INFECTION OF MILK WITH PATHOGENIC BACTERIA.
That the disease-producing, or pathogenic bacteria, are able to
infect milk supplies is shown by the fact that numerous epidemics of
contagious disease have been directly traced to milk infection. Milk
is generally consumed in a raw state, and as a considerable number
of this class of organisms are able not only to live but actually
grow in milk, which is such an ideal culture-medium for the
development of most bacteria, it is not surprising that disease
processes should be traced to this source. The organisms in milk
capable of causing disease do not alter or change its physical
properties sufficiently to enable their presence to be detected by a
physical examination.
=Origin of pathogenic bacteria in milk.= Disease-producing bacteria
may be grouped, with reference to their relation toward milk, into
two classes, depending upon the manner in which infection occurs:
Class I. Disease-producing bacteria capable of being transmitted
directly from a diseased animal to man through the medium of
infected milk.
Class II. Bacteria pathogenic for man but not for cattle, which are
capable of thriving in milk after it is drawn from the animal.
In the first group, the disease produced by the specific organism
must be common to both cattle and man. The organism must live a
parasitic life in the animal, developing in the udder, and so infect
the udder. It may, of course, happen that diseases toward which
domestic animals alone are susceptible may be spread from one animal
to another in this way without affecting human beings.
In the second group the bacterial species live a saprophytic
existence, growing in milk, as in any other nutrient medium, if it
happens to find its way therein. In such cases, milk indirectly
serves as an agent in the dissemination of disease, by giving
conditions favorable to the growth of the disease germ.
By far the most important of diseases that may be transmitted
directly from animal to man through a milk supply is tuberculosis,
but in addition to this, foot and mouth disease (aphthous fever in
children), Malta fever, and acute enteric troubles have also been
traced to a similar source of infection.
The most important specific diseases that are disseminated through
subsequent infection of the milk are typhoid fever, diphtheria,
scarlet fever, and cholera, but, of course, the possibility exists
that any disease germ capable of living and thriving in milk may be
spread in this way. In addition to these diseases that are caused by
the introduction of specific organisms (the causal organism of
scarlet fever has not yet been definitely determined), there are a
large number of more or less illy defined troubles of an intestinal
character that occur especially in infants and young children that
are undoubtedly attributable to the activity of micro-organisms that
gain access to milk during and subsequent to the milking, and which
produce changes in milk before or after its ingestion that result
in the formation of toxic products.
=Tuberculosis.= This disease is by far the most important bacterial
malady that affects man and beast. In man, it assumes a wide variety
of phases, ranging from consumption, tuberculosis of the lungs,
which is by far the most common type, to scrofulous glands in the
neck, cold abscesses, hip-joint, and bone diseases, as well as
affection of the bowels. These various manifestations are all
produced by the inroads of the specific organism, Bacillus
tuberculosis. The bovine, as well as swine, fowls, and other
warm-blooded animals, are also affected with similar diseases. In
man, the importance of the malady is recognized when it appears that
fully one-seventh of the human race die of this scourge. In cattle,
the disease is equally widespread, particularly in those countries
where live stock has been intensively developed. In the northern
countries of Europe, such as Denmark, Germany, England, France, and
the Netherlands, as well as in Canada, and this country, this
disease has been most widely disseminated. This has been occasioned,
in large measure, because of the exceedingly insidious nature of the
disease in cattle, thereby permitting interchange of such diseased
stock without the disease being recognized. Tuberculosis is found
more abundantly in this country in dairy than in beef stock. Dairy
cattle are, however, not more susceptible, but the closer
environment in which milch cattle are kept, and the fact that there
has been greater activity in the matter of introducing improved
strains, accounts for the larger percentage of affected animals.
It has been a disputed question for some years whether the organisms
producing bovine and human tuberculosis are identical or from the
practical standpoint, whether the bovine type of disease is
transmitted under natural conditions to man. The bacteriologist can
readily detect differences in appearance, in growth of cultures, and
in disease-producing properties between the two strains. Of the two,
the bovine is much the more virulent when inoculated into
experimental animals. In a considerable number of cases, record of
accidental infection from cattle to man has been observed. These
have occurred in persons making postmortem examination on
tuberculous animals, and the tubercular nature of the wound proven
by excision and inoculation.
More recently, since the agitation by Robert Koch of Germany, a
number of scientific commissions have studied particularly the
problem of transmission. It is now estimated that perhaps seven per
cent of the tuberculosis in man is of bovine origin. This is almost
wholly confined to children. The portions of the body that become
diseased, when the infection has resulted from the use of milk, are
the glands of the neck and of the abdomen.
=Manner of infection in man.= In the main, the source of the malady
may be traced either to air infection or to the food, if one
disregards the comparatively small number of cases of wound
infection. Air is frequently a medium by which the germ is
transferred from one person to another. The sputum is exceedingly
rich in tubercle bacilli and since this material is carelessly
distributed by tubercular people, the air of the cities, villages
and public buildings will frequently contain tubercle organisms.
Some of the organisms in the air find their way into the lungs,
there to develop and produce consumption. The organisms in the air
may be deposited in the nasal passages and throat, and ultimately
find their way into the tissues of the body by penetrating the walls
of the throat or of the intestine. It is probable that the tubercle
bacilli thus introduced may find their way to the lungs and there
develop without leaving any trace of their path.
Food may also possibly serve as a medium of infection. The
contamination of solid food from flies and other sources is, of
course, a possibility, but tuberculous meat from cattle and swine is
much more likely to occur, although it must be said that the
processes of preparing such food for use (roasting, frying, and
boiling) are sufficient to destroy the vitality of the causal
organism. The fact that most food products of this character are now
inspected renders this possibility less likely to occur.
Unquestionably, the likelihood of ingesting tubercle organisms is
much greater with milk than with any other food supply, as milk is
consumed usually in an uncooked state, and as microscopic and
physiologic tests indicate that not infrequently milk from
tuberculous animals contains these organisms.
=Distribution of the disease in animals.= As practically any organ of
the body may be affected with tuberculosis, it naturally follows
that the lesions of this disease are widely distributed. The disease
germ is introduced, in the main, through the lymph and not the blood
system; consequently, in the initial stages the evidence of
tuberculosis is often comparatively slight, and the lesion is
restricted in its development. Where such a condition obtains, it is
known as "closed," in contradistinction to "open" tuberculosis,
where the diseased tissue is more or less broken down and is
discharging into the circulation, or elsewhere. Manifestly, the
danger of spreading not only in the affected animal itself, but to
the outside, is much greater in the case of the open lesion.
Especially is this true where the disease is present in the lungs or
organs that have an exterior opening so that the material containing
the organisms is discharged from the body in the sputum, manure,
urine or milk. The intestines themselves are rarely affected, but
the lymph glands associated with the intestinal tract are not
infrequently involved.
=Infection of milk with tubercle bacilli.= In a small percentage of
cases, the udder itself becomes involved. Where this condition
obtains, one or more hard lumps are formed, which slowly increase in
size, usually being restricted to one quarter of the udder.
Sometimes the affected quarter may develop to an enormous size,
producing a hard, painless tumor. Not often does the affected tissue
break down into pus; consequently, no abnormal appearance is to be
noted in the milk secretion until the disease has made very extended
progress, in which case the percentage of fat generally diminishes.
Whenever the udder shows physical manifestation of this disease, the
milk almost invariably is rich in tubercle bacilli.
Tubercle organisms may also appear in milk of animals in which no
physical symptoms of the disease are to be found. This fact has been
demonstrated by microscopic and animal experiments, but it is also
abundantly confirmed by the frequent contraction of the disease by
calves and hogs when fed on factory by-products. This latter class
of animals is particularly dangerous, because there is no way in
which the danger can be recognized.
[Illustration: Fig. 17.--a Tuberculous Animal.
The animal appears perfectly healthy although she has had the
disease for five years.]
It has also been proven that milk may become infected through the
feces. In coughing up material from the lungs and associated glands,
the matter is swallowed, instead of expectorated, as in man. The
organisms retain their vitality in the intestine, and are voided in
the feces. Under ordinary conditions, the flanks and udder become
more or less polluted with such filth, and the evidence is
conclusive that infection of milk is not infrequently occasioned in
this way. The fact that hogs following tuberculous steers in the
feeding lots are very likely to acquire the disease is explained by
the presence of tubercle organisms in the manure of such animals.
[Illustration: Fig. 18.--a Tuberculous Animal.
The last stages of generalized tuberculosis. Note the emaciated
condition.]
It must be kept in mind that many animals may be infected with
tubercle bacilli and therefore have tuberculosis in the incipient
stages, without their being able to disseminate the disease to
others. In the early stages, they are bacillus-carriers without
being necessarily dangerous at that particular time, but the
possibility always exists, as the disease develops in the system,
that the trouble may assume a more formidable character, and that
slowly developing chronic lesions may become acute, and "open," in
which case, the affected animal becomes a positive menace to the
herd. As the time when the lesions change from the "closed" to the
"open" type and the animal becomes a source of danger cannot be
determined, the only safe way to do is to exclude the milk of all
tuberculous animals from the general supply, whether for direct
consumption, or for manufacture into dairy products and to look upon
every diseased animal as a menace to the herd. This is rendered all
the more necessary when the milk is used for the feeding of
children, who are relatively more susceptible to intestinal
infection than the adult. The early stages of the disease in cattle
are, however, so insidious that no reliance can be placed upon the
detection of the malady by physical means. Fortunately, in the
tuberculin test, a method is at hand, which in a simple, but
effective manner, enables the disease to be distinguished in even
the early stages, long before recognition is possible in any other
way.
=Tubercle bacilli in dairy products.= When infected milk is used for
the preparation of butter and cheese, the organisms inevitably are
incorporated in them. In the separation of milk a relatively large
part of the tubercle organisms in the milk appear in the cream. In
the making of cheese even more of the organisms are held in the
curd. In butter and cheese, as in milk, no growth of the organism
can take place; however, the vitality of the organism is retained
for a considerable number of months. It is not believed that these
products are of much importance in the spread of tuberculosis in the
human family, since they are not consumed by children to any extent.
Cream is to be considered as a means of distribution since it is
often used by children.
=Treatment of tuberculous milk.= It is easily possible to treat milk
or factory by-products so as to render them positively safe. The
process of pasteurization or sterilization is applicable to whole
milk, and when effectively done destroys entirely the vitality of
any tubercle bacilli. In making such exposure, care should be taken
to prevent the formation of the "scalded layer," as the resistance
of the organism toward heat is greatly increased under these
conditions. In a closed receptacle, 140 deg. F. for 15 to 20 minutes
has been found thoroughly effective in destroying this organism. A
momentary exposure at 176 deg. F. is likewise sufficient. This is the
method that is almost universally used in Denmark in the manufacture
of the finest butter.
In the treatment of factory by-products, heat should also be
employed. In Denmark, compulsory pasteurization at not less than
176 deg. F. is required. This treatment prevents not only the
dissemination of tuberculosis among hogs and young cattle, but is
equally efficacious in preventing the spread of foot and mouth
disease.
The per cent of tuberculous milch cows varies widely in different
sections of the country, being greatest in the older dairy sections,
and in those supplying milk to the cities, on account of the
constant buying and selling of animals, thus giving more frequent
opportunity of introducing the disease into the herds. Throughout
the country at large, probably less than ten per cent of the cows
are tuberculous, and it is estimated that at least one per cent of
the diseased animals have tuberculous udders. It has been suggested
that the dilution of the milk of such animals with that of healthy
cows would remove a great part of the danger from milk. In the
case where the milk of a large number of herds is mixed, this may be
of some importance, but in no case is it safe to assume that
dilution of the milk of tuberculous cows is any guarantee of safety.
It has been shown that milk, perfectly normal in appearance, coming
from a tuberculous udder could be diluted a million times and still
produce the disease on inoculation into experimental animals. In the
case of swine, the susceptibility is so great that a single feeding
of infected milk, even in a very dilute condition, causes with
certainty the production of the disease.
Some observers maintain that the contamination of the milk with the
manure of tuberculous animals is of greater hygienic importance,
than that coming from diseased udders, since the number of animals
having tuberculosis of the lungs and intestines is far greater than
those with diseased udders.
=Economic aspects of bovine tuberculosis.= Not only is this disease
invested with much importance because of its inter-relation with the
human, but from an economic point of view alone, it is undoubtedly
the greatest scourge that affects the dairyman. Its insidiousness
makes it exceedingly difficult to recognize. The consequence is that
many fine herds become seriously involved before its presence is
recognized. In the main, the disease is introduced into a herd by
purchase, often by buying in pure-bred stock to improve the quality
of the herd. Where the disease has been established in a region for
some time, there is also danger that unheated factory by-products,
as skim milk and whey, may function in its spread. Where such
conditions prevail, the spread of the disease in the creamery
district is exceedingly rapid. When once introduced into a herd,
the disease sooner or later spreads from the originally affected
animal to others in the herd. Close contact, and close confinement
in ill ventilated stables facilitate the spread of the disease, and
sooner or later, other animals acquire the trouble. This may all
occur while all animals appear in a healthy condition.
The symptoms of the disease in the earlier stages are quite
indefinite. As the disease progresses, the nutritive functions
appear to be disturbed, and sooner or later, the body weight begins
to decline, and finally marked emaciation ensues. Accompanying this
condition, especially when the disease is in the lungs, is a cough,
which is generally aggravated with active exercise. While the
run-down condition permits frequently of the detection of the
disease in the advanced stages, it is wholly impossible with any
accuracy to diagnose the trouble in the incipient stages. It is at
this stage that the tuberculin test comes to the aid of the
stockman.
_Tuberculin test._ This test is made by the injecting beneath the
skin of the animal a small quantity (about 2 c. c.) of tuberculin,
and noting the temperature of the animal, before and after the
injection. Tuberculin, a product of the growth of the tubercle
bacillus, when injected into the body causes a marked rise in
temperature, in the case of an animal affected with the disease, and
no such elevation in the case of a healthy animal. The process of
preparing tuberculin makes it absolutely free from danger, so far as
liability of producing the disease, or in any way injuring the
animal, is concerned. Fig. 19 shows the temperature range of both
reacting and non-reacting animals. While the test is not
absolutely infallible, it is so far superior to any and all other
methods of diagnosis that it should take precedence over them.
=Miscellaneous diseases.= There are a number of diseases that affect
both human beings and cattle, the causal organisms of which may be
transmitted through the milk. Foot and mouth disease is one wide
spread in European countries but which has not yet gained a
permanent foothold in this country. The ingestion of the milk, which
always contains the causal organism, produces the disease in both
humans and cattle. In the human the disease is very similar to that
in cattle; it may end in death. Vesicles are produced in the mouth,
on the lips, nose and fingers. The causal organism, which has not
yet been demonstrated, may occur in butter or cheese. It is easily
destroyed by pasteurizing the milk.
[Illustration: Fig. 19.--Temperature Curves.
1, the temperature curve of a healthy animal after injection with
tuberculin; 2 and 3, the temperature curves of tuberculous animals
after injection with tuberculin. (After Moore.)]
Anthrax, actinomycosis (lumpy jaw), rabies, and malta fever are
diseases the organisms of which have been found in the milk of
affected animals. In case of the first three, while the possibility
exists of the infection of human beings by milk, it is improbable
that such infection does normally occur. Malta fever is becoming an
important disease in portions of southern Europe. It is produced in
man by the use of milk of goats suffering from the disease.
The organism causing contagious abortion in cattle is known to be
present in the milk of the infected animal at the time of its
withdrawal from the udder. It is not probable that the organism is
of any sanitary significance as far as man is concerned. It has been
shown that the organism is able to produce a disease in guinea pigs
on artificial inoculation that is very similar, so far as the
lesions are concerned, to tuberculosis. It is also probable that the
by-products of creameries and cheese factories may serve to spread
the disease from one herd to another.
Inflammation of the udder (garget) is a frequent trouble in every
herd. It is marked by the swelling of one or more quarters, by the
appearance of fever and changes in the appearance and composition of
the milk. The inflammation may be caused by cold or injury, or by
the invasion of the udder with pus-forming bacteria. In the first
case the trouble is not likely to persist for any length of time,
and does not spread to other members of the herd. The milk may be
more or less stringy, and may show a slimy flocculent sediment. It
cannot be asserted that such milk is harmful to man but it should be
rejected on general sanitary grounds, and because it cannot always
be differentiated from that coming from an udder in which the
inflammation is produced by bacteria.
Inflammation caused by the invasion of the udder with specific
bacteria is usually of greater severity, the entire gland often
becoming involved. The secretion of milk may cease and the function
of the diseased quarters may never be restored. The milk in the less
severe cases may not be abnormal in appearance, but with increasing
severity, the nature of the milk changes, until it may be a watery
liquid. The milk of any animal suffering from any form of garget
should be rejected, as it may cause trouble, especially in children.
There is some reason to believe that organisms coming from cases of
garget have been responsible for the extensive outbreaks of septic
sore throat that have occurred in some parts of the country.
The milk of animals suffering from indigestion, diarrhea, abscesses
on any part of the body, as from those which have retained the
afterbirth should be likewise rejected. In short only the milk of
healthy animals should be used for human food; that from any animal
suffering from any disease or which is receiving medical treatment
should not be so used.
=Typhoid fever=. The most important disease germ, distributed through
the medium of milk, that is unable to produce a diseased condition
in the cow is the organism of typhoid fever. This malady is an
intestinal affliction of man, and the germ causing the same is found
abundantly in the dejecta, both solid and liquid, as well as in the
blood in certain stages of the disease. While the causal organism
does not leave the body through the expired air, it is found
abundantly in both the urine and feces. Therefore, the dejecta, and
any articles that may be soiled with the same become a positive
menace.
Many different methods of transmitting the contagion exist, such as
water, food infected in various ways, contact with infected persons,
and through the medium of milk. Milk is not so frequently the cause
of dissemination as the other factors, but where milk supplies
become contaminated, epidemics of considerable magnitude are wont to
occur. The danger from milk is also aggravated by the fact that the
typhoid bacillus is capable of withstanding considerable amounts of
acid, and consequently finds, even in raw milk containing the normal
lactic acid bacteria, conditions favorable for its growth. In a
considerable percentage of cases, the disease is not sufficiently
severe to cause the patient to take to his bed. These so-called
"walking typhoid" cases are particularly dangerous, because they
serve to spread the disease organism more widely.
A very considerable proportion of the people that recover from
typhoid fever still continue to harbor the typhoid bacillus in their
urinary and gall bladders. This condition may obtain for years, and
since such individuals are in perfect health and are ignorant of
their own condition, and since they give off the organisms more or
less constantly, they are often the cause of extensive milk borne
epidemics. Such persons are known as "typhoid carriers" and
constitute one of the gravest problems the public official has to
contend with in his struggle to prevent the spread of typhoid fever.
Where outbreaks are caused by milk, they can readily be traced by
means of the milk route, as there are always a sufficient number of
susceptible persons, so that outbreaks of epidemic proportions
develop. In the Stamford, Conn., outbreak in 1895, 386 cases
developed on one milk route. In this case it was shown that the
carrying cans were thoroughly washed, but were later rinsed out with
_cold_ water from a polluted shallow well.
The mode of infection of milk varies, but in general, the original
pollution is occasioned by the use of infected water in washing the
utensils, or a case of "walking typhoid" or bacillus carrier, who
directly infects the milk. In case of sickness in rural families,
some member of the household may serve in the dual capacity of nurse
and milkmaid, thus establishing the necessary connection. Busey and
Kober report twenty-one outbreaks, in which dairy employees also
acted in the capacity of nurses. The fact that the urine of a
convalescent may retain the typhoid germ in large numbers for some
weeks renders the danger from this source in reality greater than
from feces, as, naturally, much less care is exercised in the
disposition of the urine.
The house fly is now regarded as one of the important means of
spreading typhoid fever, indeed it is often called the "typhoid
fly." The infectious material deposited in an open vault may serve
as a source from which the fly carries the organisms to milk and
other foods in the house or elsewhere. The protection of vaults and
the screening of every place where human food is handled or prepared
is the only protection.
It should be emphasized that in the case of the tubercle organism,
no growth ever occurs in milk, but with the typhoid bacillus growth
is possible. It thus needs but the contamination of the milk with
the smallest particle of material containing them to seed the
milk. By the time it is consumed it may contain myriads of the
disease-producing organisms.
=Diphtheria.= This is a highly infectious disease, affecting children
primarily and is characterized by the formation of membranous
exudates in the throat and air passages, which are teeming with the
causal organism, the diphtheria bacillus. This organism is capable
of forming highly toxic products, and it is to the effect of these
poisons that its fatal result is generally due. The organism is
thrown out from the body, in the main, through the mouth, the
surroundings of the patient being infected directly from the air,
and indirectly, by contact with polluted hands, lips, etc. Thus, the
germ deposited from the lips of a case of the disease, on the common
drinking cup, slate, lead pencils, toys, and the like, may easily
pass from child to child. Not infrequently, the causal organism
persists in the throat long after all evidence of membranous growth
has subsided, and so the child itself may act as a "bacillus
carrier."
Not so many epidemics of diphtheria as of typhoid have been traced
to milk, but the evidence is sufficient to indict milk as a
disseminator of contagion. In several cases, the diphtheria germ has
actually been isolated from infected milk supplies. Actual growth of
the diphtheria germ is said to take place in raw milk more rapidly
than in sterilized.
=Scarlet fever.= While the germ of scarlet fever has not yet been
isolated, and therefore its life history in relation to milk cannot
be depicted so accurately, yet milk-borne epidemics of this disease
are sufficiently abundant to leave no doubt but that this food
medium may sometimes serve as a means of disseminating such
troubles. Infection of the milk doubtless comes in the case of this
disease from direct contact with a person suffering from the malady.
=Cholera.= While this disease is of no practical importance in
America, owing to its relative infrequency, yet outbreaks of cholera
have been traced to milk, in spite of the fact that the causal
organism is more sensitive to the action of acids than most
disease-producing bacteria. In several outbreaks in India, milk has
been the medium through which the disease was spread. Generally,
infection of the milk has been traced to the use of polluted water.
=Children's diseases.= An exceedingly high mortality exists among
infants and young children in the more congested centers, especially
during the summer months. In the main, the cause of these troubles
is due to intestinal disturbances, and unquestionably, the character
of the food enters largely into the problem. As milk constitutes
such a large proportion of the diet of the young, and is so
susceptible to bacterial invasion, it would appear probable that
much of the trouble of this character is due to the condition of
this food supply. This is rendered more probable when it is
remembered that bottle-fed infants suffer a much higher mortality
than breast-fed children, due probably to the fact that the
lengthened period between the time the milk is drawn and consumed
permits of abundant bacterial growth. Much carelessness also
prevails among the poor in cities, relative to the care of utensils
used in feeding children. Nursing bottles often serve to infect the
milk. Where milk is pasteurized, or properly heated, it has been
found that the mortality rate has been greatly reduced, thus
indicating that the condition of the milk was directly responsible
for the death rate. In fact, the mortality from these indefinite
intestinal troubles probably exceeds that from all of the specific
infectious diseases combined. Improved care in handling this
sensitive food supply will do much to better conditions in this
direction.
=Ptomaine poisoning.= Acute poisoning affecting adults as well as
children, not infrequently occurs from the use of foods of various
kinds. Cases of poisoning arising from the use of shell fish, canned
meats, ice cream, cheese, and other dairy products, are from time to
time reported. These troubles are due to the production of toxic
compounds, in the main, probably caused by bacterial decompositions.
Often such troubles may affect a number of persons, as at banquets
and such gatherings, thereby giving the semblance of an epidemic.
While such troubles are doubtless to be ascribed to bacterial
activity, they are not transmissible from person to person.
In the case of troubles arising from ice cream and such confections,
the probable cause is due to the storage of milk or cream under
refrigerator conditions, where germ growth can go on in the product,
and yet the temperature be sufficiently low to prevent the usual
acid fermentations.