T. W. COLE AND WM. M. MACKELLAR.
TICK fever is marked by high fever, destruction of red corpuscles, enlarged spleen, engorged liver, thick and flaky bile, jaundice, emaciation, and death in 10 percent of the chronic cases and up to 90 percent of the acute cases.
Tick fever is caused by the development and activity of minute animal parasites known as piroplasms, which are conveyed to the animals by the cattle fever tick.
A peculiarity of the disease is that animals responsible for its spread are apparently healthy, while those that become diseased do not as a rule convey the infection, except as carriers and distributors of the cattle fever tick.
Tick fever probably was introduced into the United States by cattle imported from the West Indies during the early Spanish colonization of Mexico and the southern parts of the United States.
The disease caused large losses year after year during the early history of this country. In the summer of 1796 a severe outbreak in Lancaster County, Pa., was attributed to cattle brought from South Carolina. An investigation indicated that native cattle contracted tick fever when they mingled with the southern cattle. In one instance the Pennsylvania cattle got sick when they used a pasture previously occupied by the southern cattle. This strange malady always seemed to occur in summer and disappear after the first heavy frost. It was observed that the southern cattle apparently responsible for spreading the disease remained in good health.
Alarming reports of this trouble came from the West and Southwest in the mid- 1800's, when many cattle were trailed out of Texas to Western and Midwestern States. The heavy death losses that followed in the wake of the trail movements from Texas led to the name "Texas fever," a misleading name which wrongly gives the impression that the disease originated in Texas or was confined to that State.
Several States made laws and regulations in undertaking to control the movements and prevent the spread of the disease. Nevertheless the disease advanced northward. It became evident that the problem was of national interest if the cattle industry in the North were to receive effective protection, the Federal Government would have to establish control measures by dividing the infected areas from the free areas and by regulating the movement of all cattle from the regions where the disease existed.
A survey was undertaken in 1883 to locate the northern limits of the infection. By order of the Secretary of Agriculture, a Federal quarantine line was established on July 3, 1889, and regulations were issued that permitted only the shipment of southern cattle under special sanitary restrictions to northern markets for immediate slaughter. The quarantine and the regulations checked the spread of the disease, but they did little to improve conditions in the quarantined area, except to emphasize the need to combat the disease where it was established.
While field surveys, were in progress to determine the limits of the territory involved, scientists were conducting investigations to solve the baffling problems posed by the disease. In 1889 investigators of the Department of Agriculture recognized and described as protozoa the minute parasites that are found in the red blood cells and are the direct causative agents of tick fever.
The theory that Texas fever was caused by ticks, which were carried to and scattered on northern pastures by southern cattle, had been advanced by some observing western cattlemen. Others considered the idea ridiculous.
The Department's chief of animal disease eradication activities decided to investigate the theory, and experiments conducted by two of its scientists, Theobald Smith and Fred L. Kilborne, in 1889-1890 established that transmission of the disease depended on infestations of cattle ticks. They demonstrated that an infectious disease could be transmitted by an intermediate host or carrier from one animal to another a notable advance in medicine.
Cooper Curtice, another Department veterinarian, made noteworthy contributions by his studies and description of the life history and characteristics of the cattle tick. The combined work of these three scientists pointed the way for studies that later resolved similar problems with respect to parasitic vectors that spread such human diseases as malaria, yellow fever, typhus, and Rocky Mountain spotted fever.
THE DESIRABILITY of eradicating the ticks became apparent when investigators found that without the tick the disease would soon die out. So in 1906 a campaign was started to eliminate cattle ticks from the United States. The work, carried out by State officials, livestock owners, and Department representatives, has continued to this time.
At the outset, 985 counties in 15 Southern and Southwestern States nearly one-fourth of the United States were under Federal quarantine because of tick infestation.
The only known tick-infested area in 1956 was a narrow buffer area in parts of eight counties adjacent to the lower Rio Grande River in Texas, where re-infestations continue to occur through stray and smuggled animals from Mexico. The area has remained under quarantine as a protective measure.
In order to free the United States of cattle tick fever, workers had to eradicate the North American fever tick (Boophilus annulatus), which formerly infested most of the quarantined area, and the tropical variety of the same tick B. annulatus var. microplus, which occurred in Florida, Puerto Rico, and areas near the gulf coast. The ability of the tropical species to perpetuate itself on deer complicated the established procedures and delayed the completion of tick eradication in several of the larger swampy areas and game preserves in Florida.