Australian tick typhus

Clinical presentation and systemic manifestations

The pathogenetic basis of tick-borne spotted fevers is generalized panvasculitis. Meanwhile, changes in most internal organs are relatively mild.

The most characteristic and early sign of vascular bed dysfunction is persistent arterial hypotension, which persists until the very end of the disease. In the early stages, pulse dicrotomy may be detected due to decreased blood pressure. At the same time, severe vascular collapse is completely atypical for this group of infections (unlike, for example, classical epidemic typhus), which is explained by less involvement of the adrenal glands and the sympathetic nervous system in the pathological process.

Central nervous system disturbances are exclusively functional and reversible in nature. Patients complain of intense diffuse headaches, persistent insomnia, general lethargy, and psycho-emotional dullness. Focal neurological symptoms and meningeal syndrome are not observed in moderate severity courses, and impaired consciousness is possible only in extremely severe forms of the infection. Changes in the respiratory system are minimal: pulmonary inflammation is clinically almost unnoticeable, although acute laryngotracheobronchitis is registered in one-tenth of patients, and lung root densification due to enlarged perihilar lymph nodes can be seen on chest X-rays. The digestive system reacts with a mild dyspeptic syndrome, a tendency toward constipation, moderate hepatomegaly (in more than half of the patients), and mild pain in the right iliac region due to mesadenitis. The kidneys are practically not involved in the pathological process.

Clinical forms and specific symptoms

The disease always begins acutely. The leading clinical signs are pronounced intoxication syndrome, fever, myalgia, and arthralgia. Upon examination, hyperemia and puffiness of the face, injection of scleral and conjunctival vessels, and redness of the oropharyngeal mucosa draw attention.

The most important pathognomonic symptom is the formation of a primary affect at the site of the infection's entry point (the tick bite). It is accompanied by the development of regional lymphadenitis, and in some cases, lymphangitis. Following this, usually on the 2nd–4th day of fever, a characteristic abundant roseolopapular rash appears.

The course of the disease can be mild or severe:

• Mild form: the duration of low-grade fever does not exceed one week. The general condition remains satisfactory, the rash is sparse, and there are no complications.
• Severe form: characterized by high (up to 39–41°C) and prolonged (more than 10 days) fever. Severe intoxication, hemodynamic disorders, absolute bradycardia or tachycardia, and enlargement of heart borders are observed. The rash becomes abundant and acquires a hemorrhagic character. CNS involvement may manifest as serous meningitis or meningism, and delirium. The convalescence period is accompanied by severe asthenia.

Complications are rare (sometimes secondary bacterial microflora joins in, causing pneumonia or sinusitis, or articular syndrome develops). Infectious-toxic shock is a casuistry, while bedsores, gangrene, and acute psychoses are not observed at all. The prognosis is generally favorable: complete recovery occurs within a month, with no relapses. Fatal outcomes are possible exclusively in elderly individuals with severe comorbid conditions.

Diagnostic algorithm

Diagnosis is based on three pillars: a thorough collection of epidemiological history, identification of key clinical symptoms, and laboratory confirmation.

1. Epidemiological history The fact of staying in an endemic zone during the spring-autumn period and having contact with ticks 10–12 days before the appearance of the first symptoms must be taken into account.

2. Laboratory diagnostics

• Hemogram: from the 4th–5th day of the disease, normocytosis or leukopenia, aneosinophilia, absolute or relative lymphocytosis, and thrombocytopenia are recorded. ESR usually remains within normal limits or is slightly accelerated. Eosinophilia may appear during the convalescence period.
• Serological reactions: performed at the peak of the disease using paired sera.
• CFT (complement fixation test) with R. sibirica antigens: antibodies appear from the 5th–7th day. The diagnostic titer is 1:40. Anamnestic titers (1:10–1:20) can persist in convalescents for up to 3 years.
• IHA (indirect hemagglutination assay): diagnostic titer is 1:200. Antibodies are detected only during the disease and in the early convalescence period.

One should keep in mind cross-serological reactions with other rickettsioses (Mediterranean spotted fever, epidemic typhus). To determine the true causative agent, reactions are set up in parallel with different diagnostic agents — the titer to the "native" antigen will always be higher.

PCR has not found widespread application in routine practice. The biological test (isolation of the pathogen from the patient's blood using male guinea pigs) is used only in complex, clinically atypical cases.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Pathogenetic mechanisms of generalized panvasculitis in rickettsioses
• Specifics of differential diagnosis with measles and secondary syphilis
• Principles of interpreting cross-serological reactions
• Features of performing the biological test in guinea pigs
• General principles of antibacterial therapy for tick-borne spotted fevers

Other topics in the "Vector-borne infections" section

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