Tick-induced neurological disorders

Diseases transmitted by ticks have been increasing in frequency and distribution, partly due to climate change. In the last decades, new tick-borne pathogens have been discovered that cause prominent neurologic disease. In this review, the impacts created by these discoveries insofar as nervous system involvement will be addressed. The effects of climate change ensure that this is a timely topic of medical importance. Tick bite paralysis causes direct neurological injury through a neurotoxin that leads to a dangerous clinical condition. Several pathogens can be transmitted by ticks of the genus Ixodes (I. scapularis in North America, and I. ricinus in Eurasia), either singly or as co-infections. Of these, two arboviruses (tick-borne encephalitis virus and Powassan virus) cause direct neurological injury. The neurological manifestations of the borrelioses, Lyme disease, and relapsing fevers involve the central and peripheral nervous systems. A hemoprotozoan, Babesia microti, causes significant neurological involvement. Anaplasma phagocytophilum, a bacterial agent that infects neutrophils, is an infrequent cause of neurological injury. Both A. phagocytophylum and B. microti can cause severe illness in elderly and immunosuppressed patients, so their impact on morbidity and mortality can be high. Another feature for increased severity is that the Ixodes-transmitted pathogens can cause polymicrobial infections. The rickettsiae are prominent causes of neurological disease. Ehrlichia spp. infect monocytes and can cause diverse forms of neurological injury. The rickettsial agents of Rocky Mountain Spotted Fever and Mediterranean fever infect the endothelium, which leads to multiple organ involvement, including the nervous system.