Classification and transmission routes
Relapsing fevers are divided into two fundamentally different groups in terms of epidemiology:
- Epidemic (louse-borne) relapsing fever. This is a strict anthroponosis caused by Borrelia recurrentis (the pathogen was discovered by O. Obermeier in 1868). The source of infection is a febrile patient. The bacteria are transmitted by lice (primarily body lice, less commonly head and pubic lice), which become infectious from the 6th to the 28th day after a blood meal. Infection occurs through a contamination pathway: when scratching the skin, a person rubs in the hemolymph of a crushed insect. The disease is typical of periods of social distress and is currently not registered in the Russian Federation.
- Endemic (tick-borne) relapsing fever. A vector-borne zoonosis of subtropics and tropics caused by more than 20 species of borreliae (e.g., B. duttoni, B. persica). The reservoirs are rodents and argasid ticks (transovarial transmission to offspring is possible). Humans are infected directly through the bite of a tick of the genus Ornithodoros.
Pathogenesis and mechanism of relapses
Once in the body, borreliae are captured by cells of the lymphoid-macrophage system, where they actively multiply. This is followed by their massive breakthrough into the bloodstream — bacteremia. This causes a sharp rise in temperature, chills, and headache.
In response to the infection, the immune system produces antibodies. They bind and destroy the majority of the pathogens. Massive aggregates of bacteria and antibodies form in the bloodstream, upon which platelets settle. These complexes clog small capillaries, disrupting blood circulation in internal organs.
Why does the disease return? The genome of borreliae contains several dozen genes encoding various surface antigens. Initially, most of them are in a "silent" form. During the disease, chromosomal rearrangements occur: one of the inactive genes is activated, forming a new antigenic variant of the microbe. Old antibodies against it are powerless, a new population enters the blood, causing another attack.
Clinical presentation and immunity
The latent incubation period averages 7–8 days. The disease occurs as a series of attacks with fever up to 39–40 °C. Due to the constant change in the antigenic structure of the pathogen, relapses can recur from 3 to 20 times.
The prognosis for the endemic variant is favorable. In epidemic relapsing fever, the mortality rate does not exceed 1%. Humoral immunity develops after the infection, but it is short-lived. However, the indigenous population of endemic regions often exhibits resistance to locally circulating borrelia strains.
Laboratory diagnostics and treatment
The main method for identifying the pathogen is bacterioscopy. At the height of a fever attack, the patient's blood is taken, a "thick drop" smear is prepared and stained with Giemsa stain. To observe live borreliae, dark-field "hanging drop" microscopy is used, as well as Burri's negative method (with India ink) and silver impregnation.
A special place is occupied by the biological method, which allows precise differentiation of typhus species in laboratory animals:
- White mice and rats are sensitive to Borrelia recurrentis.
- Guinea pigs are sensitive to endemic typhus pathogens.
The complement fixation test (CFT) is used as an auxiliary serological method.
Prevention and treatment: A specific vaccine has not been developed. Etiotropic treatment involves antibiotics (tetracyclines, chloramphenicol, ampicillin). Non-specific prevention boils down to interrupting transmission routes: combating pediculosis for epidemic typhus and eliminating ticks and rodents in natural foci.
Briefly: answers to topical questions
What morphological and tinctorial properties are characteristic of Borrelia recurrentis?
Borrelia recurrentis is characterized by large and sparse coils.
- Morphological properties — spiral-shaped bacteria having 3–8 large and sparse coils, which distinguishes them from treponemes.
- Microscopic diagnostics — the main method includes examining a thick blood smear stained with Giemsa. Additionally, dark-field "hanging drop" microscopy, Burri's negative method, and silver impregnation are used to visualize live borreliae.
What serological reactions are used for the diagnosis of relapsing fevers?
The complement fixation test is used for the serological diagnosis of relapsing fevers.
- Complement fixation test (CFT) — used in laboratory practice as an auxiliary method.
The main method for detecting the pathogen remains bacterioscopy, which examines a thick blood smear of the patient taken at the height of the fever, since serology is only of an additional nature.
What pathogenicity factors are distinguished in relapsing fever pathogens?
The pathogenic effect of relapsing fever pathogens is associated with antigenic variation and the formation of "borrelia + antibody" aggregates.
- Antigenic variation — the borrelia genome has a large set of genes encoding different protein antigens; most genes are in an inactive form.
- As a result of rearrangements in the chromosome, a "silent" gene is activated and the surface antigen changes. Antibodies produced against the previous variant do not work on the new variant, which leads to the repeated release of borreliae into the blood and relapse.
- Aggregates of "borrelia + antibody" are formed, which are loaded with platelets. This causes capillary occlusion and circulatory disorders in organs.
To avoid confusing animals in the bioassay: in epidemic typhus, "mice and rats" are infected (a disease of cities and poor sanitation), and in endemic typhus — "guinea pigs".
Class and exam questions
Can one contract epidemic typhus directly from a louse bite?
No, infection occurs via contamination—when scratching the skin, a person rubs the hemolymph of a crushed insect into microtraumas.
What causes the alternation of fever bouts?
The reason lies in the antigenic variation of borreliae: the activation of "silent" genes leads to a change in surface proteins, making microbes invisible to already produced antibodies.
What vaccine is used against relapsing fevers?
Specific prevention (vaccination) has currently not been developed and is not carried out.
What else to study on this topic
- Genetic mechanism of activation of "silent" genes in borreliae
- Features of transovarial transmission of borreliae in ticks
- Methodology for conducting a bioassay on laboratory animals
- Romanowsky-Giemsa blood smear staining technique
- Pathophysiology of microthrombosis during the formation of "borrelia + antibody" aggregates
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