Pathogenesis and Transmission
Transstadial transmission of E. canis occurs within the Brown Dog tick, Rhipicephalus sanguineus, whereas transovarial transmission has not been confirmed so far. All tick stages can be infected while feeding on infected dogs. Subsequently these ticks drop off, molt and acquire another canine host. Ticks can obtain E. canis only if engorgement occurs during the acute phase of the disease in dogs.
Nymphal and adult Brown Dog ticks are capable of transmitting E. canis for at least 155 days following detachment from the infected host. If the tick acquires an uninfected host and harbours a sufficient number of organisms, infection of the new host may occur. It is not known how long the tick must feed to transmit the organism. But due to belonging into the same order of Rickettsiales the period for pathogen transmission is expected in the literature to be identical with other rickettsial pathogens, i.e. 4-48 hours (here for A. phagocytophilum, E. canis, E. chaffeensis, E. ewingii, R. ricketsii, R. conorii and other spotted fever group rickettsiae by Nicholson et al., 2010). Infected dogs from endemic areas may travel to non-endemic areas, but lacking suitable vectors, they are no threat to other dogs.
In the American Dog tick, Dermacentor variabilis, experimental transmission of E. canis to nymphs and experimental transmission from infected D. variabilis adults to the host was successful. The tick is suspected as additional vector in the USA, but epidemiological importance is unclear.
Finally, Ehrlichia spp. infection may also be introduced in susceptible dogs by blood transfusion.
Dissemination of Ehrlichiae
Ehrlichiae are obligate intracellular bacteria which attack white blood cells, a feature even more specialized at the species level:
- E. canis infects canine monocytes and lymphocytes but only rarely infects neutrophils;
- E. ewingii infects canine granulocytes;
- E. chaffeensis infects predominantly monocytes and macrophages.
The elementary body is the infective stage and enters the monocyte or other leucocyte types by phagocytosis. Elementary bodies are individual ehrlichiae about 1 µm in diameter and usually coccoid or ellipsoid in shape. Once they are captured inside the phagosomes, the pathogens replicate by binary fission, forming clusters of tightly packed elementary bodies termed initial bodies. Additional growth and replication leads to the formation of the morula, the configuration that typifies the genus. Rupture of the host cell releases the elementary bodies to infect new cells.
- Nicholson WL, Allen KE, McQuiston JH, et al.: The increasing recognition of rickettsial pathogens in dogs and people. Trends Parasitol. 2010, 26, 205-12