CHAPTER IX
GEOGRAPHICAL LIMITS OF YELLOW FEVER
The geographical limits of yellow fever, when first encountered by the white man, were not very wide. They were contained within a zone limited on the north by the twentieth parallel of latitude, on the south by the eighth, on the east by the sixtieth meridian, and on the west by the one hundredth. Practically, the disease was confined to the shores of the Caribbean and the southern shores of the Gulf of Mexico. Within these regions it was practically endemic at the time Columbus discovered America. The arrival of the white man with his sailing ships made the disease much easier of transmission from place to place, because man traveled much more frequently and much farther, and also because sailing ships, with their fresh-water tanks, bred the stegomyia extensively. Under these influences the endemic area began slowly to spread. Toward the north the endemic area never extended far beyond the limits it occupied when Columbus made his first voyage. Havana, in latitude 23°, is the northern endemic limit in the western hemisphere. This is rather strange, as the city is located so near the original endemic focus. The arrival of a considerable number of non-immunes in a locality is necessary for yellow fever to become endemic, and this supply has to be fairly constant. During the first hundred years after the founding of Havana, in 1511, the city was considered very healthy. In 1620, an epidemic of yellow fever occurred, the disease having been introduced from Panama (Porto Bello) by the treasure fleet on its way to Spain. In 1649 another epidemic occurred, which was extremely severe and spread to many other towns of Cuba. This epidemic lasted more or less until 1655. For a hundred years after this, nothing in the nature of an epidemic occurred. In 1762, at the time of the capture of Havana by the English colonial troops from the North American colonies, the disease again became epidemic, and after that time remained constantly in Havana as an endemic disease. At this time it was recognized as being introduced from Vera Cruz, Mexico, under the name of “black vomit.” It became endemic, and did not disappear as on former occasions, on account of the large and constant supply of non-immune persons, due to the presence of the English troops, and after their departure, to the fact that the port was thrown open to general commerce, and to a continuous supply of immigrants from Spain.
It would seem, then, that Havana, after remaining in the epidemic area for some two hundred and fifty years, finally, about the year 1762, passed permanently into the endemic area.
Havana is the most northern point to which the endemic area ever extended. Toward the south the endemic area spread much farther from the original endemic focus than on the north. While in distance from the original endemic area the disease spread further south than north, it did not in point of latitude. Rio Janeiro and Santos, the furthest points south to which the endemic area spread, are about latitude 23°, much the same as Havana on the north. Para, Manaos, Pernambuco and Bahia gradually came within the endemic area, until finally, in 1849, yellow fever took up its permanent habitat in Rio and Santos. During the year 1850, forty-one hundred and sixty persons died of yellow fever in Rio.
These cities, like Havana, were subject to epidemics of yellow fever years before they became endemic centers. Endemicity did not spread regularly over this area from north to south. The disease became endemic in Rio thirty or forty years before it did in Manaos. Other factors proved more potent than location as to latitude. The large emigration to Brazil, which set in about the middle of the nineteenth century, gave a constant and large supply of non-immunes to Rio, while Manaos did not get this supply until railroad construction commenced in the interior, about the beginning of the present century.
Strictly speaking, Pernambuco, on the eastern coast of Brazil, about longitude 35° west, is the point farthest east where yellow fever really became endemic, in the sense of remaining continuously in one city for a number of years. But on the west coast of Africa, from St. Paul de Loanda on the south to the Canary Islands on the north, yellow fever has been present all the time at some point or points, ever since its introduction in 1494, though on this coast it has never manifested itself continuously in the same town or city for any considerable number of years. But it is endemic on the coast in the sense that it no longer has to be re-introduced from the outside, but is always present at some point. We have a pretty accurate account of its first introduction into the Canary Islands in 1494 by Spaniards returning from Hispanola.
The farthest point west at which it has ever become endemic is its original home, Vera Cruz, Mexico. It has been endemic at both Panama and Guayaquil, on the west coast of America. While these points are on the west coast, and Vera Cruz is on the east coast, still, Vera Cruz is some sixteen degrees of latitude west of either Panama or Guayaquil.
The endemic limits of yellow fever at its period of greatest extent would be defined by a line drawn from Havana, commencing on the north, to the Canary Islands, down the west coast of Africa to Loanda, from Loanda west to Rio Janeiro, Brazil, from Rio Janeiro to Guayaquil, Ecuador, from Guayaquil to Panama, from Panama to Vera Cruz, Mexico, and from Vera Cruz back to Havana, a very restricted area compared with other infectious diseases.
But it is as an epidemic disease that it has caused most alarm and loss. The United States, Spain, and the West Indies have suffered most severely in this respect. It has been epidemic as far north as Quebec in North America. In 1805, some fifty-five cases occurred among the English troops stationed there. In Europe, it has been epidemic as far north as Swansea, Wales. In the year 1865, the sailing ship Hecla from Cuba, introduced yellow fever into Swansea, and twenty-two cases developed in the town in persons who had no communication with the vessel.
France has never had a yellow-fever epidemic of any consequence, though it has been demonstrated that under favorable conditions, the disease might become epidemic. At St. Nazaire, in the year 1861, yellow fever was introduced by the sailing ship Anne-Marie, and forty cases resulted, with twenty-three deaths.