Project Summary/ Abstract
African tick-bite fever caused by Rickettsia africae is probably the most common human spotted
fever group rickettsiosis. It occurs throughout Africa, wherever its major vector, Amblyomma
variegatum, is found. Other Amblyomma species can also transmit R. africae and the organism
has spread in Amblyomma loculosum on migratory birds through the Indian and Pacific Ocean
islands. Rickettsia africae also occurs in Central America in A. ovale, and in the Caribbean
islands in A. variegatum which were imported on cattle from Africa in the 1800s. Infected ticks
were spread widely around the Caribbean islands on cattle egrets, which are migratory birds
that move readily between the islands and as far as the Florida Keys.
The presence of R. africae in the nearby Caribbean islands and Central America, and Africa, the
expanding tourism and trade between these areas and the USA, the travelers entering the USA
from Africa, the Caribbean, and Central America that are infected with R. africae, and the
presence of migratory birds raises the risks of A. variegatum and/or R. africae being introduced
into the USA and becoming established. If Amblyomma species endemic in the USA could then
become infected with and maintain and transmit R. africae, African tick-bite fever could become
established in the USA causing widespread morbidity as it does in Africa. Additionally, this
would further complicate the already difficult diagnosis of American spotted fever group
Rickettsia infections, in particular Rocky Mountain spotted fever. The effects of R. africae on
American wildlife are unknown.
To establish the reservoir and vector capacity of the most relevant US Amblyomma species, A.
americanum and A. maculatum, we intend to conduct transmission experiments with guinea
pigs and calves as hosts. Amblyomma variegatum larvae/ nymphs and adults infected with R.
africae will be fed on guinea pigs or calves, respectively, at the same time as uninfected
immature and adult US Amblyomma species to determine if infections can be transferred
horizontally. If infections can be demonstrated in the US Amblyomma species, further
experiments will be performed to establish if R. africae can be transmitted vertically between
feeding stages (transtadially), transovarially through the eggs, and from each feeding stage to
the host.
Data generated from these experiments will enable American health workers to more precisely
determine the risk of African tick-bite fever becoming established in the USA and for appropriate
prevention and response strategies to be developed.