What is Tularemia?
Tularemia is an acute infectious natural focal disease with damage to the lymph nodes, skin, sometimes the eyes, throat and lungs and accompanied by severe intoxication.
In 1910, near the Lake Tulare in California, D. McCoy discovered a gopher disease in gophers, reminiscent of the clinical picture of bubonic plague. Soon, he and Ch. Chapin isolated the pathogen from sick animals, which was named Bacterium tularense (1912). Later it was found out that people are susceptible to this infection, and on the proposal of E. Francis (1921) it was called tularemia. Later, the pathogen was named after Francis, who studied it in detail.
Causes of Tularemia
The causative agent is immobile gram-negative aerobic encapsulated bacteria F. tularensis of the genus Francisella of the family Brucellaceae. Show pronounced polymorphism; most often have the form of small coccobacilli.
In bacteria, three subspecies are distinguished:
- non-arctic (African);
- Central Asian;
- Holarctic (Euro-Asian).
The latter includes three biological variants: Japanese biovar, erythromycin-sensitive and erythromycin-resistant. The intraspecific differentiation of the causative agent of tularemia is based on the differences of subspecies and biovars in a number of phenotypic characters: biochemical activity, the composition of higher fatty acids, the degree of pathogenicity for humans and animals, sensitivity to certain antibiotics, as well as environmental characteristics and the range of the pathogen. O and Vi antigens were found in bacteria. Bacteria grow on vitelline or agar media supplemented with rabbit blood or other nutrients. From laboratory animals, white mice and guinea pigs are sensitive to infection. Outside the host organism, the pathogen persists for a long time. So, in water at 4 ° C it remains viable for 1 month, on straw and grain at temperatures below 0 ° C – up to 6 months, at 20-30 ° C – up to 20 days, in the skins of animals that fell from tularemia, at 8 -12 “C – more than 1 month. Bacteria are unstable to high temperature and disinfectants. For disinfection use a 5% phenol solution, a mercuric chloride solution 1: 1000 (kills bacteria within 2-5 minutes), 1-2% formalin solution (destroys bacteria for 2 hours), 70 ° ethanol, etc. For complete disinfection of the bodies of infected animals, they should be kept for at least 1 day in a disinfectant solution, then autoclave and burn.
The reservoir and source of infection are numerous species of wild rodents, hare, birds, dogs, etc. Bacteria are isolated from 82 species of wild, as well as from domestic animals (sheep, dogs, artiodactyls). The main role in maintaining the infection in nature belongs to rodents (water rat, common vole, muskrat, etc.). A sick person is not dangerous to others.
The transmission mechanism is multiple, most often transmissible. The causative agent is preserved in nature in the cycle “tick – animal”, transmitted to agricultural animals and birds by ticks and blood-sucking insects. Specific carriers of tularemia are ixodid ticks. A person becomes infected with tularemia as a result of direct contact with animals (skinning, collection of dead rodents, etc.), as well as by alimentary route through food infected with rodents and water. Often infection occurs through blood-sucking vectors (ticks, mosquitoes, fleas, horseflies and other arthropods). Possible infection by the respiratory route (by inhaling infected dust from grain, straw, vegetables). Cases of human diseases were recorded in industries associated with the processing of natural raw materials (sugar, starch and syrup, distilleries, hemp plants, elevators, etc.), in meat processing plants, when slaughtering sheep and cattle, which had infected ticks, outskirts of cities located near natural foci. There are known cases of the introduction of infection during the transportation of products and raw materials from areas unfavorable for tularemia.
The natural susceptibility of people is high (almost 100%).
The main epidemiological signs. Tularemia is a common natural focal disease that occurs mainly in landscapes of the temperate climatic zone of the Northern Hemisphere. The wide spread of the pathogen in nature, the involvement of a large number of warm-blooded animals and arthropods in its circulation, the seeding of various environmental objects (water, food) also determine the characteristics of the epidemic process. There are various types of foci (forest, steppe, meadow-field, bog, in the river valley, etc.). Each type of foci has its own species of animals and blood-sucking arthropods involved in the transmission of the pathogen. Among patients, adults predominate; often the incidence is associated with the profession (hunters, fishermen, agricultural workers, etc.). Men get sick 2-3 times more often than women. Anthropurgic foci of tularemia occur when infected rodents migrate from their habitats to settlements where they come into contact with synanthropic rodents. Tularemia remains a disease of the countryside, however, a steady increase in the incidence of urban population is currently noted. Tularemia is recorded throughout the year, but more than 80% of cases occur in summer and autumn. In recent years, the incidence is sporadic. In some years, local transmissible, commercial, agricultural, and aquatic outbreaks are noted, less frequently other types of outbreaks. Transmissible outbreaks are caused by transmission of the pathogen by blood-sucking dipterans and occur in the foci of tularemia epizootics among rodents. Transmissive outbreaks usually begin in July or June, peak in August and cease in September-October; haymaking and harvesting contribute to an increase in the incidence.
The industrial type of outbreak is usually associated with the capture of a water rat and muskrat. Commercial outbreaks occur in spring or early summer during the flood season, and their duration depends on the harvesting period. Infection occurs by contact with animals or skins; the pathogen penetrates through lesions on the skin, in connection with which axillary buboes more often occur, often without ulcers at the site of insertion.
Water outbreaks determines the entry of pathogens into open waters. The main pollutant of water are water voles that live along the coast. Diseases usually occur in the summer with a rise in July. Diseases associated with field work and the use of drinking water from random reservoirs, wells, etc. In 1989-1999. the proportion of isolates of the causative agent of tularemia from water samples reached 46% or more, which indicates the important epidemiological significance of reservoirs as long-term reservoirs of infection.
Agricultural outbreaks occur when airborne aerosol is inhaled when working with straw, hay, grain, feed, and urine contaminated with sick rodents. Pulmonary, rarely abdominal and angina-bubonic forms predominate. The household type of outbreaks characterizes infection in the home (at home, on the estate). Infection is also possible during sweeping, bulkheading and drying of agricultural products, distributing pet food, and eating contaminated foods.