Wildlife disease specialists have confirmed the return of Eurasian Highly Pathogenic Avian Influenza H5N1 across California wild birds, following detections in Sacramento County and subsequent spread to multiple counties.
Avian Influenza Resurges Across California Wild Bird Populations
Wildlife disease specialists have confirmed the re-occurrence of the Eurasian strain of Highly Pathogenic Avian Influenza (HPAI) H5N1 in California wild birds. In late October, Canada geese found dead in Sacramento County were submitted to the California Department of Fish and Wildlife’s (CDFW) Wildlife Health Laboratory for testing. Preliminary testing at the California Animal Health and Food Safety Laboratory in Davis preceded confirmation by the U.S. Department of Agriculture’s (USDA) National Veterinary Services Laboratories in early November.
Subsequent detections appeared during mortality surveillance across Contra Costa, San Diego, Santa Clara, Solano, and Yolo counties, alongside hunter-harvest surveillance detections in Fresno, Glenn, Kern, and Yolo counties. The prior confirmed wild bird detection occurred in June 2023. This Eurasian HPAI H5N1 strain has circulated in the United States and Canada since the winter of 2021-22, arriving in California in mid-July 2022. Between July 2022 and June 2023, officials documented 354 confirmed detections across 44 California counties. Detections of the virus appeared to subside during spring and summer in California, as well as more broadly in the United States. Now, as waterfowl and other waterbirds are undertaking fall migration, cases again are on the rise. Avian influenza spreads through direct bird-to-bird contact and may also spread to birds through contaminated surfaces including hands, shoes, clothing and hunting gear. While the Centers for Disease Control considers the transmission risk of avian influenza to people to be low, residents and waterfowl hunters are advised to take precautions to protect themselves, hunting dogs, falconry birds, poultry and pet birds.
Broader Garden and Wild Bird Disease Pressures
Outbreaks of disease may occur in populations of wild birds wherever they occur, including those visiting gardens. Garden birdwatchers may occasionally come across diseased birds through local feeding stations. A number of different diseases have been reported in birds seen at garden feeding stations. Alongside avian influenza, various pathogens afflict wild birds.
Avian influenza—also known as avian flu or bird flu—is a disease caused by an influenza virus. Birds may have a swollen head, watery droppings and show neurological symptoms. Since the winter of 2021/22, avian influenza has spread through many bird species, including wildfowl (ducks, geese and swans), seabirds and birds of prey. It has caused high levels of mortality. If you find a dead wild bird, or see a suspected case of avian influenza, you can report it and assist in research into the outbreak.
Avian pox causes warty or tumour-like growths, particularly on the head, wings or legs. Colibacillosis, a disease caused by Escherichia coli (E. coli), leaves birds looking weak, lethargic and fluffed up, and may cause digestive problems. Salmonellosis, a disease caused by Salmonella bacteria, also makes birds look weak, lethargic and fluffed up, with potential digestive problems. Trichomonosis, caused by the parasite Trichomonas gallinae, prompts birds to show difficulty in swallowing, laboured breathing, and swelling of the throat and neck. Meanwhile, Usutu virus—first detected in the UK in London in summer 2020—is potentially fatal to Blackbirds and concerns are growing as the virus appears to have started to spread across south-east England.
Anecdotal evidence suggests that some bird species may be more susceptible to disease than others. Certainly, Greenfinches, Chaffinches, Siskins, House Sparrows and Collared Doves appear to be the most commonly reported victims of diseases. This may, in part, reflect the gregarious nature of these birds and their tendency to feed in flocks.
Prion Risks and Chronic Wasting Disease Monitoring
Beyond avian pathogens, animal prion diseases present separate surveillance challenges. Chronic wasting disease (CWD) is a prion disease that affects deer, elk, moose and similar animals. It has been reported in the United States, Canada, Norway, Finland, Sweden and South Korea. In the United States, CWD was first identified in the 1960s in captive deer. In 1981, the first wild deer with CWD was reported. Since then, the disease has been reported in wild deer in 36 U.S. states. It’s possible that CWD is also in other states but hasn’t been reported yet.
Once the disease is established in an area, the CWD prions can stay in the soil for years. CWD infections in people have never been reported, and so far, it’s not known if people could get infected with CWD. However, another prion disease in animals, bovine spongiform encephalopathy (BSE, or “mad cow” disease) can cause a disease in people called variant Creutzfeldt-Jakob Disease. Also, some studies in monkeys suggest they could get CWD by eating meat or brain tissues of infected deer or elk, so, in theory, CWD could be considered a risk to people. Scientists continue to look into this possibility. If CWD could spread to people, it would most likely be by eating meat from an infected animal. A CDC survey of U.S. residents showed nearly 1 in 5 said they had hunted deer or elk, while more than 6 in 10 said they had eaten venison or elk meat, making these groups likely most at risk if CWD spreads to people.
Safety Precautions and Official Recommendations
Public health agencies and wildlife specialists advise specific hygiene measures for outdoor recreationists, farmers, and hunters. If you notice anything unusual—a decrease in food or water intake, sudden deaths in the flock, breathing difficulties, strange behaviour, or other signs of disease—report it immediately. If you suspect avian influenza or find dead wild birds or mammals in or near your farm, do not wait or guess, and contact your veterinarian or local animal health authorities as soon as possible.
Residents should report dead wild birds and other wildlife to CDFW using the mortality reporting form, as all mortality reports are important and help disease specialists monitor the outbreak. You should also report sick and dead poultry. Avoid contact with potentially infected animals or handle them with protective equipment in line with good hygiene practices.
If you’re hunting in areas with CWD activity, follow these steps to reduce your risk of contact with the disease: do not shoot, handle or eat animals that look sick or act strangely, and do not touch or eat meat from animals found dead. If you’re field-dressing a deer or handling meat, wear latex or rubber gloves, avoid handling its internal organs, especially the brain or spine, and do not use knives or other tools you use in your kitchen. Always check state wildlife and health department recommendations, and apply biosecurity consistently to stop animal diseases before they spread.
