Neurotropic parasites of small companion animals: an overlooked neurological concern

The present review aims to provide a comprehensive overview of parasitic diseases impacting the nervous systems of small companion animals. Toxoplasma gondii and Neospora caninum are well-documented pathogens linked to neurological disorders; however, a variety of other parasitic agents, ranging from unicellular protozoa to complex helminths, also exhibit a tropism for the nervous system in these animals. Parasites can exert a direct influence on the nervous system through mechanisms that reflect their inherent neurotropism (e.g. Encephalitozoon cunicoli and Gurltia paralysans). Additionally, they may indirectly compromise neurological function through several distinct mechanisms: i) ectopic dislocation of larvae, eggs or adult parasites/fungi (e.g., Baylisascaris procyonis, Angiostrongylus cantonensis, Dirofilaria immitis, Spirocerca lupi, Cryptococcus spp., Cuterebra spp.); ii) immune-mediated reactions (e.g., Leishmania infantum); iii) coagulation disorders (e.g., Angiostrongylus vasorum); iv) electrolyte imbalances (e.g., Toxocara canis); and v) effects of neurotoxins (e.g., Ixodes holocyclus bite). Furthermore, neurological complications have been documented following antiparasitic interventions, ranging from direct drug toxicity in Dirofilaria immitis and Babesia spp. infections, to the potential post-treatment neuroinflammatory responses in Angiostrongylus cantonensis cases. Collectively, these mechanisms can lead to substantial neurological dysfunction as a consequence of both primary parasitic invasion and secondary pathological processes within nervous tissues. In addition, parasitic diseases injuring the nervous system usually cause neurological sequelae that could reduce life expectancy and quality of life, leading owners to elect euthanasia of their pets. In conclusion, parasitic diseases affecting the nervous system may represent a relevant health concern in companion animals and warrant further attention.