DEAR MAYO CLINIC: My 3-month old has just gotten over RSV, and our pediatrician says we now need to watch for asthma as she gets older. How are these two conditions related?

ANSWER: Although the two seem connected, the nature of the relationship between respiratory syncytial virus (RSV) and asthma is unclear. Some experts believe that a bout of severe RSV may predispose a child to developing asthma in the future. Other researchers claim that many children who develop more severe forms of RSV actually have asthma already but that it simply hasn't been diagnosed yet.

RSV is a common virus that causes infections of the lungs and respiratory tract. Research has shown that nearly everyone contracts RSV at some point, and some people get it more than once. Most identified cases of RSV infections take place during the first two years of life.

In adults and older, healthy children, RSV usually causes mild, coldlike signs and symptoms, including a congested or runny nose, dry cough, low-grade fever, sore throat, mild headache and a general feeling of malaise. But RSV can become severe in some cases -- especially in premature babies and infants with underlying health conditions -- and lead to a lower respiratory tract illness, such as inflammation of the lungs (pneumonia) or inflammation of the small airway passages entering the lungs (bronchiolitis).

Pneumonia and bronchiolitis can often be treated in a doctor's office, but these conditions sometimes become quite serious in infants, young children, individuals whose immune systems are compromised, or people with chronic heart or lung disease. If pneumonia or bronchiolitis becomes serious, hospitalization may be required so doctors can monitor and treat breathing problems and give intravenous fluids.

Bronchiolitis usually lasts from a few days to a few weeks. Symptoms typically include a severe cough, wheezing and difficulty breathing that may resemble an asthma attack. In some cases, bronchiolitis can be effectively treated with anti-asthma medications. This makes it very hard to tell if a child with RSV has bronchiolitis or if that child may actually be suffering from an asthmatic reaction to RSV.

Although the exact link between the two conditions isn't known at this time, it appears that many children who are diagnosed with asthma have previously been diagnosed with bronchiolitis due to RSV. With that in mind, it seems wise to follow the advice of your daughter's pediatrician and watch her for signs of asthma as she grows. In early childhood, those signs include a persistent nighttime cough, wheezing, or shortness of breath with colds.

To reduce RSV and its sometimes serious complications, a vaccine against the virus is needed. But attempts to develop an RSV vaccine have failed so far. The medication palivizumab (Synagis) can help protect children under age 2 who are at high risk of serious complications when they get RSV, such as those born with extreme prematurity or with severe congenital heart or lung disease. This treatment has been shown to decrease the frequency and length of hospitalization for RSV infections.

But the high cost of palivizumab, combined with its failure to prevent death from RSV, limits its usefulness to those at highest risk of complications from RSV infections. Also, the medication isn't helpful in treating RSV once it has developed. Scientists continue to work on finding a vaccine against RSV, not only in infants but also in older children and high-risk adults.

-- Robert M. Jacobson, M.D., Pediatric and Adolescent Medicine, Mayo Clinic, Rochester, Minn.

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