California Ticks Can Spread Lyme Disease… but that’s not the whole story
The risk of contracting Lyme disease from a tick bite in California has been well-documented, though there is still a long way to go in educating health providers and the broader community in the exposure risk from a tick bite. TickReport’s surveillance of ticks from California (and Oregon and Washington) goes back as far as 2006 and has expanded in recent years.
What ticks are endemic (commonly and consistently found in wild populations) to California and other West Coast states?
That’s a big question, and there are a few dozen species from different genera or families. Many of those species are specialist feeders and—if everything goes “right” in their life cycle—they will only feed on certain wild mammals, birds, or lizards and will bite humans very rarely. That’s doesn’t mean that finding one of these “specialists” attached to ourselves or a family member is impossible: it’s just much less common (and a topic we’ll try to visit soon in another post).
Our surveillance shows that the majority (91.5%) of human or human-adjacent (dogs, cats, horses, etc) tick bites are caused by the following ticks:
Ixodes pacificus (“Western black-legged tick,” a close relative of the Deer tick in the Eastern U.S.)
Dermacentor variabilis (“American dog tick”)
Dermacentor occidentalis (“Pacific Coast tick”)
Dermacentor andersoni (“Rocky Mountain Wood tick”)
Ixodes spinipalpis
Ixodes angustus
What pathogens can these ticks transmit to humans?
Vector competence (the ability of a vector like a tick to transmit a given disease-causing pathogen) tends to run along genus lines, so species within the Ixodes genus tend to be able to transmit pathogens X and Y but not Z, while Dermacentor species tend to transmit Z but not X and Y.
The most common pathogen found in California ticks is Borrelia burgdorferi, which causes Lyme disease in humans and pets. But there’s more than Lyme in those hills! If you find a tick bite from an Ixodes tick in California, it’s also important to consider possible exposure to these pathogens:
Borrelia miyamotoi: a bacterium that can cause hard tick relapsing fever—sometimes called Borrelia miyamotoi disease.
Anaplasma phagocytophilum: a bacterium that can cause Human granulocyctic Anaplasmosis.
For every two ticks we find infected with Borrelia burgdorferi (Lyme disease bacteria), we detect one (or more) of these other pathogens
It’s vital that both tick surveillance and diagnostic approaches keep these non-Lyme pathogens in mind. For every two ticks we find infected with Borrelia burgdorferi (Lyme disease bacteria), we detect one (or more) of these other pathogens, so be sure to resist Lyme Tunnel Vision when responding to a tick bite! Common diagnostic tests for Lyme disease have a specific focus on Lyme disease and will not detect infection by these other pathogens if present. Make sure you and your doctor are considering the whole story of a tick bite.