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Keywords:

  • measles;
  • outbreak investigation;
  • occupational health

Abstract

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Conclusion and implications
  7. Acknowledgements
  8. References

Objective: To describe the outbreak investigation and control measures for a cluster of measles cases involving ‘fly-in fly-out’ (FIFO) workers on an off-shore industrial vessel.

Methods: Following Australian guidelines, measles cases were interviewed and at-risk contacts on the Australian mainland received measles vaccine, immunoglobulin or health advice. For the industrial vessel: (i) exposed FIFO workers who had already left the vessel received health advice through their employer; (ii) workers remaining on the vessel were offered measles vaccine; and (iii) FIFO workers joining the vessel for 21 days following the prodrome onset of the last case of measles on the vessel were offered measles vaccine. Measles virus isolates were sent for genotype determination.

Results: Four measles cases from two Australian jurisdictions were epidemiologically linked to the retrospectively identified index case, a New Zealand FIFO worker. No further cases were detected following the institution of outbreak control measures.

Conclusion: FIFO workers congregating on large industrial projects are a discrete risk group with the potential to spread infectious diseases over large distances, both domestically and internationally.

Implications: FIFO workers’ immunisation history should be reviewed prior to deployment. Catch-up vaccination, where appropriate, would minimise transmission of vaccine-preventable diseases such as measles and help maintain a healthy, productive workforce.

Australia is free of endemic measles transmission.1,2 However, sporadic cases still occur in returned travellers or visitors infected overseas, as do small clusters of locally acquired cases linked to index cases infected overseas.3–5

On 7 December 2011, a 36-year-old nurse (Case 1) working in a General Practice (GP) surgery in Darwin, Northern Territory (NT), presented to the Royal Darwin Hospital (RDH) emergency department with a rash and a seven-day history of cough and fever. On the same day, a 32-year-old ‘fly-in fly-out’ (FIFO) personnel manager (Case 2) on a large industrial vessel working in international waters in the Timor Sea (1,500 km from Darwin) was evacuated to RDH with fever, rash and a five-day history of cough, coryza and conjunctivitis. In both cases, measles virus RNA was detected by polymerase chain reaction testing of throat swab specimens.

The NT, with a population of 225,000 and high immunisation coverage, has averaged one to two measles cases per year for the past decade.6 In this context, two seemingly independent measles presentations within 24 hours was highly unusual.

FIFO is an employment alternative to relocation: workers are flown to a work site for a number of days and are then flown back to their home for rest days. The industrial vessel had up to 340 FIFO workers aboard from more than a dozen countries, with a turnover of 30 to 40 workers each day. All workers arriving and departing the vessel transited through a heliport in Truscott, northern Western Australia (WA), and then Darwin, NT, (see Figure 1). As a condition of employment on the vessel, FIFO workers were required to complete a pre-deployment medical questionnaire and examination. Prior to this outbreak, the health assessment did not include an immunisation history or tests for immunity to vaccine-preventable diseases. Following the outbreak response, FIFO workers joining this industrial project were required to provide documentation of immunity (previous infection or immunisation) against measles virus.

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Figure 1. Location of the industrial vessel and measles cases at the time of diagnosis.

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Our report describes the epidemiology of this measles outbreak, of which these two cases were the sentinel event. We also describe the public health response initiated to limit further transmission and highlight the risk of infectious diseases spread associated with FIFO workers.

Methods

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Conclusion and implications
  7. Acknowledgements
  8. References

Following Australian guidelines,1 a detailed history regarding contacts and prior immunisation was obtained from both cases. An alert was distributed NT-wide to all health care providers requesting that they contact the NT Centre for Disease Control (CDC) regarding possible cases of measles. A media release describing the movements of Case 1 was published, advising the Darwin public of appropriate action if they suspected they had contracted measles, including the need for self-isolation and care not to expose others when seeking medical attention. As Case 2 was a resident of Perth, WA, public health authorities in that state were also informed. If indicated, contacts of cases were provided with measles-mumps-rubella (MMR) immunisation or normal human immunoglobulin, as per Australian guidelines (see Table 1).1 Measles virus isolates were sent to an accredited reference laboratory for genotype testing.

Table 1.  Australian guidelines for measles post-exposure contact management in non-pregnant adults.1
Patient immune statusMMR vaccination history
 Unknown or 0 doses1 dose MMR2 doses MMR
  1. Notes:

  2. MMR = measles mumps rubella vaccine, NHIG = normal human immunoglobulin.

Within 72 hours of first exposure to infectious case
HealthyMMRMMRNil necessary
ImmunosuppressedNHIGNHIGNHIG
73–144 hours after first exposure to infectious case
HealthyNHIGNil necessaryNil necessary
ImmunosuppressedNHIGNHIGNHIG

Due to the remoteness and transient nature of the workforce in contact with Case 2, alternative response strategies were required for the industrial vessel, as follows: (i) workers who had left the vessel during the infectious period for Case 2 were contacted by email or phone by their employer and advised that they may have been exposed to measles. They were asked to seek medical attention if they developed illness consistent with measles and to advise their employer or CDC Darwin; (ii) to disrupt further transmission cycles, all workers still on the vessel were offered MMR vaccine if they could not verify prior receipt of two measles containing vaccines or previous illness with measles; and (iii) all workers joining the vessel for 21 days following the prodrome onset of the most recent measles case on the vessel were offered MMR immunisation before deployment if they were unable to verify prior receipt of two measles containing vaccines or previous measles infection. Three hundred doses of MMR vaccines were transferred to the vessel to be used as part of the response. In addition, we requested the vessel medic to inspect the sick bay records to identify a possible index case, and to maintain surveillance for any further cases.

Results

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Conclusion and implications
  7. Acknowledgements
  8. References

Inspection of the vessel sick bay records led to the retrospective discovery of the index case – a 49-year-old New Zealand (NZ) national and FIFO worker who had travelled from his home in the Waikato region near Auckland on 7 November 2011. Arriving on the vessel on 8 November, he presented to the vessel medic on 17 November with fever and cough (see Table 2). On 21 November, he developed a rash, and was evacuated to Darwin and assessed at the GP clinic where Case 1 worked. He was not investigated for measles at that time. On 24 November, while probably still infectious, he flew back to NZ, where he consulted a medical practitioner and had blood collected but was not tested for measles. Subsequently, after his potential link to the Australian cases was recognised, the stored blood specimen was retrieved on 6 January 2012 and tested positive to measles IgM. No immunisation documentation was available for this case. Retrospectively, he reported three hours’ contact with a person coughing profusely on a public bus to Auckland airport on 7 November 2011.

Table 2.  Cases of confirmed measles.
CaseAge/SexResident locationPrior measles immunisationProdrome onset dateDayPlace of acquisitionSourceVirus genotype
  1. Notes:

  2. NZ = New Zealand, NT = Northern Territory, WA = Western Australia.

Index49 MNZUnknown17/11/20110AucklandUnknownUnknown
136 FNT1 dose30/11/201113GP surgeryIndexD4
232 MWAUnvaccinated3/12/201116VesselIndexD4
340 MWAUnvaccinated16/12/201129VesselCase 2D4
417 MWAUnknown19/12/201132Darwin hotelCase 2D4

Case 1 had one documented measles immunisation as a child. Her onset of symptoms was on 30 November 2011, following her likely exposure to the retrospectively identified index case on 21 November 2011. While infectious, she had worked at the medical practice, attended a cinema and had shopped at two major shopping centres. She had no history of travel or known exposure to other measles cases. She reported no face-to-face contact with the index case, but was at work on the day and at the time of his attendance.

Case 2 was not immunised. His prodrome onset was on 3 December 2011. He had been on the industrial vessel for the preceding four weeks and had therefore acquired measles aboard the vessel. Due to the congregate living circumstances, he reported exposure to all workers – including the index case – on the vessel. He also reported limited exposure – following his evacuation on 7 December 2011 – to people in Darwin: hotel staff and patrons; GP surgery staff and clientele; and RDH staff.

Two tertiary cases were subsequently diagnosed in WA (see Figure 1). Case 3 was a resident of a mid-west WA town and a FIFO worker who was on the industrial vessel between 29 November and 6 December. He became unwell at home on 16 December 2011. While unaware of exposure to anyone with a measles-like illness, he was almost certainly infected by Case 2 sometime between 2 and 6 December. Case 4 was a resident of a north-east WA town who reported staying at the same Darwin hotel as Case 2 on 7 December 2011, and became unwell on 19 December 2011.

The measles virus genotype for Cases 1–4 was D4, and supports the epidemiological case links. Furthermore, this was the only strain in circulation in Auckland in the period from mid-2011 (Tomasz Kiedrzynski, NZ Ministry of Health – personal communication 4 January 2012).

The public health response involved follow-up of: an estimated 180 contacts of Case 2 who left the ship between 2–7 December 2011; an estimated 300 potentially exposed contacts still aboard and those joining the vessel on 7 December 2011; 177 potentially exposed contacts for Case 1 and Case 2 in Darwin (GP surgery, RDH, hotel staff); and 14 close contacts of Cases 3 and 4 in WA, who were residents of small towns and had a limited number of identifiable contacts during their respective infectious periods. In addition to the 150 FIFO workers vaccinated on the vessel, 42 of the identified Darwin and WA contacts had post-exposure vaccination against measles and three required normal human immunoglobulin. None aboard the vessel required immunoglobulin. No further cases of measles were notified on the vessel, in the NT or WA in the following four months, indicating that there was no further transmission related to this outbreak in Australia. No information was received by the Australian National Focal Point (under International Health Regulation requirements) to indicate transmission to nationals of other countries who worked on the vessel, or from the index case after his return to NZ.

Discussion

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Conclusion and implications
  7. Acknowledgements
  8. References

We have described the public health investigation and response to a measles outbreak associated with FIFO workers on an off-shore industrial vessel serviced via Darwin in northern Australia.

The probability of two coincidental measles presentations within 24 hours in the NT is low so, despite the two cases being 1,500 kilometres apart at the time of disease onset, a common source was considered possible. The retrospective identification and confirmation of the index case and the subsequent diagnosis of cases in two distant WA rural towns illustrates the importance of investigating seemingly sporadic cases clustered in time. A similar case report of workplace spread of rubella to the general population has been described.7 However, our report, we believe, is the first to describe measles transmission associated with FIFO workers and highlights the potential of this workforce to transmit infection to colleagues and the general population, over great distances, through occupational travel. This discrete workforce group presents both challenges and opportunities for the prevention, surveillance and control of vaccine-preventable and other infectious disease outbreaks.

During 2011, there was a sustained measles outbreak in Europe associated with virus genotype D4.8,9 The same strain was responsible for the 2011 NZ outbreak, and our index case travelled through Auckland during the peak of the outbreak – 490 measles cases were notified in Auckland between 20 May 2011 and 24 July 2012.10

The past decade has borne witness to unprecedented industrial development in north-west Australia linked to the resources boom. Large-scale industrial projects are staffed largely by FIFO workers. The mobility of this group, some of whom originate from or travel through countries with endemic measles transmission and other infectious diseases that are uncommon in Australia, exposes them to a greater risk of acquiring and transmitting infections. In Australia, population immunity to measles for those born prior to the introduction of measles vaccine in 1968 is high (98%).1 Australian FIFO workers are mostly in the age cohort that is: (i) too young to have acquired wild measles infection prior to the introduction of measles vaccine; and (ii) too old to have participated in the two-dose measles vaccine schedule, introduced in Australia from November 1994.1,11 However, the limited number of measles cases in subsequent generations and the rapid termination of further transmission reflects a high herd immunity to measles in the Australian population.2,12

While the impact upon industrial operations in this measles cluster was negligible, there was a significant cost and some workplace disruption associated with the evacuations by air of the index case, Case 2 and the emergency immunisation of 150 workers. Moreover, other vaccine-preventable conditions, such as seasonal influenza or pertussis, may also pose a significant risk to productivity in FIFO worksites, as employees work and cohabit in close confines, often in remote sites, for extended periods. Given the overall economic value of these resource projects, the high salaries commanded by FIFO workers, and the importance of maintaining a healthy workforce, appropriate pre-deployment immunisation is likely to be cost-effective.13,14 Similarly, as demonstrated by Case 1 in our report, who had received only one measles-containing vaccine as a child, it is imperative that all health care workers should have had either two measles-containing immunisations or documented immunity to the measles virus.1

Conclusion and implications

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Conclusion and implications
  7. Acknowledgements
  8. References

FIFO workers on large industrial projects are a discrete risk group capable of spreading vaccine-preventable conditions quickly over large distances. The previous immunisation history of FIFO workers should be routinely reviewed prior to deployment, and immunisation against relevant diseases such as measles and influenza should be recommended to those without an adequate history or demonstrated immunity. Not only should this help to maintain a healthy and productive workforce, but the risk of transmission to the wider community will also be reduced.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Conclusion and implications
  7. Acknowledgements
  8. References

We thank our public health and laboratory colleagues in both the Northern Territory and Western Australia who assisted with the investigation of and response to this measles outbreak. We also acknowledge the help of Dr Tomasz Kiedrzynski from the New Zealand Ministry of Health, and Dr Dell Hood from the Population Health Service, Waikato District Health Board in New Zealand.

References

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Conclusion and implications
  7. Acknowledgements
  8. References