Legionella spp. Colonization on Non-Passenger Ships Calling at Belgian Ports ^†

Abstract

The association of cases of Legionnaires’ disease and ships has been well documented. Results from potable water samples collected for microbiological analysis during SSC inspections conducted from 2010 to 2018 at Belgian ports were analyzed in order to evaluate the level of colonization on non-passenger ships. Results indicate a high degree of colonization (77.2% of the ships were found to be colonized with Legionella spp. at least once) and further analysis is recommended to examine possible factors associated with colonization. Inspections for issuance of SSC should focus on water safety and prevention of Legionnaires’ disease.

1. Introduction

From 1977 to 2001 about 200 cases of Legionnaires’ disease (LD) were associated with ships [1,2]. Since then, many other cases have been reported [3,4,5,6,7,8,9,10,11,12,13,14]. The majority of these events occurred on passenger ships (cruise ships, ferries and river cruise ships) and only a small number occurred on non-passenger ships. These included oil drilling platforms, training ships, naval ships, fishing vessels and cargo ships [1,9,12,14,15].

A systematic review of the literature conducted in 2015 showed that eight ships (one river ship and seven cruise ships) were linked to nine events of Legionnaires’ disease and involved a total of 83 cases [11]. In almost all of the outbreaks, Legionella positive samples were associated with high colony forming units per liter (>1000 CFU/L).

There are a number of studies investigating the prevalence of Legionella spp. on passenger ships and ferries [16,17,18]. To our knowledge only two studies investigated Legionella spp. colonization on non-passenger ships. The first one investigated the occurrence of Legionella in Norwegian naval vessels [9] and the second study examined the microbiological quality of non-passenger merchant vessels [19].

In this study, the results from samples collected during the inspection of non-passenger ships from 2010 to 2018 by the port health authorities in Belgium, are used in order to assess the level of Legionella spp. colonization on this type of ships.

2. Materials and Methods

The results of the microbiological analysis along with other information including name of the ship, sample location, temperature and pH, were provided by the Belgian port health authority. Information related to the ships, including ship type, year build, flag, were acquired from an online database [20]. Classification of the ship types followed the categories used in the EU Common Ship Sanitation Database [21].

3. Results

From 2010 to 2018 the port health authorities collected 777 potable water samples for Legionella analysis during 536 SSC inspections performed on 401 different ships calling at Belgian ports. Most of the samples for Legionella were collected from 2015 to 2018 (81.5%).

From the 401 ships, 132 (32.9%) were general cargo ships, 91 (22.7%) were chemical tankers and 65 (16.2%) were container ships. Half of the ships (50%) were registered in a port that belonged to a country that was declared as Flag of Convenience (FoC) by the International Transport Workers’ Federation [22]. A large proportion of the ships (38%) were registered in EU countries and the rest in non-EU countries that are not considered as FoC. Half of the ships (50.4%) were one to 10 years old when the samples were collected, 32.9% of the ships were 11 to 20 years old, 13.5% of the ships were 21–30 years old and 3.2% of the ships were more than 30 years old.

The potable water samples were collected from the hospital (29.1%), the galley (23.8%), the cabins (16.3%), the bridge (14.3%), and other places on the ship. A total of 162 (20.8%) samples were collected from the cold water line, 187 (24.1%) were collected from the hot water line and for 428 (55.1%) of the samples this information could not be determined.

The pH ranged from 4.93 to 10.56 (median = 7.9, SD = 0.97). The cold water line temperatures ranged from eight to 39 °C with an average of 25.1 °C (SD = 5.3 °C, n = 187), while the hot water line temperatures ranged from 26 to 75 °C with an average of 48.4 °C (SD = 9.5 °C, n = 162). The presence of heterotrophic plate count (HPC) bacteria at 22 °C ranged from 0 to 3001 CFU/100 mL with a median of 34 CFU/100 mL (SD = 1031.3 CFU/100 mL), while the HPC at 37 °C ranged from 0 to 3001 CFU/100 mL with a median of 125 CFU/100 mL (SD = 1061.7 CFU/100 mL).

From the total number of samples, 396 (51.0%) were positive (≥1 CFU/L) for Legionella spp. and 381 (49.0%) were negative. Figure 1 shows the number of samples per Legionella spp. count levels and per serogroup.

From the 536 different samplings conducted during inspections, 302 (56.3%) had at least one positive sample for Legionella spp. From the 302 ships, 233 (77.2%) were found to be colonised at least once, with at least one sample positive. Samples from 133 ships had low levels of Legionella spp. (<1000 CFU/L), 107 had ≥1000 CFU/L and <10,000 CFU/L, and 55 were found with ≥10,000 CFU/L.

4. Discussion and Conclusions

Information on whether the samples were collected directly or after flushing was not available and thus it is not possible to determine whether the colonization for these ships was local or systemic. Additionally, for most of the samples it was unclear whether the sample was collected from the hot water line or the cold water line. The median pH was close to the upper limit for effective chlorination (pH < 8) [1]. Information regarding the disinfection method used for the potable system or measurements of the chlorine levels was not available.

Almost half of the samples collected were positive for Legionella spp. and more than 2/3 of the ships were found to test positive at least once. The level of colonization on board the ship found here is slightly higher than the level of colonization presented in a study that examined the microbiological quality of non-passenger merchant vessels [19]. In that study, 803 samples collected by port health officers from 2013 to 2016 during routine inspections from 360 ships were cultured for Legionella spp. and 48.6% of the samples (corresponding to 53.8% of the vessels) were found positive. The level of colonization was reported to be even less in a study investigating the occurrence of Legionella in Norwegian naval vessels, where seven out of 41 vessels were found to be positive [9]. A high level of colonization has also been reported on ferries, while cruise ships were not found to be colonized [16,17,18]. It can be safely assumed that in most cases the ships to be sampled were selected arbitrarily and not after finding major deficiencies.

In this study, the level of Legionella spp. colonization of non-passenger ships was assessed by analyzing results of samples collected by the port health authorities of Belgian ports retrospectively, and a high level of colonization was observed. Additional analysis of the data may reveal association of colonization with specific characteristic of the ship. Finally, analysis of the data recorded in the EU Common Ship Sanitation Database may provide further insight into the relationship between the inspection findings and the level of colonization.