Approximately 90 "backyard flock" birds in Northern California affected by avian flu

Roughly 90 birds in two Northern California backyard flocks have tested positive for avian influenza in October, the U.S. Department of Agriculture said.

The department said that approximately 40 birds in a backyard flock in Sacramento County were confirmed positive for the disease on Oct. 5, and roughly 50 birds in a Solano County backyard flock were confirmed positive on Oct. 20.

Avian flu, caused by influenza type A viruses, can vary in severity depending on factors like the species affected and the strain of the disease, according to the USDA. Symptoms can range from lack of appetite, coughing and sneezing, purple discoloration of the comb, legs, and wattles, to sudden death with no prior visible symptoms.

Highly pathogenic strains are the most dangerous and can affect entire flocks in a short amount of time. Birds that are infected with low pathogenicity strains typically show few or no signs of illness, but some low pathogenic strains can become more pathogenic in poultry, according to the USDA. 

Avian influenza A viruses are not known to infect people regularly, according to the department.