In Europe, Tick-borne encephalitis (TBE) and Lyme disease (Lyme borreliosis) are the most frequent diseases transmitted by ticks. However, ticks also bear a multitude of other human pathogenic viruses, bacteria, and parasites. In European ticks e.g. viruses from 6 families comprising 8 genera and 35 species are found, the most important ones being the families Flavi-, Bunya- and Reoviridae with the genera flavivirus, nairovirus and orbivirus. The most important virus diseases are TBE, Omsk haemorrhagic fever, Louping-ill disease, and Crimean Congo haemorrhagic fever.
As examples for bacterial and parasitic tick-borne diseases occurring in Central Europe Lyme disease, Relapsing fever, Tularaemia, Anaplasmosis, Ehrlichioses, and Rickettsioses, including Q-fever, as well as Babesiosis must be mentioned.
Ticks belong to the Arachnidae (spiders) and within this family to the acarids (mites). Taxonomically, they can be classified into two families, the so-called Ixodidae (hard ticks) which have a dorsal scutum, and the so-called Argasidae (soft ticks). The prototype for the first family is Ixodes ricinus, which has very effective sense organs and mouth parts giving it amazing capabilities. With regard to the transmission of pathogens it is the most important tick species. In the Baltic States and in Eastern Europe Ixodes persulcatus (Taiga tick) is of special importance. There are numerous other Ixodes species which however are seldom. Some of them are closely adapted to certain hosts, like I. lividus to the sand martin (Riparia riparia).
Ixodes ticks need one blood meal for their development from the larval to the nymphal stage and another one to reach the adult stage.
In recent years an increasing number of autochthonous Dermacentor reticulatus ticks have been found in Germany. However, some more tick species introduced from the Mediterranean region can occur in Germany. They can survive at least temporarily under specific circumstances, Rhipicephalus sanguineus for example in kennels or Hyalomma species during hot summers.
The prototype for the second family of ticks is the pigeon tick Argas reflexus, which from above looks like an undifferentiated membranaceous sack and has ventral mouth parts. For human and veterinary medicine the Ixodidae are by far more important.
In Europe, TBE – a classical viral zoonosis - is the most important virus disease transmitted by ticks. With the exception of the United Kingdom and the Iberian Peninsula, TBE occurs autochthonously in all European countries. Its incidence varies considerably between the individual risk areas. Its causative agent is a human pathogenic flavivirus, in most cases transmitted by ticks. Other flaviviruses, e.g. the yellow fever virus, also are transmitted by mosquitoes. In Germany, yellow fever is of importance with regard to travel medicine.
In Germany, TBE is a notifiable disease. Approximately 250 TBE patients are registered annually (with considerable fluctuations from year to year), approximately 30% show a severe clinical course. In these cases the first influenza-like clinical course progresses to a second stage with neurological symptoms following some afebrile days. Meningitis, encephalitis, and radiculitis or clinical mixed forms of these are the consequence. So far, most clinical human cases have occurred in Bavaria and Baden-Württemberg. However, single autochthonous cases are seen in many other federal states. Every year the Robert Koch-Institut publishes a map of current TBE risk areas in Germany, based on human cases.
In veterinary medicine, clinical cases of TBE with neurological symptoms have been known in dogs for more than 30 years. In dogs, TBE is relatively rare. The clinical course may vary considerably; thus, a subclinical or a hyperacute lethal course is possible. TBE in dogs has been observed in most areas where TBE is endemic, including Germany. There is no approved vaccine for administration to dogs. Avoiding tick bites is the most important prophylactic method.
Rarely, TBE cases have been described in horses, also in sheep, goats, mouflons (Ovis ammon musimon) and monkeys (Macaca sylvanus) when exposed to tick bites.
TBE virus circulates between the virus-competent tick and the competent host in so-called natural foci or in risk areas, whose geographical extension as a rule is strictly limited and can be very small. This is in contrast to Borrelia burgdorferi s.l., the causative agent of Lyme disease, which in Germany is endemic in areas where Ixodes ricinus occurs.
Small mammals, e.g. various mouse populations (Apodemus flavicollis, A. sylvaticus, Clethrionomys glareolus), but also hedgehogs (Erinaceus europaeus) and moles (Talpa europea), which due to their distinct viraemic stage often contribute to virus transmission to the tick during the blood meal and, thus, keep the virus within the habitat, are competent hosts or represent the virus reservoir in the natural foci. Other contributing factors are transovarial transmission and the so-called co-feeding phenomenon. For the above-mentioned hosts the TBE virus is low- or non-pathogenic. During co-feeding infected and non-infected ticks take a blood meal on the same host at the same time and closely to each other. TBEV is transmitted to the non-infected ticks without viraemia of the host.
Large domestic animals such as goats, sheep and cattle are the appropriate hosts for adult stages of I. ricinus. It is assumed, that for transmission of the virus to the tick they are of minor importance. Especially goats and sheep, more rarely cattle, are of importance for the so-called alimentary TBE. During the viraemic stage the virus is excreted in milk and can then be ingested orally by consumption of non-pasteurized milk or of cream cheeses produced from raw milk. The occurrence of individual cases or of a small number of cases in humans may be the consequence. In Slovakia, Lithuania, Latvia, Poland, Russia, Hungary, and Albania alimentary TBE cases in humans caused by virus-infected milk has occurred in the past few years. After decades without alimentary TBE, in the last years alimentary TBE also occurred in Germany in some single cases.
Grazing animals (goats, sheep, cattle, horses) and wildlife animals (foxes, various small mammals) have been used as sentinels for the characterization of TBE risk areas by means of serosurveillance tests or virus detection. This is an additional tool to characterize natural TBEV foci, as in many cases TBEV prevalence in ticks is too low to identify a focus only via TBEV detection in ticks.
The development of TBE can easily be avoided by vaccination of persons who live in or travel to risk areas and are exposed to tick bites. Vaccines are available for adults and children. It is recommended to contact the family doctor for individual consultation.
However, to remove ticks as soon as possible after leaving tick habitats and to use tick repellants can reduce an infection risk.
Experts (including those in the field of veterinary medicine) count Lyme disease among the „emerging bacterial zoonoses“.
The knowledge on clinics and diagnostics in the field of veterinary medicine is limited. Therefore, it is possible that the numbers indicated for animals are too low.
Ixodes ricinus is an ectoparasite widely distributed in natural foci, which can transmit Borrelia burgdorferi s.I. and other pathogens to its vertebrate host during its blood meal. Its main reservoir hosts are small mammals and birds which keep the Borreliae within the habitat. In addition, ticks also attack wildlife animals which often become seropositive. This seropositive status, however, usually does not permit a precise prediction with regard to the pathophysiological effects caused by Borreliae.
In addition, domesticated animals and livestock, especially grazing animals, but also pets often are attacked by Borrelia-contaminated ticks. Here, the situation must be seen in a more differentiated way with regard to the development of the disease and to an impaired performance. Clinical cases of the disease as well as seropositive animals which do not develop clinical symptoms are reported. An assessment often is difficult as to the non-specific symptoms.
According to the present knowledge, which is considerably more limited in veterinary medicine than in human medicine, clinical cases of the disease caused by Borrelia burgdorferi s.l. occur in horses (donkeys), dogs and cattle. The situation in sheep, cats and zoo animals is unclear and has not been well investigated. In goats, antibodies have been found, a clinical course, however, has not been observed yet. Serologically positive wild boars have been reported, however, also without pathophysiological signs. Pheasants, like other birds, are reservoir hosts; it can be assumed that there is no clinical picture.
The data on the clinical picture of borreliosis in animals are too limited to draw reliable conclusions; in most cases arthritis (lameness) is observed. This and other clinical effects are discussed controversially, in experimentally infected dogs swelling of the regional lymph nodes has been described, in dogs Erythema migrans has been reported, kidneys and heart can be affected by the disease.
Horses show arthritis and an affection of the heart, more rarely infections of the eyes and encephalitis (neuroborreliosis).
In cattle, Lyme disease is associated with lameness, weight loss and abortions, in some cases isolation of the pathogen from clinical material is successful (Borrelia burgdorferi sensu.stricto., Borrelia afzelii), cases of seroconversion have been shown, also good results of therapy with tetracycline.
For the reasons mentioned above knowledge about clinical course, availability and interpretation of laboratory parameters and therapeutic options is too limited for a case definition of Lyme disease in animals.
Data on the transmission of Borrelia burgdorferi s.l. from animals to humans are not available. The relevance of Borrelia burgdorferi s.I. in milk is unknown. It is assumed that the milk of infected cattle is not infectious.
In human medicine the diagnosis Lyme disease is established clinically and is confirmed by laboratory testing. This should also be done in veterinary medicine. However, due to insufficient clinical experience this is certainly impossible at present. Thus, in veterinary medicine laboratory testing is of greater relevance, although in many cases the laboratory results do not provide any pathophysiological information. However, for a diagnosis it is recommended to take into account clinical symptoms as well as infestation of ticks.
- Advice on the biology of ticks, prophylaxis of tick bites
- Advice on tick-borne diseases
- Detection of TBE virus in ticks (realtime RT-qPCR) and TBEV-antibody titres in animal sera (after prior consultation)
- Advice on specific diagnostic issues (PCR, sequencing, serology)
- Supply of not commercially available diagnostic reagents