Ticking All the Boxes: the Amazing Variety of Ticks in San Mateo County
At the District, the majority of our tick surveillance focuses on three tick species, Ixodes pacificus, (western black-legged tick), Dermacentor varabilis, (American dog tick) and Dermacentor occidentalis, (Pacific coast tick). These ticks quest for hosts in the vegetation along trails, and are easily picked up by hikers and dogs. While these three species present the greatest risk of disease transmission to humans and pets, there are many more species of ticks that live in San Mateo County that we do not collect as part of our usual surveillance. This may be because these ticks species are not known to transmit disease, or they are very rarely encountered because they live exclusively in the nests of their host animals. Nonetheless, these ticks can play a role in maintaining diseases by circulating them amongst reservoirs (animals that are regularly infected with a disease but do not die from it).
Below are some of the tick species that we occasionally encounter in San Mateo County parks and open spaces:
This nest-dwelling tick feeds on small rodents such as mice, shrews and woodrats. They are sometimes collected when we flag near rodent burrows looking for nymphs of Ixodes pacificus. This tick can transmit the bacteria that causes Lyme disease, but rarely bites humans.
This tick is often found in the nests of its host but occasionally we will find one while collecting questing ticks along trails. Like Ixodes angustus, it feeds on rodents and may transmit the bacterium that causes Lyme disease. Immature Ix. spinipalpiscan also be found feeding on birds.
This tick is also called the rabbit tick because it prefers to feed on rabbits and hares. We often collect these ticks in the nymphal or larval stages from leaf litter and other vegetation near the ground. While rabbit ticks can vector the bacterium that causes tularemia, they do not usually feed on humans.
This tick is also called the brown dog tick, and unlike the other species, it is generally found in structures and may infest homes and animal shelters. Indoor infestations of this tick can be large and take several months to eliminate. As the name suggests, it prefers to feed on dogs, although it will bite humans and other mammals occasionally. These ticks are capable of transmitting Rocky Mountain spotted fever. We do not collect this tick during regular surveillance, but it has occasionally been submitted as a service request. While it is a major pest in the southern United States and around the world, it is uncommon in San Mateo County.