Our nurses, epidemiologists, investigators, physicians, veterinarians, and administrative staff serve as “disease detectives” working to protect King County residents from infectious diseases of public health significance. Our section:

• Identifies and promotes the most effective prevention measures (such as vaccination and infection control measures)
• Monitors the occurrence of diseases in the community and describes the affected populations
• Identifies health disparities to help prioritize resources to communities most in need
• Takes action to stop the spread of infections from contaminated food, beverages, environmental sources (e.g., animals, water), or contact with ill individuals
• Helps people who have been exposed to anything infectious minimize their risk of getting sick and/or spreading infection to others
• Provides information to the public, health care providers, hospitals and long-term care facilities, schools, early care and education programs (such as childcare centers), and businesses to help identify, manage, and prevent infections
• Connects patients to treatment
• Investigates and responds to emerging public health threats

Cases of infectious disease in King County residents summarized in this report represent only a fraction of the true number of cases. Approximately 40% of the reports we investigate are not ultimately classified as confirmed or probable cases. Typically, they aren’t confirmed because either lab testing did not support the diagnosis, the case was diagnosed as a different disease, or the clinical illness did not meet the surveillance case definition. Many patients may also not seek medical care, particularly those with mild symptoms. This may also contribute to the underestimation of confirmed or probable cases.

Following the initial COVID-19 pandemic response in 2020, several communicable diseases in 2021 returned to similar case levels seen before the COVID-19 pandemic. A decrease in pandemic restrictions, re-opening of businesses and recreational establishments, increased travel, and changes in healthcare seeking and access to testing resources may be reasons for the return to pre-pandemic levels.

However, the exception to this in King County was the notable decrease in the number of vaccine preventable disease cases since 2020, particularly among cases of mumps and pertussis. Reported cases may have continued to stay lower in part because King County residents continued masking and following pandemic precautions. These actions may have directly reduced transmission or reduced the number of people who sought testing and were therefore ill but undiagnosed.

King County investigated 3,833 confirmed, probable, or suspect communicable disease reports in 2021, up 13% from the previous year. Aside from COVID-19, the most common reports in 2021 were:

• chronic hepatitis C
• chronic hepatitis B
• campylobacteriosis
• shigellosis, and
• giardiasis.

There are notable differences in the distribution of cases for these conditions by race and ethnicity. For example, the rate of enteric (intestinal) infections such as shigellosis was highest among American Indian or Alaska Native residents (52 cases per 100,000) and non-Hispanic Black residents (35 cases per 100,000) and lowest among Asian residents (3 cases per 100,000). Rates of chronic hepatitis C were highest among American Indian or Alaska Native residents (222 cases per 100,000) whereas rates of chronic hepatitis B were highest among Native Hawaiian or Pacific Islander residents (58 cases per 100,000) and Asian residents (53 cases per 100,000).

• Rates of chronic hepatitis C were highest in Seattle (59 per 100,000 residents) and South King County (58 cases per 100,000 residents).
• Rates of chronic hepatitis B were highest in South King County (34 cases per 100,000 residents) and East King County (32 cases per 100,000 residents).
• Rates of campylobacteriosis were highest in North King County (36 cases per 100,000 residents).

The dashboard allows further exploration of regional differences on the tab labeled "Population Trends."

Enteric bacteria typically enter the body through the mouth. People are exposed through contaminated food and water, by contact with animals or their environments, or by contact with the feces of an infected person.

Case count trends for reportable enteric conditions were increasing prior to 2020, partly due to an increased use of tests, culture independent diagnostic tests (CIDT), that rapidly identify pathogens compared to traditional methods. Following a decrease in 2020, in 2021 reports of campylobacteriosis (the most common enteric infection in Washington State), Shiga toxin-producing E. coli, shigellosis, cyclosporiasis, and vibriosis substantially increased.

Public Health investigated an outbreak of shigellosis that began in October 2020 and continued into 2021. This outbreak primarily affected people living unsheltered (street, tents, encampments) in the Seattle area. One hundred fifty-two additional outbreak-related cases of shigellosis were identified in 2021. The outbreak peaked in December 2021 with 48 cases reported. A significant proportion of cases infected with Shigella were co-infected with other fecal-oral pathogens, particularly Cryptosporidium, Campylobacter, and Giardia. Six percent of cases infected with Shigella also had COVID-19 infections at the same time.

Multiple factors likely contributed to the outbreaks, made worse by the COVID-19 pandemic, including:

• reduced capacity of shelters
• increasing numbers of people living homeless (especially within encampments)
• limited availability of places for people to get clean drinking water, use the toilet, or wash hands with soap and water
• increased reliance on alcohol-based sanitizers rather than soap and water
• lack of appropriate use and/or completion of antibiotics which reduce infectiousness (especially with multiple types of antibiotic-resistant strains circulating)
• decreased care-seeking behaviors, leading to more untreated cases, further continuing the spread of infection

The Centers for Disease Control and Prevention (CDC) consider Carbapenemase-producing organisms (CPOs) such as Enterobacterales and other carbapenem-resistant Gram-negative bacteria (Pseudomonas aeruginosa and Acinetobacter baumannii) an urgent threat. They can spread in healthcare settings and contribute to carbapenem antibiotic resistance. Carbapenem antibiotics are considered a last resort antibiotic when common antibiotics no longer work. This means that carbapenem resistance makes these organisms very difficult to treat.

CPOs often live in the intestine but can spread outside the gut and cause serious infections, such as urinary tract infections, bloodstream infections, wound infections, and pneumonia. Since 2014, when Public Health began tracking CPOs, case counts have remained low. In 2021, 14 CPO cases were identified, slightly lower relative to a five-year average of 14 cases but still at a higher level than yearly cases seen prior to 2018. Increasing case counts may be attributed to more reporting due to heightened awareness, active case finding through screening activities, and enhanced detection though the Antibiotic Resistance Laboratory Network resource established in 2016.

Overall, reports of vaccine-preventable diseases dropped substantially in 2020 and remained low in 2021 relative to prior years. As described above, continued measures to prevent COVID-19 transmission may have contributed to a reduction in transmission of other respiratory pathogens. The number of cases of measles peaked in 2019, returned to baseline in 2020 and remained at the baseline level in 2021 (baseline equal to the King County 10-year average of 4 cases). The number of cases of mumps peaked in 2016 and 2017, owing to large statewide outbreaks, and has been on a decreasing trend. For Neisseria meningitides (meningococcal disease) zero cases were reported in 2021, a substantial decrease compared to the 10-year average of 4 cases. Notably, the number of pertussis cases in 2021 was 97% lower relative to the 10-year average.

After we received no reports of influenza deaths or outbreaks in long-term care facilities (LTCFs) during the 2020-21 flu season, LTCFs reported five influenza deaths and six outbreaks in LTCFs during the 2021-22 flu season; however, these numbers are lower relative to the five-year average of 45 deaths and 38 outbreaks in LTCFs. Community mitigation measures recommended for prevention of COVID-19 transmission in these settings likely contributed to the observed decrease in influenza outbreaks in long-term care facilities in 2021. More information about influenza in King County.

The number of hepatitis A cases peaked in 2020 relative to the prior 10-years due to an outbreak among people unstably housed or experiencing homelessness and people who use drugs. However, reported cases decreased substantially in 2021, falling to lower-than-baseline levels. The hepatitis A outbreak was declared over in November 2021. Disease investigation and contact tracing, mobilization of vaccination efforts, and strategic coordination with both city and community partners contributed to the decline of cases and return to baseline counts by the close of 2020 and into 2021. Visit our hepatitis A dashboard to view the case reporting data for this outbreak.

Chronic hepatitis B and C infections continue to comprise the largest proportion of communicable diseases reported to PHSKC annually:

• roughly 750 and 1200 newly diagnosed cases reported in 2021, respectively;
• chronic hepatitis C accounts for 31% (n=1194) of all communicable disease cases reported to King County, and
• chronic hepatitis B accounts for 19% (n=744) of all cases.

Reports of acute hepatitis B and C infections increased between 2016 and 2019. The number of acute infections dropped in 2020 and remained at a similar level in 2021, which may reflect changes in care-seeking behaviors (and therefore testing and diagnosis) in response to the COVID-19 pandemic, rather than a true decrease.

The COVID-19 pandemic caused major disruptions to routine public health activities, including distribution of sterile syringes and harm reduction supplies, and access to medical care, such as screening and linkage to care for hepatitis C. In 2021, King County continued to see high rates of acute hepatitis C among younger people who use drugs, with particularly high rates of infection among males and people experiencing homelessness. To address these disparities, PHSKC has partnered with local healthcare facilities, community-based organizations, and outreach providers to increase access to screening and low-barrier hepatitis C treatment options.

Our Perinatal Hepatitis B Prevention Program (PHBPP) tracks hepatitis cases in pregnant people to prevent transmission of hepatitis B to their infants by ensuring the infants receive appropriate preventive treatment. In 2021, the program ensured 95% (n=127) of pregnant people with Hepatitis B were enrolled, eligible infants were tested on time, and none of these infants were infected with hepatitis B virus. More information about PHBPP.

The overall number of reports of animal bites and potential rabies exposures reported has been on an increasing trend since 2018, though reported cases decreased in 2021 relative to cases from 2018-2020.

However, relative to 2020, case reports of legionellosis and malaria increased in 2021. As people are more likely to acquire these diseases through travel, and with COVID-19 travel restrictions reduced, renewed travel may factor into the increase in these diseases. Additionally, prolonged closures of buildings during COVID-19 may have led to stagnant or slow-moving water which can contribute to the growth of biofilm associated bacteria, like legionella, in water systems.

PHSKC investigated 32 cases of legionellosis in 2021, almost twice the average number of cases reported from the previous 10 years. Fifty percent of cases traveled outside of King County during their incubation period, significantly higher than previous years, even when excluding 2020. The mean age of case-patients in 2021 was also significantly lower than previous years [56 years (2021) vs 64 years (2016-2020)]. However, no common exposures or travel locations were discovered.