Rickettsiae: Infection, Transmission & Diseases

Diseases caused by rickettsiae were common in ancient times. Also, for example, during Napoleon’s wars, more than 125,000 soldiers died from spotted fever transmitted by lice. Today, rickettsioses – infectious diseases caused by rickettsiae – often occur in the context of poverty and poor sanitation.

What are rickettsial infections?

Rickettsiae are gram-negative rod-shaped bacteria. They live and multiply in the intestinal cells of vector animals. These are usually arthropods (lice, ticks, mites, and fleas). The pathogens belong to the bacterial species that have very short DNA strands (1.12 to 1.6 million base pairs). Rickettsiae form their own family (Rickettsiaceae) and are alphaproteobacteria. They were named after their discoverer, the U.S. physician H. T. Ricketts, who himself was affected by rickettsiosis in 1910. Depending on the infections they cause, rickettsiae are classified in the spotted fever, tick-bite fever and tsutsugamushi fever groups. The infected arthropods attach to the skin of animals and humans. Infection with the rickettsial infections occurs after the bite or sting via secretion of saliva. Inhalation of dried flea feces can also lead to infection. The different types of rickettsiae produce different types of infectious diseases. In addition, the bacteria use different vectors to spread. For example, Rickettsia prowazekii is usually transmitted by clothes lice and causes epidemic spotted fever (typhus). The rod-shaped bacteria are mainly found in the warmer regions of the world. In Germany, the diseases are often imported. In Central Europe, rickettsial diseases are mostly transmitted by ticks. The tick-borne

Rickettsioses usually have a lower morbidity and mortality rate than those transmitted by lice.

Occurrence, distribution, and characteristics

Rickettsiae grow to 0.3 to 2 micrometers in size, depending on the species. The gram-negative rod-shaped bacteria have very short DNA and live in the intestinal epithelial cells of ticks, lice, mites, and fleas. They cause diseases that are grouped under the generic term rickettsioses. The pathogens occur worldwide, preferably in warm climatic zones. In Germany, mainly Rickettsia rickettsii, Rickettsia conorii and Rickettsia helvetica have been detected so far. Until recently, physicians had difficulty diagnosing rickettsiosis because infected patients only show general symptoms of infection in the early stages of the disease. Only recently have ticks, long considered to be only vectors of Lyme disease and TBE, become the focus of researcher interest. According to recent studies, 10% of the ticks found in Germany are infected with rickettsiae, which are specialized for humans. According to the Robert Koch Institute (2009), 50% to 80% of floodplain ticks carry the rod-shaped bacterium Rickettsia helvetica, depending on the area of distribution. The rapid reproduction of the Auwald tick is problematic. Recently, scientists succeeded in developing a highly efficient, specific and yet easy-to-perform rapid molecular-genetic test with which individual rickettsioses can be identified beyond doubt. In the process, the physicians even discovered a completely unknown bacterial species (Rickettsia raoultii) in certain Dermacentor ticks. The test can also be used in a conventional medical practice. ELISA or indirect immunofluorescence detection from blood serum is normally used to diagnose rickettsiosis. In the test, which is performed at 3-week intervals, samples are tested 2-fold for IgM and IgG antibodies. An antibiogram is then prepared, which is used to determine the causative pathogen species. Treatment of rickettsialpox is usually with a proven Lyme disease drug, the antibiotic doxycycline.

Diseases and symptoms

The patient infected by the sting or bite of a vector initially exhibits only nonspecific symptoms of inflammation. Shortly after the puncture/bite, a small ulcer develops at the site of inflammation below the skin surface. European ticks leave behind an approximately pea-sized infection site covered by a blackish crust.This is followed by swelling of the lymph nodes, drowsiness, fever, headache and the reddish skin rash (macular exanthema) typical of rickettsiosis, which starts on the palms of the hands and feet. It results from the leakage of red blood cells from damaged capillary vessels. The rashes also show raised papules and tiny hemorrhages (petechiae). The infected person does not experience any pain. However, as the disease progresses, complications such as lung, heart and brain damage can occur. For example, some patients with rickettsialpox develop pulmonary edema, while others develop cardiac arrhythmias and brain inflammation (encephalitis). In particularly severe cases, gastrointestinal bleeding and thrombosis also occur. In Rocky Mountain Spotted Fever (RMSF), caused by Rickettsia rickettsii, the incubation period is 2 to 14 days. Mortality from the disease transmitted by Dermacentor and Rhipicephalus ticks is 20%. Rickettsia helvetica – originally detected only in Switzerland, but now also found in France and Slovenia – can cause pericarditis and is associated with weakness, myalgias (muscle pain), prolonged fever, and headache. The causative agent, Rickettsia conorii, causes middle spotted fever and is transmitted by shield ticks found throughout the Mediterranean region. Rickettsia slovaca infects with TIBOLA (tick-borne lymphadenopathy syndrome). TIBOLA is a lymph node disease with muscle pain, headache, and fever. Baldness often develops at the injection site on the head. Children under 10 years of age and patients with already weakened immune systems often show a worse course of the disease. Vaccinations against tick-borne TBE (early summer meningoencephalitis) are ineffective against rickettsiosis.