[Combating infectious disease using mathematical modelling]

When determining interventions against threatening infectious diseases such as HIV-infection, severe acute respiratory syndrome (SARS), smallpox and pandemic influenza, the use of mathematical models of the spread of infectious diseases is becoming increasingly popular. These models contribute to the structuring of the knowledge already available in various disciplines, to finding epidemiological connnections, to demonstrating lacunas within the pool of knowledge and to the comparison of the expected effects and costs of preventative and intervention measures. The use of models leads to a 'made-to-measure' analysis ofthe effects and costs of preventative and intervention measures which takes account of the specific characteristics of infectious diseases. The integration of knowledge from various disciplines can be supported by more research into the theoretical epidemiology of infectious disease and by better integration of mathematical models into policy development. The resulting and better foundations of this policy that are achieved by means of infectious disease modelling translate into more effective combating of infectious disease.

[Legionella outbreak in Amsterdam: a cooling tower as the source]

During the period 6-28 July 2006, 30 confirmed cases of Legionella infection were identified in Amsterdam, 2 of which were fatal. All had a positive urinary antigen test, by which Legionella pneumophila serogroup I could be demonstrated. Consultations between the parties involved in the control of infectious diseases started on July 7th, as soon as it became clear that there was an outbreak. On July 10th it was established that relatively many of these patients lived in the eastern part of the city centre. After a study of the prevailing winds during the past 3 weeks, the search for installations containing water was started. A cooling tower in the town centre was closed on July 11th by way of precaution. During the following week, this tower was proven to be the source of the outbreak.

Effectiveness of a hepatitis A vaccination program for migrant children in Amsterdam, The Netherlands (1992-2004)

OBJECTIVE

To evaluate the impact and effectiveness of risk-group vaccination against hepatitis A targeted at migrant children living in a country with low endemicity of hepatitis A.

METHODS

Retrospective population based data analysis. Routinely collected data on hepatitis A incidence in migrant children and other risk groups in Amsterdam from 1 January 1992 to 2004 were analyzed and related to exposure, immunity and vaccination coverage in migrant children.

RESULTS

The overall hepatitis A incidence in Amsterdam declined after a pediatric vaccine was introduced in 1997. This decline was seen in migrant children traveling to hepatitis A-endemic countries, contacts with hepatitis A patients, primary school students, injecting drug users, and persons with unknown source of infection, but not in men who have sex with men (MSM) or in travelers to endemic countries other than migrant children.

CONCLUSION

The hepatitis A vaccination campaigns are effective: they reduce both import and secondary HAV cases. The campaigns could be more efficient and cost-effective if the hepatitis B vaccinations currently given to these groups were replaced by a combined hepatitis A and B vaccine. This would increase the hepatitis A vaccination coverage considerably and further reduce the hepatitis A incidence.

Tattooing, permanent makeup and piercing in Amsterdam; guidelines, legislation and monitoring

Tattooing, body piercing and permanent makeup are increasing in popularity. Here, we describe the procedures involved in these practices, their risks, the content of guidelines developed by the Municipal Health Service in Amsterdam (the Netherlands) to reduce infection risks, the legislation in the city of Amsterdam, and results of monitoring in tattoo and piercing studios.

Tattooing, permanent makeup and piercing in Amsterdam; guidelines, legislation and monitoring

Tattooing, body piercing and permanent makeup are increasing in popularity. Here, we describe the procedures involved in these practices, their risks, the content of guidelines developed by the Municipal Health Service in Amsterdam (the Netherlands) to reduce infection risks, the legislation in the city of Amsterdam, and results of monitoring in tattoo and piercing studios.

Sexual risk behaviour among Surinamese and Antillean migrants travelling to their countries of origin

OBJECTIVES

To examine travel related sexual risk behaviour among migrants living in Amsterdam.

METHODS

People originating from Surinam (n = 798) and the Netherlands Antilles (n = 227) were recruited in order to study the heterosexual spread of HIV within ethnic groups. Log binomial regression was used to study determinants for homeland travel over the past 5 years; logistic regression was used to study determinants of unprotected sex on these visits.

RESULTS

Of the migrants, 38% of men and 42% of women visited their homeland. Visits were most likely among men who had lived > or =7 years in the Netherlands, were employed, had a high educational level and were/had been married. For women, visiting was associated with older age and living in the Netherlands for > or =8 years. Of migrants visiting their homeland, 47% of men and 11% of women acquired a local sexual partner. For male travellers, Surinamese origin (adjusted OR 10.66; 95% CI 1.72 to 104.48) and a history of > or =1 sexually transmitted infection (STI) (adjusted OR 12.51; 95% CI 3.75 to 46.95) were associated with having unprotected sex with local partners. For women, having >1 partner in the past 5 years (OR 13.57; 95% CI 2.57 to 250.28) was associated with unprotected sex with local partners.

CONCLUSION

Migrants are at substantial risk for HIV and STIs while visiting their homeland. It is important to reach migrants, who are likely to engage in unprotected sex during visits, for pretravel health education. Additional research on risk behaviour in the homeland and the Netherlands is needed to identify migrants with high risk behaviour.

[Avian influenza H5NI in Europe: little risk as yet to health in the Netherlands]

Since 2004, outbreaks of fowl plague caused by a highly pathogenic avian influenza virus of the subtype A/H5NI have been reported from various countries in Southeast Asia. To date, 118 cases with 61 deaths have been documented in humans, due to close contact with infected poultry or raw poultry meat. Although efficient human-to-human transmission has not occurred, in a few cases transmission to blood relatives could not be ruled out. In October 2005, outbreaks of A/H5NI in poultry and wild fowl have been confirmed from Turkey, Romania and Russia, due probably to infection via migratory birds. The direct risk of infection in humans in Europe is very low and is associated with direct exposure to infected poultry. In order to address the long-term risk of a pandemic due to recombination of human and avian viruses or to mutations in the avian virus itself, guidelines for pandemic preparedness have been developed and implemented in the Netherlands.

Molecular epidemiology of hepatitis A in Noord-Brabant, The Netherlands

BACKGROUND

Previous studies on the molecular epidemiology of hepatitis A virus (HAV) in Amsterdam, The Netherlands, show that subgenotype 1A is mainly seen among homosexual men practising anonymous oral-anal sex in saunas and darkrooms, while subgenotype 1B is usually detected among children originating from Morocco, and subgenotype 3A is mostly found among travellers to Pakistan.

OBJECTIVE

We studied the genotype distribution in a more rural area of The Netherlands, Noord-Brabant, and compared it with Amsterdam.

STUDY DESIGN

We collected blood and feces samples from 34 HAV IgM(+) individuals who were reported from August 2001-March 2003 at the Municipal Health Service (MHS) Heart for Brabant (Brabant). We also collected feces samples from nine household contacts of whom the HAV IgM status was not known. HAV RNA was isolated and subsequently amplified by reverse transcriptase polymerase chain reaction (RT-PCR) at the VP1-P2a and the VP3-VP1 region, sequenced and analysed.

RESULTS AND CONCLUSIONS

In most cases, relations between risk groups and HAV subgenotypes in Noord-Brabant were similar to those in Amsterdam. Next to genotypes 1 and 3 we also detected a genotype 2/7 strain in a Noord-Brabant case. Also, in contrast to the Amsterdam study, sporadic transmission occurred among various risk groups. Children involved in a school-related outbreak were infected with strains identical to one that was previously isolated from a man who has sex with men (MSM). Also, Dutch patients having no epidemiological link with Turkish or Moroccan children harboured strains imported from high-endemic countries. Furthermore, we report a special case in which HAV may be causally involved in meningitis. The results of this study show that the molecular epidemiology of HAV in The Netherlands can be more complicated than previously anticipated and that HAV phylogenetic studies can provide important information for the design of appropriate public health measures.

Increase of sexually transmitted infections, but not HIV, among young homosexual men in Amsterdam: are STIs still reliable markers for HIV transmission?

OBJECTIVES

The incidence of HIV and STIs increased among men who have sex with men (MSM) visiting our STI clinic in Amsterdam. Interestingly, HIV increased mainly among older (> or =35 years) MSM, whereas infection rates of rectal gonorrhoea increased mainly in younger men. To explore this discrepancy we compared trends in STIs and HIV in a cohort of young HIV negative homosexual men from 1984 until 2002.

METHODS

The study population included 863 men enrolled at < or =30 years of age from 1984 onward in the Amsterdam Cohort Studies (ACS). They had attended at least one of the 6 monthly follow up ACS visits at which they completed a questionnaire (including self reported gonorrhoea and syphilis episodes) and were tested for syphilis and HIV. Yearly trends in HIV and STI incidence and risk factors were analysed using Poisson regression.

RESULTS

Mean age at enrollment was 25 years. The median follow up time was 4 years. Until 1995 trends in HIV and STI incidence were concurrent, however since 1995 there was a significant (p<0.05) increase in syphilis (0 to 1.4/100 person years (PY)) and gonorrhoea incidence (1.1 to 6.0/100 PY), but no change in HIV incidence (1.1 and 1.3/100 PY).

CONCLUSIONS

The incidence of syphilis and gonorrhoea has increased among young homosexual men since 1995, while HIV incidence has remained stable. Increasing STI incidence underscores the potential for HIV spread among young homosexual men. However, several years of increasing STIs without HIV, makes the relation between STI incidence and HIV transmission a subject for debate.

An outbreak of hepatitis A among homeless drug users in Rotterdam, The Netherlands

From the end of January to mid-June 2004 (weeks 5-24) a hepatitis A virus (HAV) outbreak occurred among a homeless and drug user community in Rotterdam, The Netherlands. To prevent further spread of the virus within this group and to the general population, the Municipal Health Service of Rotterdam organized a mass vaccination campaign during which 83% (1,515/1,800) of the homeless people were vaccinated. As part of a national HAV typing study, blood and/or fecal samples of 30 Rotterdam HAV IgM+ patients who fell ill during the period of 1 September 2003-1 December 2004 were tested. The tests included RT-PCR and sequencing at the VP3-VP1 and VP1-P2a regions of the HAV genome. It was found that 12 homeless people, one family member of a homeless person and two people without a known risk were infected with a unique subtype 3a strain. Four of the homeless patients became ill after vaccination and were probably infected at the time. This study shows that Dutch homeless people and drug users involved in HAV outbreaks should be offered HAV vaccine actively to prevent further spread of the infection. Furthermore, it was shown by molecular techniques that the unique subtype 3a strain was not found before the Rotterdam outbreak or afterwards, indicating that the mass vaccination campaign was successful.

[Less refusal to participate in HIV screening among pregnant women in the Amsterdam region since the introduction of standard HIV screening using the opting-out method]

In 2003 the Municipal Health Service in Amsterdam started to screen pregnant women for HIV according to the opting-out method. In this method the HIV test is routinely included in the prenatal screening along with hepatitis B virus (HBV) and syphilis. If the woman does not want to be tested for HIV then she must actively opt out of this test. This screening method was chosen because in the universal screening method used in 2002, women had to give their explicit consent to test for HIV and this led to a high refusal rate (13.6%), especially among women from AIDS-endemic countries. After the introduction of the opting-out method, the refusal rate fell from 3% in the first quarter of 2003 to 1.4% in the last quarter of 2003. None of the women refused to be tested for HBV or syphilis. In 2003, the HIV prevalence among pregnant women was 0.3% (35/13.621). The experiences with this screening method in Amsterdam were used to implement the national opting-out method for HIV screening in pregnant women, which was introduced on 1 January 2004.

Performance of routine syphilis serology in the Ethiopian cohort on HIV/AIDS

OBJECTIVES

To assess the performance of routine syphilis screening during 5 year follow up of Ethiopian factory workers, participating in a cohort study on HIV/AIDS.

METHODS

Syphilis serology test results of factory workers, who each donated at least six blood samples were evaluated. Screening in 1997-8 had been performed by the Treponema pallidum particle agglutination (TPPA) assay and in 1999-2001 by the rapid plasma reagin (RPR) test. TPPA had been followed by RPR or RPR by TPPA, in case of a positive screening result. Samples of study subjects showing inconsistent sequential TPPA and/or RPR results were retested independently by three laboratory technicians.

RESULTS

A total of 540 cohort participants (8.3% HIV positive at enrollment) donated 4,376 blood samples (mean 8.3 per subject). From 93 of the 176 participants with at least one positive TPPA result during follow up, 152 samples were retested by RPR and/or TPPA. Based on the revised syphilis test results, the 540 cohort participants were classified as having no (70.5%), past (20.6%), prevalent (6.9%), or incident (2.0%) syphilis. The RPR screening test was difficult to interpret and yielded 8.2% biological false positive (BFP) RPR results, or 3.2% if weak positive results were excluded. There was no correlation between HIV infection and BFP RPR reactions. Sample mix-ups were detected in 1.2%.

CONCLUSION

Evaluation of routine syphilis screening as performed in a long term cohort study on HIV/AIDS in Ethiopia showed difficulties encountered in syphilis screening programmes such as a high percentage of BFP RPR, inconsistencies in interpretation of the RPR test, and sample mix ups. The findings stress the need to develop a syphilis screening assay that is easy to perform and interpret and to implement quality assurance programmes.

[HIV-screening among pregnant women in the region of Amsterdam in 2002]

In 2001, the Municipal Health Service started offering HIV tests to all pregnant women in and around Amsterdam. All midwives and all hospitals but one participated in this HIV screening. In 2002, of the 10,752 pregnant women offered HIV testing 13.6% refused. HIV antibodies were detected in 24 women (0.26%). The HIV prevalence was doubled compared to the prevalence in 1990-1991, but the number of newly HIV-infected pregnant women remained stable (0.1%). The high number of refusals, as a result of which many HIV-infected women might be missed, is alarming. For this reason, in January 2003, the Municipal Health Service started HIV testing by the opting-out approach. Under the opting-out approach, women are notified that an HIV test will be included in the standard test battery, together with tests for hepatitis B and syphilis, unless she explicitly refuses. It is expected that the opting-out approach for HIV screening of pregnant women will be implemented nationwide in 2003.

[HIV-infection and AIDS in the Netherlands: prevalence and incidence, 1987-2001]

OBJECTIVE

To describe the results of HIV-surveillance activities in the Netherlands between 1987 and 2001.

DESIGN

Descriptive.

METHOD

Data were obtained from HIV-surveillance at STI-clinics, laboratory-surveillance in the region Arnhem, surveillance among injecting drug users, the AIDS-notification, STI-registration and the Amsterdam cohort studies on HIV/AIDS.

RESULTS

In the Netherlands, the highest HIV-prevalences were found among injecting drug users (1-26%) and homo- and bisexual men (0-17%). In these high-risk populations, an increase in HIV-prevalence and--incidence, respectively, was found among injecting drug users in Heerlen and homosexual men (> 35 years of age) in Amsterdam. The HIV-prevalence was lower among heterosexuals in the Netherlands (0-2%). However, in certain local populations an increase was seen. In both Amsterdam and Rotterdam, the HIV-prevalence was higher in individuals tested anonymously than in those tested by name.

CONCLUSION

Local increases in HIV-infections have been observed recently, in both high- and medium-risk populations.

Hepatitis B virus transmission from a surgeon to patients during high-risk as well as low-risk procedures: unnoticed transmissions over a 4-year period

Background

Three cases of reported acute hepatitis B virus (HBV) led to the identification of a chronically HBV-infected surgeon, with a high viral load, who was a non-responder after vaccination.

Methods

A retrospective cohort study was conducted on 1564 patients operated on by this surgeon over a 4-year period.

Results

Forty-nine patients (3·1 per cent) were positive for HBV markers. Based on HBV DNA sequencing and epidemiological data, transmission from the surgeon was confirmed in eight, probable in two, possible in 18 and excluded in 21 patients. One case of secondary transmission was identified from a patient to his wife, who died, presumably from fulminant hepatitis. A case–control study identified the duration of operation, the potential risk of the surgical procedure and the occurrence of complications during or after surgery as significant risk factors for HBV infection. At least eight of the 28 patients were infected during low-risk procedures.

Conclusion

Transmission from surgeons to patients at a low rate can remain unnoticed for a long period of time. Prevention requires a more stringent strategy for vaccination and testing of surgeons. It also requires revision of policies allowing HBV-infected surgeons to perform presumed ‘low-risk’ procedures.

[The 14th International Aids Conference: 'Knowledge, commitment and action']

During the world AIDS conference in Barcelona it became clear that the worst-case scenario of 10 years ago has become a reality: the HIV epidemic is continuing to spread. Also in industrialised countries the incidence of HIV infections among homosexual men is once again increasing. An HIV vaccine is still not available. Although the development of antiretroviral therapy continues, HIV inhibitors do not result in an eradication of HIV. It is still not clear as to when therapy can best be started and what the consequences are of temporarily withdrawing therapy. In countries where HIV inhibitors are widely available, the life expectancy of HIV-infected persons is increasing. The life expectancy of HIV patients will possibly decrease again due to an increased resistance towards the currently available antiretroviral drugs.

Low incidence of syphilis among factory workers in Ethiopia: effect of an intervention based on education and counselling

BACKGROUND

The prevalence and incidence of syphilis infection were examined in a cohort study of factory workers in Ethiopia.

METHOD

Between February 1997 and March 1999, 409 men and 348 women were enrolled and followed in the cohort study.

RESULTS

The prevalence (95% CI) of past/current syphilis (positive TPPA serology) was 28.9% (25.7% to 32.3%), and factors associated with past/current syphilis were markers of risky sexual behaviours including HIV infection. In this cohort of factory workers subject to public information/education meetings, testing for HIV antibodies, and individual counselling, the incidence (97.5% one sided CI) of new syphilis infections was 0/691 = 0 (0 to 0.5) per 100 person years.

CONCLUSION

This study has documented a reduction in risky sexual behaviours and a low syphilis incidence among factory workers participating in a cohort study on HIV infection progression in Addis Ababa.

[Syphilis epidemic and an increase of the number of HIV infections among homosexual men attending the Amsterdam venereal disease clinic]

The registered number of cases of early infectious syphilis and of (ano)genital gonorrhoea among the attendees of the outpatient clinic for sexually transmitted diseases of the Amsterdam municipal health service shows a strong increase for both diagnoses in the period 1990-2001, notably in the last few years. Nearly all of this increase is accounted for by homosexual men. Syphilis increased mostly among men aged 35 years and over, gonorrhoea mostly among younger men. The population of older men also showed a distinct increase since 1997 in HIV incidence.

[Post-exposure treatment against HIV outside of the hospital in Amsterdam, January-December 2000]

The Infectious Diseases Department of the Amsterdam Public Health Service regularly sees people who have possibly been exposed to human body fluids infected with hepatitis B virus (HBV), hepatitis C virus (HCV) or human immunodeficiency virus (HIV), as a result of incidents in either a non-hospital-related occupation or during their leisure time. Until January 2000, people reporting possible exposure were referred to hospitals for HIV post-exposure prophylaxis (HIV-PEP), whereas for HBV and HCV, the tracing of the infection source and the follow-up took place at the Public Health Service clinic. Since the introduction of a new protocol in January 2000, the Public Health Service as well as a hospital can establish an indication for HIV-PEP and prescribe it. In 2000, 322 persons reported to the Public Health Service. The source was tested in 194 incidents and an HIV test was carried out on 104 occasions. In 19 cases HIV antibodies were found or the source was known to be HIV positive. A total of 50 people were prescribed HIV-PEP; 16 times after a needle-stick accident, 22 times after a sexual accident, 6 times after a bite-wound and 6 times after direct exposure to human blood. The sources of 30 PEP recipients could be tested and after this 16 recipients could stop with the treatment due to a negative HIV test. In 4 cases PEP treatment was stopped following a review of the indication. Therefore, 30 people had an indication to complete the PEP treatment. Of these, more than 80% reported side effects and 3 people had to withdraw from the treatment at an early stage because of this. The implementation of the protocol has resulted in less confusion because the follow-up now takes place at one location. Compared to other studies, the average time between exposure and treatment is shorter and compliance is good. In the period up to 15 May 2001, no seroconversions had been observed.

[The advances of HIV in the Netherlands: indications but still no definite answers]

Investigations in the province of Limburg demonstrate an increasing HIV prevalence and a high prevalence of injecting and sexual risk behaviour as well as a substantial risk of HIV transmission to the general population. It is recommended that HIV surveillance be directed towards 'bridge populations' such as prostitutes, their customers and immigrants from HIV endemic countries, instead of just drug users. In view of the apparent increase in sexual risk behaviour and sexually transmitted diseases in homosexual men (possibly due to a reduced fear of the disease because of the availability of anti-HIV medication) surveillance should be intensified in that area as well. Only then will it be possible to ascertain whether or not the incidence of HIV in the Netherlands is increasing.

A summary report from Amsterdam: increase in sexually transmitted diseases and risky sexual behaviour among homosexual men in relation to the introduction of new anti-HIV drugs

The worrying increase of sexually transmitted diseases (STDs) in Amsterdam was investigated in two studies carried out by the department of AIDS research of the municipal health service. The results indicate that the introduction of Highly Active Antiretroviral Therapies (HAART) may have had an influence on the increase of STDs and risky sexual behaviours in Amsterdam.

Molecular epidemiology of hepatitis B virus in Amsterdam 1992-1997

To gain insight into the spread of hepatitis B among various risk groups in Amsterdam a 6-year (1992-1997) retrospective DNA sequencing study was carried out on isolates from stored sera from reported primary cases of acute hepatitis B infection. Cases were classified according to risk behavior, as determined in interviews. Of the available serum, a selected region of hepatitis B-virus-DNA was amplified and sequenced. The nucleotide alignments were subjected to phylogenetic tree analysis. When nucleotide alignments were subjected to phylogenetic analysis, the strains of 54 isolates, 26% of the 204 reported primary cases, clustered in five genotypes: A, C, D, E, and F. In genotype A, a cluster related to men having sex with men was identified. In genotype D, two subclusters could be identified: one was related to injecting drug use and another was related to the Moroccan population in Amsterdam. The remaining strains showed a high genetic variability within three different genotypes: F, E, and C. Of the 14 identical isolates in the "homosexual men cluster," one was isolated from a female heterosexual. Of the 14 identical strains in the "drug users strain," six were from non-drug using heterosexual active individuals. In the cluster of twelve isolates related to hepatitis B-endemic areas, probable modes of transmission were varied. Sequence analysis provides important insight into the spread of hepatitis B among various high-risk groups. The analysis indicates that the prevention strategy in The Netherlands fails to stop transmission of hepatitis B from persistently infected individuals originating from hepatitis B endemic countries.

Carrier rate of zidovudine-resistant HIV-1: the impact of failing therapy on transmission of resistant strains

OBJECTIVE

Because maintenance of treatment success in HIV-1 infection requires viruses to remain therapy sensitive in drug-naive seropositive persons, we looked at the primary infections caused by drug-resistant HIV-1 over time. Furthermore, to study the coverage rate of therapy and therapy failure in relation to the transmission of resistant viruses a mathematical model was developed.

DESIGN

The reverse transcriptase and protease genes of viruses were analysed in newly infected people in the period 1990-1998 in the Amsterdam Cohort Study on HIV infection and AIDS in homosexual men.

METHODS

The mathematical model was based on the coverage of drug regimens selecting zidovudine (ZDV) resistance, the lag time in which resistance is gained or lost, the death rate of people infected with resistant virus, and the replacement of resistance-selecting regimens by more potent treatments that substantially reduce viral load and mortality.

RESULTS

Of 43 individuals with a primary HIV-infection, three (7%) harboured ZDV-resistant viruses. The first of the ZDV-resistant strains was transmitted in 1995, the last two in 1996. The build-up of ZDV resistance was described by the mathematical model indicating that the equilibrium level of resistance due to treatment depends only on the treatment rate and the outflow rate of patients with resistance virus.

CONCLUSIONS

Our model indicates that the frequency of viral resistance in a population is determined largely by the number of individuals on insufficient or failing therapy and is influenced only modestly by secondary transmission of ZDV-resistant strains.