Etiology and epidemiology of tick-borne borreliosis
Tick-borne borreliosis is a classic natural focal vector-borne infection. The causative agent of this disease is the spirochete Borrelia burgdorferi.
The epidemiology of Lyme disease has certain features: the prevalence of the infection in natural foci is extremely variable. Tick infestation rates can range from 5–10% to 70–90% depending on the specific area. In the Russian Federation, the Tver Region is traditionally considered one of the most prominent endemic regions for this disease.
Since the vectors are the same species of ticks as in tick-borne encephalitis, mixed infections are not uncommon in clinical practice, where a patient is infected with multiple pathogens simultaneously.
Infection vectors: order Parasitiformes
The primary vectors of Lyme disease are mesostigmatid and ixodid ticks (order Parasitiformes), specifically representatives of the family Ixodidae (Ixodidae). These include:
- Castor bean tick (Ixodes ricinus). In addition to tick-borne borreliosis, this species is capable of transmitting taiga (spring-summer) and louping ill encephalitis, Q fever, and tularemia.
- Taiga tick (Ixodes persulcatus). It is a vector of Lyme disease, taiga encephalitis, and tularemia.
To understand the general epidemiological picture, it is important to remember that the family Ixodidae also includes pasture ticks of the genus Dermacentor (Dermacentor pictus, Dermacentor marginatus). They do not transmit borreliosis, but they actively spread tick-borne rickettsiosis (pathogen: Rickettsia sibirica), Omsk hemorrhagic fever, tularemia, and taiga encephalitis.
Separately within the order Parasitiformes is the family of soft ticks (Argasidae). A prominent representative is the settlement tick (Ornithodoros papillipes). It transmits tick-borne relapsing fever (tick-borne spirochetoses), which is also caused by spirochetes of the genus Borrelia, but is a completely different disease.
Epidemiological significance of hard ticks
Hard ticks are of enormous medical and epidemiological significance as they serve as reservoirs and vectors for a wide range of pathogens from various classes:
- Spirochetes: cause tick-borne borreliosis (Lyme disease).
- Rickettsiae: provoke the development of tick-borne rickettsiosis, Mediterranean spotted fever, Q fever, and Rocky Mountain spotted fever.
- Bacteria: cause such severe infections as tularemia, plague, brucellosis, pseudotuberculosis, salmonellosis, and a number of others.
Such a variety of pathogens requires high vigilance from the physician when taking an epidemiological history from a patient with a history of a tick bite.
Clinical presentation and target organs
Lyme disease is insidious due to its pronounced tendency toward chronicity and regular relapses. The clinical picture is characterized by severe polysystemic involvement. The pathological process involves the following organs and systems:
- Skin (up to the development of a specific complication — chronic atrophic acrodermatitis).
- Nervous system (development of neuroborreliosis with involvement of central nervous system structures).
- Musculoskeletal system (primarily affecting the joints).
- Heart (cardiac manifestations of the infection).
PCR diagnostics of tick-borne borreliosis (TBB)
Molecular genetic testing (PCR) is recommended for absolutely all patients with suspected Lyme disease. The purpose of the study is the precise identification of the pathogen (Borrelia burgdorferi sensu lato) and differential diagnosis. The strength of recommendation is C (level of evidence reliability — 5).
Indications and strict timing for biomaterial collection:
- Blood: collected strictly on days 7–14 from the moment of the tick bite. It is during this period that spirochetemia develops (active circulation of the pathogen in the bloodstream).
- Cerebrospinal fluid (CSF): examined when neuroborreliosis with central nervous system involvement is suspected. The material is obtained exclusively in the presence of strict clinical indications for lumbar puncture.
- Skin biopsy: collected for laboratory confirmation of borrelial etiology when chronic atrophic acrodermatitis (CAA) is suspected.
To easily remember the target organs in Lyme disease, use the acronym SNCS: Skin (acrodermatitis), Nervous system (neuroborreliosis), Cartilage/joints (musculoskeletal system), Seart (Heart).
Class and exam questions
Within what timeframe after a tick bite should blood be tested for PCR?
Blood for PCR testing must be drawn strictly on days 7–14 from the moment of the tick bite. During this period, spirochetemia is observed in the bloodstream, which allows reliable detection of the pathogen's DNA.
Why can a single tick bite result in infection with multiple diseases at once?
Hard ticks (e.g., taiga and castor bean ticks) are common vectors for multiple pathogens. Therefore, Lyme disease often occurs as a mixed infection along with tick-borne encephalitis or tularemia.
In what cases is a skin biopsy taken for the diagnosis of borreliosis?
Examination of a skin biopsy by PCR is indicated when chronic atrophic acrodermatitis (CAA) is suspected. This allows confirmation of the borrelial etiology of skin lesions in the late stages of the disease.
Which ticks transmit Lyme disease?
The primary vectors are hard ticks: the castor bean tick (Ixodes ricinus) and the taiga tick (Ixodes persulcatus). Pasture ticks and soft ticks do not transmit this infection.
What else to study on this topic
- Tick-borne spring-summer encephalitis: clinical presentation and diagnostics
- Chronic atrophic acrodermatitis (CAA) in Lyme disease
- Neuroborreliosis: CNS involvement and indications for lumbar puncture
- Differential diagnosis of tick-borne rickettsioses
- Tick-borne relapsing fever and soft ticks
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