The influenza season is only a memory now, but the threat of other respiratory viruses is far from over. Although cases of severe acute respiratory syndrome (SARS) and human avian influenza early this year have been limited to the Far East, SARS spread rapidly to many U.S. cities last year, indicating the need to be aware of global health threats.
In 2003, 8,422 cases of SARS and 908 deaths occurred in 29 countries, according to the World Health Organization. WHO announced last July that the last chain of human-to-human SARS transmission had been broken, but as of Jan. 31, 2004, four more SARS cases were identified in Guangdong, China, the source of last year's epidemic (www.who.int/gb/EB_WHA/PDF/EB113/eeb11333.pdf).
The etiology of these other cases is unclear, according to WHO and the Centers for Disease Control and Prevention. In one case, a worker and palm civet in the same restaurant were infected, but the strains were not identical and the direction of transmission was unclear. The most recent case was in a physician, raising the specter of occupational infection (Emerg Infect Dis February 2004 www.cdc.gov/ncidod/EID/vol10no2/03-1032.htm).
SARS is caused by a corona virus. It is spread by respiratory droplets and contact with fomites, feces, and other body fluids. Disease in children is mild, while mortality rates are highest in the elderly and those with chronic disease. Patients are most infectious at day 10 of illness (when they are likely to be hospitalized, giving rise to nosocomial transmission). The incubation period for SARS is from two to 10 days, and the disease origin is unclear, although similar viruses have been found in cats, ferrets, chickens, dogs, and pigs (Emerg Infect Dis February 2004).
In early illness, fever, chills, rigors, myalgias, diarrhea, sore throat, or rhinorrhea occur (Associated Press, Thailand warns of second bird flu onslaught, Feb. 9, 2004, http://story.news.yahoo.com/news?tmpl=story&u=/ap/20040209/ap_on_he_me/bird_flu_3). Lower respiratory symptoms such as non-productive cough and shortness of breath begin on days two to seven. Productive cough or gastrointestinal symptoms occur in about 25 percent of cases, according to the AP news story.
Symptoms of SARS
Symptoms are atypical in the elderly and in those with chronic disease. Upon admission to the hospital, 40 percent to 75 percent of patients exhibit tachypnea, tachycardia, or hypoxia, although these rates are surely lower in those evaluated earlier in their course. Lung ausculatory findings are variable, and may not be present. Leukocytes and platelet counts are normal to low, while the activated partial thromboplastin time is elevated in about 50 percent. Most patients will have an abnormal chest x-ray at some point in their illness, about 75 percent by day three and almost 100 percent by day seven. First, isolated focal lesions in the peripheral lungs are noted, which progress to involve multiple lobes.
These findings are obviously similar to a number of other respiratory diseases, making epidemiologic features critical to identification. In patients with these findings, a history of travel to China, Hong Kong, or Taiwan, employment in an occupation associated with a risk of SARS exposure, exposure to a SARS patient, or being part of a pneumonia outbreak must be elicited (MMWR 2003:52[49]:1202).
If a provisional diagnosis of SARS is made based on epidemiologic and clinical features, the patient must be isolated (contact and airborne), tested for SARS-CoV by serology, and have other potential diagnoses excluded. RNA detection though RT-PCR is 32% sensitive at time of initial illness and 68% sensitive at day 14. Real-time testing for antibodies through PCR has a significant false positive rate and can't be used if there is a low SARS risk (for screening). Serology is positive starting at days 10 to 14 and is 93% sensitive at day 28 (Emerg Infect Dis February 2004). For U.S. patients suspected of having SARS in 2003, the most common alternate diagnoses were picorna viruses (enterovirus/rhinovirus), influenza, and M. pneumoniae(Emerg Infect Dis March-July 2003 www.cdc.gov/ncidod/EID/vol10no2/03-0752.htm). SARS is now a reportable disease, and there is no specific treatment for it.
Avian Influenza
Between mid-December 2003 and this February in Vietnam, 19 cases of human avian influenza have been reported, with 13 deaths, according to an Associated Press report, which said up to 23 cases, with at least five deaths, are suspected in Thailand. Human illness also may have occurred in China. Type A influenza infects many animal species, but birds are particularly important, as are all types of influenza A circulate in wild birds.
Influenza viruses are typed into either class A or B. Within type A, subtypes are determined by surface proteins, hemagluttinin (HA), and neuroaminidase (NA). There are 15 HA subtypes. The current avian influenza is caused by the H5N1 influenza subtype. Normally, only influenza with three types of HA (H1, H2, and H3) and two types of NA (N1, N2) circulate widely in humans. Avian influenza has been transmitted directly from birds to humans, and person-to-person transmission is very rare.
The major concern with avian influenza is its ability to create a pandemic. All influenza viruses have the potential to mutate. Avian and human influenza virus can exchange genes when a person is infected by human and avian strains simultaneously. This gene swapping could result in a new virus strain with potential for easy human-to-human spread, according to WHO. Because avian influenza is highly pathogenic, because almost no immune protection against it exists in humans, and because current vaccines would be ineffective, it could be a public health disaster in the making. It is thought that the 1918 influenza pandemic, which killed 20 to 50 million people worldwide originated in birds, according to the London Times (Feb. 6, 2004).
Symptoms of avian influenza in humans can range from typical influenza-like symptoms to conjunctivitis to more severe respiratory complications such as pneumonia and acute respiratory distress. Cambodia, China, Indonesia, Japan, Laos, Pakistan, South Korea, Taiwan, Thailand, and Vietnam have battled bird flu in recent weeks, and officials in the United States confirmed an outbreak of H7 virus in Delaware on Feb. 6. In Delaware, 12,000 chickens were destroyed (The Guardian Feb. 6, 2004, www.guardian.co.uk/international/story/0,3604,1143758,00.html).
Five previous instances of avian influenza viruses in humans have been confirmed by the CDC (www.cdc.gov/flu/avian/facts.htm). The earliest was in 1997 in Hong Kong when 18 patients with the H5 N1 strain were hospitalized. Six died, and infection was confirmed in chickens. H9N2 was found in Hong Kong in 1999 in two children, both of whom recovered. Poultry were suggested as the infection source. In 2003, 80 cases of human H7N7 were found in poultry in the Netherlands. That year, two human cases of H5N1 were identified in China (one death), and one non-fatal case of H9N2 was confirmed in Hong Kong.
Although SARS and avian influenza appear to be isolated from the U.S. by geography and species, we know from previous experience that we live in an age of rapidly spreading global infections, some of those from zoonotic sources. An awareness of these diseases and their current impact is important. Up-to-date information can be obtained from the World Health Organization (www.who.org) and the Centers for Disease Control and Prevention (www.cdc.gov).
© 2004 Lippincott Williams & Wilkins, Inc.