By Dana Hinesly

Each year, hundreds of thousands of Americans suffer from respiratory illnesses brought on by various strains of the influenza virus. As common an occurrence as it may be, the flu is serious business.

“The influenza virus is a significant cause of morbidity and mortality each year and has caused three major pandemics in the 20th century, and there’s growing concern that there could be another global outbreak,” says Jeff Papi, vice president of sales and marketing, Remel Inc in Lenexa, Kan. “Because of the serious nature of this disease and the epidemiology of the influenza virus, there’s a tremendous ongoing effort relative to research and development of new antivirals and vaccines.”

Though often buried in “cold and flu season” discussions, the respiratory implications of influenza viruses are more serious than those from their relatively tame partner. A cold tends to be short-lived and mild, especially when compared to the flu, which claims an average of 36,000 American lives every year.1

The Centers for Disease Control and Prevention (CDC) estimates that each year, between 5% and 20% of US residents contract the flu; and on average, more than 200,000 people are hospitalized for respiratory and heart illnesses associated with influenza virus infections.2

Though many different influenza strains currently circulate worldwide in animals, the flu epidemics suffered annually in humans are attributed exclusively to three different strains of influenza A—influenza A (H1N1), influenza A (H2N2), and influenza A (H3N2)—and influenza B.

Both A and B viruses are known to mutate slowly over time. Called antigenic drift, this evolution requires reformulated vaccines to be created each year. A more rapid change—referred to as antigenic shift—is possible with the two variations of influenza A only.

Testing for Differences
Several types of diagnostic tests currently exist for identifying flu viruses, including serology, polymerase chain reaction (PCR), immunofluorescence assays, and traditional viral cultures. There are varying degrees of specificity and sensitivity based on the individual test employed, which lab performs it, and the type of specimen collected.3

Perhaps the most widely used tests available are rapid antigen tests. Results from this type of test are typically between 70% and 95% sensitive, depending on the population and the test itself.

The trade-off for these decreased levels of accuracy is speed. Results from rapid antigen tests are typically produced in less than 30 minutes. This real-time information enables physicians “to make a rapid and specific diagnosis and prescribe appropriate antiviral treatment for the patient, and to initiate measures to prevent the spread to close contacts,” says Papi.

Test sensitivity and specificity also varies based on what point of the flu season it is when the test is taken. Generally speaking, negative results obtained during the peak of flu season should be double-checked using a molecular assay, which is much more sensitive than a viral culture.3 The same rule of thumb applies for positive results returned during the season’s waning periods.

“Rapid antigen-based diagnostics lose their positive predictive value in the off-season,” says Kelly Henrickson, MD, professor of pediatrics and microbiology, Medical College of Wisconsin, and founder of Prodesse Inc in Waukesha, Wis. “In general, I recommend their use in outpatient medicine only during the respiratory virus season.”

And while rapid antigen-based diagnostics can identify the presence of either influenza A or influenza B, or both viruses together, the only way to differentiate between various subtypes of influenza A are with molecular diagnostics.4 The use of molecular diagnostics like PCR is probably the fastest-growing area for testing for respiratory viruses in hospitalized patients. This methodology is fast and accurate, but it is much more expensive than rapid antigen assays.

To Test Or Not To Test
The CDC generally recommends testing until the presence of influenza in the community is determined, at which point physicians are advised to use individual judgment to determine the necessity of further testing. These initial tests go a long way toward helping the CDC identify and manage incidence levels across the country.

“The CDC is obviously concerned about epidemics,” says Graham Lidgard, senior vice president, research and development, Nanogen (San Diego). “If the flu appears in particular areas, then they can begin managing and monitoring it.”

In addition to work done by the CDC, regional epidemiology performed by local laboratories can help identify patterns of viral activity. Not only does this lend itself to managing epidemics, but these labs can help advise area clinicians about which tests are most appropriate.

“Every community needs a laboratory doing year-round surveillance so that they can tell practitioners when viruses are present,” says Henrickson. She prefers that such labs employ molecular techniques because of their superior sensitivity and specificity. “Once flu is in the community, it’s arguable whether you need to do rapid antigen testing on any specific patient, because those tests are only slightly better than a clinical exam,” she says.

Regular information about regional influenza incidents also increases the cost-efficiency of both testing and treatment.

“If there is a local laboratory letting people know they’re seeing only influenza A in the community, then doing the rapid antigen testing solely to determine if it’s A or B is probably not cost-effective,” says Henrickson. She adds that additional testing may be warranted once multiple strains are detected in the area. “Of course, if you don’t have a local laboratory conducting epidemiology, you may want to do A and B testing all the time, because otherwise you don’t know what you’re dealing with.”

The Importance of Testing
In some patient populations, the importance of testing outweighs competing interests, such as monetary concerns.

“There are evidence-based studies that demonstrate the cost-effectiveness of rapid viral diagnosis for decreasing length of stay in the hospitals, decreasing inappropriate antibiotic use, decreasing other microbiologic testing, and for directing specific antiviral therapy,” Henrickson says.

Speed of results is critical when prescribing antivirals, which must be administered within 2 days after flu symptoms start in order to be effective.4

“Our rapid diagnostic test plays a critical role because it is able to provide an accurate result in as little as 15 minutes. This is a tremendous benefit in the whole diagnostic scheme,” says Papi. “The value of a high-quality rapid test is that it allows the physician to initiate appropriate therapy immediately.”

Fast-acting tests are especially important in areas where infected patients reside in a high-risk population, such as in hospitals or nursing homes, where there is often a need to isolate individuals before the virus is transmitted to other patients.

“In any kind of tertiary care center, an outbreak could be severe, especially if patients are debilitated,” says Papi. “The testing for such facilities will typically be performed at hospitals or clinics using rapid diagnostic methods.”

On the Horizon
Simplified, the goal of most viral testing manufacturers is, “faster and better.”

“The ideal clinical diagnostic would be a point-of-care molecular test that allows you to get a panel of viruses tested within a few hours of seeing the patient,” says Lidgard. “That would be the absolute: ultimate sensitivity with rapid turnaround.”

Achieving this ultimate test seems to be just a matter of time.

“Creating this type of test requires bringing all the pieces together into a reasonable cost-disposable,” says Lidgard. “The technology is certainly there; now it’s a matter of companies spending the money and putting the program together.”

Henrickson concurs that just such a test is likely not far off. “Molecular diagnostics is a very exciting field, and there will continue to be advancements on a steady basis,” he predicts. “Types of testing, speed, reliability, ease of use—it’s all advancing at a very rapid pace.”

A Look at Avian Influenza
More than just a passing sound bite on the nightly news, avian influenza (also known as bird flu) is worthy of the significant attention it’s garnering from medical communities globally.

“It started in ducks and chickens, but concern grew rapidly when the same strain was detected in humans,” says Jeff Papi, vice president of sales and marketing, Remel Inc. “Once that crossover occurred, concern mounted quickly that spread of the virus within the general human population could be devastating.”

A Familiar Threat
Discovered more than a century ago in Italy, this extremely contagious virus is now found in all areas of the world. The influenza virus in general is particularly threatening because of its ability to mutate not only from year to year, but from species to species.

“Usually, diseases get less severe as time goes on, but the flu is different,” says Norman Moore, PhD, director of new technologies, Binax (Scarborough, Me). “There are always new strains being generated, and the flu is one of those unusual viruses that can attack many different species.”

This is exactly what has been happening. In 1997, officials in Hong Kong documented the first case of an avian flu infecting humans.1 The so-called “Hong Kong flu” hospitalized 18 people, killing six of them. It is believed that the Hong Kong virus was transmitted directly between a chicken and humans.

Once the virus is passed from the animal into the human body, a new strain is created. This new strain has the potential to be not only easily transmittable between people, but it can also take the form of a mutation to which most people lack natural immunity.

“If someone gets the flu from a chicken, it can be more severe,” says Moore. “But when that virus can be passed from person to person, it gets scary.”

Such an instance could be similar to previous outbreaks, most notably the influenza pandemic of 1918-19, when a virus often referred to as the Spanish flu ravaged the globe.

Transmitted along trade routes and shipping lines, it was particularly fatal in adults 20 to 40 years of age who are usually best-suited to fight illness. The final death toll estimates range from 20 to 50 million people worldwide, with Americans accounting for more than half a million of the dead.

Since then, smaller pandemics have occurred, one in 1957–’58 and another in 1968–’69. Though more than 35 years have passed since the last incident, experts believe it’s only a matter of time until the next one strikes.

While the timing and exact outcome is impossible to predict, the Centers for Disease Control and Prevention (CDC) estimates that even a medium–level pandemic could cause 89,000 to 207,000 deaths, between 314,000 and 734,000 hospitalizations, 18 to 42 million outpatient visits, and another 20 to 47 million people being sick.2

As of August 2005, the World Health Organization reported the total number of avian influenza cases in humans since December 2003 at 112, with 57 of those cases resulting in death.3 The number includes only laboratory-confirmed cases.

“The avian influenza situation in Southeast Asia is a significant global threat, and people are taking it very seriously,” says Kelly Henrickson, MD, professor of pediatrics and microbiology, Medical College of Wisconsin (Milwaukee). “This virus has a significant potential for becoming a pandemic threat to human beings.”

Organizations such as the CDC, the National Institute of Allergy and Infectious Diseases, and National Institutes of Health are actively pursuing pandemic response and preparedness plans, including the production and clinical testing of vaccines specific to the avian flu.

Testing Plays a Vital Role
Keeping a pandemic at bay is a monumental effort. In the case of the Hong Kong flu, the quick culling of every bird in the region—approximately 1.5 million animals in 3 days—is credited with averting a global outbreak. Rapid and reliable laboratory testing made this quick response possible.

Many existing laboratory tests have the specificity and sensitivity necessary to identify the avian flu, but today’s rapid tests fail to provide the exactness required to comprehensively ascertain the spread of the infection, either bird-to-human or human-to-human transmission.1 The good news is that work is under way to change that.

“There’s considerable effort right now regarding research of avian flu,” says Papi, “in terms of qualifying test kits and ensuring that if there was a global pandemic of avian flu, a high-quality diagnostic test such as ours could detect the strain the same way it detects human strains of influenza virus.”

And fortunately, progress is being made.

“Our panel test can currently detect all influenza, including the bird flu,” says Moore. “We’ve already tested it and sent samples to Taiwan, where it performed well on identifying bird flu patients.”

References:
1. World Health Organization official site; Avian influenza Fact Sheet; “Avian influenza (“bird flu”) and the significance of its transmission to humans”; updated January 15, 2004; Available at: http://www.who.int/mediacentre/factsheets/avian_influenza/en/index.html.   Accessed August 8, 2005.
2. Center for Disease Control and Prevention official site; Avian Flu: Information About Influenza Pandemics; Available at: http://www.cdc.gov/flu/avian/gen-info/pandemics.htm.   Accessed August 8, 2005.
3. World Health Organization official site; Communicable Disease Surveillance and Response (CSR); Diseases covered by CSR; Avian influenza; Confirmed Human Cases of Avian Influenza A(H5N1); Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO; updated August 5, 2005; Available at: http://www.who.int/csr/disease/avian_influenza/country/ cases_table_2005_08_05/en/index.html.  Accessed August 8, 2005.

Dana Hinesly is a contributing writer for Clinical Lab Products.

References
1. Centers for Disease Control and Prevention official site: Flu: Questions & Answers: The Disease; Updated September 22, 2004. Available at: http://www.cdc.gov/flu/about/qa/disease.htm.   Accessed August 8, 2005.
2. Centers for Disease Control and Prevention official site: Flu: Questions and Answers: Influenza-Associated Hospitalizations in the United States; Updated September 22, 2004. Available at: http://www.cdc.gov/flu/about/qa/hospital.htm.   Accessed August 8, 2005.
3. Centers for Disease Control and Prevention official site: Flu: Lab Diagnosis, dated July 13, 2005; text taken from Prevention and Control of Influenza: Recommendations of the Advisory Committee on Immunization Practices (ACIP) (MMWR 29 July 2005;54[RR08]: 1-40). Available at: http://www.cdc.gov/flu/professionals/labdiagnosis.htm.   Accessed August 8, 2005.
4. United States Department of Health and Human Services official site; National Vaccine Program Office; Updated August 4, 2004. Available at: http://www.dhhs.gov/nvpo/pandemics/flu4.htm.   Accessed August 8, 2005.