The effect of co-infecting Anaplasma phagocytophilum on replication of Langat Virus, a model for Louping ill virus, in Ixodes spp. tick cells

Tick borne fever (TBF) caused by Anaplasma phagocytophilum (Ap), and louping ill caused by the flavivirus louping ill virus (LIV) are the two most economically important vector-borne diseases in UK sheep populations. Both pathogens are transmitted by the tick Ixodes ricinus, which also harbours protozoan parasites and Borrelia spp. spirochetes. I. ricinus ticks may be co-infected with multiple microorganisms and potentially transmit more than one pathogen to hosts during blood feeding.

Ap infection is not limited to sheep, causing pasture fever in cattle and granulocytic anaplasmosis in horses, dogs and humans. There is no vaccine available for TBF, and disease control relies on tick control and antibiotic treatment. LIV mainly infects ruminants, but can also infect other livestock including horses, pigs, alpacas and llamas. Since 2016, the LIV vaccine has been unavailable and there is no alternative prophylactic treatment for livestock. Both Ap and LIV are zoonotic diseases with occasional human cases reported in the UK.

Importantly, Ap infection leads to host immunosuppression and consequently increases vulnerability to secondary infections. Both Ap and LIV have been studied as single infections in tick and mammalian cells. However the dynamics and implications of co-infections within the arthropod vector or mammalian host have, to date, not been fully explored.

Using embryo-derived Ixodes spp. cell lines infected with Langat virus (a BSL-2 model for LIV) and Ap, we examined the effect of co-infection on viral RNA replication by qRT-PCR and bacterial growth by qPCR. The results of this study will be presented and discussed.