ENDEMIC DISEASES. "Endemic disease" may denote a disease that repeatedly appears in small numbers of a population while the vast majority escape without evidence of illness. This type of endemicity, which most often results from human-to-human transfer of the agent of disease, is favored by crowding, extensive human contact, and the presence of widespread immunity in the population. Travel increases the chance of infection, and isolation reduces it. The incidence of such diseases has been causally related to changes in patterns of immigration, social mobility, urbanization, and transportation. In Texas, such endemic diseases have been similar to those encountered in other advanced nations of the world. The same diseases occur in Dallas or Houston as appear in New York, London, or Paris. Among these are tuberculosis, syphilis, gonorrhea, bacillary dysentery, influenza, and various types of pneumonia. Influenza, for example, attacked approximately one of a hundred Texans in 1984, while hepatitis of all types attacked 4,500 individuals.
The phrase may also refer to a disease that is peculiar to a region. Factors that favor endemicity of this type are isolation, rural occupations, distinctive geographic features, locally high concentrations of chemicals (fluorides, selenium, radioactive substances), climatic conditions, the presence of specific vectors of disease (mosquitoes, fleas, lice, and ticks), and reservoirs of infection in animals with intermittent transmission to humans. First among conditions in Texas arising from physical factors are those that result from prolonged exposure to intense sunlight. Nearly all places in the state receive from 60 to 70 percent sunshine out of all possible sunlit hours. The most common result is severe sunburn, sometimes requiring hospitalization. This condition is more common along the Gulf Coast, but it can occur anywhere in the state when human beings expose considerable parts of their bodies to direct rays. Additionally, the sustained heat of the Texas summer causes insolation, manifested as either heat exhaustion or sunstroke. While these conditions occur more commonly in the elderly and debilitated, a few instances of fatal heat stroke have occurred among young athletes, particularly in fall training programs for football. Texas also lies in that area of the United States sometimes referred to as "Tornado Alley." Violent storms are not uncommon, and injuries and fatalities occur intermittently (see TORNADOES). Similarly, the Gulf Coast has been a frequent site of hurricanes with attendant injuries and death.
A condition formerly common among Texans who were raised on the High Plains was mottled enamel of the teeth. Stains were caused by high concentration of fluorides in well water. With extension of city water supplies and reduction of fluorides, this condition has largely disappeared.
Infectious diseases of limited geographic distribution are those conveyed by vectors such as mosquitoes, those transmitted from animals to man, and those produced by agents that live in the soil. Since the initiation of effective mosquito-control programs, yellow fever and malaria have almost disappeared from Texas. In 1984 only seventy-five cases of yellow fever were reported to the Texas Department of Human Services. Relapsing fever, once prevalent in cattle, is transmitted to man through ticks. Though it was formerly more prevalent along the Mexican border, diseased cattle from that region were often driven north or mixed with other herds. In 1984 there were no reported cases of this disease.
Psittacosis (ornithosis) is transmitted from birds or domestic fowls to man. Since Texans operate a number of large turkey farms, episodes of this disease have been reported. During 1984, however, there were only seven reported cases. Parrots imported from South America also cause isolated cases of psittacosis. Tularemia occurs sporadically in Texas. Eight cases were reported in 1984. Humans are usually inoculated when skinning or cleaning infected rabbits. A febrile disease, originally designated Bullis fever, attacked a high proportion of military personnel stationed at Camp Bullis, near San Antonio, during World War II. Evidence accumulated that the disease was transmitted by tick bites. This disease appears very infrequently and is mostly limited to hunters or to individuals engaged in agricultural activities in the Camp Bullis area.
Rocky Mountain Spotted Fever appears in Texas as an occasional infection. The disease is transmitted by ticks. Occurrence is likely only in those who frequent wooded areas. Murine typhus, conveyed by fleas, has persisted in Texas for many years. At one time it was especially common along the Rio Grande, where it was known as "tabardillo," but control of rats, on which the fleas are parasitic, has reduced the incidence of this disease. During 1984, only thirty-five cases were reported. The condition appears sporadically throughout the state, and there is evidence that fleas from other rodents may be infectious.
Of organisms present in the soil and distributed by its disturbance, two fungi, Histoplasma capsulatum and Coccidioides immitis, are present in Texas. Histoplasma capsulatum may be encountered at any place where considerable accumulation of droppings from birds or bats occurs. Not infrequently, histoplasmosis occurs in subclinical forms and is not reported. Only eight cases were reported in 1984. These probably represent the most serious cases of chronic pulmonary histoplasmosis, which resembles tuberculosis. Coccidioides immitis, which infects desert rodents, is distributed by dust storms and by vehicles that have operated in contaminated areas. It also produces a mild, self-limited disease in humans. The organism is widely distributed throughout the lower western part of the state, along the Rio Grande, but it also appears as far upstate as Midland-Odessa, San Angelo, and south of San Antonio. First identified as San Joaquin valley fever in California, coccidioidomycosis occurs sporadically through the lower Sonoran Life Zone of the western United States and in adjacent states of Mexico.
Other mycobacteria occasionally produce disease in humans. The most virulent of these organisms, Mycobacterium kansasii, produces a significant number of pulmonary infections each year. Unquestionably a large number of people temporarily harbor the organism without resulting illness. The chief distribution of this organism is through the north central region of the state (Region 5 of the Texas Department of Human Services), but the infection also occurs in lower incidence in other states. Mycobacterium avium-intracellulare is prevalent throughout the Gulf Coast area. It is found chiefly in the eastern third of Texas. The organism appears to be closely linked to water, though certain types also appear in soil samples. It seems certain that although many people encounter and harbor the organism, only a small proportion become ill. This organism, along with Mycobacterium Kansasii, seems to account for as many as 10 percent of pulmonary infections originally thought from X-ray analysis to be tuberculosis.
Texas Preventable Disease News, January 26, 1985.