By a series of perfectly conclusive experiments carried on near Washington, D.C., Smith and Kilbourne showed that this organism was carried from Southern cattle to non-immune animals by the so-called Southern cattle tick, _Boophilus annulatus_ (= _Margaropus annulatus_) (fig. 139).
Of fourteen head of native cattle placed in a field with tick-infested Northern cattle all but two contracted the disease. This experiment was repeated with similar results. Four head of native cattle kept in a plot with three North Carolina cattle which had been carefully freed from ticks remained healthy. A second experiment the same year gave similar results.
Still more conclusive was the experiment showing that fields which had not been entered by Southern cattle but which had been infected by mature ticks taken from such animals would produce Texas fever in native cattle. On September 13, 1889, several thousand ticks collected from cattle in North Carolina three and four days before, were scattered in a small field near Washington. Three out of four native animals placed in this field contracted the disease. The fourth animal was not examined as to its blood but it showed no external symptoms of the disease.
[Ill.u.s.tration: 140. Hyalomma aegyptic.u.m. After Nuttall and Warburton.]
In these earlier experiments it was believed that the cattle tick acted as a carrier of the disease between the Southern cattle and the _soil_ of the Northern pastures. "It was believed that the tick obtained the parasite from the blood of its host and in its dissolution on the pasture a certain resistant spore form was set free which produced the disease when taken in with the food." The feeding of one animal for some time with gra.s.s from the most abundantly infected field, without any appearance of the disease, made this hypothesis untenable.
In the experimental work in 1890 the astonishing fact was brought out that the disease was conveyed neither by infected ticks disintegrating nor by their directly transferring the parasite, but that it was conveyed by the young hatched from eggs of infected ticks. In other words, the disease was hereditarily transferred to ticks of the second generation and they alone were capable of conveying it.
Thus was explained the fact that Texas fever did not appear immediately along the route of Southern cattle being driven to Northern markets but that after a certain definite period it manifested itself. It was conveyed by the progeny of ticks which had dropped from the Southern cattle and deposited their eggs on the ground.
These results have been fully confirmed by workers in different parts of the world,--notably by Koch, in Africa, and by Pound, in Australia.
The disease is apparently transmitted by _Boophilus annulatus_ alone, in the United States, but it, or an almost identical disease, is conveyed by _Ixodes hexagonus_ in Norway, _Ixodes ricinus_ in Finland and France and by the three species, _Boophilus decoloratus_, _Hyalomma aegyptic.u.m_ (fig. 140 and 141), and _Haemaphysalis punctata_ in Africa.
[Ill.u.s.tration: 141. Hyalomma aegyptic.u.m. Capitulum of female; (_a_) dorsal, (_b_) ventral aspect.]
In spite of the detailed study which it has received, the life cycle of _Babesia bovis_ has not been satisfactorily worked out. The as.e.xual reproduction in the blood of the vertebrate host has been described but the cycle in the tick is practically unknown.
More successful attempts have been made to work out the life cycle of a related species, _Babesia canis_, which causes malignant jaundice in dogs in Africa and parts of Southern Europe. In this instance, also, the disease is transmitted by heredity to the ticks of the second generation. Yet the larval, or "seed ticks," from an infected female are not capable of conveying the disease, but only the nymphs and adults.
Still more complicated is the condition in the case of _Babesia ovis_ of sheep, which Motas has shown can be conveyed solely by the adult, s.e.xually mature ticks of the second generation.
In _Babesia canis_, Christopher (1907) observed developmental stages in the tick. He found in the stomach of adult ticks, large motile club-shaped bodies which he considered as ookinetes. These bodies pa.s.s to the ovaries of the tick and enter the eggs where they become globular in form and probably represent an oocyst. This breaks up into a number of sporoblasts which enter the tissues of the developing tick and give rise to numerous sporozoites, which collect in the salivary glands and thence are transferred to the vertebrate host. A number of other species of _Babesia_ are known to infest vertebrates and in all the cases where the method has been worked out it has been found that the conveyal was by ticks. We shall not consider the cases more fully here, as we are concerned especially with the method of transfer of human diseases.
TICKS AND ROCKY MOUNTAIN SPOTTED FEVER OF MAN--Ever since 1873 there has been known in Montana and Idaho a peculiar febrile disease of man, which has gained the name of "Rocky Mountain spotted fever." Its onset is marked by chills and fever which rapidly become acute. In about four to seven days there appears a characteristic eruption on the wrists, ankles or back, which quickly covers the body.
McClintic (1912) states that the disease has now been reported from practically all of the Rocky Mountain States, including Arizona, California, Colorado, Idaho, Montana, Nevada, Oregon, Utah, Washington, and Wyoming. "Although the disease is far more prevalent in Montana and Idaho than in any of the other States, its spread has a.s.sumed such proportions in the last decade as to call for the gravest consideration on the part of both the state and national health authorities. In fact, the disease has so spread from state to state that it has undoubtedly become a very serious interstate problem demanding the inst.i.tution of measures for its control and suppression."
A peculiar feature of the Rocky Mountain spotted fever is a marked variation in its severity in different localities. In Montana, and especially in the famous Bitter Root Valley, from 33 per cent to 75 per cent of the cases result fatally. On the other hand, the fatality does not exceed four per cent in Idaho.
In 1902, Wilson and Chowning reported the causative organism of spotted fever to be a blood parasite akin to the _Babesia_ of Texas fever, and made the suggestion that the disease was tick-borne. The careful studies of Stiles (1905) failed to confirm the supposed discovery of the organism, and the disease is now generally cla.s.sed as due to an invisible virus. On the other hand, the acc.u.mulated evidence has fully substantiated the hypothesis that it is tick-borne.
According to Ricketts (1907) the experimental evidence in support of this hypothesis was first afforded by Dr. L. P. McCalla and Dr. H. A.
Brereton, in 1905. These investigators transmitted the disease from man to man in two experiments. "The tick was obtained "from the chest of a man very ill with spotted fever" and "applied to the arm of a man who had been in the hospital for two months and a half, and had lost both feet from gangrene due to freezing." On the eighth day the patient became very ill and pa.s.sed through a mild course of spotted fever, leaving a characteristic eruption. The experiment was repeated by placing the tick on a woman"s leg and she likewise was infected with spotted fever."
The most detailed studies were those of the late Dr. H. T. Ricketts, and it was he who clearly established the tick hypothesis. In the summer of 1906 he found that guinea pigs and monkeys are very susceptible to spotted fever and can readily be infected by inoculation of blood from patients suffering from the disease. This opened the way to experimental work on tick transmission. A female tick was fed upon an infected guinea pig for two days, removed and isolated for two days and then placed upon a healthy guinea pig. After an incubation period of three and a half days the experimental animal contracted a well-marked case of the disease.
A similar result was obtained at the same time by King, and later in the season Ricketts proved that the male tick was also capable of transmitting the disease. He found that there was a very intimate relation of the virus to the tick and that the transmission must be regarded as biological throughout. Ticks remained infective as long as they lived and would feed for a period of several months. If they acquired the disease in the larval or nymphal stage they retained it during molting and were infective in the subsequent stages. In a few cases the larvae from an infected female were infective.
The evidence indicated that the tick suffers from a relatively harmless, generalized infection and the virus proliferates in its body. The disease probably is transferred through the salivary secretion of the tick since inoculation experiments show that the salivary glands of the infected adult contain the virus.
It is probable that in nature the reservoir of the virus of spotted fever is some one or more of the native small animals. Infected ticks have been found in nature, and as various wild animals are susceptible to the disease, it is obvious that it may exist among them unnoticed.
Wilson and Chowning suggested that the ground squirrel plays the princ.i.p.al role.
Unfortunately, much confusion exists regarding the correct name of the tick which normally conveys the disease. In the medical literature it is usually referred to as _Dermacentor occidentalis_, but students of the group now agree that it is specifically distinct. Banks has designated it as _Dermacentor venustus_ and this name is used in the publications of the Bureau of Entomology. On the other hand, Stiles maintains that the common tick of the Bitter Root Valley, and the form which has been collected by the authors who have worked on Rocky Mountain spotted fever in that region, is separable from _D. venustus_, and he has described it under the name of _Dermacentor andersoni_.
Mayer (1911) has shown experimentally that spotted fever may be transmitted by several different species of ticks, notably _Dermacentor marginatus_, _Dermacentor variabilis_ and _Amblyomma americanum_. This being the case, the question of the exact systematic status of the species experimented upon in the Bitter Root Valley becomes less important, for since _Dermacentor occidentalis_, _Dermacentor venustus_ and _Dermacentor andersoni_ all readily attack man, it is probable that either species would readily disseminate the disease if it should spread into their range.
Hunter and Bishop (1911) have emphasized the fact that in the eastern and southern United States there occur several species which attack man, and any one of which might transmit the disease from animal to animal and from animal to man. The following species, they state, would probably be of princ.i.p.al importance in the Southern and Eastern States: the lone star tick (_Amblyomma americanum_); the American dog tick (_Dermacentor variabilis_); and the gulf-coast tick (_Amblyomma maculatum_). In the extreme southern portions of Texas, _Amblyomma cajennense_, is a common pest of man.
Since the evidence all indicates that Rocky Mountain spotted fever is transmitted solely by the tick, and that some of the wild animals serve as reservoirs of the virus, it is obvious that personal prophylaxis consists in avoiding the ticks as fully as possible, and in quickly removing those which do attack. General measures along the line of tick eradication must be carried out if the disease is to be controlled. That such measures are feasible has been shown by the work which has been done in controlling the tick-borne Texas fever of cattle, and by such work as has already been done against the spotted fever tick, which occurs on both wild and domestic animals. Detailed consideration of these measures is to be found in the publications of the Public Health and Marine Hospital Service, and the Bureau of Entomology. Hunter and Bishopp give the following summarized recommendations for control or eradication measures in the Bitter Root Valley.
(1) A campaign of education, whereby all the residents of the valley will be made thoroughly familiar with the feasibility of the plan of eradication, and with what it will mean in the development of the valley.
(2) The obtaining of legislation to make it possible to dip or oil all live stock in the Bitter Root Valley.
(3) The obtaining of an accurate census of the horses, cattle, sheep, mules, and dogs in the valley.
(4) The construction of ten or more dipping vats.
(5) The providing of materials to be used in the dipping mixture.
(6) The organization of a corps of workers to carry on the operations.
(7) The systematic dipping of the horses, cattle, sheep, and dogs of the valley on a definite weekly schedule from approximately March 10 to June 9.
(8) The treatment by hand of the animals in localities remote from vats, on the same schedule.
They estimate that after three seasons" operations a very small annual expenditure would provide against reinfestation of the valley by the incoming of cattle from other places.
Supplementary measures consist in the killing of wild mammals which may harbor the tick; systematic burning of the brush and debris on the mountain side; and in clearing, since the tick is seldom found on land under cultivation.
CHAPTER X
ARTHROPODS AS ESSENTIAL HOSTS OF PATHOGENIC PROTOZOA [_Continued_]
ARTHROPODS AND SPIROCHaeTOSES OF MAN AND ANIMALS
The term spirochaetoses is applied to diseases of man or animals which are due to protistan parasites belonging to the group of slender, spiral organisms known as spirochaetes.
There has been much discussion concerning the relationship Of the spirochaetes. Formerly, they were regarded as bacteria closely related to the forms grouped in the genus _Spirillum_. The results of the detailed study which the spirochaetes have received in recent years, have led most of the workers to consider them as belonging to the protozoa. The merits of the discussion we are not concerned with here, but rather with the fact that a number of diseases caused by spirochaetes are arthropod-borne. The better known of these we shall discuss.
AFRICAN RELAPSING FEVER OF MAN--It has long been known to the natives of Africa and to travelers in that country, that the bite of a certain tick, _Ornithodoros moubata_, may be followed by severe or even fatal fever of the relapsing type. Until recent years, it was supposed that the effect was due to some special virulence of the tick, just as nagana of cattle was attributed to the direct effect of the bite of the tsetse-fly. The disease is commonly known as "tick-fever" or by the various native names of the tick.
In 1904, Ross and Milne, in Uganda, and Dutton and Todd on the Congo, discovered that the cause of the disease is a spirochaete which is transmitted by the tick. This organism has been designated by Novy and Knapp as _Spirochaeta duttoni_.
_Ornithodoros moubata_ (fig. 142), the carrier of African relapsing fever, or "tick-fever," is widely distributed in tropical Africa, and occurs in great numbers in the huts of natives, in the dust, cracks and crevices of the dirt floors, or the walls. It feeds voraciously on man as well as upon birds and mammals. Like others of the _Argasidae_, it resembles the bed-bug in its habit of feeding primarily at night. Dutton and Todd observed that the larval stage is undergone in the egg and that the first free stage is that of the octopod nymph.
[Ill.u.s.tration: 142. Ornithodoros moubata. (_a_) Anterior part of venter; (_b_) second stage nymph; (_c_) capitulum; (_d_) dorsal and (_e_) ventral aspect of female; (_f_) ventral aspect of nymph; (_g_) capitulum of nymph. After Nuttall and Warburton.]