lague has always been one of the most dreaded diseases, and when we read of its ravages in the old world and the utter helplessness of the people before it we do not wonder that the very word filled them with horror. One of the greatest scourges ever known began in Egypt about A.D. 542, and spread along the shores of the Mediterranean to Europe and Asia. It lasted for sixty years, appearing again and again in the same place and decimating whole communities.

Another great pandemic, beginning in 1364, spread over the whole of the then known world and appeared in its most virulent form. On account of diffuse subcutaneous hemorrhages it came to be known as the "black death" and of course spread terror in all the communities where it appeared. Whole villages and districts were depopulated. The death-rate was very high, one authority placing the total mortality at twenty-five million.

During this time new centers of infection were established, and since then it has been carried by the commerce of the nations to all parts of the world. It is not restricted, as many other epidemic diseases, to the tropics or semi-tropics, although as a matter of fact we find it is more prevalent in these regions on account of the sanitary conditions.

HOW PLAGUE WAS CONTROLLED IN SAN FRANCISCO

Attention is called to these things in order that we may compare past conditions with present. During the last few years San Francisco has been fighting an outbreak of plague that in other days would have been nothing less than a national calamity. But with modern methods of handling it, based on knowing what it is, what causes it and how it is spread, the authorities there have been able not only to hold the disease in check, but practically to stamp it out with the loss of comparatively few lives.

Dr. Blue of the Public Health and Marine Hospital Service and his co-workers directed their whole energy toward controlling the rats. A small army of men were employed, catching rats in every quarter of the city. Dr. Rucker reports that fully a million rats were slain in this campaign. Their breeding-places were destroyed by making cellars, woodsheds, warehouses, etc., rat-proof and removing all old rubbish. Garbage cans were installed in all parts of the city, as it was required that all garbage be stored where rats could not feed upon it, and altogether every effort was made to make it as uncomfortable as possible for the rats.

The marked success attending this work abundantly confirms the soundness of the theory upon which it was based, and serves as another example of the way in which science is teaching us how to prevent or control many of our most serious diseases.

THE INDIAN PLAGUE COMMISSION

In 1896, what proved to be a very serious outbreak of plague, occurred in Bombay and spread to other parts of India. In 1898, a commission was appointed to inquire into the origin of the different outbreaks, the manner in which the disease is communicated, etc. This was known as the Indian Plague Commission, and its exhaustive report, together with the minutes of the evidence presented to the committee, represents a stupendous amount of work on this subject and is the basis for much of the later investigation that has been undertaken.

After the consideration of the evidence from various sources the commission decided that the principal mode of infection both for man and rats was through some sort of an abrasion in the skin, although it recognized also the possibility of infection through the nose and throat, and possibly, very rarely, through the intestinal tract or other places.

Considerable time was spent in considering Dr. Simond's claim, made in 1898, that fleas which have been parasitic on plague-infected rats migrate on the death of their hosts and convey the infection to healthy men and rats. Dr. Simond sought to establish the following:

After reviewing the experiments which had been made to establish these claims the commission believed that sufficient precaution had not been taken to prevent infection from other sources and that not enough definite evidence was produced. Against this claim much negative evidence was considered and the final conclusion was "that suctorial insects do not come under consideration in connection with the spread of plague."

In 1905 another body of men known as the Advisory Committee was appointed to arrange for further studies in India and other places, particularly in relation to the mode of dissemination of the disease. They at once appointed a new working commission who immediately began their studies and experiments. The preliminary reports of their work, which are still known as the Reports of the Indian Plague Commission, as well as the reports of contributing investigations that are being made from time to time, have served to establish entirely Dr. Simond's claims and have completely revolutionized the methods of fighting plague.

There are several different types of plague, seeming to depend largely on the manner of infection. The most common type is that known as the bubonic plague which is characterized by buboes or swellings in various parts of the body. This form of infection is usually received through the skin in some manner or other. Only rarely does direct man-to-man infection occur though there is always the possibility of it. The investigations have shown that the flea is the most common agent in transferring the disease from rat to rat or from rat to man. This may be accomplished by the flea transferring the bacilli directly from one host to another on its proboscis, or they may be carried in the alimentary canal of the flea and gain an entrance into the skin through an abrasion of some kind when the flea is crushed as it is biting, or when some of the bacilli are left on the skin in the excreta of the insect.

RESULTS OF VERJBITSKI'S EXPERIMENTS

A very important series of experiments bearing directly on this subject was made in 1902 and 1903 by Dr. D.T. Verjbitski. The paper giving the results of this work was not published in any scientific journal until 1908 when the Advisory Committee published it in one of their reports. The experiments were so well planned and executed and the results so definite that I think it is worth while to give in full his summary of results. The bugs referred to are bedbugs.

RESULTS OF VARIOUS INVESTIGATIONS

Various other plague commissions from other countries as well as many individuals have investigated the same subject, and the results all point conclusively to the fact that the rats and the fleas are at least the most important factors in the spread of the disease. The evidence from many sources and from many experiments may be briefly summed up as follows: The disease is caused by the presence in the system of minute bacteria, Bacillus pestis. It is probable that plague is primarily a disease of rats and only secondarily and accidentally, as it were, a disease of man.

Rats are subject to the plague and are often killed by it in great numbers. An outbreak of plague among men is often preceded by a very noticeable outbreak among rats.

Rats dying of the plague have their blood filled with the plague bacillus. Fleas or other suctorial insects feeding on such rats take myriads of these bacilli into their stomach and get many on their proboscis.

The fleas usually leave a rat as soon as it dies and of course seek some other source of food. When such infected fleas are permitted to bite other rats or guinea-pigs these animals often develop the disease. Several of the species of fleas that infest rats will bite man also, and in the cases of many plague patients it can be definitely shown that they had recently been bitten by fleas.

STRUCTURE AND HABITS OF FLEAS

A study of the structure and habits of fleas shows that in many respects they are particularly adapted for spreading such a disease as bubonic plague. The piercing proboscis consists of three long needle-like organs, the epipharynx and mandibles, and a lower lip or labium. The mandibles have the sides serrate like a two-edged saw. The labium is divided close to its base so that it really consists of two slender four-segmented organs which lie close together and form a groove in which the piercing organs lie. When the flea is feeding, the epipharynx and mandibles are thrust into the skin of the victim, the labium serving as a guide. As the sharp cutting organs are thrust deeper and deeper the labium doubles back like a bow and does not enter the skin. Saliva is then poured into the wound through minute grooves in the mandibles, and the blood is sucked up into the mouth by the sucking organ which lies in the head at the base of the mouth-parts. Just above this piercing proboscis is a pair of flat, obtuse, somewhat triangular pieces, the maxillary blades or maxillÆ. When the proboscis is fully inserted into the skin the tips of these maxillÆ may also be embedded in the tissue and perhaps help to make the wound larger. Attached to these maxillÆ is a pair of rather stout, four-jointed appendages, the palpi. They probably act as feelers.

If the flea chances to be feeding on a plague-infected rat or person many of the plague bacilli will get on the mouth-parts and myriads of them are of course sucked up into the stomach with the blood. Those on the proboscis may be transferred directly to the next victim that it is thrust into, and those in the stomach may be carried for some time and finally liberated when the flea is feeding again or when it is crushed by the annoyed host. The latter is probably the most common method of infection, for the bacilli that are liberated when the flea is crushed may readily be rubbed into the wound made by the flea bite or into abrasions of the skin due to the scratching. Kill the flea, but don't "rub it in."

During the recent outbreak in San Francisco many thousand fleas that were infesting man, rats, mice, cats, and dogs, squirrels and other animals have been studied and it has been found that while each flea species has its particular host upon which it is principally found, few if any of them will hesitate to leave this host when it is dead and attack man or any other animal that may be convenient.

COMMON SPECIES OF FLEAS

Throughout India and in all the warm climates where plague frequently occurs the most common flea found on rats has come to be known as the plague flea (LÆmopsylla cheopus) (Figs. 105, 106), and is doubtless the principal species that is concerned in carrying the disease in those climates. It now occurs quite commonly on the rats in the San Francisco Bay region and is occasionally found there on man also. In the United States, Great Britain and other temperate regions another larger species, Ceratophyllus fasciatus is by far the most common flea found on rats, and is commonly known as the rat flea. It occurs on both the brown and the black rats Mus norvegicus and M. rattus, on the house mouse and frequently on man. It has also been taken in California on pocket gophers and on a skunk.

The common human flea (Pulex irritans) (Figs. 108, 109), is found in all parts of the inhabited world. Although we regard it primarily as a pest of human beings it often occurs very abundantly on cats, dogs, mice and rats as well as on some wild mammals such as badgers, foxes and others and has occasionally been found on birds.

Most entomologists regard the fleas commonly found on cats and dogs as belonging to one species Ctenocephalus canis. Others believe them to be distinct species and call the cat flea Ctenocephalus felis. So far as our personal comfort and safety is concerned it makes but little difference to us whether the flea that bites us is called canis or felis for they both look very much alike, and act alike and the bite of one hurts just as much as the bite of the other. Although cats and dogs are their normal hosts they are very often troublesome household pests, sometimes making a house almost uninhabitable. They are frequently found on rats, and therefore may carry the plague bacillus from rat to rat or from rat to man.

GROUND-SQUIRRELS AND PLAGUE

As early as 1903 Dr. Blue, in charge of the plague suppressive measures in San Francisco, became impressed with the possibility of the common California ground-squirrels (Otospermophilus beecheyi), acting as an agent in the transmission of plague. It was rumored at that time that some epidemic disease was killing the squirrels in some of the counties surrounding San Francisco Bay, notably in Contra Costa County. None of the squirrels were examined at that time, but since then many thousand have been carefully studied and it has been definitely shown that many of them are plague-infected. Just how the plague got started among them will probably never be really known. There is little doubt, however, but that it was transferred in some way from the rats to the squirrels. The trains and the bay and river steamers running out from San Francisco would afford abundant opportunity for the rats to go from the city to the warehouses all along the shore. Once there they would use the same runways as the squirrels about the warehouses and in the near-by fields. In harvest time the rats migrate to the fields and make constant use of the squirrel holes. The farmers in some sections report that they frequently catch more rats than squirrels in traps set in squirrel holes at that season of the year.

This close association of the rats and the squirrels affords a good opportunity for the fleas infesting them to pass from one host to the other.

So far only two species of fleas have been recorded from the ground-squirrels. One, Ceratophyllus acutus, is very common, sometimes literally swarming over the squirrels, particularly if a squirrel is sick or weak from any cause. The other species, Hoplopsyllus anomalus, is less abundant but still quite common. Both of these species infest rats also, so the chain of evidence is practically complete. We have only to assume that at sometime one or more of the plague-infected rats found their way into the region where the squirrels were, and the fleas passing from the rats to the squirrels would carry the plague with them.

The fact that the plague already has such a start among the squirrels opens a new and very serious phase of the problem of suppressing the disease. All who have hunted the ground-squirrels will testify to the readiness with which the fleas from them will bite those who are handling them. As it is the sick or weak squirrels that are most often taken there is always a chance that plague may be transferred from them to human beings. The records of the plague cases in California show at least three cases in which there seems to be very little doubt that the disease resulted from handling plague-infected squirrels.

A still more serious thing is the possibility of the disease remaining in a more or less virulent form among the squirrels for some time, possibly for years, and then breaking out again in some locality where the rats or men may become infected. As long as there is a trace of the disease among the squirrels there is always the chance of it spreading, so that new areas may become infested. Those in charge of the plague-suppressive measures are fully aware of these dangers and are making a careful study of the situation and will doubtless be able to cope with it successfully. It may be that the squirrels will have to be exterminated in the infected regions. This would be a long and difficult task, but the success attending the fight against the rats in a great city shows what can be done when the determination to do it is there.

REMEDIES FOR FLEAS

We have seen how a great city set to work to rid itself of the plague-sick rats. As a matter of fact it was not the rats that they were after primarily. If the rats had not harbored fleas the city would have been glad to let the disease take its course and destroy as many rats as possible. But it was found that the only way to get rid of the fleas that might possibly be infected with the plague was to kill their rat hosts.

General cleaning-up measures will of course very materially lessen the number of fleas about the private dwellings, but there often remains a number of fleas in the house that are a source of great annoyance even if the danger is eliminated.

Particularly is this apt to be so in places where cats or dogs are members of the household. These animals almost always harbor at least a few fleas, and where there are a few there is always a possibility, even a great probability, that there will be many more unless an effort is made to get rid of them.

In some sections of the country it is the cat and dog flea that is the most troublesome to man. The minute white eggs of the fleas are usually laid about the sleeping-places of these animals and the slender active larvÆ that hatch from them feed upon any kind of organic matter that they can find in the dust or in the cracks and crevices. About eight or ten days after hatching the larvÆ spin delicate brownish cocoons in which they pass the pupal stage, issuing a few days later as the adult fleas.

It will at once appear, then, that it is important to provide the cats and dogs with sleeping-places that can be kept clean. If they have a mat or blanket to sleep on this can be taken up and shaken frequently and the dust swept up and burned. In this way many of the eggs or larvÆ may be destroyed. Very often the dust under a carpet that has not been taken up and dusted for some time will be found to be harboring a multitude of fleas or their larvÆ. In such cases a thorough cleaning of the carpet and the floors will bring relief. Houses that are unused for some time during the summer months are often found to be overrun with fleas in the fall, for the fleas have had an unmolested opportunity to breed and multiply. Such rooms of course require a thorough cleaning or it is sometimes possible to kill the fleas by a liberal use of pyrethrum powder or benzine or to fumigate. In this connection, Dr. Skinner's note in the Journal of Economic Entomology is worth repeating.