(b) This class of carriers offers a difficult problem in the prevention of infectious diseases since they may continue to give off the organisms indefinitely and thus infect others. Typhoid carriers have been known to do so for fifty-five years. Cholera, diphtheria, meningitis and other carriers are well known in human practice. Carriers among animals have not been so frequently demonstrated, but there is every reason for thinking that hog-cholera, distemper, roup, influenza and other carriers are common. Carriers furnish the explanation for many of the so-called “spontaneous” outbreaks of disease among men and animals.
It is the general rule that those who are sick cease to carry the organisms on recovery and it is the occasional ones who do not that are the exceptions. In those diseases in which the organism is known it can be determined by examination of the patient or his discharges how long he continues to give off the causative agent. In those in which the cause is unknown (in human beings, the commonest and most easily transmitted diseases, scarlet fever, measles, German measles, mumps, chicken-pox, small-pox, influenza), no such check is possible. It is not known how long such individuals remain carriers. Hence isolation and quarantine of such convalescents is based partly on experience and partly on theory. It is highly probable that in the diseases just mentioned transmission occurs in the early stages only, except in small-pox and chicken-pox where the organism seems to be in the pustules and transmission by means of material from these is possible, though only by direct contact with it.
The fact that such individuals are known to have had the disease is a guide for control. The methods to be used are essentially the same as for the sick, (a), though obviously such human carriers are much more difficult to deal with since they are well.
(c) Another class of carriers is those who have never had the disease. Such individuals are common and are very dangerous sources of infection. Many of them have associated with the sick or with convalescents and these should always be suspected of harboring the organisms. Their control differs in no way from that of class (b). Unfortunately a history of such association is too often not available. Modern transportation and modern social habits are largely responsible for the nearly universal distribution of this type of carrier. Their detection is probably the largest single problem in the prevention of infectious diseases. A partial solution would be universal bacteriological examination. In our present stage of progress this is impossible and would not detect carriers of diseases of unknown cause.
The various classes of carriers just discussed are in a large part responsible for the continued presence of the commoner diseases throughout the country. The difficulties in control have been mentioned. A complete solution of the problem is not yet obtained. The army experience of the past few years in the control of infectious diseases shows what may be done.
There is another class of carriers which might be called the “universal carrier,” i.e., there are certain organisms which seem to be constantly or almost constantly present in or on the human body. These are micrococci, streptococci and pneumococci, all Gram positive organisms. They are ordinarily harmless parasites, but on occasion may give rise to serious, even fatal, infection. Infected wounds, pimples, boils, “common colds,” most “sore throats,” bronchitis, pneumonia are pathological conditions that come in this class. Such infections are usually autogenous. There is a constant interchange of these organisms among individuals closely associated, so that all of a group usually harbor the same type though no one individual can be called the carrier. Whenever, for any reason, the resistance of an individual (see [Chaps. XXV] et seq.) is lowered either locally or generally some of these organisms are liable to gain a foothold and cause infection. It sometimes happens that a strain of dangerous organisms may be developed in an individual in this way which is passed around to others with its virulence increased and thus cause an epidemic. Or, since all of the group are living under the same conditions the resistance of all or many of them may be lowered from the same general cause and an epidemic result from the organism common to all (pneumonia after measles, scarlet fever and influenza in camps). Protection of the individual is chiefly a personal question, i.e., by keeping up the “normal healthy tone” in all possible ways: The use of protective vaccines ([Chap. XXX]) appears to be advisable in such instances (colds, pneumonia after measles and influenza, inflammation of throat and middle ear following scarlet fever and measles). Results obtained in this country during the recent influenza epidemic have been conflicting but on the whole appear to show that preventive vaccination against pneumonia liable to follow should be practiced.
It would seem that among groups of individuals where infection may be expected the proper procedure would be to prepare autogenous vaccines ([Chapter XXX]) from members of the group and vaccinate all with the object of protecting them.
II. B.
(d) In this class come the “accidental carriers” like flies, fleas, lice, bed-bugs, ticks, and other biting and blood-sucking insects, vultures, buzzards, foxes, rats, and carrion-eating animals generally; pet animals in the household, etc. Here the animals are not susceptible to the given disease but become contaminated with the organisms and then through defilement of the food or drink or contact with individuals or with utensils pass the organisms on to the susceptible. Some biting and blood-sucking insects transmit the organisms through biting infected and non-infected animals successively. The spirilloses and trypanosomiases seem to be transmitted in this way, though there is evidence accumulating which may place these diseases in the next class. Anthrax is considered in some instances to be transmitted by flies and by vultures in the southern United States. Transmission of typhoid, dysentery, cholera and other diseases by flies is well established in man. Why not hog-cholera from farm to farm by flies, English sparrows, pigeons feeding, or by turkey buzzards? Though this would not be easy to prove, it seems reasonable.
Preventing contact of such animals with the discharges or with the carcasses of those dead of the disease, destruction of insect carriers, screening and prevention of fly breeding are obvious protective measures.