Institute of Bacterial Infections and Zoonoses (IBIZ)

Working group Rickettsiology

Rickettsiae are obligate intracellular bacteria and transmitted to humans by various vectors such as ticks, lice or fleas. They are distributed worldwide and divided into four different groups: Spotted fever group (SFG), Typhoid group (TG) and transitional group (TRG) and ancestral group (AG).

The SFG includes more than 15 different species that cause tick-borne spotted fever. They infect a variety of tick species and are geographically distributed according to their vector. Some rickettsiae can be transmitted within the tick to the next developmental stage (transstadial) and/or the next generation (transovarial). SFG includes R. conorii, the causative agent of Mediterranean spotted fever (MSF), which occurs in Europe, and R. slovaca and R. raoultii, the causative agents of tick-borne lymphadenopathy or dermacentor-borne necrosis erythema lymphadenopathy (TIBOLA/DEBONEL). MSF is transmitted by the tick Rhipicephalus sanguineus, which is found in southern and eastern Europe. Infection is accompanied by flu-like symptoms, lymphadenopathy, a rash of varying severity over the entire body, and the typical eschar at the bite site. R. slovaca and R raoultii are transmitted by the ticks Dermacentor marginatus and D. reticulatus, which are also native to Germany, and cause local lymphadenopathy and scabbing at the bite site.

Other species, such as R. massiliae or R. aeschlimannii, transmitted by the tick Rhipicephalus sanguineus or Rhipicephalus spp. and Hyalomma spp. have been described in France. R. helvetica is transmitted by the tick species Ixodes ricinus, although the pathogenic potential is largely unknown.

Non-tick-borne TG rickettsiae, R. prowazekii (lice) and R. typhi (fleas), have been detected in a few regions of Europe. They cause febrile illness accompanied by swelling of the local lymph node. The occurence of an eschar or rash is highly variable.

Domestic animals are considered insensitive to infection by rickettsiae. However, dogs may show symptoms similar to those described for humans when infected with R. rickettsii or R. conorii.

Anaplasmae, the causative agents of anaplasmosis, are also tick-borne bacteria. They are obligate intracellular and multiply in the cells of the hematopoietic system. Three species have been detected in Europe so far, A. phagocytophilum, A. ovis and A. marginale.

A. phagocytophilum has a zoonotic potential and causes febrile diseases in humans and animals (dogs, horses, ruminants). Due to replication of the pathogen in neutrophilic granulocytes, the disease is also called granulocytic anaplasmosis (HGA) in humans or tick fever in ruminants. The bacterium is transmitted by I. ricinus and is mainly found in northern and central Europe.

A. ovis is transmitted by Dermacentor spp. and is common in Mediterranean areas but also in Central Europe. In small ruminants but also wild ruminants, infection causes severe anemia, due to multiplication of the pathogen in the erythrocytes, as well as weakness and anorexia. Subclinically infected goats and sheep are so-called carrier animals and are considered reservoirs. Infections of humans with A. ovis are highly rare.

A. marginale, the causative agent of bovine anaplasmosis, has been detected sporadically in Switzerland, Austria, and Hungary. Similar to A. ovis, the pathogen replicates in erythrocytes. The severity of infection depends, among other things, on the age of the animal. Subclinically infected lifelong carrier animals may occur.

Climate change, renaturation and the near-natural design of green spaces, but also the reduced use of insecticides in organic farming influences the spread of ticks and human-tick encounters. New tick species can migrate and bring new pathogens with them. Livestock and domestic animals but also wild animals can be a reservoir. It is therefore important to consider all factors (environment, pathogen, vector, reservoir and human) in a One Health approach to develop strategies to prevent infections in humans and animals.