Outbreak of metam sodium poisoning at a flower farm: Central Uganda, October 2016

Summary

In October, 2016, an out of a mysterious disease, presenting with fainting and vomiting, affected 80 workers at flower farm X in Central Uganda. We investigated to identify the disease’s nature and recommend evidence-based prevention measures. We defined a probable case as sudden onset in a flower farm X worker of ≥1 of the following in October: shortness of breath, dizziness, fainting, vomiting. We identified cases through record review and in-person interviews. We conducted a retrospective cohort study to compare risks between 197 workers working in Green House 7 (GH7, suspected exposure place) and 403 workers not working at GH7 during the outbreak days. Cases increased sharply and peaked on 14 October, and gradually declined but remained high until 20 October. During 14- 20 October, 56% (110/197) of workers working at GH7 com- pared with 13% (54/403) of workers working elsewhere developed a case (RR=4.2; 95%CI=3.2-5.5). The soil in GH7 was reportedly fumigated the night of 13 October by mixing metam sodium with soil and covering the ground with plastic sheeting afterwards. However, in violation of protocol, workers poorly covered the ground, potentially allowing metam sodium fumes to escape into GH7 for several days; also, no post-fumigation checking was done. No workers wore masks while working because masks were not provided. In conclusion, this outbreak was caused by inhalation of metam sodium fumes, which escaped from the soil due to protocol violation. We recommended training employees on strict adherence to protocol, and providing masks to workers and enforcing their use.

Introduction

On 25 October 2016, a mysterious disease outbreak, presenting with symptoms of vomiting, sudden onset of diarrhea and fainting reportedly affected 80 workers at a flower farm in Nsangi sub-county, Wakiso District in Central Uganda. Reports indicated that the affected workers entered a greenhouse which had been recently fumigated and the chemical used was metam sodium (sodium N-Methyldithiocarbamate), a non-selective soil fumigant commonly used in flower farms. Metam sodium is in liquid form and one of the major hazards associated with its use is that it has a degradation product called Methyl isothiocyanate (MITC) which turns into a gas. MITC may cause a chemically induced asthma-like condition known as reactive airways dysfunction syndrome [1]. We conducted investigation to identify cause, determine the magnitude of the problem, establish the source of the outbreak and recommend evidence-based prevention measures for future out- breaks.

Methods

We defined a suspected case as sudden onset in a flower farm worker of ≥1 of the following in October

Shortness of breath, dizziness, fainting, vomiting, headache, itching eyes, itching throat, itching nose, wheezing, diarrhea; and a probable case as sudden onset in a flower farm worker of ≥1 of the following in October 2016: Shortness of breath, dizziness, fainting, vomiting. We reviewed medical records and actively searched for cases using face-to-face interviews. We conducted a retrospective cohort study to compare risks between exposed and non-exposed workers which included flower pickers, packers and supervisors. The exposed category worked at Greenhouse No 7 during the exposure period (10^th to 23^rd October, 2016); mean- while the non-exposed controls did not. An assessment of the fumigation store and procedures was also done.

Results

Attack rates were 41% among sprayers, 34% among pickers and 30.8% among growing support ladies workers. Of the 9 green- houses, 58% (81/139) of the case persons reported having worked in Greenhouse 7 (GH7) in October 2016.

The epicurve suggests continuous common source exposure. Cases with features suggestive of poisoning started on the 14^th October, a day after GH7 was fumigated and peaked around 14^th and 15^th October but steadily reduced

Figure I: Distribution of cases by time

Retrospective cohort study findings

During 14-20 October, 56% (110/197) of workers working at GH7 compared with 13% (54/403) of workers working elsewhere developed a case (RR=4.2; 95%CI=3.2-5.5). Further investigations revealed that, the ground was poorly covered with plastic sheeting after the fumigation process thus allowing the gaseous fumes of metam sodium to escape into the air overnight and for several additional days inside GH7. In addition, it was also likely that post fumigation double checking was not well done.

Risk of developing symptoms by date worked in GH7*

The higher the total scores for symptom severity, the more severe symptoms the case persons experienced. All (100%) who were in the highest category of total scores (6-10) based on symptom severity were exposed to GH7. Three of the 4 case persons who got the full total score of 10 were from team 12, a working team which was exposed to GH7 less than 24 hours after fumigation. Overall, the findings from the investigation revealed that there was a strong association be- tween developing symptoms of poisoning and being exposed to GH7 as shown in the table below.

Conclusions

The likely source of the outbreak was inhalation of metam sodium fumes, which escaped from the soil due to protocol violation. The violated protocol was not following the SOPs of carefully sealing off the ground with plastic sheeting after fumigation. Also, there was no evidence to show that the post fumigation double checking was well conducted.

Finally, none of the case persons had nasal or mouth protection as part of Personal Protective Equipment. The public health actions taken include: Presentation of the findings at the National Task Force; and generation of a policy brief to inform policy decisions. The recommendations to the Farm Management was amending Standard Operating Procedures (SOPs) to provide more detailed guidance on pre, actual and post fumigation protocols and identify forms of nasal and mouth protection for the pickers of flower cuttings for example masks.

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