Tuesday, May 31, 2016

Barcoding Bird–Tick Interactions

In the tropics, ticks parasitize many classes of vertebrate hosts. However, because many tropical tick species are only identifiable in the adult stage, and these adults usually parasitize mammals, most attention on the ecology of tick-host interactions has focused on mammalian hosts. In contrast, immature Neotropical ticks are often found on wild birds, yet difficulties in identifying immatures hinder studies of birds’ role in tropical tick ecology and tick-borne disease transmission.

In the American tropics the most dangerous tick-borne disease for human is caused by Rickettsia rickettsii, the etiological agent of Rocky Mountain Spotted Fever, which is also known as blue disease. It is the most lethal (20-40% fatality rate) and most frequently reported rickettsial illness in the United States. However, it has been diagnosed throughout the Americas. There is a fairly good understanding what vector species transmit the disease, e.g. in Panama there are four tick species that are confirmed intermediate hosts. Due to our inability to identify immature ticks we lack knowledge on bird-tick associations which in turn hampers our understanding of their role in disease transmission.

As a new study by colleagues in Panama shows DNA barcoding can be used to close this knowledge gap. The colleagues collected adult ticks for their reference library and generated DNA barcodes for 20 species. This library was subsequently used to identify immature ticks collected from a variety of different bird species (93 species). The authors also studied potential host specificity but were unable to find any indication for that. Only 6.5% of all birds examined for the study carried ticks but given the range of bird species identified indicates that wild birds may play an important role in the life history of tick species and have the potential to play a role in the transmission of tick-borne diseases.

The authors estimate based on typical avian densities recorded for Panama, that as many as 96,000,000 Panamanian wild birds might be infested with ticks. but although certain avian ecological traits are positively associated with parasitism, we found no evidence that individual tick species show specificity to particular avian host ecological traits. Finally, our data suggest that the four principal vectors of Rocky Mountain Spotted Fever in the Neotropics rarely, if ever, parasitize Panamanian birds. However, other tick species that harbor newly–discovered rickettsial parasites of unknown pathogenicity are frequently found on these birds. Given our discovery of broad interaction between Panamanian tick and avian biodiversity, future work on tick ecology and the dynamics of emerging tropical tick-borne pathogens should explicitly consider wild bird as hosts.

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