ANIMAL PARASITES
Not only pathogenic bacteria but certain kinds of animal parasites
sometimes enter the human body in or upon articles of food. One of the
most important of these is the parasite causing trichiniasis.
TRICHINIASIS
Trichiniasis or trichinosis is a disease characterized by fever,
muscular pains, an enormous increase in the eosinophil blood corpuscles,
and other more or less well-defined symptoms; at the onset it is
sometimes mistaken by physicians for typhoid fever. The responsible
parasite is a roundworm (_Trichinella spiralis_, formerly known as
_Trichina_) which is swallowed while in its encysted larval stage in raw
or imperfectly cooked pork.[90] The cysts or envelopes in which the
parasites live are dissolved by the digestive fluids and the young
larvae which are liberated develop in the small intestine to the adult
worm, usually within two days. The young embryos, which are produced in
great numbers by the mature worms, gain entrance to the lymph channels
and blood stream, and after about ten days begin to invade the
muscles--a procedure which gives rise to many of the most characteristic
symptoms of the infection. It is estimated that in severe cases as many
as fifty million embryos may enter the circulation. The parasites
finally quiet down and become encysted in the muscle tissue and the
symptoms, as a rule, gradually subside. Ingestion of a large number of
parasites at one time often results fatally, the mortality from
trichiniasis being on the average somewhat over 5 per cent and rising in
some outbreaks to a much higher figure (30 per cent). On the other hand,
many infections are so light as to pass unnoticed. Williams[91] found
_Trichinella_ embryos present in 5.4 per cent of the bodies of persons
dying from other causes. Such findings are considered to indicate that
occasional slight _Trichinella_ infections even in the United States are
quite common. This might indeed be expected from the frequent occurrence
of infection in swine, about 6 per cent of these animals being found to
harbor the parasite.
[Illustration: FIG. 7.--Trichinae encysted in intercostal muscle of pig.
(About 35x1.) (After Neumann and Mayer.)]
The specific symptoms (such as the muscular pain) of trichiniasis may be
due in part to mechanical damage of the muscle tissue, but it is also
probable that they are partly due to toxic products exuded by the worms
and partly to the introduction of alien protein material--the protein of
the worm--into the tissues. Secondary bacterial infection is also a
possibility, but there is little evidence to prove that this is an
important factor in most cases of trichiniasis. The various stages
observed in the progress of the disease are plainly connected with the
different phases of the worm's development--the initial localization in
the intestines, the invasion of the muscles, and the final encystment.
Swine become infected with this parasite by eating scraps of infected
meat, or the offal of their own kind, or by eating infected rats. The
rat, through its cannibalistic propensities, becomes infected
frequently, and is one of the chief factors in the wide dissemination of
the disease. Human infection is practically accidental and self-limited;
biologically speaking, man as a host does not enter into the
calculations of the parasite.
Treatment of established trichiniasis infection is palliative, not truly
remedial. The parasites, once inside the body, cannot be materially
affected by the administration of any drug. While cure of trichiniasis
is thus difficult, if not impossible, prevention is very simple. The
thorough cooking of all food is sufficient to preclude infection. This
relatively simple means of destroying the larvae is a more certain as
well as less expensive method of preventing infection than is the
laborious microscopic examination of the tissues of every slaughtered
hog. In Germany between 1881 and 1898 over 32 per cent of 6,329 cases of
trichinosis that were investigated were traced to meat that had been
microscopically examined and passed as free from trichinae.[92] On the
other hand, thorough cooking removes all possibility of danger.
TENIASIS
Various tapeworm or cestode infections are contracted by eating meat
containing the parasite. Particular species of tapeworm usually infest
the flesh of specific hosts, as _Tenia saginata_ in the beef and _Tenia
solium_ in the hog. The dwarf tapeworm, _Hymenolepis nana_, develops in
rats, and the human infections with this parasite occasionally observed
are probably caused by contamination of food by these animals.
[Illustration: FIG. 8.--_Cysticercus cellulosae_ in pig's tongue. (After
Neumann and Mayer.)]
Sometimes the existence of the tapeworm in man is restricted to the
alimentary tract and the symptoms vary from trivial to severe, but
sometimes (_Tenia solium_) the larval stage of the tapeworm invades the
tissues and becomes encysted in various organs (brain, eye, etc.),
where, as in the case of cerebral infection, it may result fatally. The
encysted larva of _Tenia solium_ was at one time regarded as an
independent animal species and named _Cysticercus cellulosae_. The
condition known as "measly pork" is produced by the occurrence of this
encysted parasite.
So-called hydatid disease is due to the cystic growth produced by the
larva of a species of tapeworm (_Echinococcus_) inhabiting the intestine
of the dog. Human infection may be caused by contaminated food as well
as more directly by hands soiled with petting infected dogs. Several
varieties of tapeworms infesting fish, especially certain fresh-water
species, may be introduced into the human body in raw or partly cooked
fish.
Methods for the prevention of tapeworm infection include the destruction
of the larvae by heat--that is, the thorough cooking of all meat and
fish--and the minimization of close contact with those animals, such as
the dog and cat, that are likely to harbor parasites. Cleanliness in the
preparation and serving of food, and attention to hand-washing before
meals, and especially after touching pet animals, are necessary
corollaries.
UNCINARIASIS
Hookworm infection (uncinariasis, ankylostomiasis) is commonly caused by
infection through the skin of the feet, but the possibility of mouth
infection cannot be disregarded, and in regions where hookworm disease
exists methods of guarding against food contamination should be
practiced, as well as other precautions. Billings and Hickey[93] believe
that hookworm disease is contracted by unconscious coprophagy (from raw
vegetables) much more frequently than is generally supposed.
OTHER PARASITES
A number of other parasitic worms (e.g., _Strongyloides_, _Ascaris_ or
eelworm, and _Oxyuria_ or pinworm) may conceivably enter the human body
in contaminated food, and while, as in hookworm disease, other modes of
infection are probably more important, the liability to occasional
infection by uncooked food must not be overlooked.
[Illustration: FIG. 9.--_Lamblia intestinalis._ (After Neumann and
Mayer.)]
Various forms of dysentery or diarrhea have been attributed to infection
with _Giardia (Lamblia) intestinalis_. Observations made by Fantham and
Porter[94] upon cases contracted in Gallipoli and Flanders have given
support to this view. Strains of this parasite of human origin have been
shown to be pathogenic for mice and kittens. It is considered possible
that these animals may act as reservoirs of infection and spread the
disease by contamination of human food.
FOOTNOTES:
[90] The consumption of raw sausage made with pig meat is particularly
likely to give rise to trichiniasis.
[91] _Jour. Med. Research_, VI (1901), 64.
[92] Edelmann, Mohler, and Eichhorn, _Meat Hygiene_, 1916, p. 182.
[93] _Jour. Amer. Med. Assoc._, LXVII (1916), 1908.
[94] _Brit. Med. Jour._, II (1916), 139.