This has a reference to both man and beast and involves measures of
private and public hygiene alike. In man, as we have seen, milk is the
source of greatest danger, being habitually taken uncooked, whereas meat
has been usually subjected to a high temperature before it comes to the
table.
_Milk._ The unsterilized milk of the tuberculous cow must always be
regarded with grave suspicion. Especially is the drinking of milk warm
from the cow to be strongly condemned. The feeding experiments of
Vilemin, Gerlach and others, in 1866 to 1869, demonstrated the virulence
on the lower animals of the milk of infected cows, and many accidental
infections have shown the same for man. (See relation of the bacillus of
man to that of animals). This conceded, the question arose as to the
virulence of milk drawn from the sound udders of tuberculous cows. Many
experiments of Nocard, Galtier, McFadyean and others seemed to decide
against such infection. Others have had a different experience, and
especially when the test was made by inoculation. Ernst found tubercle
in 30 per cent. of the cows he examined, though no affection of the
udder could be made out. Theobald Smith, Hirschberger, Bang and others
found tubercle bacilli in milk from sound udders. Pearson, injecting the
milk from sound udders intraperitoneally into Guinea pigs had ten in
sixty-three affected. Rabinowitsch and Kemper found tubercle bacilli in
ten out of fifteen cows with sound udders. While it must be allowed that
often a very small proportion of tuberculous cows with sound udders pass
the bacilli in the milk yet a sufficient number of exceptions are found
to deter one from endorsing such milk as safe. Much more is this
conclusion justified by the consideration that commencing mammary
tuberculosis is in many cases unrecognizable by ordinary examination
intravitam or post mortem. Again when the bacilli are circulating in the
blood (generalized tuberculosis) they can escape from such a vascular
tissue as the mamma without the formation of a local lesion, as they can
pass through the intestinal walls or lungs and colonize the adjacent
lymph glands. Nocard’s injections of the bacilli into the veins seem to
show that they disappear from the blood in 4 to 6 days, but with a
generalization of the infection from within, the presumption is against
a single isolated entrance of bacilli, and in favor of a continuous
introduction. In such generalization therefore the bacilli circulate in
all vascular tissues, and are liable to escape with any normal
secretion, but especially with the milk or urine on account of the great
vascularity of the glands.
When the udder is itself tuberculous the case is incomparably worse. The
milk can scarcely fail to be infecting, and the bacilli, grown in these
highly vascular tissues, apart from the air, are usually of a very
virulent type. Of these Martin writes in the report of the Royal
Commission (England): “The milk of cows with tuberculosis of the udder
possesses a virulence which can only be described as extraordinary. All
animals inoculated showed tuberculosis in its most rapid form.” Woodhead
is equally positive in this position. Galtier says “we should absolutely
avoid the consumption of milk from cows in which the udder is
tuberculous, and boil before using, the milk of all cows affected with,
or suspected of tuberculosis in any form.” Finally we should never
consume without boiling, milk of which we do not certainly know the
origin, and especially in cities, we ought not to omit this precaution
in the case of milk furnished by milkmen.” Rabinowitsch and Kemper add:
“Milk from cows that react to tuberculin must be suspected of being
tuberculous in every case.”
It may well be allowed that the mixed milk from a large dairy,
containing but one or two tuberculous cows, is much less infecting than
that of the tubercular cow herself, and that skim milk that has been
passed through the separator has been robbed of many bacilli which have
been precipitated in the albuminoid material that collects on the inner
side of the bowl. But these mean dilution not purification: they reduce
the danger but do not altogether remove it, and sanitary police should
aim at sterilization in every case where available.
_Butter_, _cheese_, _whey_, and other dairy products have been proved
virulent in different cases (Galtier, Heim, Lasar, Bang, Obermüller,
Roth, Groëning, Gasperimi, Petri, Rabinowitsch). This was true for
butter 120 days old, and cheese 330 days. In a number of cases
Rabinowitsch found a bacillus, similar to the tubercle bacillus and
producing tubercle-like lesions, but differing in its staining,
morphology and culture, and she supposed that other observers had
mistaken this for the tubercle bacillus. Petri, Morgenroth and Hormann,
found the bacillus of Rabinowitsch in company with the real tubercle
bacillus in butter, and Rabinowitsch, later, in fifteen samples of
butter found the bacillus tuberculosis in two.
Bacilli coming from the mouth, bowels, or lungs, are also liable to get
into the milk, through the floating dust of the stable, or from the
teats, udder, tail, etc., (Gaffky).
_Oleomargarine_ cannot be considered as free from this indictment, for
though the tuberculous glands, etc., from the mesentery, may be
sterilized in the preliminary heating process, yet the subsequent mixing
with milk is liable to convey the infection.
The ideal course with milk, if the herds cannot be purified from
tuberculosis, would be to compulsorily remove from the herd every cow
that shows objective symptoms of tuberculosis, or any internal disease
of the udder, and to subject all the milk of the remainder to
Pasteurization at a temperature of 155° F. for twenty to thirty minutes.
This, however, requires skilled and faithful management to avoid renewed
contamination from the lips of the vessel which may have escaped the
heat, or from hands, vessels and objects that were in contact with the
milk before. The boiling temperature for fifteen minutes would be a
safer resort, as requiring less careful handling, yet even this may be
contaminated afterward under poor management. It has the further
drawback of the boiled milk taste and the coagulation of the albumen.
But the outlay for such careful sterilization will soon amount to more
than will the tuberculin test.
_Meat._ For various reasons meat must be held less virulent than milk,
but mainly because it is less frequently the seat of tubercle than the
udder, and because it is usually cooked before being eaten. The muscular
tissue of the ox appears to be unfavorable to colonization by the
bacillus, and although the intermuscular lymph glands do not partake of
this inherent resistance, yet as the glands are very frequently affected
from the tissues which they drain, they necessarily partake in some
degree of the comparative immunity of the muscles. This immunity is,
however, far from complete, as the frequent implication of the
intermuscular glands (prescapular, axillary, prefemoral, etc.),
sufficiently show. Again in estimating the virulence of meat we must
never forget that in the great majority of cases, in ordinary infected
herds, the tubercle is still essentially local; no generalization has
taken place. The muscle is vascular throughout, and in cases of
generalized tuberculosis is infecting, yet a single transient escape of
tubercle bacilli into the blood does not ensure permanent infection of
that fluid, which can usually purify itself in six days or less
(Nocard). On the contrary, when the escape of bacilli into the blood is
constant, so becomes of necessity the virulence of the blood and a rapid
generalization ensues. Such continuous escape may occur in actively
advancing tuberculosis at any point, but it is more certain if the
degenerating tubercle has opened through the walls of the vessels
(capillaries), so that the infecting bacilli can pass into the blood in
a continuous stream. The pus and other microbes, in complex infections,
hasten this degeneration and contribute to generalization of the
tubercle and emaciation. The transient infection of the blood with the
pure tubercle bacillus does not, however, lead to emaciation and
marasmus, and hence the frequency of high condition in spite of
extensive tuberculosis.
Ostertag gives the following as indicating blood infection: “When with
emaciation there is evidence of recent blood infection, enlargement of
spleen and all lymph glands, miliary tubercles of the lungs, liver,
spleen and kidneys.” In the absence of these indications, though there
may be numerous old standing tubercles, caseated, calcified or
sclerosed, he considers that there is no reason to dread infection of
the blood and carcass. Even a tuberculous intermuscular gland does not,
in his opinion, condemn the adjacent muscle. When vomicæ (caverns) are
present in the lungs and internal organs the flesh may still be used “if
embolic tubercles of different ages, indicating repeated eruptions of
tubercle bacilli into the blood stream are absent from the spleen and
kidneys.” This meat is, however, to be sterilized before marketing
(Ostertag).
The United States Bureau of Animal Industry orders the destruction of
all cases of “extensive or generalized tuberculosis;” “any disease or
injury causing elevation of temperature or affecting the system of the
animal to a degree which would make the flesh unfit for human food;”
also “any organ or part of a carcass which is * * * affected by
tuberculosis * *.”
In most countries of Europe even emaciated animals are used for food for
animals or even man, provided the wasting is not too extreme. The
carcass must, however, first be subjected to a temperature of 230° F.
for a period of three hours. This is sold in a special market as low
priced meat and labelled as such. Ordinary cooking does not always
sterilize, as Martin and Woodhead, like Vilemin, found living bacilli in
the centre of a cooked six-pound roast.
TREMBLING IN SHEEP. LOUPING-ILL. INFECTIVE MYELO-MENINGITIS. IXODIC
TOXÆMIA.
Definition: infective, tick-borne disease, characterized by
meningo-myelitis. Animals susceptible: sheep, and possibly swine
(Meek, Greig-Smith) and cattle (Williams). Known in North Britain only
in spring (in Skye also in autumn), on rough pasture with much brush,
and wood ticks, (Ixodes ricinus, erinaceous, marginatus or other).
Experimental infection. Bacteriology: Bacterium fluorescens β and γ
found in exudate and infecting. Accessory causes: dried grass of
previous year, brush, low condition, cold, youth. Symptoms: incubation
10 to 30 days; impaired innervation, hyperæsthesia, timidity,
excitability, trembling, jerking, lack of coördination and balance,
falling, convulsive struggling, jumping, rolling of eyes, stiffness,
opisthotonos, paresis, paralysis of hind—later of fore limbs, apathy.
Wry neck, arched back, stiff joints. Diagnosis from _myelo-meningitis_
by its enzoötic appearance, in spring, on tick infested ground: from
_paralytic rabies_, also by absence of that disease locally; from
_tetanus_ by its general prevalence, the absence of tonic spasm, and
presence of palsy; from _braxy_ by the lack of emphysematous
swellings, and of speedy sepsis; from _anthrax_ by usually healthy
spleen and its confinement to sheep. Lesions: cerebral meningitis with
encrease of subarachnoid fluid, of myelon, reddened, softened, also of
other serosæ, stomach, bowels, liver and kidneys. Prevention: destroy
ticks in winter by burning grass and brush, by ploughing and cropping;
or fence off ½ the pasture one year and the other half next; or lime
soil; or dip repeatedly in April, May and June to keep off ticks;
avoid moving sheep in these months: Give liberal feeding. Mortality 10
to 20 per cent.
_Definition._ All infective disease of sheep, inoculated by ticks, and
producing a meningo-myelitis, with drowsiness, hyperæsthesia,
irritability, paresis and other nervous disorders.
_Animals susceptible._ This is almost exclusively a disease of sheep,
yet Meek and Greig-Smith claim to have seen it in swine that have eaten
the raw carcasses of _louping-ill_ sheep, or that have ranged the tick
infested pastures, and in rabbits inoculated with the microbe from the
wound caused by the tick. W. Williams claims to have seen well marked
cases in cattle, and heard of cases in horses and swine. He speaks,
however, rather obscurely of “_the tick disease_” and seemingly includes
in this all affections inoculated by ticks.
_Geographical Distribution._ This disease has been hitherto described as
existing in the northern part of Great Britain only, but given the
presence of the tick, and of the infection which it carries and the
malady might easily be extended indefinitely. It is known to prevail in
Northumberland north of the Tyne, in Kirkcudbright, Dumfries, Ayrshire,
Lanark, Peebles, Roxburgh, Berwick, Argyle, Inverness, and Ross. In
Berwick it is less prevalent than in the other countries named, while in
the Western Isles it is not only widely spread rising on the hills 2000
feet or as high as the sheep range, but in Skye there are two distinct
outbreaks, in spring and autumn respectively, apparently coinciding with
the appearance of two successive generations of ticks. This may be due
to the prevalence of warm winds from the Gulf Stream. Further
investigation will doubtless show a much wider distribution—the author
has seen an affection bearing the same general characters on the spurs
of the Lammermoors in East Lothian, and the supposed adaptation of the
Norse word _hloupa_ (staggering) suggests that it is probably not
unknown in Scandinavia. W. Williams notes its prevalence on the Silurian
formation, but ticks confine themselves to no geologic stratum, and the
tick is the main agent in carrying infection.
_Causes. 1st, Sheep Ticks._ The sheep tick is not the _ked_ (Melophagus
ovinus) which is common on long wooled sheep everywhere and is an
example of a wingless, degraded dipterous insect.
The sheep ticks, on the other hand, are true ixodes, and of the same
family with our common wood tick and of the cattle tick (boöphilus
bovis) of the Southern States and the West Indies. The ticks collected
from the sheep by W. Williams were identified by Mr. Moore, of the
British Museum, as ixodes ricinus, ixodes erinaceous and ixodes
marginatus. Those obtained by Meek and Greig-Smith showed the following
characters: The male is 2.48 mm. long, by 1.30 mm. broad; the female is
5 to 5.5 mm. long, by 3 mm. broad, or when gorged with blood, 10 mm.
long, by 7 mm. broad. The fasting female is yellowish green, and when
full of blood, blue. Following the rule of their genus, the mature tick
has eight legs, the larva but six. The fasting larva has head, legs and
dorsal plate (scutellum) brown, the remainder of the body yellow.
Scattered hairs appear on the body, legs and maxillæ.
In all ticks the rostrum is a characteristic feature. It consists
centrally and interiorly of a dart covered below and on each side by
rows of teeth turned backward, which, when imbedded in the skin, hold so
firmly that the parasite may be pulled in two in any attempt to pull it
out. Above this dart and on the two sides lie the cheliferæ (horns),
each furnished with three or four teeth turned outwardly and more or
less recurved, by which the dart is worked into the skin. Finally, on
the lateral sides of this central apparatus, are the two maxillary
palpi, which are not inserted into the skin, but applied against it and
operate as feelers prior to and during the insertion of the dart and
cheliferæ. The maxillary palpi are club-shaped and soft.
Ticks pass through three moultings before they attain to the sexually
mature eight-legged form, and though the hexapod larvæ attach themselves
to animals and irritate the skin by their bites, it is only the mature,
impregnated egg-bearing female that lives exclusively on blood and sucks
this to excess.
While given species of ticks show a preference for particular genera of
animals, yet ticks generally in their vagabond life will leave the long
grass and brush where they have been hatched to become temporarily the
guests of any passing animal. It is, therefore, premature to seek to
identify any single species of tick as the only bearer of the infection,
and it is quite possible that any one of several species may contribute
to its propagation.
_Experimental Infection by the Tick._ W. Williams muzzled four sheep
from a healthy district and turned them for several hours a day on a
tick-infested field, and two sickened—one on the eighth day and one on
the sixteenth. Twelve ticks sent out of the district and put on a
healthy sheep caused illness on the tenth day. In a second experiment
with ten sheep, during a colder spring, when there were fewer ticks, no
deaths occurred.
Meek and Greig-Smith turned twenty sheep on a tick-infested louping-ill
pasture, six of the number wearing muzzles to prevent grazing, seven
having been dressed with a mixture of sweet oil, 2 quarts; castor oil, 1
quart; train oil, 1 quart; pitch oil, 3 gills, and cade oil, 1 gill,
while the remaining seven were unmuzzled and undressed. The muzzled
sheep were regularly taken out and fed on food from a healthy locality.
At the end of a fortnight oils were reapplied on the second lot (seven
sheep). No ticks appeared on these sheep, while many were found on the
undressed ones. On the fifteenth one of the unmuzzled and untreated
sheep sickened and died with lesions of louping-ill. On the
twenty-second day one of the muzzled sheep sickened, and died of
louping-ill next morning. On the thirty-first day another of the muzzled
sheep took ill and on the thirty-eighth was killed. It showed
characteristic symptoms and lesions. The seven salved sheep, on which no
ticks could at any time be found, remained healthy throughout.
_Bacteriology._ W. Williams describes a bacillus which he figures as
forming filaments of very uneven breadth, with frequent branching
(contrary to the habit of bacilli), and forming spores in clusters.
These were obtained from the coagulum of the cerebro-spinal fluid and
Meek and Greig-Smith conclude that the alleged mycelium was but the
filaments of fibrin.
McFadyean found pus microbes.
Meek and Greig-Smith (Veterinarian, 1896–7) found in the black bloody
swellings under the skin, where the ticks had inserted their
proboscides, a variety of microbes which in pure cultures did not
provoke louping-ill. These included staphylococcus cereus albus, sarcina
lutea, bacterium putridum, bacterium coli commune, micrococcus
sulphurous, micrococcus bicolor, and micrococcus caudicans. He also
found penicillium glaucum. Two organisms allied to the bacterium
fluorescens and designated as β and γ (G) respectively, were found in
these sores and produced in rabbits and sheep nervous disorders and
lesions which could be fairly identified with louping-ill. These are
about 1.5 to 1.7μ long by 0.7μ broad, and chromogenic with a special
fluorescent appearance. Microbe β inoculated on a rabbit caused on the
second day rhythmic movements of the head downward and to one side, the
eyelids closing as the head dropped, as if the animal were constantly
falling asleep. In another rabbit it caused stiffness of the legs only
and in a third it had no effect. Microbe γ when cultivated ærobically
was harmless to rabbits, but when grown anærobically on mutton bouillon
to which a drop of blood had been added, it caused on the second day
general paralysis of the neck and limbs, spasmodic twitching of the
muscles, dyspnœa and feeble heart action. Two rabbits which survived the
early effects developed large axillary and inguinal abscesses four
months later.
A lamb inoculated with the bouillon culture of γ showed after thirteen
days, lameness of one hind limb, with paresis, and a disposition to fall
to one side or the other. Two months later, when it had greatly
improved, a second inoculation of microbe γ grown anærobically in
bouillon and blood, caused, on the third day, a severe aggravation of
the lameness.
_Accessory Causes._ Much depends on the abundance of old dried grasses
of the previous year in which the larval ticks may hibernate. Land that
has been burnt over in winter, that is cleared of brush, or in which the
aftermath has been killed by free salting or liming will be largely
cleared of the ticks.
Whatever disturbs or undermines the general health lays the system open
to the disease and many flockmasters in tick-infested districts have
succeeded in greatly reducing the mortality by feeding sound hay and
oats in winter. Sudden changes of weather have long been noticed to
coincide with outbreaks of the disease. A change to cold and wet is
especially dangerous as causing a chill and robbing the system of its
tone and vigor. But a sudden access of warm spring weather, especially
if at the same time moist, may have a decidedly predisposing effect by
lowering the general tone. A fatal paresis common in the flocks of New
York, in the absence of ticks, shows a similar tendency to select the
atonic animal. This occurs mainly in spring, when the sheep have been
shut up in close confinement for weeks or months, with flaccid muscles
and fatty livers, and above all if they are in advanced pregnancy with
twin lambs, and if their fleeces are extra heavy. In both affections the
majority of the flock escape, while those that are specially predisposed
succumb. Lambs suffer most, doubtless because of relative weakness, and
on account of their innate and unexhausted susceptibility.
_Symptoms._ After an incubation varying from ten days to thirty some
impairment or disorder of the innervation is shown, varying widely,
however, in different cases.
The two names “trembling” and “louping-ill” long used by shepherds as
characteristic of the disease indicate spasmodic disorder of a clonic
kind, the paresis which is essentially passive having been very
naturally overlooked, or held to be subordinate. There is hyperthermia
the temperature rising at times to 105° or higher, and often marked
hyperæsthesia and excitability at the outset. On approaching the patient
it is very much frightened, and when caught, struggles and twitches in a
remarkable manner or trembles violently. If merely raised or disturbed
the trembling or clonic spasms are very marked, the nose is jerked
forward and upward from the contraction of the muscles attached to the
occiput; the legs may be lifted jerkingly as in stringhalt; when raised
they are moved stiffly or sway uncertainly before the foot is once more
planted; or the sheep loses its balance, falls to the ground and
struggles convulsively in its efforts to get up and escape. As the
result, it will in certain cases jump to its feet, rising meanwhile to
an undue height in the air. In other cases there is squinting or rolling
of the eyes, and movements of the jaws with frothing from the mouth. Or
the spasms may be tonic affecting especially the muscles of the back and
loins, and causing extreme stiffness or rigidity or even opisthotonos.
Lambs are unable to suck.
But whether the early spastic symptoms are well marked or not, paresis
and even paralysis set in sooner or later. This usually begins as
paraplegia, or exceptionally one limb only may be affected at first,
causing the animal to walk on three legs. For a time the fore limbs may
be free, and the patient attempts to move by dragging the hind limbs,
which are extended backward. When the fore limbs become involved the
animal remains down helpless and after awhile apathetic. Temperature and
sensibility are both greatly lowered in the paralyzed limbs. Sometimes
the spasms are lateral and the head may be drawn to one side.
In the animals that survive the early attack, there is likely to remain
some lasting deformity, such as wry neck, arched back, stiff or swollen
joints. Abscesses, which appeared in the inoculated rabbits after a
lapse of four months, are a not uncommon sequel in sheep, the pus
collecting in the neighborhood of a joint, or of the lymph glands of the
axilla, inguinal region, breast or shoulder.
The succession of symptoms are in the main such as are observed in other
cases of myelo-meningitis, first exalted function and later depressed
and abolished.
_Diagnosis._ From other forms of myelo-meningitis it may be
distinguished by its enzoötic occurrence on tick-infested pastures,
which already have a reputation for causing this malady, by its
appearance only in the season of the development of the tick, by the
presence of the tick or of its sores on the skin, and by its entire
absence from adjacent fields which are free from ticks. From paralytic
rabies it is differentiated by the same conditions, and as a rule by the
absence of rabies from the district and of any evidence of a bite. In
tetanus the tonic persistent nature of the spasms, the absence of
paralysis, the marked spasms of the muscles of the eye, and the usually
isolated condition of the case should prevent any confusion. Braxy is to
be distinguished by its emphysematous swellings near the surface of the
body, and by the comparative absence of hyperæsthesia, spasm, or
paralysis. The carcass in braxy undergoes much more rapid decomposition.
Anthrax is more rapidly fatal, shows no such marked nervous disorder,
has a dark, nonoxygenated and often incoagulable blood, an enlarged
sanguineous spleen, and the characteristic anthrax bacilli. It attacks
the larger herbivora as readily as the small.
_Lesions._ The most constant and striking lesions are found in the nerve
centres. In many cases there have been found cerebral meningitis,
involving the choroid plexus (Fair, Hamilton, Klein, Murray, McFadyean)
with an encrease of the ventricular and subarachnoid fluid (Murray,
Hamilton, Williams). The exudate may be yellow or rosy from contained
blood globules (Klein). The meninges are thickened and the seat of
ramified redness. In the region of the spine inflammation is found not
only in the meninges but also in the cord, which may be blood-stained,
softened (Mathewson, Goodwin, Robertson), or in older cases indurated
(Robertson, Hamilton, Williams, Young). In this last condition there is
a sclerotic condition of the neuroglia, and it may be a distinct
atrophy. The exudate is usually abundant and more or less coagulated
into a soft, diffluent jelly.
In many cases there is inflammation of the serous membranes of the chest
(pleura, pericardium, endocardium), and even of the lungs (Fair,
Hamilton, Klein). In some instances there has been inflammation of the
stomach and intestines (McFadyean), liver and kidneys (Klein), and
enlargement of the spleen has been noted. The most constant lesions
appear to be those of the nerve centres, but the wide variety of organs
involved in different cases sufficiently accounts for the variability of
symptoms.
_Prevention._ As the ticks are the chief media of infection, the disease
may be eradicated by their destruction. The burning of all withered
grass and brush during the winter months will do much in this direction.
Their destruction is rendered even more complete by ploughing and
putting the land under a series of cultivated crops. By this means not
only is the winter shelter of the tick removed, but the animal host
which it requires for its complete development is denied it, and it must
perish before the land is again seeded to grass. When the land is
unsuited to cultivation, the same end may be in some measure secured by
fencing off half the pasture, and leaving it unpastured for a season,
meanwhile burning the dry grass or temporarily suppressing it by a
liberal application of salt. The following year the pasture so treated
may be restored to pasturage, and the other half subjected to the same
course of treatment. In the absence of such thorough treatment, a
liberal application of lime to a virgin soil will often bring a growth
so fresh and appetizing that the stock keeps it closely cropped and thus
removes the shelter for the offensive ticks. Finally, the ticks may be
prevented from attacking the animals, by repeated use during April, May
and June of a dip in which tar oil, cade oil, heavy petroleum or other
odorous insecticide forms a component part (see the dip of Meek and
Greig-Smith).
A very obvious precaution is to avoid the movement of sheep during
April, May and June, from tick-infested pastures to others which furnish
rank grass, brush or other suitable shelter for the preservation of the
parasite.
It has been noticed that sheep indigenous to the tick-infested and
louping-ill pastures are less susceptible than those that have been
introduced from outside, but, as yet, no attempt appears to have been
made to secure immunity by the use of sterilized products of the
microbe, nor therapy by the resort to antitoxin. A liberal and tonic
diet is an important element in prevention. Grain and hay should
therefore be allowed whenever necessary to bring the sheep to early
summer in good condition.
_Treatment_ can hardly be said to have been attempted, though mild cases
are allowed by the shepherds to recover. According to Meek, the deaths
often average 10 to 20 per cent. of the flock.