More chicken soup for the flu

February 2001

Although this year's influenza season is not yet through the halfway mark, it appears so far that we may have lucked out this year. Virtually all of the influenza viruses isolated in the United States during the current season have been influenza A(H1N1) or influenza B, one or the other strain predominating depending on the geographic region of the country; there have been only a handful of influenza A(H3N2) isolates. After four consecutive years of epidemic influenza A(H3N2), we were due for a break!

Both the influenza A(H1N1) and the influenza B isolates are well covered by the strains in the current vaccine. Neither influenza A(H1N1) nor influenza B has historically been associated with significant excess mortality, and no excess mortality has been documented this far in the current season. There have been pockets of high level school and industrial absenteeism, to be sure, but on balance this has been a mild to moderate influenza season.

Vaccine supply

Influenza vaccine by this time is available in good supply. It now seems a bit ironic, in retrospect, that all the pleading from the ACIP to allow the elderly and other high-risk folks to be immunized first (when vaccine was in short supply) was not only unheeded but also probably unnecessary, at least in retrospect. What is clear is that the public confidence in immunization programs has taken another "hit." After years of public education leading to acceptance and even expectation of influenza vaccine, I suspect that overall vaccine use this year will be sharply lower than previous years, and it may take some years of meeting expectations for public confidence in influenza immunization to reach previous levels.

As I alluded to two months ago, another action to be taken as a result of this year's problems is a full investigation of the vaccine distribution system in this country; does it need more control, especially in the event of a threatened influenza pandemic?

Grandma's remedy

Meanwhile, as we in infectious diseases continue to explore new approaches to influenza immunization and new neuraminidase-inhibiting drugs, our brothers and sisters in the discipline of pulmonary diseases were also busy investigating the therapeutic potential of another well-known remedy - chicken soup! In a paper that is best described as a cross between serious science and a complete spoof, Barbara Rennard and her colleagues in the pulmonary and critical care section, Nebraska Medical Center in Omaha showed that chicken soup inhibits neutrophil chemotaxis in vitro (CHEST 2000; 118: 1150-57). After brief discussion of the historical background and possible beneficial effects of chicken soup, these investigators sought to evaluate the possibility that chicken soup had some kind of attenuating effect on the immune response.

They identified and prepared a traditional chicken soup, identified only as "Grandma's soup," and obtained 19 samples at various stages in the preparation process. These samples were then tested for their ability to inhibit neutrophil chemotaxis in a standard assay using a Boyden blindwell chemotaxis chamber, and using two different chemoattractants, zymosan-activated serum, and fMet-Leu-Phe. The results showed a clear and significant inhibition of chemotaxis that was concentration-dependent, and interestingly, seemingly limited to the non-particulate part of the soup. The soup itself was not cytotoxic, hence the inhibition of chemotaxis could not be explained on that basis.

The investigators went on to examine 13 other commercially available chicken soup products, at identical dilutions, and found that there was wide variation in inhibition of chemotaxis. Indeed, five products were more potent inhibitors than "Grandma's soup;" others were less to far less active. The investigators carefully pointed out that Omaha tap water had no activity! Thus, they concluded that their study is evidence that chicken soup has anti-inflammatory activity, although they could not identify the active ingredient(s) responsible for this activity. What happens to the inflammatory/immune response when one actually ingests chicken soup is unknown as this was purely an in vitro study!

Several other issues were raised in the discussion. Some hazards of chicken soup have been reported previously, including anaphylaxis, aspiration, including aspiration of chicken bone, and hypernatremia. None of these seem to be especially surprising, and are based on known mechanisms. If, however, there is an in vivo suppression of neutrophil chemotaxis (which has never been demonstrated), one could theoretically increase the risk of secondary infection. (For whatever it may be worth, the source of financial support for this study was not identified!)

Naturally beneficial

In a delightful accompanying editorial, quaintly entitled "Barbara, what's a nice girl like you doing writing an article like this?" (CHEST 2000; 118: 887-8), Bradley Bender of the University of Florida took a rather light-hearted approach to this work. He wrote that chicken soup, historically, appears to be an ideal remedy for several reasons. First, it is generally consumed sitting down, thus enforcing some rest period from normal activity. Furthermore, if the soup is prepared in a traditional "grandmother" fashion, additional down time is assured. Second, inhaling the warm steam raises the temperature of the airways and assists in loosening secretions. Finally, since the soup contains a large amount of liquid, hydration is well maintained. All these effects are distinctly beneficial, even absent any effect on the host immune response!

Thus, the definitive study of chicken soup remains to be carried out! As Bender points out, one approach would be to measure neutrophil chemotaxis in a group of volunteers in a fasting state, feed them chicken soup, and then repeat the chemotaxis studies. Even better, use experimentally infected subjects in such a study! He concludes: "Volunteers for this study should form a line behind me."

And then - who knows - perhaps there might eventually prove to be a synergistic effect of chicken soup and neuraminidase inhibitors!