The pickled past: Tropical brown dog tick distribution over time using ancient DNA

The movement of vectors over time influences epidemiology, disease history, and impacts of climate change on vector-borne diseases. The brown dog tick (BDT) is a blood feeding parasite of dogs which is found globally. In Arizona, the Southwest, and Mexico, this tick carries a deadly canine and human disease called Rocky Mountain Spotted fever (RMSF). The BDT has been in Arizona for at least a century, but RMSF outbreaks did not begin in the state until 2003. The BDT has two genetic lineages, called tropical and temperate, which are morphologically identical but different in their mitochondrial genomes. Arizona has both lineages, with the tropical lineage in southern counties and the temperate lineage in northern counties. BDT taxonomy is still debated, but the lineages differ in their behaviors and vectorial capacities for certain diseases. Villareal et al. (2018) hypothesized that the tropical lineage has been moving north over time, suggesting that the arrival of the tropical tick may have caused Arizona’s first outbreaks. To look at this temporal relationship, we used the 12S mitochondrial gene to determine the lineages of BDT collected between 1940 and 1954. DNA was extracted from alcohol specimens archived in the University of Arizona Insect Collection, and diagnostic parts of the 12S gene were amplified with nested PCR and aligned with modern specimens. The data show that tropical ticks have been in Arizona since at least the 1950s, so the tropical tick’s arrival was not the main reason for Arizona’s first outbreaks of RMSF.