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Genetic Search Uncovers Multiple Pathogens in Many Individual Ticks

NEW YORK (GenomeWeb) – Co-infection by multiple pathogens or parasites may be quite common in adult forms of the castor bean tick Ixodes ricinus, according to a study appearing online yesterday in PLOS Neglected Tropical Diseases.

Researchers from the French National Institute for Agricultural Research (INRA) and elsewhere used real-time reverse transcription PCR (rRT-PCR) to search for genetic material from several endosymbionts and more than three-dozen bacterial pathogens, viral pathogens, or parasites in hundreds of female I. ricinus ticks collected in the Ardennes region of France.

Almost half of the ticks contained at least one possible disease-causing culprit and the presence of one pathogen coincided with additional infection(s) some 45 percent of the time.

"The high co-occurrence of symbionts and pathogens in ticks reveals the necessity to account for these interactions in the development of new alternative strategies to control ticks and tick-borne diseases," INRA's Muriel Vayssier-Taussat, senior author on the study, said in a statement.

The presence of multiple pathogens in any given tick points to the possibility that a single tick bite might lead to more than one type of infection in an unfortunate host, Vayssier-Taussat and co-authors wrote. Beyond that, though, there are hints that pathogens or endosymbionts may impact tick biology in ways that influence their disease-transmitting capabilities.

In an effort to understand the prevalence of co-infection in ticks, the researchers used Fluidigm's BioMark high-throughput rRT-PCR system to test 267 adult female I. ricinus ticks, searching for DNA or RNA from 38 pathogens such as Borrelia and Rickettsia bacteria, Babesia and Coxiella burnetii parasites, and viruses behind conditions such as tick-borne encephalitis. The ticks in question were collected in 2012 in a region in France where rodent-borne hantaviruses are endemic, they noted.

The approach uncovered DNA or RNA from at least one pathogen in 120 ticks — nearly 45 percent of those tested — and Lyme disease-causing Borrelia bacteria were especially common. And the presence of one pathogen or parasite was often not the end of the story: at least 54 of the 120 infected ticks contained more than one pathogen or parasite species.

The researchers searched for endosymbiotic organisms in a subset of 255 ticks, which all seemed to harbor endosymbionts. The most common endosymbiont, Midichloria mitochondrii, was present in just over three-quarters of ticks. Almost 65 percent of the ticks had genetic material from Spiroplasma species and 19 percent appeared to carry the well-known endosymbiont Wolbachia.

The team went on to evaluate combinations of pathogens and pathogen-endosymbiont groups that tend to turn up together, picking up collections of commonly co-occurring pathogens but no clear pathogen-endosymbiont associations.

"Even though additional larger studies of nymph co-infection are yet to be completed," the researchers wrote, "this primary result raises questions about the possibilities of co-transmission of these agents to humans (or animals), their prevalence, the effects of these co-infections on the severity of symptoms, the efficacy of treatments, and how to develop new diagnostic tests better adapted to tick-borne diseases."