Upsurge of Hepatitis A in the Netherlands early 1998

Far more cases of hepatitis A were identified in the Netherlands in the first five months of 1998 than in the same period of 1997.(1) In order to investigate the reasons for this upsurge we compared the notified cases of hepatitis A in the first five months of 1998 with those seen in the same months of the preceding five years in terms of their age, sex, nationality, and suspected country of infection.

The highest cases of hepatitis A for the decade

Hepatitis A has been a notifiable disease in the Netherlands since 1951. Cases are reported by physicians and municipal health services. The case definition comprises clinical cases either confirmed serologically (hepatitis A IgM) or linked epidemiologically to a laboratory confirmed case (2). Data concerning age, sex, residence, date of notification and, since 1993 the most likely country of contraction, nationality, date of diagnosis, and date of onset of clinical signs are registered. The extent of under-reporting is unknown but may be as high as 70% (3). There is no evidence, however, that under-reporting has changed over time so that the notification data can therefore be used to study trends in the incidence of hepatitis A.

More than twice as many cases (669) of hepatitis A were notified from January to May 1998 in the Netherlands as in the corresponding period in 1997 (313 cases). This was the highest total since 1988 (594 cases).

Notifications of hepatitis A from January 1993 to 1997 have shown that a peak in October is followed by a gradual decline broken by a second and third peak until the following autumn. The 1998 upsurge appeared to be a large, prolonged, secondary peak that followed a relatively high autumn peak in 1997. It has been shown that all autumn peaks in the years 1993 to 1997 were related to infections contracted originally in foreign countries, notably Morocco and Turkey (4). The 1997-98 wave was also preceded by a large peak of infections contracted in Turkey and Morocco.

In the first five months of 1998 most infections appear to have been contracted in the Netherlands (83.7%) mainly by Dutch nationals of all ages (more men than women) and children (under 15 years of age) of non-Dutch nationality. A small proportion of infections were contracted in Morocco or Turkey (9.2%). The same subgroups were also over-represented in the preceding year. When the same comparison is done by region in the Netherlands, the regions substantially contributing to the 1998 upsurge were mainly the Hague, Rotterdam, and Amsterdam. Fewer cases were notified from the Utrecht region in 1998 than in earlier years.

Increased transmission along the regular routes

As during the preceding years, the recent upsurge of infections contracted in the Netherlands, might therefore be the result of secondary transmission from a relatively high 1997 autumn peak. This assumption is supported by the observed distribution of cases among subgroups maintained, which is similar to that seen in the January to May of the preceding years. Moreover, the upsurge appears not to have been concentrated in one particular geographical area.

The present upsurge cannot be ascribed to a local explosion in a particular population group (for instance young children in a day care centre, homosexual men, elderly people) but suggests increased transmission along the regular routes. The consumption of food or water (shellfish, raw vegetables, or fruit) contaminated with hepatitis A virus is an implausible explanation for the observed upsurge, because we would then have expected all ages to have been affected equally. Furthermore, contaminated food or water is an uncommon route of transmission for hepatitis A in the Netherlands (3). There is no evidence for a changing reporting pattern, but the possibility of notification bias cannot be ruled out.

The observed increase in notifications early in 1998 indicates the potential risk of epidemic peaks of hepatitis A and the possible role of importation of the infection from abroad. The potential risk of epidemic peaks is plausible in view of waning natural immunity in the Netherlands since the 1940s (4). Human normal immunoglobulin (IgG) for passive immunisation is used in the control of outbreaks at schools and day care centres. Immunisation is also recommended for travellers to endemic countries (passive) and for non-Dutch children visiting their country of origin (active) (5). The many infections contracted abroad each year suggest that not all individuals of the target groups receive immunisation. The extension of our immunisation policy, including financial allowances, is currently under debate and cost-effectiveness analyses will be required in due course.

References

1. 1. Termorshuizen F, Laar van de MJW, Verheffing van hepatitis A in het eerste kwartaal van 1998; 9: 97-8.
2. CDC. Case definitions for infectious conditions under public health surveillance. Hepatitis, viral, acute (revised 9/96). MMWR Morb Mortal Wkly Rep 1997; 46 (RR-10); 18-19.
3. Leentvaar-Kuijpers A. Wie komt in aanmerking voor immunisatie met een geïnfecteerd hepatitis A-vaccin? Ned Tijdschr Geneeskd 1994; 138: 941-2.
4. Termorshuizen F, Laar van de MJW. De epidemiologie van hepatitis A in Nederland. 1957-1998. Ned Tijdschr Geneeskd 1998 (in press).
5. LCI-protocol Hepatitis A. Rijswijk, LCI 1998.