We're seeing a lot of new viruses lately, but that's nothing new.
The novel pandemic H1N1 flu is just one of many emerging viruses that we're seeing clinically, although we may not always recognize them. Metapneumovirus, bocavirus, and norovirus are three others. But new viruses have been emerging since time began. One of my favorite books, Jared Diamond's “Guns, Germs, and Steel: The Fates of Human Societies” (New York: W.W. Norton & Co., 1997), describes how the Europeans who conquered the New World were aided in large part by the diseases they brought with them to a vulnerable population, a weapon at least as successful as those designed for warfare.
Diamond, a geography professor at the University of California, Los Angeles, who won a Pulitzer prize for his book, also points out that from the beginning of time, humans have acquired mutated germs from animals, resulting in disease of varying severity. The Europeans conquered by spreading new disease.
Of course, the current pandemic influenza A(H1N1) strain that we're dealing with now didn't come from human conquerors, but it did come from animals—more than one type, in fact. The virus was originally referred to as swine flu because laboratory testing showed that many of its genes were similar to those of influenza viruses that normally occur in pigs in North America.
However, now it is clear that this new virus is different from that which normally circulates in North American pigs, and actually includes genes from influenza viruses that normally circulate in pigs in Europe and Asia, along with avian genes and human genes, according to the Centers for Disease Control and Prevention.
Although this influenza strain surprised us in a couple of ways—it didn't come from birds and it isn't as virulent as we would have expected from a genetically “shifted” virus—the fact that a novel strain has arisen and is being transmitted from human to human is not a surprise.
Clinically, we are hoping that we have a safe and effective vaccine against the new H1N1 strain and that the supply will be sufficient to allow us to vaccinate all of our patients in a timely manner. In the meantime, the CDC's Advisory Committee on Immunization Practices has drafted new recommendations for the use of antivirals in the upcoming influenza season.
A second emerging virus, human metapneumovirus, was first isolated just 8 years ago, in previously virus-negative nasopharyngeal aspirates from children with respiratory tract infections. Since then, it has been seen worldwide, mainly circulating during the winter and spring. It is closely related to respiratory syncytial virus (RSV), and its clinical appearance resembles that of RSV in many ways, ranging from mild upper respiratory tract infections to wheezing to bronchiolitis, particularly in children less than 1 year of age. Metapneumovirus is generally milder than RSV, although the two infections often occur together.
The two infections are also essentially managed the same way—supportively, or with oxygen if the child becomes hypoxic. But this approach is far less likely with metapneumovirus than with RSV.
In a child with a clinical picture suggesting viral bronchiolitis in the hospital setting, a rapid test for RSV can help to determine whether the child can room with another child who also has RSV. If the test is negative, assume that you're dealing with metapneumovirus alone, and keep the child away from RSV-infected children. In the ambulatory setting, such testing is unlikely to be helpful.
Be aware that like RSV, metapneumovirus can also exacerbate asthma symptoms.
Bocavirus, another newly identified viral pathogen, is closely related to the parvovirus that pediatricians know as the cause of Fifth disease. Clinically, bocavirus is another RSV mimic. Children often present with wheezing in the context of an upper respiratory infection, which can easily be mistaken for asthma. In terms of severity, it probably ranks about the same as metapneumovirus.
Finally, norovirus is an emerging gastrointestinal virus that's been in the news a lot in recent years as the cause of gastritis on cruise ships. Symptoms include diarrhea, abdominal pain, and vomiting. In young children, it's fast surpassing rotavirus as the most common cause of this clinical picture, now that rotavirus vaccination is routine. Like rotavirus, norovirus is highly contagious. It may be transmitted through food, and is the likely culprit when more than one family member is affected. On the bright side, the course of illness for norovirus is shorter than that of rotavirus. Symptoms are usually gone after 1–2 days, as opposed to 5–7 days for rotavirus.