New Swedish chlamydia strain ‘possible emerging threat to public health’

A European-wide investigation into a possible outbreak of a new, often undetected variant of Chlamydia trachomatis – first seen in Sweden last year – has found that so far the infection has only spread to a handful of individuals elsewhere in Europe, according to several reports in the October 2007 issue of Eurosurveillance monthly. However, an accompanying editorial warns, “it is of public health importance to assess the risk of possibly widespread undetected chlamydial infections in Europe” even if it “puts an extra burden on chlamydia control programmes in many countries that already have to face continuous increasing trends.”

Diagnosis is the key

Chlamydia – caused by bacteria called Chlamydia trachomatis – is the most frequently reported sexually transmitted infection (STI) both in the UK, and throughout Europe.

Although symptoms of chlamydia normally occur one to three weeks after infection, many people are unaware that they have the infection; as many as 75% of women with chlamydia, and 50% of men with chlamydia are asymptomatic.

Undiagnosed and untreated chlamydia can lead to pelvic inflammatory disease (PID) in women, which can cause ectopic pregnancy, infertility, and even death in extreme cases. Men are less likely to develop serious complications, although undiagnosed and untreated chlamydia may cause infertility. Both men and women may develop arthritis as a consequence of undiagnosed and untreated chlamydia.

However, if diagnosed in a timely fashion, chlamydia is easily treated with a single dose of antibiotics.

Outbreak of new Swedish variant due to diagnostic failure

In 2006, a new strain of chlamydia was discovered in Sweden. It is called either the Swedish CT variant (swCT variant) or new variant of Chlamydia trachomatis (nvCT).

This new strain has a mutation (a 377 base pair deletion in the cryptic plasmid) in the exact region targeted by three chlamydia PCR diagnostic assays, two manufactured by Roche and one by Abbott. Until recently, these assays were utilised by chlamydia screening laboratories in 13 of the 21 counties in Sweden.

Consequently, anyone infected with this strain would have been given a false negative result by a lab that used either of these assays. This resulted in an outbreak of undiagnosed, untreated chlamydia in parts of Sweden.

By March 2007, all labs in all parts of Sweden had switched to a different brand of diagnostic assay that is able to recognise the new strain.

The Swedish Institute for Infectious Disease Control reports that chlamydia diagnoses in Sweden increased by 38% for the first two months of 2007 compared with the first two months of 2006. Further investigation, however, suggests that this was only partially due to the increased prevalence of chlamydia due to the lack of diagnosis of the new chlamydia strain (and which accounted for between 10% to 65% of all diagnoses, depending on county).

It is also possible, suggests the report, that more active testing and the underestimated rates for 2006 (due to lack of correct diagnosis) are also likely to have contributed to the large increase in diagnoses in Sweden in January and February 2007.

What does this mean for the rest of Europe?

With the increase in international travel, particularly among young sexually active adults, it was anticipated that the new Swedish strain would spread throughout Europe, and take hold in countries that used the three diagnostic assays unable to detect it.

However, investigations by the European network for the surveillance of STI (ESSTI) and the European Centre for Disease Prevention and Control (ECDC) suggest that, with a few exceptions, the new variant has so far been confined to Sweden.

Despite increased vigilance and testing so far only four other European countries – Denmark, France, Ireland and Norway – have far reported cases, involving one or two individuals in each country. Although most of the identified cases appear to be epidemiologically linked with Sweden, epidemiological characteristics were not available for all cases.

The ESSTI and ECDC investigators also found that Malta and Iceland were the only countries that used the Roche or Abbott tests exclusively, although there was widespread use of these test in individual laboratories in other countries.

An international workshop on sexual networks and the chlamydia epidemic held earlier this year in Sweden discussed why the spread of the new Swedish variant appeared to be “surprisingly limited” given that “while every fourth [chlamydia] case in Malmö, Sweden, is swCT, only a single case of swCT has so far been detected in neighbouring Copenhagen, Denmark, although the two cities are connected by a bridge and share a large number of commuters.”

The workshop report suggests that “this phenomenon might be explained by the fact that the [diagnostic] test system by Becton Dickinson which is able to detect swCT, dominates in Denmark. So the swCT was not selected for in Denmark, whereas Sweden, by not treating the undetected cases, selected for swCT.”

The workshop report also notes that gay men, “who tend to have more international contacts and could have spread the infection abroad, did not contribute to the spread of this particular variant because swCT belongs to genotype E which is rare in [gay men] in Sweden.”

Lessons to be learned

An editorial by Marita van de Laar of the European Centre for Disease Prevention and Control and Servaas Morré of VU University Medical Centre, Amsterdam, suggests that “the collaboration and rapid reaction of the STI community to this possible emerging threat to public health can serve as a good example. The sharing of information facilitates action and inventing solutions of the problem.”

This should mean, they argue, that “news of another [swCT] discovery will travel fast.”

They write that “several diagnostic lessons can be learned” from the Swedish outbreak. “Developing diagnostic assays based on essential genes only will reduce the chance of diagnostically escaped new CT variants,” they note. They also recommend using dual target tests, which “could also circumvent the problem of missing new variants.”

They conclude that “it is of public health importance to assess the risk of possibly widespread undetected chlamydial infections in Europe” and warn that “the detection of this swCT variant puts an extra burden on chlamydia control programmes in many countries that already have to face continuous increasing trends.”

The authors of the ESSTI and ECDC Europe-wide investigation say that “experts in all EU Member States should remain vigilant.”

They conclude by noting that “it is too early to tell whether the variant will remain confined to Sweden or whether the number of cases will significantly increase. Enhanced surveillance will need to be continued to address these concerns. ESSTI and ECDC aim to repeat the survey at the end of the year to determine if the picture across Europe remains the same.”