"FILTER - PASSING Germs. - The discovery by Pasteur of the significance of microbic life in the phenomena of fermentation, putrefaction and disease and the development by Koch of an appropriate technique for the new science of bacteriology had already led in a comparatively few years to the determination of the causation of many infectious diseases of man and animals. There remained, however, a number of diseases of man and animals - and amongst these some common maladies such as typhus, measles, smallpox, foot and mouth disease, swine fever, rabies and cattle plague - in which the cause had not been discoverable by the methods of microscopical examination and cultivation which proved successful in so many cases. It was suggested by Pasteur, who searched in vain for the infective agent of rabies, that some microbes were too small to be visible with the optical apparatus at disposal. There is no reason a priori to suppose that the lower limit of size of microbic organisms should be of a dimension at present discernible, and the question raised by Pasteur was answered ten years later by the discovery made by Loeffler in 1898 that the virus of foot and mouth disease was invisible. The limit of visibility of a particle is in the last instance conditioned by the wave length of light. With the best modern microscopes employing white light of which the average wave length is 0.55A (µ = i o ov mm.) this limit is rather less than 02 1 /,. If ultra-violet light of half this wave length is used, photographs of objects of about o1µ in diameter can be made. The existence of particles of much smaller size can be demonstrated by the method of dark-ground illumination (Tyndall phenomenon), when they appear as bright points. The limit appears to depend upon the intensity of illumination and with direct sunlight is 0 004/1.
Invisibility of a microbe commonly handicaps every effort at its isolation, propagation and identification, but does not render them impossible of achievement, for the existence of a living virus in an optically clear liquid may still be demonstrated by its power to infect an animal or plant, or produce recognizable chemical changes in a medium. The existence of these ultramicroscopic viruses was brought to light unexpectedly through the instrumentality of so-called " bacterial filters." These are constructed of fine-grained unglazed porcelain, clay or infusorial earth. They are commonly moulded in the shape of hollow candles and fired at a high temperature. Liquid is made to pass through the walls of the filters, which vary from a quarter to half an inch in thickness, by hydrostatic pressure or by suction with some form of exhaust pump. Use of such filters to separate bacteria from the products of their activity was first made in 1871. Since that time they have become a usual part of the equipment of a bacteriological laboratory and have been extensively employed to free water from microbes for domestic use. In structure the wall of a filter resembles a bed, of sand on a diminutive scale, with crevices of variable size between the particles and a good many splits and holes of larger dimensions throughout the matrix. The different types of filter vary in their permeability according to the fineness of the pores and thickness of the wall. The smallest passages are of the order of 0 2 to 0.5 in the case of the porcelain filters and 02 to o8 in those made of infusorial earth. The size of the smaller pores is of the same order of magnitude as that of the smallest bacteria, and the power of a filter to hold back these microbes depends upon the walls being of sufficient thickness to ensure that a bacterium will become impacted in one of the smaller passages through which the liquid in which it is suspended has to pass. If a filter through which a liquid containing bacteria in suspension has been filtered be allowed to remain for a few days immersed in a nutrient fluid, the bacteria caught in the interstices divide and multiply and generally manage to grow through the walls of the filter, for, during growth, the cells can adapt themselves to the size and shape of the crevices. On this account filters cannot be relied upon to render drinking-water secure unless removed and sterilized by heat at least every second day. The similarity in magnitude of filter-pore and bacterium is not a coincidence, but due to selection of material for the manufacture of the filters of such size of grain as to afford a bacteria-free filtrate and at the same time the maximal flow of liquid. In other words, bacterial filters have been made to fit the known microbes.
The first discovery of an ultra-microscopic or filterable virus was made by Loeffler in 1898 in the course of some experiments upon foot and mouth disease in which a filter of infusorial earth was being used to remove ordinary recognizable bacteria from the diluted contents of the superficial vesicles which are characteristic of this disease. The filtrate was free from any particles visible by the microscope and no bacteria developed in it on cultivation. Nevertheless, injection of this filtrate into animals caused the disease. Material removed from the vesicles of the animal so infected and filtered again reproduced the disease in a fresh animal. Similar experiments were carried out through a number of generations of experimental animals, so that there was no doubt that a virus capable of propagation was contained in the filtrates. In the next few years the filterability of the virus was established in the case of infectious pleuro-pneumonia of cattle, South African horse-sickness, fowl plague and mosaic disease of the tobacco plant, in which patches of discoloration occur in the leaves and, spreading rapidly, destroy them. With the exception of the virus of pleuro-pneumonia, which is just on the margin of visibility, all are invisible.
The first virus of a human disease which was found to be small enough to pass a bacterial filter was that of yellow fever. The observation was made by the American commission to study yellow fever in Havana in 1901. The cause of yellow fever has recently been shown to be an organism which, owing to its thinness and motility, can pass through a bacterial filter.
Up to the year 1921 the virus of 38 diseases of man, animals or plants had been found to pass through a bacterial filter by some reliable observer. The more important of these are the following: foot and mouth disease, contagious bovine pleuropneumonia, mosaic or spotted disease of the tobacco plant, African horse-sickness, fowl plague, yellow fever, cattle plague, sheep-pox, epithelioma contagiosum of birds, swine fever, rabies, cow-pox (vaccinia), molluscum contagiosum of man, equine infectious pernicious anaemia, canine distemper, " blue tongue " of sheep, dengue fever, papataci or sand-fly fever, smallpox, trachoma, poliomyelitis, scarlatina, measles, typhus fever, and trench fever. There are many observations indicating that the primary cause of pandemic influenza may be a filterable virus.
Two filterable viruses fall into a class apart. They are distinguished from the others in that they do not seem to produce disease directly by their poisonous activities but rather to stimulate certain cells of the body in the neighbourhood of their inoculation to excessive and anarchical development. The injection of these viruses into certain varieties of fowls leads to sarcomatous new-growths of great malignity. In the one case it is a pure sarcoma, in the other a mixed tumour, an osteo-chondro sarcoma. These malignant tumours spread not only by proliferation at the point of origin but some of the cells of which they are composed, boring their way into blood vessels and lymphatics, are carried all over the body, giving rise to similar tumours in internal organs. The metastases as well as the primary tumours contain the virus, and if extracted with water and filtered through a bacterial filter, whereby all the cells of which the tumour is composed are held back, the filtrate may be dried and powdered and the powder retains its original property of exciting the formation of these malignant tumours in suitable animals into which it is injected. The experiment may be repeated indefinitely through generations of young chicks, showing that the virus propagates itself and appears to live in some sort of symbiotic manner with the particular cells it excites to inordinate development.
It frequently happens that one observer succeeds in passing a particular virus through a filter, whilst another fails. When the particles of a virus are of the same order of magnitude as the crevices of the filter this may well happen, as in any case the majority will be retained in the wall of the filter. Even when the size of the particles of virus are much less than that of the smaller pores, they are liable to be deposited upon the walls of the minute passages under the influence of surface action and the filtrate becomes thereby greatly reduced in concentration.
In addition to size, shape, and rigidity of particles, the conditions under which filtration is carried out, pressure, temperature and amount of liquid passed through the filter, exert an influence. With high pressure some may be forced through which would otherwise be obstructed. This is particularly the case when the particles are of unequal dimensions in two planes. The nature and reaction of the liquid in which the virus is suspended is also of importance. If suspended in a colloidal solution such as blood serum or a slimy emulsion of nerve tissue, high dilution is necessary for the colloidal particles of the solution or emulsion are themselves deposited on the walls of the pores and rapidly reduce the permeability of the filter. Reaction plays a part by modifying the electric charge on the particles and thereby facilitating or hindering their aggregation and deposition on the surface of the filter pores which is also charged.
The fact that a virus, under certain circumstances, traverses a bacterial filter, does not tell us any more as to its nature than that it is very small, or at least very thin, and of the order of 0.1 to 0.2 p or less in its smallest diameter. It is not necessary to assume a contagium fluidum in the case of those viruses which are outside the range of visibility under the best optical arrangements at present available. The particulate nature of the infective agent of rabies, fowl plague, variola and vaccinia, is indicated by the fact that the upper layer of a liquid containing them may be deprived of infectivity by prolonged subjection to a powerful centrifugal force.
Little is known about most of these filterable viruses. They appear to be of various natures, and the only property common to them is minuteness. The parasite responsible for yellow fever is a small spirochaete, those occasioning bovine pleuro-pneumonia and human poliomyelitis are just on the margin of visibility and have been cultivated in artificial media. Some of them occur in the blood of the patient during the acute stage of the illness and are transported to a fresh host by the bite of blood-sucking insects. The infections of yellow fever and dengue are conve y ed by the mosquito (Stegomyia fasciata). That of papataci fever is transmitted by the sandfly (Phlebotomus papatasii), and that of typhus and trench fevers by lice. In each case some days elapse before the insect is capable of handing on the infection, indicating that an interval for the multiplication of the parasite is necessary. It is possible that a stage in the life-history of the parasite can only occur in the body of the insect host. Some filterable viruses, such as smallpox, cowpox, foot-andmouth disease, and molluscum contagiosum give rise to superficial lesions, and are spread by contact; others occasion catarrh of the respiratory passages and are distributed by coughing and intimate contact, as in distemper, measles, scarlet fever and pleuro-pneumo nia. In many cases the precise method of infection had still been undetermined in 1921. (C. J. M.*)
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