Infectious Disease Investigation
in Travis County
"Health care is vital to all of us some of the time, but public health is vital to us all of the time" - C. Everett Koop
Introduction
Introduction
Disease surveillance is a critical component of public health. The Epidemiology and Disease Surveillance Unit (EDSU) within Austin Public Health monitors all notifiable diseases and conditions reported in Travis County excluding sexually-transmitted diseases, HIV/AIDS, perinatal Hepatitis B, tuberculosis, lead, controlled substance overdose, pesticide poisoning, and prion diseases. Additional EDSU core functions include:
- Monitor disease trends over time
- Detect disease outbreaks
- Gather information about risk factors that contribute to disease
- Collaborate with health providers and the community to prevent the spread of infection
Behind the Scenes
Under Texas law, healthcare providers and laboratories are required to report cases of nearly 80 diseases and conditions to their local public health agency. Any person suspected of having a notifiable condition is considered a case until it is ruled out. Timely reporting allows agencies to respond to potential disease outbreaks and mitigate transmission of disease.
After receiving a report, EDSU staff will:
- Conduct a Case Investigation – This may include interviewing the case to collect additional information, consulting with the healthcare provider, reviewing medical records, and collecting additional specimens for testing.
- Determine Case Status - Cases are classified as either "Confirmed", "Probable", "Suspect", or "Not a Case" depending on the case definition. Cases classified as"Not a Case" are considered not to represent true episodes of the disease.
- Provide Infection Control Guidance - This may include providing education on disease control to reduce the spread of disease and preventively treating family members and other close contacts.
- Report Case Data - Case data is entered into a state database. This data is used for a wide variety of purposes, including analyzing epidemiological trends, identifying high risk groups, allocating resources, developing policies, and designing prevention programs.
Public Health Reporting
Local public health department reporting to the state and CDC generally reflects cases that are considered confirmed or probable. However, these numbers only represent a portion of the total number of cases investigated. By examining all cases, regardless of case status, a better understanding is gained of the actual work involved in conducting disease investigations.
The following are a series of charts visualizing EDSU investigation data between 2013 and 2018. Drop-down menus (when available) located below the chart titles may be used to filter the data by year.
Investigations By Year
Every case reported to EDSU requires an investigation by staff, which may range from a simple review of laboratory results to a more in-depth interview with the case and/or healthcare provider. The number of cases that EDSU staff investigate each year in Travis County has remained relatively steady, with a significant drop in cases from 2017 to 2018. The highest number of investigations occurred in 2017 (2,409) and the lowest number of investigations occurred in 2013 (1,907).
In the following chart, "MMWR Year" corresponds with the calendar year in which the case is first entered and counted for MMWR (Morbidity and Mortality Weekly Report) publication.
Duration of Investigation by Year
Duration of investigation was calculated by subtracting the investigation start date from the first date that notification was sent to the state. The average duration of investigation increased each year from 2013 to 2015 and has been decreasing each year since 2015. 2018 had the shortest average investigation duration of 6.56 days.
Duration of investigation varies considerably when stratified by case status. For example, in 2018, cases that were ultimately classified as "Not a Case" averaged 11.5 investigation days, whereas confirmed cases averaged just 6.36 investigation days. A case might be classified as suspected or probable while waiting for the laboratory results to become available. Once the laboratory provides the report, the case can be reclassified as either "confirmed" or “not a case,” depending on the results.
Average Duration of Investigation by Week
On average, the longest duration of investigation (21.9 days) occurs in MMWR Week 25 of the year, which typically falls in the middle of June. The shortest duration of investigation (6.9 days) occurs in MMWR Week 13, which typically falls in the end of March or beginning of April.
"MMWR Week" corresponds with the week of the calendar year in which the case is first entered and counted for MMWR (Morbidity and Mortality Weekly Report) publication. MMWR weeks do not fall on the same dates every year and may range from 1 to 53 depending on how the days of the week fall across years.
Investigations by Case Status
Case status is determined by case definition. A case definition is condition-specific and consists of clinical criteria which usually includes laboratory confirmation. This is not always necessary, however; some diseases have no distinctive laboratory findings.
39% of all cases reported between 2013 and 2018 were ruled out and considered "Not a Case". Case status varies widely by condition. For example, 96% of all reported streptococcus pneumoniae invasive disease cases were confirmed, while only 25% of pertussis cases were classified as confirmed or probable after investigation.
Investigations by Program Area
52% of all cases reported between 2013 and 2018 were related to immunizations, more than the other 8 program areas combined. Top conditions in this program area include:
- Hepatitis B
- Mumps
- Pertussis
- Streptococcus pneumoniae, invasive disease (IPD)
- Varicella
Foodborne is the second most prevalent program area. Top conditions in this program area include:
- Cyclosporiasis
- Salmonellosis
- Shiga toxin-producing E. coli
- Vibriosis
Waterborne is the third most prevalent program area. Top conditions in this program area include:
- Amebiasis
- Cryptosporidiosis
- Shigellosis
Top 10 Conditions Investigated
Between 2013 and 2018, Pertussis was the most frequently investigated condition. However, only 25.5% of all Pertussis cases reported to EDSU met confirmed or probable case definition. 74.5% of all Pertussis cases were determined not to meet case definition.
Conclusions: Process Improvement
Analyzing disease investigation trends will enable EDSU to improve work processes and utilize resources more efficiently. For example, in 2017, data visualizations showed that cases classified as "Not a Case" increased by more than 50%. Upon further examination, staff discovered that investigations of Pertussis had doubled from the previous year due to new lab testing processes at a local hospital. This change led to unnecessary investigation of cases with negative lab results. EDSU staff worked with the hospital to narrow their reporting criteria by only reporting cases who had targeted testing done. This freed up staff time to focus on investigating more important cases.
Tip of the Iceberg
The investigation of cases at the local level extends beyond what is reported to the state and CDC. While it is important to accurately report true episodes of disease, it is also important to understand what is not reported to the public. EDSU staff must investigate every report of a notifiable condition. Frequently, they still have to interview the case, consult with the healthcare provider, and review medical records and test results. In some cases, the investigation for cases that were eventually ruled out lasted longer than for cases that were confirmed. By continuing to look at disease investigation trends, EDSU will be better able to anticipate workload and make best use of staff resources.
Looking at all cases investigated builds a more complete picture of local disease surveillance and investigation, a vital part of keeping Travis County a safe and healthy community.