Feature Article

Risk of Parasitic Infection from Consumption of Raw Marine or Freshwater Fish

Background

 Recently, there were discussions about whether rainbow trout (Oncorhynchus mykiss), a freshwater fish, can be called “salmon” as it belongs to the Salmonidae family, which includes the Atlantic salmon. Meanwhile, a local TV programme introduced grass carp “yu sang” (or raw fish) as a delicacy. The community is concerned about the food safety of freshwater fish sashimi.

 Fish-borne Parasites Can Infect Humans

 Fish-borne parasites are common in both marine and freshwater fish. Some of these parasites, such as nematodes and trematodes, can infect humans. Anisakis is a common species of nematodes in marine fish, while the trematode Clonorchis sinensis (the Chinese liver fluke) (Figure 1) is commonly associated with freshwater carp and the related species. Clonorchiasis, or the Chinese liver fluke disease, is endemic in Southeast Asia and East Asia. The affected patients are mostly in China.

Figure 1: Clonorchis sinensis under microscope
(Source: Dr Samson S.Y. WONG, Assistant Professor (Clinical), Department of Microbiology, University of Hong Kong)

Fish-borne parasites have complex life cycles involving multiple intermediate hosts. Unfortunately, some commercially important fish like grass carp are intermediate hosts to these parasites. During the rearing period, these fish may acquire the eggs or cysts of the parasites through the contaminated environment or the feed ingested. People will be infected if they consume raw or undercooked fish containing the parasites.

Figure 2: Good aquaculture practices help reduce the risk of parasitic infestation.

 Good Aquaculture Practices Can Greatly Reduce the Risk of Parasitic Infestation

 Some European countries (e.g. Norway) have developed special aquaculture practices to control parasitic infection in farmed fish (Figure 2). It has been accepted by the European Union (EU) that farmed fish reared in a parasite-free environment since the embryo stage and fed on parasite-controlled diets (e.g. feed that has been heat-treated) have a much lower risk of parasitic infection. Even so, it is hard to prevent the presence of parasites in farmed fish and their products must be subject to regular inspection. However, fish cultured in Southeast Asia, such as grass carp, are not traditionally raised in such a controlled environment.

Killing Parasites in Fish by Freezing

To deal with parasites in fish, the EU and the Food and Drug Administration of the United States require fishery products intended to be consumed raw to go through freezing treatment, so as to reduce the risk of parasitic infection. Only fish products produced in a properly controlled environment are exempted.

 In short, we should choose the proper ways to eat fishery products. Marine and freshwater fish and their products can be contaminated with parasites if the fish are not reared in a controlled environment or their products are not treated properly (Figure 3). Therefore, it poses a risk of contracting parasites when fishery products are consumed raw.

Figure 3: Raw fish found with a suspected parasite in a food complaint case.

Key Points to Note

1. Both marine and freshwater fish may carry a variety of parasites.
2. There is an inherent microbiological risk in consuming raw fish.
3. Good aquaculture practices and freezing treatment can greatly reduce the risk of parasitic infection.

Advice to the Trade

• Obtain from importers an official health certificate issued by the place of origin. Only source fish for raw consumption from reliable suppliers that raise their fish in a controlled environment and process fishery products properly (e.g. freezing treatment).
• Obtain a relevant licence/permit from the Food and Environmental Hygiene Department for manufacturing and/or sale of sushi and sashimi.
•  Do not serve Chinese “Yu Sang” dishes.

Readers' Corner

 Food Ordering Apps and Food Safety

Food ordering mobile applications (food ordering apps) are becoming more popular in recent years. Unlike the conventional way of ordering foods and requiring delivery service by the food premises, these apps allow customers to place food orders with premises in a wider area, with food delivery arranged by the apps service providers.

Compared with the conventional mode of meal delivery, the new mode requires a delivery person to pick up foods from a restaurant. The time pending food delivery may thus be longer. Besides, some food items may be purchased in bulk and delivered in insulation containers to an outdoor location for pick up by individual customers, which will add to the time of food delivery and distribution. Delivered meals usually include perishable foods. If there is no proper time and temperature control during food delivery, microorganisms (including pathogens) that may be present in foods can multiply rapidly and make people sick. No matter it is the new or the conventional mode of delivery, food safety risks will increase in the absence of proper time or temperature control. To address potential food safety risks arising from delivered meals, it is important to maintain proper time and temperature control for foods.

Time and Temperature Control for Safe Consumption of Delivered Foods

 To ensure that delivered foods are safe for consumption, it is advisable to keep hot foods at above 60°C and cold foods at/below 4°C. Most pathogenic bacteria proliferate at temperatures between 4°C and 60°C. This range of temperature is known as the “temperature danger zone”. To keep delivered foods out of this danger zone, we can take steps such as using insulated containers, keeping food in a warmer or cooler before delivery, and avoiding preparing food too early.

Food safety has a connection with time and temperature control. If foods are delivered within the temperature danger zone, the only safeguard will be proper time control in advance. There are two time limits for keeping perishable foods, namely the two-hour limit and the four-hour limit. 

• Prepared foods should be discarded if they have been held at room temperatures for more than four hours;
• Prepared foods kept at room temperatures for not more than two hours can be refrigerated for later use or consumed within the four-hour limit.

 The longer the time foods are delivered without temperature control, the shorter the time is left for safe consumption by customers.

Time Within Temperature Danger Zone

The time foods are kept within the temperature danger zone is cumulative, i.e. each period of time that foods have been left under ambient conditions, including food handling, storage and transportation, should be added up to calculate the total time of exposure (see the above illustration). Individual ready-to-eat ingredients may be prepared in advance and kept aside at ambient temperatures. If these ingredients do not need further processing or no means is taken to control microbial growth, the time that the ingredients have been exposed to the temperature danger zone should be added up with the two-hour and four-hour principles in view. With respect to food delivery, both the food handler and the delivery person should work out arrangements together to fulfil the time and temperature control requirements for food safety.

Key Points to Note

1. Delivered meals usually include perishable foods.  Microorganisms that may be present in foods can proliferate and cause illnesses if foods are delivered without proper time and temperature control.
2. Keep hot food at above 60°C and cold food at/below 4°C to ensure safe consumption of delivered foods.  If perishable foods are kept within the temperature danger zone, the two-hour and four-hour principles should be followed.
3. Add up all periods of time that foods, including individual ingredients, have been exposed to the temperature danger zone to calculate the total time of exposure.

Advice to the Trade

• Food premises and delivery agents are advised to work together to meet the time and temperature control requirements for delivered foods.
• Shorten the time that delivered foods are left without temperature control and remind customers to consume the delivered foods as soon as possible where appropriate.
• Keep records and follow the time and temperature requirements, especially in handling foods for delivery.

News on New Dishes

Meatballs and Spaghetti in Tomato Sauce

Meatballs and spaghetti is a classic combination in western cuisine.  In this issue, we bring you an Italian dish “Meatballs and Spaghetti in Tomato Sauce” prepared by Mr Robin Harart, the chef of Invisible Kitchen, a signatory of the Food Safety Charter.  This is a savoury dish which many people would like to have for a main course.  You can give it a try too.

Tips from the chef:

1. Choose beef rump for its low fat content.  It is a healthy ingredient for a delicious dish.
2. Placing a piece of parchment paper on top of the saucepan can prevent excessive evaporation of water in sauce preparation.
3. Use honey and water instead of red wine if the food is prepared for children.

Food Safety Plan Corner

Meatballs and Spaghetti in Tomato Sauce

Ingredients:

Minced beef, egg, cheese, parsley, carrot, spaghetti, purple onion, yellow onion, garlic, bay leaves, bread crumbs, paprika powder, tomato paste, oregano, red wine.

Steps:

1. Take out minced beef, egg, cheese, parsley and carrot that are refrigerated at 4ºC or below.  Rinse the parsley and carrot under clean running water.  Finely chop the parsley leaves.  Peel and dice the carrot for later use. 
   
2. Get purple onion, yellow onion, garlic, bay leaves, bread crumbs, paprika powder, tomato paste, oregano and red wine from the dry goods shelves.  Rinse and peel the purple onion, yellow onion and garlic.  Finely chop the purple onion and section the yellow onion.  Cut the garlic into fine pieces and slices.  
3. Place minced beef, egg, parsley, chopped purple onion and garlic, bread crumbs and paprika powder into a mixing bowl.  Blend well and slap the mixture until sticky. 
4. Roll the mixture into balls.  Coat the meatballs with corn flour, pan-fry until golden brown.  Set aside for later use.
5. Heat up the saucepan and add in some oil.  Stir-fry yellow onion sections and garlic slices for about 30 seconds.  Add in carrot, bay leaves, tomato paste, oregano and red wine to prepare the sauce.  Bring the sauce to boil.  Put the meatballs into the sauce and add cheese.  Switch to medium fire and braise for about 2 hours. 
6. ring water to boil, add in the spaghetti and cook for about 8 minutes.  Serve the meatballs and sauce over the cooked spaghetti. 

Production Process

Briefing of Activities

Trade Consultation Forum

The Centre for Food Safety (CFS) conducts the Trade Consultation Forum regularly for representatives from various food trade associations, food manufacturers, food importers, wholesalers and retailers to exchange views on food safety matters, discuss food safety control measures and provide feedback on risk communication activities.

The 67th meeting of the Forum was held on 14 December 2018.  The CFS representatives briefed the trade on the results of the study “Sodium, Total Fat and Trans Fat Contents in Local Bread” and the control measures on the import and sales of foods that contain cannabis/tetrahydrocannabinol.  The CFS also advised the trade on the guidance on control of histamine in fish and fish products, as well as the latest revisions of the guidelines on disposal of recalled foods.

 Members of the trade enthusiastically expressed their views on various issues at the forum.  The CFS would like to thank all the attendees for their active participation.

For details of the discussions at the meeting, please browse the website at:
http://www.cfs.gov.hk/english/committee/committee_tcf.html.

Upcoming Activities

Food Safety Seminar for Trade

The biennial Food Safety Seminar conducted by the CFS will be held in the latter part of 2019.  The seminar aims at providing a forum for the Government and the food trade to exchange information and views on current and important food safety issues in Hong Kong.  It is envisaged that through this event, the culture of continuous improvement and collaboration on food safety control can be further enhanced among the local food trade.