Rats and other reservoir hosts can carry leptospirosis without illness, shedding bacteria into the environment while dogs and people become the visible victims of infection, explains Jane Sykes, BVSc (Hons), DACVIM (SAIM), PhD, MPH, MBA, FNAP.
In this interview tied to her 2026 Veterinary Meeting & Expo session, Jane Sykes, BVSc (Hons), DACVIM (SAIM), PhD, MPH, MBA, FNAP, a professor at the University of California, Davis, breaks down how leptospirosis moves through animal and human populations. She explains how reservoir species, such as rats, can carry the bacteria without appearing ill while still shedding high concentrations into the environment. Incidental hosts, on the other hand, such as dogs and people, often develop acute and sometimes life-threatening disease but are not typically infectious to other incidental hosts.
Below is the video transcript, lightly edited for clarity.
Sykes: My name is Jane Sykes. I'm a professor of small animal internal medicine at the University of California, Davis, and I study infectious diseases of public health significance.
dvm360: Can you explain the difference between reservoir hosts and incidental hosts in the transmission of leptospirosis?
[Leptospirosis] is a really fascinating bug because it achieves either [a] reservoir host status with a host or an incidental host status. If the spirochete is well adapted to a particular reservoir host—for example, we think about serovar Icterohaemorrhagiae as being really well adapted to rats—then it sits in the renal tubules. It forms [a thick layer of] biofilm in the renal tubules…[which] allows it to be shed chronically in the urine in high concentrations without causing any clinical signs.
So that reservoir host status is this chronic, persistent, subclinical infection that promotes contamination of the environment and transmission to other hosts. Animals or people that are acting as incidental hosts have not established that degree of premunition, and they mount a strong inflammatory reaction to the particular strain of [leptospirosis] that's infecting them, and that inflammation causes multiorgan failure and illness. That's when there's not that state of adaptation. So you see a very sick dog or a very sick human that has acute kidney injury and maybe dysfunction of other organs because [leptospirosis] doesn't actually cause liver failure—it causes hepatic dysfunction, and that's cholestatic—so that multiorgan illness is what is characteristic of the incidental host state.
Typically, incidental hosts don't shed very many organisms in their urine, and it's very hard for an incidental host to transmit infection to another incidental host. So we don't see one dog with [leptospirosis] transmitting [leptospirosis] and causing illness in a person [or another dog] very often.
That might have been a little different in the Los Angeles, [California], outbreak, but that's pretty typical in general for [leptospirosis]. It's mostly transmitted by reservoir hosts, which shed these large numbers of organisms in their urine without showing signs.