A reader (hat tip River) sent me a link to a New York Times piece quoting a physician who recently saw swine flu cases in Mexico City. He called attention to what seemed like an anomalous clinical presentation of many cases. Besides a higher proportion of gastrointestinal symptoms (nausea, vomiting, diarrhea), Virginia Commonwealth infectious disease specialist Dr. Richard P. Wenzel was surprised that many cases, even severely ill ones, did not have fever:
Many people suffering from swine influenza, even those who are severely ill, do not have fever, an odd feature of the new virus that could increase the difficulty of controlling the epidemic, said a leading American infectious-disease expert who examined cases in Mexico last week.
Fever is a hallmark of influenza, often rising abruptly to 104 degrees at the onset of illness. Because many infectious-disease experts consider fever the most important sign of the disease, the presence of fever is a critical part of screening patients.
But about a third of the patients at two hospitals in Mexico City where the American expert, Dr. Richard P. Wenzel, consulted for four days last week had no fever when screened, he said. (Lawrence Altman, New York Times)
High fever is indeed a characteristic finding of influenza illness, but many cases of seasonal flu are asymptomatic. After reading this I did a quick search of the literature to see what proportion it is. One of my virology textbooks says as many as half of flu infections are asymptomatic, while a recent systematic review of the literature of 56 different volunteer challenge studies estimates it to be about a third. In the same review, the frequency of fever in influenza A infections was about 40%, not too different from Dr. Wenzel’s clinical impression. Since subjects in volunteer challenge studies are mainly young, healthy adults, this comparison is probably appropriate. The authors of the review conclude:
Pessimistically, viral shedding peaked rapidly, infections were rarely “typical,” and symptoms or signs widely used for influenza case definitions (e.g., fever or cough) would be unreliable for identifying infectious individuals. (Carrat et al.,”Time Lines of Infection and Disease in Human Influenza: A Review of Volunteer Challenge Studies,” American Journal of Epidemiology 2008 167(7):775-785; doi:10.1093/aje/kwm375)
In other words, the absence of fever might not be particularly unusual. On the other hand, the relatively high prevalence of gastrointestinal symptoms is a worry:
Also, about 12 percent of patients at the two Mexican hospitals had severe diarrhea in addition to respiratory symptoms like coughing and breathing difficulty, said Dr. Wenzel, who is also a former president of the International Society for Infectious Diseases. He said many such patients had six bowel movements a day for three days.
Dr. Wenzel said he had urged his Mexican colleagues to test the stools for the presence of the swine virus, named A(H1N1). ?If the A(H1N1) virus goes from person to person and there is virus in the stool, infection control will be much more difficult,? particularly if it spreads in poor countries, he said. (NYT)
Gastrointestinal symptoms are also seen in H5N1 patients. The question whether there is intestinal infection and carriage is of importance. We know little about the distribution of appropriate viral receptors in tissues outside the respiratory tract, and some data suggests that flu virus can successfully make the passage through the acid environment of the upper g.i. tract. Is ingestion a possible route for influenza infection? Conventional wisdom says, “no.”
But influenza is the surprise that keeps surprising.