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This report summarizes communicable disease surveillance data reported to the Public Health – Seattle & King County (PHSKC) Communicable Disease Epidemiology and Immunization Section by healthcare providers, laboratories, and the public. It includes a table with notifiable condition data from the past ten years, as well as trends by demographic characteristics and geography. Additional information about the clinical features of each condition is available on our website.

This report does not include a summary of COVID-19 data. Please refer to our COVID-19 data and reports pages for COVID-19 specific surveillance summaries.

Information about the conditions below is available from their respective program websites:

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. We do this by:

  • Identifying and promoting the most effective prevention measures (such as vaccination and infection control measures)
  • Monitoring the occurrence of diseases in the community and describing the affected populations
  • Identifying health disparities to help prioritize allocation of resources to communities most in need
  • Taking action to stop the spread of infections from contaminated food, beverages, environmental sources (e.g., animals, water), or contact with ill individuals
  • Helping people who have been exposed to infectious agents minimize their risk of getting sick and/or spreading infection to others
  • Providing information to the public, health care providers, hospitals and long term care facilities, schools, child care centers and businesses to help identify, manage and prevent infections
  • Connecting patients to treatment
  • Investigating and responding to emerging public health threats

Cases of notifiable conditions 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 because either lab testing did not support the diagnosis, another diagnosis was established, or the clinical illness did not meet the surveillance case definition. The number of cases that end up being classified as confirmed or probable is underestimated, as many patients, particularly those with mild disease, do not present themselves for medical care.

During the COVID-19 pandemic response in 2020, there was a substantial decrease in reports across many communicable diseases. This decrease was especially notable among foodborne and enteric diseases. Changes in healthcare-seeking behavior, masking, physical distancing, more rigorous adherence to handwashing and environmental cleaning, food establishment and recreational water closures, and travel restrictions all contributed to the decrease, either by directly reducing transmission, or by reducing the number of people who sought testing and were therefore ill but undiagnosed.

King County investigated 3,722 confirmed, probable or suspect communicable disease reports in 2020, down 18% from the previous year. Aside from COVID-19, the most common reports in 2020 were chronic hepatitis C, chronic hepatitis B, campylobacteriosis, giardiasis and salmonellosis. There are notable differences in the distribution of cases for these conditions by race and ethnicity. For example, the rate of animal bites and potential rabies exposures in 2020 was 3-fold greater among White residents (9 cases per 100,000) relative to Asian residents (3.0 cases per 100,000), who had the second-highest rate of these reports. The rate of enteric infections such as salmonellosis was highest among American Indian or Alaska Native residents (29.5 cases per 100,000) and non-Hispanic Black residents (11.5 cases per 100,000) and lowest among Asian residents (4.4 cases per 100,000). Rates of chronic hepatitis C were highest among American Indian or Alaska Native residents (147.6 cases per 100,000) whereas rates of chronic hepatitis B were highest among Asian residents (57.6 cases per 100,000) and Native Hawaiian or Pacific Islander residents (52.7 cases per 100,000). Rates of hepatitis A were highest among non-Hispanic White residents (22.1 cases per 100,000), more than double the rates of other race/ethnicity groups.

Rates of communicable disease reports also varied by geography. For example, rates of chronic hepatitis C were highest in South King County (62.3 per 100,000 residents) and Seattle (61.3 cases per 100,000 residents), whereas rates of chronic hepatitis B were highest in East King County (37.3 cases per 100,000 residents). Rates of animal bites and potential rabies exposures were highest in North King County (10.6 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. They are acquired through contaminated food and water, by contact with animals or their environments, by contact with the feces of an infected person.

Case counts for reportable enteric conditions were on an increasing trend prior to 2020, partly due to an increased use of culture-independent laboratory testing; these tests offer rapid identification of pathogens compared to tradition methods. However, relative to 2019, there was a substantial decrease in reports of campylobacteriosis (the most common enteric infection in Washington State), cryptosporidiosis, giardiasis, salmonellosis (the second most common enteric infection in Washington State), Shiga toxin-producing E. coli, shigellosis and vibriosis in 2020.

PHSKC investigated outbreaks of shigellosis and cryptosporidiosis that began in October 2020 and continued into 2021. These outbreaks primarily affected persons living unsheltered (street, tents, encampments) in the Seattle area. Forty-two outbreak-related cases of shigellosis and six outbreak-related cases of cryptosporidiosis were identified in 2020, with case counts continuing to grow in 2021. A significant proportion of cases infected with Shigella were co-infected with other fecal-oral pathogens, particularly cryptosporidiosis and diarrheagenic E. coli (enterotoxigenic E. coli [ETEC], enteropathogenic E. coli [EPEC], and enteroaggregative E. coli [EAEC]). The outbreaks are likely attributed to multiple factors that were exacerbated by the COVID-19 pandemic, including de-intensification 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 present); and decreased care-seeking behaviors, leading to a larger number of untreated cases who were able to continue to chain of transmission.

Carbapenemase-producing organisms (CPOs) such as Enterobacterales and other carbapenem-resistant Gram-negative bacteria (Pseudomonas aeruginosa and Acinetobacter baumannii), are considered an urgent threat by the US Centers for Disease Control and Prevention due to their ability to spread in healthcare settings and contribute to antibiotic resistance by inactivating carbapenems, one of the most powerful classes of antibiotics. Antibiotic resistance makes these organisms very difficult to treat. CPOs often reside 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 PHSKC began tracking CPO, case counts have remained low, but a steady increase has been observed. In 2020, 15 CPO cases were identified, relative to a five-year average of 12.6 cases. 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 relative to prior years. As described above, measures to prevent COVID-19 transmission, including school closures, may have contributed to a reduction in transmission of other respiratory pathogens. The number of cases of measles peaked in 2019, and returned to baseline in 2020 (typically, there are fewer than 2 cases annually in King County). The number of cases of mumps peaked in 2016 and 2017, owing to large statewide outbreaks, and has been on a decreasing trend thereafter. Notably, the number of pertussis cases in 2020 was 80% lower relative to the 10-year average. A shift in testing methodologies from nasopharyngeal swabs to a greater reliance on antibody testing may have also contributed to the decreased number of pertussis reports in 2020.

PHSKC did not receive any reports of influenza deaths or outbreaks in long-term care or healthcare facilities during the 2020-21 flu season, relative to a 5-year average of 48.2 deaths and 49.2 outbreaks in long-term care facilities. 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 2020.

The number of hepatitis A cases was higher in 2020 relative to the prior 10-year period. A hepatitis A outbreak began in 2019 and peaked in 2020 with 150 confirmed cases, 78% of whom reported unstable housing or drug use. In response to this outbreak, PHSKC administered 2,800 doses of hepatitis A (or combined hepatitis A and B) vaccine to high-risk persons living in or accessing encampments, permanent supportive housing, shelters and day centers, needle exchanges, correctional facilities, and other non-medical service sites. Additional doses were also provided by community partners. Disease investigation, mobilization of vaccination efforts, and strategic coordination with both city and community partners contributed to a decline of cases and return to baseline counts by the close of 2020. Visit our hepatitis A dashboard for the latest 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, with roughly 800 and 1200 newly diagnosed cases reported in 2020, respectively (chronic hepatitis C accounts for 32% of all communicable disease cases reported to King County, and chronic hepatitis B accounts for 21% of all cases). Reports of acute hepatitis B and C infections increased between 2016 and 2019. The number of acute infections dropped in 2020, 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. PHSKC's ability to distribute syringes, screen, and provide linkage to care was restricted during 2020 as resources were diverted to managing the pandemic. In 2020, 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 persons to prevent hepatitis B in their infants by ensuring the infants receive appropriate preventive treatment. In 2020, the program ensured 82% (n=106) of enrolled and eligible infants were tested on time and none of these infants were infected with hepatitis B virus. More information about PHBPP.

The number of reports of animal bites and potential rabies exposures has been on an increasing trend since 2018.

In contrast, relative to the previous 5 years, there was a decrease in reports of malaria, arboviral diseases, and Lyme disease in 2020. As these diseases that are more likely to be acquired through travel outside of Washington State, this decrease is most likely due to travel restrictions in place during the COVID-19 pandemic.

In late 2020, PHSKC investigated two separate Brucella canis infections in dogs brought into Washington State as rescues, one from another country and one from another state. These two dogs may have exposed up to 76 people as well as other dogs through potential inhalation during medical or laboratory procedures or due to contact with infectious material. PHSKC provided guidance and preventive treatment recommendations as indicated to those potentially exposed in laboratories and veterinary clinics. No further transmission to humans or animals was identified.

PHSKC investigated 18 cases of legionellosis in 2020. Because stagnant or standing water in plumbing systems can increase the risk for growth and spread of Legionella, the Centers for Disease Control and Prevention issued guidance in 2020 aimed at minimizing risk of Legionella for buildings and businesses that are reopening after COVID-related closures.