard to tuberculosis was
recently made the disease was found among ordinary cattle wherever animals
from this herd had been introduced, and that this herd, which had been
looked upon as one of the greatest benefits to the farming community, was
really a danger, because it disseminated tuberculosis among the farmers'
herds. Still another well-known herd recently attracted attention because
four animals from it offered for export to the United States were all
tuberculous.
From December 23, 1900, to February 19, 1901, the period that the
department inspector tested all Canadian cattle intended for shipment to
the United States, 140 purebred Shorthorns and 3 Shorthorn grades were
tested, and of the total number 26, or 18 per cent, reacted. During the
first month that this inspection was enforced, and when it may be assumed
that the condition of the cattle most nearly represented what it had
previously been, 74 cattle were offered for importation, and 18, or 24.3
per cent, were found tuberculous.
In justice to Shorthorn cattle it should be said in this connection that
they are probably no more susceptible to tuberculosis than are other
breeds, but the disease has been allowed to spread in certain herds and
families to such an extent as to give a wrong impression concerning the
breed as a whole.
The slaughterhouse statistics of Prussia show 14.6 per cent of the cattle
and 2.14 per cent of the hogs to be tuberculous. In Saxony the percentage
is 29.13 with cattle and 3.10 with hogs. In the city of Leipzig the figures
are 36.4 for cattle and 2.17 for hogs. (Siedamgrotzky.) Of 20,850 animals
in Belgium tested with tuberculin in 1896, 48.88 per cent reacted.
(Stubbe.) Of 25,439 tested in Denmark from 1893 to 1895, 49.3 per cent
reacted; and of 67,263 tested from 1896 to 1898, 32.8 per cent reacted.
(Bang.)
Figures available in the United States allow us to make a reliable estimate
of the extent of tuberculosis of cattle. The following summary is
presented:
_Statistics of tests for tuberculosis in the United States, 1917 to March
1, 1922._
-----------------------+---------+------------+-------------
State. | Number | Number | Per cent
| tested. |tuberculous.|tuberculous.
-----------------------+---------+------------+-------------
Alabama | 68,772 | 1,071 | 1.6
Arkansas | 5,917 | 98 | 1.7
Colorado | 1,959 | 76 | 3.9
Connecticut | 29,286 | 4,695 | 16.0
Delaware | 19,003 | 2,132 | 11.2
Florida | 56,533 | 1,438 | 2.5
Georgia | 46,522 | 998 | 2.1
Idaho | 57,731 | 1,052 | 1.8
Illinois | 92,781 | 6,112 | 6.6
Indiana | 142,833 | 3,991 | 2.8
Iowa | 158,514 | 9,958 | 6.3
Kansas | 64,341 | 1,796 | 2.8
Kentucky | 66,839 | 1,492 | 2.2
Louisiana | 36,391 | 981 | 2.7
Maine | 67,406 | 1,792 | 2.7
Maryland | 65,888 | 5,491 | 8.3
Massachusetts | 26,297 | 2,371 | 9.0
Michigan | 163,323 | 5,361 | 3.3
Minnesota | 240,888 | 7,555 | 3.1
Mississippi | 99,245 | 503 | .5
Missouri | 196,208 | 2,587 | 1.3
Montana | 165,640 | 3,346 | 2.0
Nebraska | 125,162 | 3,947 | 3.2
Nevada | 29,541 | 1,042 | 3.5
New Hampshire | 16,623 | 1,697 | 10.2
New Jersey | 32,184 | 2,542 | 7.9
New Mexico | 3,897 | 39 | 1.0
New York | 167,852 | 23,071 | 13.7
North Carolina | 64,008 | 1,098 | 1.7
North Dakota | 139,501 | 4,142 | 3.0
Ohio | 97,612 | 4,470 | 4.6
Oklahoma | 67,522 | 2,453 | 3.6
Oregon | 123,792 | 2,581 | 2.1
Pennsylvania | 102,465 | 6,322 | 6.2
Rhode Island | 3,458 | 338 | 9.8
South Carolina | 41,868 | 740 | 1.8
South Dakota | 43,433 | 2,353 | 5.4
Tennessee | 63,631 | 956 | 1.5
Texas | 61,956 | 1,256 | 2.0
Utah | 59,711 | 586 | 1.0
Vermont | 160,361 | 11,486 | 7.2
Virginia | 135,677 | 3,881 | 2.9
Washington | 154,292 | 3,864 | 2.5
West Virginia | 36,603 | 798 | 2.2
Wisconsin | 285,269 | 8,166 | 2.9
Wyoming | 22,811 | 322 | 1.4
-----------------------+---------+------------+-------------
Reports of tuberculin tests made on 400,000 cattle in the United States
during the years 1893 to 1908 by Federal, State, and other officers with
tuberculin prepared by the Bureau of Animal Industry show 37,008 reactions,
or 9.25 per cent. These were mostly dairy cattle, and in some cases herds
were suspected of being diseased.
Later reports of tuberculin tests made in the United States from July 1,
1917, to March 1, 1922, on 3,911,546 cattle by State, county, and Federal
officers engaged in cooperative tuberculosis eradication work showed
153,046 reactions, or 3.9 per cent.
All cattle in the District of Columbia, numbering 1,701, were tested with
tuberculin in 1909-10, and 18.87 per cent reacted. In 1909-11 herds in
Maryland and Virginia supplying milk to the District of Columbia were
tested, with 19.03 and 15.38 per cent of reactions, respectively, among
4,501 cattle.
All cattle in the District of Columbia were tuberculin tested in 1920-21,
numbering 1,313, and 5 animals reacted, or 0.4 per cent, demonstrating that
tuberculosis may be eradicated from all the herds in a circumscribed area.
The beef cattle of the United States show a much smaller proportion of the
disease than dairy cattle, though the percentage of cattle found
tuberculous in the Government meat-inspection service has increased
considerably in recent years. This increase is due partly, but not wholly,
to more stringent inspection. Of 7,781,030 adult cattle slaughtered under
Federal inspection during the fiscal year ended June 30, 1911, 76,448 were
found tuberculous, a percentage of 0.98.
From the statistics above referred to, and other data, it appears that in
the more densely populated areas of Europe and America from 5 to 50 per
cent of the dairy cattle are more or less affected with tuberculosis, while
the proportion of beef cattle affected is distinctly less, ranging from
0.14 to 30 per cent. This difference is due to a number of causes. Beef
cattle average younger when slaughtered. They are not so frequently
stabled, and are for that reason less liable to infection, and as the males
constitute a large proportion of this class of animals the effect of milk
secretion in lowering the vital forces is not so apparent. In the United
States it has been estimated that about 10 per cent of the dairy cattle are
tuberculous, while only about 2 per cent of the beef cattle are so
infected.
_Cause and nature of the disease._--The cause of tuberculosis is the
tubercle bacillus, which gains entrance to the body, lodges somewhere in
the tissues, and begins to grow and multiply at that point. As this
bacillus vegetates and increases in numbers it excretes substances which
act as irritants and poisons and which lead to the formation of a small
nodule, called a tubercle, at the point of irritation. As the bacilli are
disseminated through the animal body they affect many parts and cause the
formation of an enormous number of tubercles. By the union of such
tubercles, masses of tubercular material are formed, which in some cases
are of great size. The disease is called tuberculosis because it is
characterized by the formation of these peculiar nodules, and the bacillus
which causes the disease is for the same reason known technically as the
_Mycobacterium tuberculosis._
There are undoubtedly predisposing conditions which contribute toward the
development of the disease; some of these are found in the animal body and
others in the environment. An enfeebled condition caused by insufficient
feed, exposure to great extremes of atmospheric temperature and insanitary
surroundings, or the drain occasioned by heavy production of milk, appear
to aid the development of the bacillus, and there is also a special
individual susceptibility in some cases which may be otherwise described as
an inability of the animal tissues to resist and destroy the bacilli when
they have penetrated to the inner recesses of the body.
Among the conditions of environment which aid the development of
tuberculosis may be mentioned stabling with lack of ventilation, damp
buildings, the keeping of many animals together, drafts of air which cause
colds and catarrhs, and, in general, everything which prevents the animals
from developing and maintaining the highest condition of health. None of
these conditions of body or environment are sufficient to cause the
disease, however, unless the animals are exposed to the _Mycobacterium
tuberculosis_ and it penetrates the tissues of their bodies.
The ways in which the tubercle bacilli find their way into the body may be
considered under four heads: (1) By inhalation into the lungs; (2) by
taking into the digestive tract in the milk of tuberculous cows or with
other contaminated feed; (3) during coition when the sexual organs are
tuberculous; (4) from the tuberculous mother to the fetus in the uterus.
The bacilli can reach the lungs by inhalation only when the bacilli are
thoroughly dried and pulverized and in condition to be carried by currents
of air.
It is well known that the bacilli withstand drying for months before they
lose their power of producing disease. They leave the bodies of diseased
animals in several ways. There may be a little discharge occasionally
coughed up as a spray from the diseased lungs, or this material may be
swallowed and the bacilli carried off with the excrement, or milk may be
spilt, or there may be a discharge from the vagina when the genital organs
are tuberculous. There may also be ulcers of the intestines, from which
many bacilli escape with the feces. The bacilli from these sources may
become dried and pulverized and carried in the air of the stable and into
the lungs of still healthy cattle, where the disease then develops.
The disease of the stomach, intestines, and mesenteric glands is very
probably the result of feed infection. Tubercle bacilli may have been
scattered upon the feed by diseased animals, but the most common source of
such infection is the milk of tuberculous cows. Calves may become infected
in this way. The disease may remain latent until the animal becomes older.
The not-infrequent occurrence of tuberculosis of the uterus and ovaries
makes it probable that the disease may be transmitted by a diseased bull or
carried by a healthy one from a diseased cow to a number of healthy cows.
The source of infection is always some previous case of the disease, for
the disease can never rise spontaneously; hence, in those stables in which
there is frequent change of cattle the introduction of tuberculosis by
cattle coming from other infected stables is the most frequent source of
infection. Since the bacilli when dried can be carried by the air, it is
not necessary that healthy animals come in direct contact with cases of
disease to become infected. In general, the greatest number of cases occur
in the immediate environment of cities, where there are not only abundant
opportunities for infection, owing to the frequent introduction of new
animals into herds, but where the sanitary conditions may be regarded as
the poorest.
The bacillus of tuberculosis was discovered by Robert Koch in 1882. It is a
slender, rodlike body (see Pl. XXVIII, fig. 6) from one-third to two-thirds
the diameter of a red blood corpuscle in length. As already explained, when
the bacillus has become lodged in any organ or tissue it begins to
multiply, and thereby causes an irritation in the tissue around it, which
leads to the formation of the so-called tubercle. The tubercle, when it has
reached its full growth, is a little nodule about the size of a millet
seed. It is composed of several kinds of tissue cells. Soon a change takes
place within the tubercle. Disintegration begins, and a soft, cheesy
substance is formed in the center, which may contain particles of lime
salts. When these tubercles continue to form in large numbers they run
together, forming masses of various sizes. The disintegration which attacks
them leads to the formation of large cheesy masses of a yellowish color,
containing more or less of lime salts in the form of gritty particles.
These large tuberculous masses are surrounded by or embedded in layers of
fibrous tissue which in some cases becomes very dense and thick.
The disease is thus a development of these tubercles in one or more organs
of the body. The distribution and number of the tubercles determine the
course of the disease.
In a large number of cases the changes are limited to the lungs and the
serous membranes[4] of the thorax and abdomen. Pathologists have been in
the habit of calling the lung disease tuberculosis and the disease of the
serous membranes "pearly disease." Statistics have shown that in about
one-half of the cases both lungs and serous membranes are diseased, in
one-third only the lungs, and in one-fifth only the serous membranes. At
the same time the lymphatic glands near the diseased organs are usually
involved. Other organs, such as the liver, not infrequently contain
tubercles. Though the disease may remain restricted to a single organ, it
now and then is found generalized, affecting all organs of the body.
In the lungs (Pl. XXXIV) the changes observed vary according to the age and
intensity of the disease process. They usually begin with the appearance of
very minute tubercles. These may appear in large numbers on the surface of
the lungs or within the lung tissue. Later the contents become cheesy and
partly calcified. When these tubercles are sufficiently numerous to become
confluent, large masses may be formed, which undergo the same retrogressive
changes of caseation and calcification. In addition to the formation of
tubercles in the lung tissue, certain other changes take place. There is
usually bronchitis with abundant catarrhal secretion; this plugs up the
smaller air tubes, and the lung tissue supplied with air by the tubes
collapses. Subsequently it becomes filled with yellowish, cheesy matter,
which greatly distends the small air tubes and air vesicles
(bronchopneumonia). The connective tissue between the lung lobules, around
the tubercles, and around the air tubes becomes thickened and indurated. In
the larynx and the bronchi tubercles may vegetate upon the mucous membrane,
and ulcers may result from their breaking down. The inflammatory irritation
which the growth of the tubercles on the surface of the lungs arouses gives
rise to adhesion of the lungs to the ribs and diaphragm. This adhesion is
sometimes so firm and extensive that the lungs appear grown to the chest
wall.
When, therefore, the lungs in advanced stages of the disease are cut open
we observe large yellowish masses, from one-quarter to three-quarters of an
inch in diameter, of a cheesy texture, in which calcified, gritty particles
are embedded and which are surrounded by very firm connective tissue. The
neighboring lung tissue, when collapsed and involved in bronchopneumonia,
has the color and consistency of pale-red flesh. The air tubes, large and
small, stand out prominently on the cut surface. They are distended with a
pasty, yellowish, cheesy mass, surrounded and enveloped in thick mucus, and
their walls greatly thickened. The larger bronchi may be sacculated, owing
to the distention produced by the cheesy contents.
The disease usually attacks the bronchial glands, which are situated on the
trachea and bronchial tubes at the bifurcation. The changes in the glands
are the same as those going on in the lung tissue, and they frequently
reach an enormous size.