The U.S. public health system does a remarkable job of preventing, tracking and controlling infectious diseases. Local, state and federal public health agencies collaborate with private medical providers to monitor disease trends in populations, to detect any novel and emerging threat, and to mount an effective response.
When a new — or newly recognized — microbial disease comes along, these agencies must quickly find answers to several questions, including: What microbe (bacterium, virus, etc.) is causing this disease? How long does it take for a person to develop symptoms after they're exposed? When, during the course of their infection, are they infectious for other people? How is the microbe transmitted: By contact? Through food, water, air or inanimate objects? How long does the microbe remain viable and infectious in the environment? Among all those who are infected, what is the spectrum of disease severity; i.e., what portion will never develop symptoms, how many will have mild-to-moderate disease, and what fraction will have a severe or fatal illness? Who has the greatest risk of severe disease? What treatment options, if any, are available to help infected people and to stop the transmission of the microbe?
To answer these questions, public health professionals use many tools.
Epidemiology provides the science base for public health practice, and it relies heavily on data collection, analysis and statistical inference to identify disease trends and patterns. Every state has a disease surveillance system that gathers ongoing reports from clinicians and laboratories, and these data "roll up" from local health departments to the state public health agency, and onward to the federal Centers for Disease Control and Prevention (CDC).
Analysis of these data can reveal case clusters and outbreaks in need of investigation and control measures. If a new and unusual disease affects a population, epidemiologists will likely be the first to notice.
Another critical tool for controlling infectious diseases is laboratory testing. Medical labs provide much of the data used by epidemiologists to track and monitor diseases. Government public health labs test samples from patients seen in public clinics — many of whom have a high risk for certain infectious diseases. Public health labs also provide specialized testing that's unavailable in private labs and they serve as reference labs for confirming test results, especially for diseases of community health importance.
The current outbreak of coronavirus disease (COVID-19) caused by the novel coronavirus (SARS-CoV-2,) illustrates the crucial role of public health labs in identifying and characterizing newly emerging microbes. During the winter months, dozens of respiratory viruses circulate in human populations, and the diseases they cause are often clinically similar. These include influenza, parainfluenza, rhinoviruses, coronaviruses, respiratory syncytial virus, adenoviruses and others. These viral diseases range from mild colds in the upper respiratory tract to life-threatening pneumonia in the lungs. Without lab testing, it would be impossible to identify — or perhaps even notice — a newly circulating virus, based purely on clinical signs and symptoms, against the background of ongoing seasonal infections.
Fortunately, during about the past 15 years, lab methods for diagnosing viral diseases have improved dramatically. In the past, viruses in patients' respiratory secretions had to be grown in cell cultures or embryonated eggs, and then identified using laborious and expensive methods. Now, because of the molecular biology revolution, labs can quickly look for the presence of the genes of a specific virus in a patient's sample, without having to grow the virus.
Viruses aren't cellular organisms like plants, animals, bacteria, fungi and protozoa — all of which use DNA as their genetic material. Instead, viruses are simply packets of information encoded in either DNA or RNA, surrounded by a protein coat, sometimes with a lipid envelope on the outside.
Viruses, unlike cellular organisms, can't multiply by themselves or grow on culture media in Petri dishes like bacteria. They must invade the cell of an organism, take over the cell's operations, and force it to make a large number of new virus particles instead of its normal cellular components. (The analogy of a computer virus to a biological virus is almost perfect.)
Under the virus's control, the cell makes new viral RNA or DNA, as well as the proteins needed for its coat, and some viruses acquire a surrounding envelope by capturing part of the cell's outside membrane when the newly made virus particles bud through the membrane, as they leave the cell to seek other cells to infect.
So, in the case of the novel coronavirus, SARS-CoV-2, labs can look for segments of RNA in respiratory secretions that, if present, mean that the patient specifically has COVID-19.
This process takes three to four hours, is automated for high analytical throughput, and is so sensitive that it can theoretically detect a single RNA (or DNA) target sequence in a sample. The same method is used for influenza viruses, norovirus and many others, as well as numerous bacteria, such as those causing gonorrhea, chlamydial infection, whooping cough and tuberculosis.
No one can predict the future of the COVID-19 epidemic, and the number of U.S. cases will undoubtedly continue to increase now that lab testing is being applied to more and more patients with respiratory diseases. But the public should be confident that we have a system in place that has responded effectively to many disease threats in the past, and new lab technologies are providing faster, more accurate information for epidemiologists and clinicians.
As microbes move through humans and other animals they can change quickly, and there will always be new infectious agents emerging. Fortunately, our country has a dynamic, professional and robust system that is up to the challenge.
Michael Skeels, PhD, MPH is the retired director of the Oregon State Public Health Laboratory and former administrator of the Oregon Health Division.
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