30 March 2012

New strain blamed for whooping cough epidemic

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An emerging strain of whooping cough has been identified, and is thought to be driving the latest outbreak of infection in Australia.
whooping cough health strain Australia

Australia’s prolonged whooping cough epidemic has entered a worrying new phase, with a recent study showing a new strain or genotype capable of evading the vaccine may be responsible for the sharp rise in the number of cases. Credit: iStockPhoto

LONDON: An emerging strain of whooping cough has been identified, and is thought to be driving the latest outbreak of infection in Australia.

Scientists claim the new variant has significantly increased the incidence and severity of the disease, and has developed due to selection pressures from the current vaccine, resulting in the emergence of new and potentially more dangerous clones.

“The prolonged whooping cough epidemic in Australia that began during 2008 has been predominantly caused by the new genotype,” said Ruiting Lan from the School of Biotechnology and Biomolecular Sciences at the University of New South Wales in Sydney, and lead author of the study recently published in the Journal of Infectious Diseases. “These variants have also been found in other countries, suggesting that they have the potential to cause epidemics elsewhere.”

Threat to public health

In Australia, an ongoing epidemic of the respiratory disease has caused a significant number of deaths, especially among babies under six months old. A national immunisation program has been in place since the 1950s, which has used the acellular vaccine (ACV) since 2000 to combat the disease. However, despite national campaigns ensuring a high vaccine uptake, the disease has recently reemerged as a serious public health threat.

The new findings indicate that there has been a significant increase in variants of the disease with a particular genetic signature. “This genotype was responsible for 31% of cases in the 10 years before the epidemic, and that’s now jumped to 84% – a nearly three-fold increase, indicating it has gained a selective advantage under the current vaccination regime,” said Lan.

Lan’s research team examined samples of whooping cough bacteria collected from sufferers across the country between 2008 and 2010, using genetic analysis techniques to identify specific markers present in each of the 194 cases. By identifying patterns in the genetic fingerprints and comparing those to evidence collected prior to the start of the epidemic, the scientists were able to determine which strains have dominated the latest outbreak.

Protecting the population

The group discovered that the majority of cases were caused by two closely related strains of the disease, carrying a specific combination of two gene alleles, prn2 and ptxP3. Lan argues that this may be due to a mismatch between the ACV vaccine and the new variants, which has allowed the bacteria to sweep across Australia over the last four years.

“The prn2-ptxP3 isolates have the potential not only to evade the protective effects of ACV but also to increase disease severity as a double act of adaptation,” said Lan. “The vaccine is still the best way to reduce transmission of the disease and reduce cases, but it appears to be less effective against the new strain and immunity wanes more rapidly. We need to look at changes to the vaccine itself or increase the number of boosters.”

Commenting on the findings, Mark Walker from the Australian Infectious Diseases Research Centre at the University of Queensland said, “This study will provide useful information in the consideration of vaccine policy in Australia.” However, he does not feel the evidence warrants a change in the national immunisation strategy. “At this stage, whooping cough is still well below the levels seen prior to the introduction of whooping cough vaccination, and thus the vaccine program must continue to protect the population,” he said.

Increase in diagnosis

Peter McIntyre, director of the National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases at Westmead in Sydney, disagreed that Lan’s findings are evidence of increased incidence and severity of the disease.

He argued that the increase in notified cases among children is due to improvements in diagnosis techniques, and that there is currently no evidence to suggest the vaccine is less effective against newer strains of the disease. However, he has begun a National Health and Medical Research Council-funded research project in collaboration with Lan to address these questions.

Walker also highlighted the importance of further research on the disease. “Analysis of the epidemiology of whooping cough isolates from across the nation needs to be undertaken before discussing any possible change in vaccine strategy and formulation,” he said.

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