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Medical Warning!  
This article is from the 1911 Encyclopaedia Britannica. Medical science has made many leaps forward since it has been written. This is not a site for medical advice, when you need information on a medical condition, consult a professional instead.  
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YELLOW FEVER, a specific infective tropical fever, the germ of which is transmitted by the Stegomyia fasciata or domestic mosquito, occurring endemically in certain limited areas. The: area of distribution includes the West Indies, Mexico, part of Central America, the W. coast of Africa and Brazil.

The first authentic account of yellow fever comes from Bridgetown, Barbados, in 1647, where it was recognized as a "nova pestis" that was unaccountable in its origin, except that Ligon,, the historian of the colony, who was then on the spot, connected. it with the arrival of ships. It was the same new pestilence that Dutertre, writing in 5667, described as having occurred in the French colony of Guadeloupe in 1635 and 1640; it recurred at Guadeloupe in 1648, and broke out in a peculiarly disastrous form at St Kitts the same year, and again in 5652; in 1655 it was at Port Royal, Jamaica; and from those years onwards. it became familiar at many harbours in the West Indies and Spanish Main. It appeared at the Brazilian ports in 1849. In 1853 it appeared in Peru and in 1820 on the W. coast of Africa. In Georgetown (British Guiana) 69%69% of the garrison died in 5840.

During the great period of yellow fever (1793-1805), and for some years afterwards, the disease found its way time after time to various ports of Spain. Cadiz suffered five epidemics in the 18th century, Malaga one and Lisbon one; but from 1800 down. to 1821 the disease assumed much more alarming proportions,. Cadiz being still its chief seat, while Seville, Malaga, Cartagena,. Barcelona, Palma, Gibraltar and other shipping places suffered severely, as well as some of the country districts nearest to theports. In the severe epidemic at Barcelona in the summer of 1821, 5000 persons died. At Lisbon in 1857 upwards of 6000 died in a few weeks. In New Orleans 7970 people died in 1853,, 3 0 93 in 1867, and 4056 in 1878. In Rio 4160 died in 1850, 1 943 in 1852, and 1397 in 1886.

Certain distinct conditions have seemed to be necessary for an outbreak. Foremost we may notice a high atmospheric temperature, one of 75° F. or over. As the thermometer sinks, the disease ceases to spread. Moisture favours the spread of yellow fever, and epidemics in the tropics have usually occurred about the rainy season. Seaport towns are most affected. In many instances the elevated airy and hygienic quarters of a town may escape, while the shore districts are decimated. Usually the disease does not spread to villages or sparsely populated districts. Certain houses become hotbeds of the disease, case after case occurring in them; and it is usually in houses that the disease is contracted. A house may be said to be infected when it contains infected mosquitoes, whether therebe a yellow-fever patient there or not. Ships become infected in the same way, the old wooden trading ships affording an ideal hiding-place to the Stegomyia in a way that the modern and airy steamship does not.

The incubation period of yellow fever is generally four or five days, but it may be as short as twenty-four hours. There areusually three marked stages: (1) the febrile period, (2) the period of remission or lull, (3) in severe cases, the period of reaction. The illness usually starts with languor, chilliness, headache, and muscular pains, which might be the precursors of any febrile attack. These are followed by a peculiar look of the eyes and face,. which is characteristic: the face is flushed, and the eyes suffused at first and then congested or ferrety, the nostrils and lips red,, and the tongue scarlet - these being the most obvious signs of: universal congestion of the skin, mucous membranes and organs. Meanwhile the temperature has risen to fever heat, and may reach a very high figure (maximum of 110 Fahr., it is said); the pulse is quick, strong and full, but may not keep up in these characters with the high temperature throughout. There are all the usual accompaniments of high fever, including hot skin, failure of appetite, thirst, nausea, restlessness and delirium (which may or may not be violent); albumen will nearly always be found in the urine. The fever is continued; but the febrile excitement comes to an end after two or three days. In a certain class of ambulatory or masked cases the febrile reaction may never come out, and the shock of the infection after a brief interval may lead unexpectedly and directly to prostration and death. The cessation of the paroxysm makes the stadium, or lull, characteristic of yellow fever. The hitherto militant or violent symptoms cease, and prostration or collapse ensues. The internal heat falls below the normal; the action of the heart (pulse) becomes slow and feeble, the skin cold and of a lemon-yellow tint, the act of vomiting effortless, like that of an infant, the first vomit being clear fluid, but afterwards black from an admixture of blood. It is at this period that the prospect of recovery or of a fatal issue declares itself. The prostration following the paroxysm of fever may be no more than the weakness of commencing recovery, with copious flow of urine, which even then is very dark-coloured from the presence of blood. The prostration will be all the more profound according to the height reached by the temperature during the acute paroxysm. Much blood in the vomit and in the stools, together with all other haemorrhagic signs, is of evil omen. Death may also be ushered in by suppression of urine, coma and convulsions, or by fainting from failure of the heart. In severe types of the disease an apoplectic, an algid and a choleraic form have been described.

The case mortality averages from 12 to 80%. In Rio in 1898 it reached the appalling height of 94.5%. In cities where it is endemic the case mortality is usually lower. In 269 cases observed by Sternberg, the mean mortality was 27.7%. In 158 cases of yellow fever in Vera Cruz in 1905 there were 91 deaths. The death-rate, however, tends to vary in different epidemics. In the epidemic occurring in Zacapa, Mexico, in 1905 in a population of 6000 there were 700 cases, and the mortality among the infected was 40%. Treatment. - The patient should be removed from the focus of infection and nursed in a well-ventilated room, screened from mosquitoes. The further treatment is symtomatic. A purgative, followed by hot baths, is useful in the early stages to relieve congestion, high temperature may be controlled by sponging; vomiting, by ice; or, if haemorrhagic, by ergot, perchloride of iron or other styptics; and pilocarpine may be given if the urine be scanty. Sternberg has introduced a system of treatment by alkalis to counteract the hyperacidity of the intestinal contents and increase the flow of urine. Of 301 whites treated by this method only 7.3% died, and of 72 blacks all recovered.

## Causation

The pathology of the disease is discussed in the article Parasitic Diseases. In 1881 Dr Charles Finlay, of Havana, propounded the theory that mosquitoes were the carriers of the infection. Numerous theories had previously been brought forward, notably that of the Bacillus icteroides, described by Sanarelli; but it is now certain that this organism is not the cause. Other authorities held that the disease was spread by contagion, by miasmata, or some other of the vague agencies which have always been put forward in the absence of exact knowledge. Finlay's mosquito theory remained in abeyance until attention was again drawn to it by the demonstration in recent years of the part played by these insects in the causation of other tropical diseases. The mosquito selected by Finlay was the Stegomyia fasciata, a black insect with silvery markings on the thorax, which is exceedingly common in the endemic area. It frequents towns, and breeds in any stagnant water about houses. Specimens were caught, fed upon yellow-fever patients, kept for a fortnight, and then allowed to bite susceptible persons established in a special camp with other susceptible persons as a control. Those bitten developed the fever, the others did not. An American commission was appointed in 1900, consisting of Walter Reed, James Carroll, A. Agramonte and Lazear, and its conclusions were: that the Stegomyia fasciata is the agent of infection, that the virus of yellow fever is present in the blood during the first three days of the fever, and is generally absent on the fourth; that the germ is so small that it can pass through a Chamberland porcelain filter; that the bite of all infected Stegomyia does not produce yellow fever (about 35% of the experiments proving negative); that mosquitoes fed on yellow-fever blood were not capable of giving rise to infection until after a lapse of twelve or fourteen days, but the insects retained their infective power for at least fifty-seven days. It can therefore be concluded that the virus of yellow fever is a parasite, requiring as in malaria an alternate passage through a vertebrate and an insect host, the analogy to malaria being very complete. E. Marchoux and P. L. Simond, of the French Yellow Fever Commission to Rio de Janeiro, 1906, have observed an interesting fact in connexion with the S. fasciata.  In order to lay her eggs she must first have a feed of blood, three days after which she lays them. Before she lays her eggs she strikes both day and night, after that period at night only. Persons bitten in the day-time, therefore, do not develop yellow fever, while those bitten at night do. This may explain the impunity with which Europeans may visit an infected district in the day-time provided that they are careful not to sleep there at night. It was stated by Marchoux and Simond that an infected mosquito transmits the parasite to her eggs, the progeny proving infective.

## Prophylaxis

Following on the publication of these experiments there was instituted a vigorous campaign against mosquitoes in Havana in 1901, based on the methods applied to the suppression of malaria, and carried out under the direction of Major W. C. Gorgas of the United States army, chief sanitary officer of Havana. The work was begun on the 27th of February 1901. An order was issued that all receptacles containing water were to be kept mosquito-proof; sanitary inspectors were told off for each district to maintain a constant house-to-house inspection, and to treat all puddles, &c., with oil; receptacles found to contain larvae were destroyed and their owners fined; breeding-grounds near the town were treated by draining and oil; hospitals and houses containing yellow-fever patients were screened; infected and adjacent buildings were fumigated with pyrethrum powder. The results exceeded all expectation, and after January 1902 the disease entirely ceased to originate in Havana. Cases occasionally now come into Havana from Mexican ports, but are treated under screens with impunity in ordinary city hospitals and never at any time infect the city. Thus in 1907 there was one death from yellow fever, and the general death-rate of Havana from all diseases was 17 per thousand. In the Bulletin of Public Health and Charities of Cuba it is stated there only occurred between 1905-9 a total of 345 cases of yellow fever in all Cuba, where formerly they numbered many thousands, and in April 1910 the republic was declared to be entirely free from the disease.

Among other modern outbreaks in which sanitary measures have triumphed in the suppression of yellow fever were the outbreak in New Orleans in 1905, in which a medical staff of 50 with subordinates to the number of 1203 started immediately on the outbreak to clean up the city; the outbreak in Belize, British Honduras, in 1905; the anti-yellow-fever campaign undertaken in the British W. Indies in 1906-9. As soon as the Isthmian Canal commissioners took over the administration of the Panama Canal Zone they undertook a vigorous campaign against the mosquito, as the result of which yellow fever was successfully banished. Colonel Gorgas in his 1908 report wrote: "It is now three years since a case of yellow fever has developed in the Isthmus, the last being in November 1905." Rio de Janeiro, which had lost 28,078 inhabitants in 13 years by yellow fever, and Santo, have also waged war against the disease; as a result of the anti-Stegomyia policy the deaths from yellow fever in Rio fell to 42 in 1906, 39 in 1907, 4 in 1908, and o in 1909.

See Sir P. Manson, Tropical Diseases (1907); article "Yellow Fever" in Allbutt and Rolleston's System of Medicine; Sir R. Boyce, Report on Yellow Fever in Honduras (1906), and Health and Administration in the West Indies (1910); Bulletins of the U.S. Yellow Fever Institute; Annales de l'Institut Pasteur (January 1906).