LONDON (Reuters Health) - A public health doctor has uncovered what he believes may be evidence of a mutant mumps virus circulating in Britain.

Dr. Brendan Crowley, from Liverpool Public Health Laboratory, said two recent unusual cases of mumps infection raise the possibility that a new strain has developed that may be resistant to the current mumps vaccine.

In a report in the journal Communicable Disease and Public Health, Crowley suggests the emergence of mutant forms of the virus could account for some of the recent resurgence in mumps infection in the UK. He also believes it's possible that, after years of exposure to the strain used to make mumps vaccines, this mutant form may have learned to evade detection by the body's immune system.

The report describes the case of a mother and son who both developed the infection, even though earlier tests confirmed she was immune and her 11-year-old child had already been vaccinated or infected.

"I'm just putting this idea forward because there was something very different about the strain involved," he told Reuters Health in an interview. "We need to ask whether it is possible it could be a mutant strain."

Mumps was once a common infection in Britain and is characterised by swelling of the lymph nodes on the side of the neck, making the face look round and swollen.

In many cases, the infection is mild but it can also cause fever, headache and muscle pains. In the worst instances, it can lead to deafness and inflammation of the testicles, ovaries or pancreas.

Before vaccinations were introduced, it was also one of the leading causes of life-threatening viral meningitis in children.

Over the last 18 months, there have been a number of mumps outbreaks, mainly involving teenagers who missed out on the measles, mumps, rubella vaccine when it was introduced in 1988 or failed to get enough doses of single mumps vaccine to develop immunity.

The latest figures from the Public Health Laboratory Service show there were 131 cases of mumps in England and Wales in the first quarter of 1992, the last period for which figures are available, compared to 115 in the same quarter of 2001.

Around 70% of the cases involved teenagers born between 1983 and 1990.

Crowley raised the alarm after discovering "an unusual antibody response" in the mother of the infected child.

The 11-year-old boy had been admitted to hospital, where doctors confirmed mumps infection and discharged him after a short course of treatment. Ten days later his 44-year-old mother developed facial swelling, which her doctor confirmed was also mumps.

Blood tests showed the boy had either been vaccinated or infected already, which should have meant he had some protection against reinfection.

Meanwhile, the mother, a healthcare worker, had given a blood sample a year earlier that showed she too was carrying mumps antibodies, probably as a result of earlier infection.

"I was really surprised because when we tested the mother's first blood sample for antibodies it was positive," Crowley told Reuters Health. "She should have been immune to mumps but she still developed the symptoms."

Further tests also showed her immune system responded completely differently to the virus than her son's--even though they were infected with the same strain. She produced high levels of antibodies to a strain of mumps known as the G genotype--which is not included in the current vaccine.

"It may simply reflect genetics because people vary in their response to viral infections. But it's also possible there's something very different about this strain," Crowley said. "The G genotype is one that's only appeared in the last seven or eight years and it's certainly not in the vaccine strain."

Dr. Mary Ramsay, consultant epidemiologist at the Public Health Laboratory Service, said there was no evidence that mutant mumps strains were responsible for the rise in cases. However, she said it is possible there are mutant strains circulating that are not particularly virulent and cause very few infections.

"There will always be some element of mutation in a virus. What matters is whether it persists in the general population and that does not seem to be a problem with this infection."

SOURCE: Communicable Disease and Public Health 2002;5:311-313.

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