Nobel Lecture*

Investigations on Typhus

I am going to give you an account of how I arrived at the results for which I have received the Nobel Prize for Medicine. I shall also summarize these results.

It did not seem likely that I was destined to undertake research on typhus. I was born, first studied medicine and undertook my first research work in a French province from which typhus had disappeared since 1814. It is true that I came across a few imported cases at Rouen in 1889. They made no particular impression on me.

My arrival in Tunisia placed me immediately in contact with typhus. Ten days after my arrival, at the beginning of January 1903, I saw a few cases in natives living in a suburb of Tunis.

In those days the disease flared up each winter in the rural districts of Tunisia. From these remote districts it spread to the doss houses, the prisons and the outskirts of towns. The native districts of Tunis and the prisons were regularly stricken. The epidemic receded in June; it drew back into the remote country districts and was not heard of again until the end of the year.

Of all the problems which were open to me for study, typhus was the most urgent and the most unexplored. We knew nothing of the way in which contagion spread. The field of experimental study was virgin ground. We were scarcely able to conclude, from the results of debatable experiments, that it was possible to inoculate the disease from one man to another by means of the blood.

In June of 1903 I was determined to carry out a preliminary study. At that time typhus was raging in a native prison, 80 kilometres South of Tunis, Djouggar. I requested the doctor in charge of this establishment to allow me to accompany him on his weekly visits. We made an appointment. The evening before I had a haemoptysis. If it had not been for this accident, my first contact with typhus would undoubtedly have been my last. My colleague, Motheau, and his servant went to Djouggar; they spent the night there, contracted typhus and both died.

Most of the doctors in the Tunisian administration, especially those in country districts, contracted typhus and approximately one third of them died of it. The fact that I was fortunate enough to escape contagion, in spite of frequent, sometimes daily contacts with the disease, was because I soon guessed how it spread.

The native hospital in Tunis was the focal point of my research. Often, when going to the hospital, I had to step over the bodies of typhus patients who were awaiting admission to the hospital and had fallen exhausted at the door. We had observed a certain phenomenon at the hospital, of which no one recognized the significance, and which drew my attention. In those days typhus patients were accommodated in the open medical wards. Before reaching the door of the wards they spread contagion. They transmitted the disease to the families that sheltered them, and doctors visiting them were also infected. The administrative staff admitting the patients, the personnel responsible for taking their clothes and linen, and the laundry staff were also contaminated. In spite of this, once admitted to the general ward the typhus patient did not contaminate any of the other patients, the nurses or the doctors.

I took this observation as my guide. I asked myself what happened between the entrance to the hospital and the wards. This is what happened: the typhus patient was stripped of his clothes and linen, shaved and washed. The contagious agent was therefore something attached to his skin and clothing, something which soap and water could remove. It could only be the louse. It was the louse.

Even if it had not been possible to reproduce the disease in animals and consequently to verify the hypothesis, this simple observation would have been sufficient to demonstrate the way in which the disease was propagated.

Fortunately, it was also possible to provide experimental proof.

My first attempts to transmit typhus to laboratory animals, including the smaller species of monkeys, had failed, as had those of my predecessors, for reasons which I can easily supply today.

I asked my teacher, E. Roux, to get me a chimpanzee, thinking that an anthropoid might be more susceptible to infection than animals of other species. The day I received the chimpanzee I inoculated it with the blood of a patient from Dr. Broc’s department at the hospital in Rabta. The chimpanzee contracted a fever. I inoculated a macaco (M. sinicus) with blood from the chimpanzee taken during the fever, and he also developed a fever. I cultivated lice on the macaco, which I then transported to other macacos. The latter became infected and subsequently proved to be vaccinated against a test inoculation of the virus.

These decisive experiments did not take very long. I had reproduced typhus in the chimpanzee in June, 1909; I demonstrated the role played by the louse in August. I published these results in September with Charles Comte and Ernest Conseil. This was the yield for the year 1909. During the years that followed I undertook with Conseil and Alfred Conor, and later with Georges Blanc, a more detailed experimental study of the disease and the conditions of transmission.

At the beginning of my research the only animals I knew to be susceptible to the disease were monkeys. All species of monkeys can be infected, provided that a sufficient quantity of virus is inoculated through the peritoneum. The monkey was well suited for preserving the virus. But the monkey is an expensive animal. During the first two years, therefore, I had to be satisfied with studying typhus during the months of its seasonal expansion, experimenting from man to monkey and, exceptionally, from one monkey to another. The discovery that I soon made that the guinea pig was also susceptible to infection made it possible for me, from the third year on, to preserve the virus on this animal. It was then easy for me to conduct my research independently of epidemics, that is, all the time. From the practical point of view, the susceptibility to infection of the guinea pig proved to be the most useful step forward. Today, all laboratories use this animal for preserving the virus.

In man typhus is characterized by a triade of symptoms: fever, rash, nervous symptoms. In animals, fever is the only sign of infection. The fact that the virus is localized more particularly in the brain explains the nervous symptoms to be found in our species.

I demonstrated the characteristics of experimental fever. It appears after an incubation period which is never less than five days. It follows the same pattern as natural fever in man, but is of shorter duration and less pronounced. There follows a period of hypothermia, clearly apparent in the monkey, but less obvious in the guinea pig. In the monkey there are minor general symptoms and subsequent loss of weight is usual. In the guinea pig the disease would be inapparent if the temperature were not noted. At the time when I was conducting my research there was no known method for taking the guinea pig’s temperature. I demonstrated a technique which is now widely used.

I have already mentioned that the virus could be kept indefinitely by injecting guinea pigs. For this purpose, brain tissue gives more constant results than blood.

It can sometimes happen, especially when blood is used, that out of a group of guinea pigs inoculated with the same dose of the same product, certain of them show no signs of fever. At first I attributed this fact to a technical fault or to greater individual resistance. Repeated negative results precluded my continuing to accept these oversimplified explanations. Animals for which the virus is pathogenic present a whole scale of degrees of susceptibility – from the grave form, often fatal in adult Europeans, down to the fever, revealed only by the thermometer, in the guinea pig, passing through all the intermediate stages: typhus of the native adult, benign typhus in children, still more benign in monkeys. I wondered if there was not, below the very slight susceptibility of the guinea pig, an even lesser degree in which the only sign of infection was the virulence of the blood during the period when more susceptible animals showed signs of the characteristic fever. This was indeed the case. I was able to ascertain this fact in 1911. A little later I went back to study this problem with Charles Lebailly. It led me to the conception of what I have called inapparent infections. Apyretic typhus, without symptoms, the inapparent typhus presented by the guinea pig, is the first case described and the most well known.

Inapparent typhus in the guinea pig may be a primary infection typhus, as was the case I discovered. The cause in this case is the inoculation of a quantity of virus insufficient to produce fever. Blood from a guinea pig who has contracted inapparent typhus will always produce pyretic typhus in another guinea pig if the dose inoculated is sufficient. The positive result of this inoculation proves in fact that the first guinea pig had contracted typhus, in spite of the lack of fever.

Having ascertained inapparent typhus in cases of primary infection, it was then easy for me to demonstrate the existence of apyretic typhus in certain guinea pigs that had been reinoculated a fairly long time after contraction of a primary infection of pyretic typhus.

Lebailly and I then went on to demonstrate that inapparent primary infection typhus, which is exceptional in inoculated guinea pigs, is the only form which the disease takes experimentally in the rat and the mouse. This curious disease, which presents no symptoms, which has an incubation period and a period when the blood is virulent, and which confers a certain degree of immunity, can be transmitted from one rat to another. On two occasions I effected twelve such transmissions. At the twelfth, brain tissue from the rats induced pyretic typhus in the guinea pig.

This new concept of inapparent infections that I introduced to pathology is, without a doubt, the most important of the discoveries that I was able to make. In the absence of fever, experimental work on typhus must take into account the possible existence of inapparent forms. We shall see that this concept alone can explain the preservation of the virus in nature and the reappearance of seasonal epidemics. I applied this new concept to measles and certain spirochaete infections, and Georges Blanc has just discovered the existence of inapparent dengue in man, as well as in the monkey and the guinea pig.

Thus my research opened a new chapter, subpathology, which is doubtless a vast field where almost everything remains to be discovered.

We have known for a long time that primary infection of typhus confers immunity in man in almost all cases and that this immunity lasts a lifetime. I established that laboratory animals were subject to a similar immunity; but I also demonstrated that this was of shorter duration than was generally thought before the discovery of inapparent typhus in secondary infections. Although benign, inapparent typhus also confers a certain degree of immunity.

Lucien Raynaud (of Algiers) and E. Legrain (of Bougie) had used serum from convalescent typhus patients empirically in treating typhus. Conseil and I established that this method was not effective. On the other hand, we demonstrated that serum from convalescent patients and from animals that had recovered from the disease had preventive properties as regards subsequent inoculation of the virus. Following these discoveries, we were able to institute a preventive method against typhus by using serum from convalescents. This method gives sure protection to persons in contact with typhus patients, to doctors and to nurses. It proved to be particularly effective, in our hands, in protecting persons contaminated by lice from typhus patients and who would otherwise certainly have contracted the disease. The immunity conferred by the serum is of short duration, but inoculation may be repeated where necessary.

The knowledge of the preventive properties contained in serum from convalescent typhus patients led me, with Conseil, to use serum taken from children cured of measles to prevent this disease. This method spread from Tunis throughout the whole world and has saved thousands of lives.

I was less successful in my attempts to effect preventive vaccination against typhus by using the virus and in trying to produce large quantities of serum using large animals.

I did, certainly, succeed in vaccinating a number of people by injecting very small, repeated doses of virulent blood (with Conseil) and achieved rather better results with brain tissue from the guinea pig (with Hélène Sparrow). However, the method is unreliable and an element of danger precludes any recommendation to generalize it.

Prevention of typhus by serum from convalescent patients presupposes the existence of such patients; moreover, the quantity of serum provided by a convalescent is very small. Although useful, therefore, this method can be considered only as a makeshift. If it were possible to produce these preventive properties in a large animal, the quantity of serum obtained would be unlimited and, by preserving the virus in guinea pigs, it would thus be possible to produce the serum as and when required.

I had realized that repeated virulent inoculations of the donkey did not cause appreciable preventive properties to be developed in the blood of this animal, but in attempting to find a solution to the problem, I tried to ascertain whether the donkey, apparently resistant to the disease, could not contract inapparent typhus and I found that this was sometimes the case. As inapparent typhus produced no preventive properties, I then endeavoured to produce pyretic typhus in the donkey. Working first alone and later with Hélène Sparrow, I succeeded by intracerebral inoculation of the virus, but only in very rare cases and without being able to determine the conditions necessary to produce frequent, if not constant, results. Serum from the donkey cured of pyretic typhus does in fact possess preventive properties. Up to the present time, this is as far as we have gone in our research on this point.

I was more successful in my study of the conditions in which transmission of typhus takes place. After clearly establishing the role of the louse, I demonstrated the detailed mechanism of transmission. Two factors are involved: man and the louse.

The patient’s blood is virulent, not only during the whole duration of the fever, but also two or three days before it appears and two or three days after the temperature has fallen. During this whole period, therefore, the louse can be infected by the patient. Children play an important part in the propagation of typhus. Conseil and I have demonstrated that native children, particularly very young children, contract a benign form of typhus, sometimes so slight that it can only be detected by positive results obtained from inoculation of blood in the monkey or guinea pig.

The role of inapparent typhus in cases of re-infection is indubitably even more important. This alone explains the preservation of the virus in nature when there is no epidemic and the seasonal reappearance of the epidemics themselves. It would be impossible to understand how typhus could remain active if this depended on the louse continually finding different people to infect. And if this is self-evident in the case of typhus, it is even more so for measles. We have no doubt discovered here the most useful application of the concept of inapparent infections.

Just as the only reservoir for the typhus virus in nature is provided by man, so the only vector of infection is the louse. The bite of the louse is not virulent immediately after the infecting meal. It becomes so only towards the 7th day following infection. On the 9th and 10th days the bite is invariably virulent. It is therefore a necessary condition that the virus should multiply within the louse for it to become dangerous. I have shown that this multiplication takes place in the digestive tract and that louse faeces become virulent at the same period as does the bite. This observation was made in 1910. Developed by further, more detailed research undertaken in conjunction with Georges Blanc in 1914, it provided a guide to those who undertook to locate the pathogenic organism responsible for typhus in the intestine of the louse. Edmond Sergent (from Algiers) was the first to describe inclusion bodies, later called rickettsia, in the intestinal cells and faeces of the louse. Not being attracted to purely morphological research, I have not myself studied these inclusion bodies, the significance of which is not clearly demonstrated.

I demonstrated the more immediately important fact that typhus can be transmitted through louse faeces. By soiling the natives’ skin; they are easily inoculated by scratching or by soiling the fingers, which are then brought into contact with the conjunctiva, which is an ideal means of entry for such an active virus.

I finally demonstrated that typhus infection is not hereditary in the louse.

These observations formed the basis of typhus prophylaxis. The pioneer in this field was Conseil, Director of the Bureau of Public Health in Tunis; in three years he eradicated typhus from a town where it had raged year after year since the beginning of history. Thanks to the endeavours of our medical administrators, and in particular Gobert, Cardaliaguet, and Henry, typhus has been eliminated from the mines and prisons and has retreated to remote rural districts, where it will remain a permanent menace until it is finally eradicated. From Tunisia the method has spread over the world.

From the very beginning of the Great War steps were taken, on my instruction, to institute medical control of troops in North Africa. No native left African soil for Europe without having been previously deloused. This was the precaution, taken by all nations in similar conditions, which saved the armies from typhus. Although lice appeared and multiplied in the trenches and became a plague in themselves, for the first time in the history of man typhus did not go hand in hand with a long war.

If in 1914 we had been unaware of the mode of transmission of typhus, and if infected lice had been imported into Europe, the war would not have ended by a bloody victory. It would have ended in an unparalleled catastrophe, the most terrible in human history. Soldiers at the front, reserves, prisoners, civilians, neutrals even, the whole of humanity would have collapsed. Men would have perished in millions, as unfortunately occurred in Russia.

And this is the ultimate lesson that our knowledge of the mode of transmission of typhus has taught us: Man carries on his skin a parasite, the louse. Civilization rids him of it. Should man regress, should he allow himself to resemble a primitive beast, the louse begins to multiply again and treats man as he deserves, as a brute beast.

This conclusion would have endeared itself to the warm heart of Alfred Nobel. My contribution to it makes me feel less unworthy of the honour which you have conferred upon me in his name.

* As Professor Nicolle has been prevented by ill health from coming to Stockholm to deliver his Nobel Lecture, he has very kindly sent the text to the Editor of Les Prix Nobel for publication.