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‘A canary in the coal mine:' California dogs increasingly exposed to tick-borne bacteria
Dog owners may need to be on alert for a disease many have never heard of — anaplasmosis, spread by the same tick that transmits Lyme disease.
Anaplasmosis can make dogs feverish or lethargic, turn their nose up at food and lose weight, but many show no sign of illness at all. In rare circumstances, though, dogs can bleed from the nose, have neck pain and suffer seizures.
The percentage of dogs across California that tested positive for exposure to the bacterium that causes anaplasmosis skyrocketed in the last five years, nearly tripling between 2021 and 2024, according to data from the nonprofit Companion Animal Parasite Council.
Humans can get the disease too. Dogs get more tick bites and are tested more often for vector-borne diseases than people, so they can be sentinels for their owners.
'It's a canary in the coal mine,' said Brad Ryan, a professional services veterinarian at Antech Diagnostics, a company that offers testing and other services to vets. 'When the prevalence starts to go up in the animal population, we know that the public health risk has, by default, also gone up.'
Anaplasmosis and Lyme disease are spread by western blacklegged ticks.
More than half of California counties are labeled as high infection risk, including Santa Barbara and San Diego. The rise began after several years of decline.
In Los Angeles County, which is deemed moderate risk, about 1.2% of dogs are now testing positive — more than double the 0.5% five years ago.
The Companion Animal Parasite Council believes its numbers are an undercount, estimating they are only capturing 30% of the parasite activity in a given geographic region.
Climate change is among a cocktail of factors fueling an increase in tick-borne illnesses across the country, as warming temperatures allow parasites to march into new areas and lengthen their season. In temperate California, experts say the risk is year-round.
Western blacklegged ticks are migrating into Southern California from the northern part of the state. Meanwhile, brown dog ticks — vectors for the bacterial infection Ehrlichiosis — are trekking north from Mexico into Southern California.
The good news is that both pups and people afflicted with anaplasmosis typically recover quickly with antibiotics. But some experts say awareness of the threat is trailing in Southern California, an area not historically known for being a tick haven.
'Anaplasmosis is not just an East Coast problem; it's very much here in California,' said Dr. Sunjya Schweig, founder of the California Center for Functional Medicine.
'Unfortunately, the doctors don't always recognize that, so it's really hard sometimes for people to get the right care.'
Relative to other parts of the country, the prevalence of anaplasmosis among dogs in Southern California remains low, with the parasite council forecasting roughly 1% to 2% this year. However, there are hot spots along the state's northern coast, some with rates of 8% or more.
Western blacklegged ticks thrive in moist, cool environments. Experts disagree about the prospects for their future spread.
They're unlikely to go gangbusters in arid SoCal, in the view of Janet Foley, a professor of medicine and epidemiology in the UC Davis School of Veterinary Medicine.
They do show up in hospitable zones in places like Malibu, Ventura and San Diego, she said.
But Ryan thinks the ticks could proliferate in the region.
'I always joke, unless your vet clinic is set up right in the middle of Death Valley National Park, where you hardly get any rain at all throughout the year, there's nothing that's going to stop the establishment of these western blacklegged ticks, or deer ticks, across Southern California,' Ryan said.
A report from the California Department of Public Health shows a rise in confirmed and probable anaplasmosis cases in people in recent years. Though the raw numbers are low, the figure roughly doubled between 2020 and 2021 and has remained elevated through 2023, the year of the most recent report available.
In 2023, almost all the 21 cases — which included eight patients from L.A. County — were linked to travel to Northern California or other parts of the U.S.
Foley said areas including New England, New York, Minnesota and Michigan are seeing a dramatic increase in anaplasma. 'So we're going to see more cases here, in dogs and people, because there's going to be more travel-imported anaplasma.'
Local factors may also be linked to the trend. People are building deeper into animal habitat, slicing up terrain in ways that can upset the ecosystem, Schweig said. Predators such as bobcats may no longer be able to hunt for rodents and birds — tick hosts — in pocket parks or suburban landscapes. So the small animals and their parasites thrive.
Getting tested for illnesses spread by ticks may be easier if you walk on four legs than two.
'We have more options, honestly, on the veterinary side,' said Ryan, of Antech. He recommends that vets screen dogs for common vector-borne diseases in annual wellness exams, and that dog owners put their pets on year-round flea, tick and heartworm prevention medication.
In the human realm, Schweig described a huge fight over appropriate testing and treatment, 'like a religious battle,' between groups such as the Centers for Disease Control and the Infectious Diseases Society of America 'versus people like me' and the Bay Area Lyme Foundation, where he serves as a scientific advisory board member.
'We're saying, hey, there is a big problem. It's much more common than we know. The tests are crummy. There's a lot of sick people who aren't being diagnosed.'
He called the vet community 'more open minded' about Lyme and other tick-borne diseases. 'Unfortunately, when you go to the human doctor they do not ever routinely offer these tests, unless you, like, pull this tick off of you and you have a fever.'
Ticks are active throughout the year in California, but risk typically peaks in the spring when the biting arthropods are born, and in fall, after the first rains.
Many humans are infected when they're nipped by nymph ticks, a period in their life cycle when they're roughly the size of a poppy seed and barely visible to the naked eye.
Hiking, camping, horseback riding and really anything that involves getting into the grass will up your chances of being exposed to the bugs.
To reduce the likelihood of a tick bite:
