Rhipicephalus appendiculatus ticks transmit theileria parva from persistently infected cattle in the absence of detectable parasitemia: implications for East Coast fever epidemiology

Authors: OLDS, CASSANDRA - Washington State University; Mason, Kathleen; Scoles, Glen

Submitted to: Parasites & Vectors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2018
Publication Date: 3/2/2018
Citation: Olds, C.L., Mason, K.L., Scoles, G.A. 2018. Rhipicephalus appendiculatus ticks transmit theileria parva from persistently infected cattle in the absence of detectable parasitemia: implications for East Coast fever epidemiology. Parasites & Vectors. https://doi.org/10.1186/s13071-018-2727-6.
DOI: https://doi.org/10.1186/s13071-018-2727-6

Interpretive Summary: Diseases transmitted by ticks can cause significant losses to the livestock industry. In Africa, the brown ear tick transmits the parasite Theileria parva to cattle, causing the disease known as East Coast Fever. During acute disease Theileria parva infects the white blood cells of cattle causing the cells to rapidly divide and spread throughout the body. Once cattle have recovered from the initial infection, T. parva remains in the body of animal for many years, slowly dividing, resulting in long term persistent infection with the parasite. Brown ear ticks feeding on an infected animal acquire the parasite and can pass it on to a new uninfected cattle host the next time it feeds. This paper describes a series of experiments designed to understand the tick’s ability to acquire and transmit T. parva from persistently infected hosts. Most of what we know about transmission of T. parva has been learned by studying animals during acute infection, transmission from persistently infected cattle has rarely been studied, but may be more representative of the transmission that takes place naturally. We have shown that ticks can acquire T. parva parasites from cattle long after the acute stage of disease has passed, even if parasites are below the level of our most sensitive detection tools. The T. parva parasite levels are below the level of detection in individual ticks, but as a population, these ticks are able to transmit the parasites to a new host. These results are important for our understanding of tick-borne diseases and have implications for evaluating field infection rates and vaccine deployment.

Technical Abstract: East Coast fever is a devastating disease of cattle caused by infection with the protozoan parasite, Theileria parva. Transmitted by the three-host tick, Rhipicephalus appendiculatus, the disease is present in 11 sub-Saharan African countries and is a major constraint to improvement of livestock production in affected areas. Acquisition and transmission of T. parva occurs transtadially, with the larval or nymphal tick stages acquiring infection while feeding on an already infected host. The only effective vaccine, known as the Infection and Treatment Method (ITM), requires giving cattle an attenuated infection with live parasites. Current knowledge relating to the acquisition of T. parva by ticks has been gathered from studies done during the acute stage of infection where parasitemia is high. This is in stark contrast to the low level persistent infections that predominate in cattle naturally in the field, of which little is known. To begin to answer questions surrounding the acquisition of T. parva from clinically healthy but persistently infected cattle, ticks were fed on infected cattle in which circulating parasites could no longer be detected by nested PCR. Infection was below detectable levels in individual ticks by PCR but as a population, these ticks were able to transmit T. parva to naïve cattle. These tick transmitted infections were sub-clinical, mimicking conditions that would be seen under natural endemic stability. The results of these experiments play a critical role in helping understand the impact that ITM vaccination programs may have on T. parva epidemiology and endemic stability.