Natural Endemism

a feature of certain diseases whereby the causative agents, carriers, and animals serving as reservoirs of the pathogens exist indefinitely under natural conditions (nidi) and are unaffected by human habitation. Human beings are infected by the causative agents of diseases of wild animals as a result of a temporary or permanent stay in the area of a natural nidus. The presence of natural reservoirs of the pathogens among wild animals (chiefly rodents) and birds is characteristic of diseases contracted in a natural nidus. Natural endemism is most pronounced in the case of transmissible diseases spread by bloodsucking arthropods; for example, ticks infected by diseased animals attack healthy ones and infect them. As a result, the causative agent passes from an animal to the transmitter to another animal.

Animals susceptible to disease or to pathogens, transmitters, or intermediate hosts are members of a biocenosis associated with a particular community habitat. Naturally endemic diseases are therefore characterized by pronounced seasonality, a more limited geographic spread than anthroponoses, and distinct confinement to a particular type of landscape. For example, the nidi of tick-borne rickettsioses are characteristic of the Siberian taiga, while cutaneous leishmaniasis and tick-borne spirochetosis are characteristic of the Middle Asian deserts. A natural nidus may contain the causative agents of several diseases as well as different animal species susceptible to these diseases. The naturally endemic diseases of man, according to E. N. Pavlovskii, who introduced the concept of natural endemism (1938), include plague, tularemia, tick-borne and Japanese encephalitides, rabies, leptospirosis, cutaneous leishmaniasis, tick-borne relapsing fever, and such helminthiases as opisthorchiasis and trichinosis.

Methods of controlling these diseases include vaccination, as against tularemia and encephalitis; the wearing of special protective clothing; the use of repellents; and the eradication of rodents and insect and tick carriers in the nidus of a particular disease. Both epidemiology and medical geography study natural endemism.

Natural endemism characterizes many diseases of agricultural animals, including bacterial diseases (leptospirosis, listeriosis, swine erysipelas, and necrobacillosis) and viral diseases (rabies, African swine fever, equine encephalomyelitis, Nairobi sheep disease, and malignant catarrhal fever). Other endemic diseases of agricultural animals are caused by protozoan parasites (piro-plasmosis, trypanosomiasis, and leishmaniases). Naturally endemic diseases of helminthous origin include trichinosis, echinococcosis, and opisthorchiasis. Such diseases as anthrax, foot-and-mouth disease, brucellosis, European swine fever, and Newcastle disease, harmful to society and to the economy, have also been found capable of existing independently in a natural nidus, where an epizootic occurs naturally. In the case of anthrax, emphysematous carbuncle, infectious enterotoxemia, and some other soil infections, the causative agent circulates by means of animals’ contacts with an environment infected by the discharges of diseased animals. The causative agents of foot-and-mouth disease, European swine fever, and a number of other diseases may circulate among wild fauna, causing a typical epizootic.

Natural endemism of diseases of agricultural animals is confined chiefly to the border areas of different types of landscapes, where rodents, birds, and arthropod carriers are most numerous. Certain types of landscapes are associated with certain diseases: meadows with leptospirosis, steppes with Q fever, and savannas with trypanosomiasis. This makes it possible to predict possible outbreaks of these diseases and take appropriate preventive measures.

Natural endemism of diseases of agricultural animals may result from the recent introduction of the causative agents into natural environments. Some investigators believe that periodic epizootics of foot-and-mouth disease among such wild ungulates as Saiga antelopes (Saiga tatarica) result from importation of the causative agent from livestock farms. This may also be true of European swine fever and brucellosis.