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Health officials have warned that the number of cases tied to a deadly nationwide listeriosis outbreak is likely to rise due to the disease's long incubation period.

The outbreak has already led to the recall of 220 Maple Leaf products and the shutdown of the firm's Toronto plant as a precautionary measure.

More products, including ready-made sandwiches from Calgary-based distributor Lucerne, were pulled from shelves Monday in Saskatchewan and Alberta.

As the outbreak and recalls expand, many Canadians are looking closely at the food in their fridges and thinking twice about food safety.

What do you want to know about food-borne disease? What questions do you have about food preparation, safety and the current recalls?

Linda Corso, a registered nurse with the Food Safety Network at University of Guelph, joined us online Tuesday to answer your questions.

Your questions and Ms. Corso's answers appear below.

Linda Corso is a registered nurse in the province of Ontario. She holds a bachelor of science in nursing and a graduate degree with a focus on adult education. She has worked in public health nursing, teaching, administration, clinical research and a small business providing evidence-based information for the health sector.

The Food Safety Network provides research, commentary, policy evaluation and public information on food safety issues from "farm-to-fork." The FSN is a national repository of food safety related information, providing an extensive number of services including consumer and student outreach, information research, on-line resources, collaborative projects, evaluation and analysis. It is funded through a combination of public, private and foundation sources.

The FSN runs a bilingual information centre that answers questions via a toll-free information (1-866-503-7638) and e-mail address. For more information, visit the Network's website.

Editor's Note: globeandmail.com editors will read and allow or reject each question/comment. Comments/questions may be edited for length or clarity. We will not publish questions/comments that include personal attacks on participants in these discussions, that make false or unsubstantiated allegations, that purport to quote people or reports where the purported quote or fact cannot be easily verified, or questions/comments that include vulgar language or libellous statements. Preference will be given to readers who submit questions/comments using their full name and home town, rather than a pseudonym.

Brodie Fenlon, globeandmail.com: Thank you so much for joining us online today Ms. Corso. We've had enormous interest in this discussion — dozens and dozens of questions — suggesting there's quite a hunger for information out there. We'll try to get to as many as we can.

J W from Toronto writes: I am six weeks pregnant. As all the experts advise, I have steered clear of lunch meats since I found out I was expecting a little over two weeks ago. However, given the long incubation period for listeriosis, how worried should I be about past consumption? Is there any way to test for it's presence, and is there any treatment if it is found? Kal Nair from Canada writes: Hello, My understanding is that listeriosis is a concern to women who are currently pregnant. However, given that it has an incubation period for 90 days, should women who find out they are pregnant now or in the couple of months, or those trying to get pregnant, get themselves tested. If so, is this just a blood test that can be done through one's doctor. Thanks.

Linda Corso: There is no routine screening test for listeriosis during pregnancy, as there is for rubella and some other congenital infections. If you have symptoms such as fever or stiff neck, consult your doctor. A blood or spinal fluid test (to cultivate the bacteria) will show if you have listeriosis. During pregnancy, a blood test is the most reliable way to find out if your symptoms are due to listeriosis.

Listeriosis is treated with antibiotics. When infection occurs during pregnancy, antibiotics are given promptly to the pregnant women and can often prevent infection of the fetus or newborn. Babies with listeriosis receive the same antibiotics as adults, although a combination of antibiotics is often used until physicians are certain of the diagnosis. There is no vaccine to prevent listeriosis.

If you have symptoms (for example, diarrhea, fever, headache, sore neck) and/or are worried, consult your health care provider.

Ryan Hickman from Canada writes: I have been feeling ill for about a week now but can't for certain say it's from this recent outbreak. I have felt nauseated and sick to my stomach. Do I visit my doctor or do I just give this a couple of days to see if it passes?

Linda Corso: Have you eaten foods listed on the recall list from Maple Leaf Foods? The most current list of recalls from the Canadian Food Inspection Agency (CFIA) can be found here.

The risk of an individual person developing Listeria infection after consumption of a contaminated product is very small. If you have eaten a contaminated product and do not have any symptoms, it is not usually recommended that you have any tests or treatment, even if you are in a high-risk group. However, if you are in a high-risk group, have eaten the contaminated product, and within 2 months become ill with fever or signs of serious illness, you should contact your physician and inform him or her about this exposure.

Ian Caldwell from Canada writes: What should a person do if they have consumed some of this product? What should I watch for as symptoms? Also the last 4 days, I have been dealing with what I think is stomach flew cramps and diarrhea. I consumed this product 1 week ago.

Linda Corso: Although the risk of an individual person developing Listeria infection after consumption of a contaminated product is very small, because you are experiencing flu-like symptoms and the symptoms of foodborne listeriosis are often flu-like and include nausea, vomiting, diarrhea and fever, contact your health care provider and mention your possible exposure.

Symptoms may start suddenly and include: vomiting, nausea, cramps, diarrhea, severe headache, constipation or fever. Some infections become severe and develop into an infection of the brain or the lining of the brain and blood poisoning. Some people only experience mild flu-like symptoms. Symptoms can occur from 1-90 days after eating foods contaminated with Listeria. Certain groups such as the very young, the elderly, pregnant women and those with poorly functioning immune systems (e.g. kidney disease, diabetes, cancer patients receiving chemotherapy, AIDS patients, transplant recipients etc.) are more vulnerable but healthy individuals can sometimes get listeriosis, too.

Although a whole list of pre-cooked meat products has been recalled by Maple Leaf Foods Inc., not all of these products are actually contaminated with the bacteria. However, as a safety precaution, Maple Leaf Foods Inc. has recalled all products manufactured in the specific facility (97B) linked to the outbreak. Although the possibility of a person consuming a product that is contaminated with the bacteria is very small, there is still a possibility. Therefore, we advise everyone NOT TO CONSUME ANY pre-cooked Maple Leaf Meats from facility (97B). All affected products can be identified by the Establishment number that appears on the packages. The products made at this facility bear Establishment number (Est) 97B. The Establishment number is located near the Best Before or Packed On dates.

Roger Bottomsley from Toronto writes: I am finding the list of affected foods somewhat unhelpful because I have purchased meat products from the deli counter at the grocery store (Sobeys) where I shop. Of course, the meats at the deli counter are pre-sliced and I have no idea whether they are Maple Leaf products or another brand. The grocery store has yet to respond to my request for further information. Do I need to follow up (could these be Maple Leaf products?) Thanks.

Linda Corso: Deli counters generally have stickers that they stick onto the front of the packages which provide important information such as the brand, the UPC code, best before dates and other important information. If there is no such sticker provided on the package and the grocery store is unable to help you, it is best not to either not consume it or reheat it to at least 74°C (165°C) before consuming. Heating meat at this temperature will destroy any bacteria that may be present.

Doris Randazzo from Kingston writes: Hello Linda: How careful should we be in washing our fruits, vegetables and meat? I give everything a good rinse first, but I think I am in the minority, and take a lot of kidding about my concerns. Am I being paranoid?

Linda Corso: Your actions are not paranoid; we recommend that all fruits and vegetables be washed under potable water before consumption, especially if they are to be eaten raw. Be sure to remove all visible dirt from the outside of the produce and do not forget the creases and crevices such as the stems of apples. Leafy greens, like lettuce should be washed by removing the outer leaves and then each leaf gently rubbed under running water. Melons and other skinned vegetables like squash should be rubbed with a soft brush while rinsing before being cut into. Attached is our fact sheet about washing fruits and vegetables for more information. Rinsing meat is not recommended because you may be spreading more bacteria throughout your kitchen area than you are removing, leading to possible contamination of other foods and food containers or utensils. For more information, click here for our page on rinsing fruits and vegetables.

Austin Guerin from Canada writes: With this recent outbreak of listeria, how does one know if meat they buy, whether it be prepackaged or not, contains this bacteria? Can microwaving supposedly precooked meat kill this bacteria?

Linda Corso: The presence of Listeria monocytogenes cannot be detected by sight or smell. Harmful bacterial populations will be eliminated when the internal temperature of the meat reaches 165ºF (74ºC). Microwaving is not a suitable method of cooking as it is notorious for uneven heating; undetected cold spots that contain growing bacteria could remain in the food. Reheat any pre-cooked meats by pan-frying, grilling or boiling until the meats are steaming. Use a food thermometer when possible to ensure that the centre of the meat has reached the recommended internal temperature.

Roman Spears from St. Catharines writes: How common is this sort of contamination in other countries? What is it that can be done in the way of inspections/treatment of incoming products at the processing plant or farm, to prevent a recurrence?

Linda Corso: Listeriosis has been reported as a foodborne illness worldwide, particularly in North America, Europe and Japan. The comparison of foodborne illness incidence data between countries is made difficult by differences in the disease reporting and monitoring systems. Under-reporting presents an additional uncertainty factor; mild listeriosis is often mistaken for the flu or a stomach virus. Between 1980 and 2005, 58 cases of invasive listeriosis were reported in Canada; 408 cases were reported in the United States, and 898 cases were reported in Europe. Between 1993 and 2001, 1632 cases of gastrointestinal listeriosis were reported in Europe; 60 cases were reported in the United States; and 38 cases were reported in Japan.

Contamination of extended shelf-life refrigerated products, such as RTE (ready-to-eat) meats and soft cheeses, is the most common method of Listeria transmission. Processing often entails the application of heat, which destroys most bacteria, but contamination can occur after processing if improper handling is practised. Listeria is found everywhere in the environment. Contamination of the product after processing and before packaging can occur through numerous routes: prior contamination of packaging material, utensils, equipment, and/or other food contact surfaces with Listeria; and poor employee hygiene, as examples. Temperature abuse during distribution further exacerbates the contamination by allowing more suitable temperatures for bacterial growth.

Meat inspection and grading in Canada is regulated under federal and provincial law. The Canadian Food Inspection Agency (CFIA) is the federal agency responsible for regulating the Canadian food supply, which works in tandem with provincial governments to maintain food safety and quality. The CFIA's Food Safety Enhancement Program (FSEP) is an initiative for the development and implementation of HACCP in all federally registered establishments. HACCP (Hazard Analysis Critical Control Points) is a food safety and quality assurance program implemented in many meat processing plants, which serves to reduce or eliminate contamination risks from the processing and distribution line. Control measures are set throughout the line at points where contamination is most likely to enter. Inspection by the CFIA serves to ensure that HACCP procedures and good manufacturing practices are in place. Food safety programs are constantly improved and monitored to maintain their efficacy; however, 100% efficacy is almost impossible to maintain — mistakes are made.

Stephen Hardy from Vancouver writes: Hello I have a package of bacon that I believe may have been produced and packaged at the Maple Leaf plant. Although I understand that cooking kills the listeria bacteria, what precautions should I take between the time I open the package and when I complete cooking of the bacon? Or should I simply dispose of the unopened package? In case my refrigerator meat drawer was also contaminated, what cleaning procedures and precautions should I use, to ensure my refrigerator no longer contains listeria bacteria? Thank you.

Linda Corso: Proper food handling is important to prevent the spread of bacteria. Your hands should be washed thoroughly with soap and warm water before handling the contents of the package. Make sure all surfaces and utensils that will come into contact with the meat are clean. The meat should be refrigerated up to the time it is to be cooked. Once the package is opened, cook the meat immediately and cook thoroughly until it is piping hot. You may wish to ensure that it achieves the internal temperature of 165ºF (74ºC) with a food thermometer, if possible. If you are in doubt, throw it out.

Do not leave the cooked bacon at room temperature for longer than 2 hours to prevent re-contamination or re-growth. Wash all surfaces and utensils that have come into contact with the food with warm water and soap, including surfaces in the refrigerator. Additionally, sterilization of all surfaces and utensils can be done with a kitchen sanitizer/disinfectant.

Laurie Reader from Canada writes: How long after a jar of peanut butter or honey is opened can it be safely kept in the cupboard? Should these foods be refrigerated? I would also like to know how long mayonnaise and margarine can be kept in the refrigerator. Thank you.

Linda Corso: Opened peanut butter can be stored 2-3 months, while pasteurized honey can be stored for 18 months because of the high sugar content. These guidelines reflect a decline in quality, rather than food safety. Refrigeration is not necessary. Different brands recommend keeping mayonnaise in the fridge for varying time-lengths. Kraft Canada's guidelines state a 2 month refrigeration time. Commercial margarine can be stored unopened for 8 months, or 1-3 months if opened. Always check product labels for specific best-before date and other storage instructions.

Mr. Malcontent from Canada writes: I want to know about the issue of cross-contamination of Listeria. If I had purchased any of the items on the recall list, and I probably did, whether the original or expanded list, did I by handing it, and then took crackers out of a box, cut a piece of cheese, or what ever, transfer the live bacteria to all other foods that I will eventually consume. If I had the suspect meat in my refrigerator, did the bacteria transfer to everything else. Do we have to destroy every food item in the refrigerator, or for that matter in the entire house and have my home completely sanitized.

Linda Corso: Cross-contamination can be avoided by keeping raw foods separate from cooked ones. It is not necessary to destroy all items in your fridge. Instead, clean all areas that have come into contact with the raw, suspect meat using a kitchen sanitizer and then rinsing with water. The inside of the fridge can also be washed and disinfected to decrease the risk of cross-contamination. Before and after handling any uncooked foods, such as raw meats, wash hands, knives, cutting boards and/or work surface area.

Refrigerated meats that are properly wrapped will not contaminate other foods. If fluids drip from suspect meat onto other products that will be eaten raw/have already been cooked, they should be thrown out.

Tracy Lowe from Canada writes: Can listeria be passed through breastmilk? Is it possible to kill the bacteria by cooking the deli meat?

Linda Corso: Breast milk has been implicated in the transmission of Listeria monocytogenes to the infant, however only one case has been reported in the late 1980's. Infected mothers should receive proper antibiotics and then continue breastfeeding. You can reduce your risk of consuming L. monocytogenes by reheating all hotdogs, luncheon meats and deli meats to at least 74°C (165°C) before consuming. Listeria cannot survive at temperatures higher than this.

L C from Canada writes: Thank you for taking questions on food safety. 1. I recently purchased raw chicken (Maple Leaf brand), is there a possibility that this meat could contain the listeriosis contamination? Will cooking chicken thoroughly remove the risks? 2. When preparing pork products (chops or tenderloin), is it imperative to fully cook the pork, or is it safe to eat when pinkish? Thanks.

Linda Corso: To date raw chicken has not been implicated in any of the cases of listeriosis, however, there is a possibility that raw chicken may contain this or other pathogenic bacteria. Chicken is particularly noted for carrying E. coli and Salmonella. Always practice safe food handling techniques including, keeping raw foods and utensils used on raw foods separate from cooked foods, wash your hands before and after handing raw food materials, and keep foods out of the Temperature Danger Zone (4°C to 60°C). The bacterium Listeria monocytogenes is destroyed cooking temperatures; ensure the internal temperature of the chicken reaches 74°C for chicken pieces or 85°C for a whole bird by using a meat thermometer.

Regarding pork products, colour can not be used as an indicator of the bacteriological safety of the meat. The only indicator that has been proven to help minimize your chance of contracting a bacterial infection from food is cooking to the recommended internal temperature, for pork products this is 71°C.

Paul Cohen from Toronto Canada writes: In the Maple Leaf case, there has been extensive reporting of the bad lot codes and the process to decontaminate the processing facility. However there has been no mention of the incoming supply chain, and the analysis to determine if the bacteria originated with a raw material supplier. I am concerned that there are inadequate controls on unprocessed meat and meat products to prevent the re-contamination of the facility. Can you describe what level of inspection and quarantine of incoming material, by shipment and supplier, is required by provincial and federal Food and Health Agencies ? And, what is Maple Leaf specifically, doing in this regard ? Thank you.

Linda Corso: You make a good point about the incoming supply chain and the need to examine this area since this is one of the ways in which Listeria can be introduced into food processing facilities. I am sure this is one of the areas that is currently being investigated by the public health authorities.

The Canadian regulatory system does address inspection and quarantine of imported meat and plants. The following information may be helpful.

Quarantine of incoming material applies to meat/agricultural products imported into Canada is specified under the Health of Animals Regulations.

The National Meat and Poultry Regulations do not require quarantine of incoming material. Section 11 on standards to be achieved by meat product manufacturers follows. Source here.

PART 11 - PROCESSING AND MEAT PRODUCT STANDARDS

General

66. Every operator shall, where applicable, implement and maintain procedures to:

establish a current written recipe for every prepared meat product;

ensure that the process used in manufacturing a meat product is designed and implemented to ensure a safe product;

identify and control biological, physical and chemical factors in a production process that are critical to delivering or

manufacturing a safe product;

control and monitor the critical factors in the production process during preparation and blending to minimize risk and ensure accuracy of composition;

identify, isolate, evaluate and correct any deviations from established procedures and any defects that might affect product safety; and

evaluate and verify through sampling and testing procedures the effectiveness of controls affecting product safety.

67. An operator must ensure that meat products, ingredients, food additives and packaging materials used in the manufacturing and packaging of all meat products are handled and stored in a manner that avoids contamination of meat products.

68. The recipe for a meat product, and any food additives or nutrients added to the product in accordance with the recipe, must meet the requirements of:

the Food and Drugs Act and Regulations; and

Meat Product Standards

General

69.(1) An operator must ensure that food processing operations in an establishment are performed in a manner that produces meat products that are safe to eat when prepared and eaten in accordance with their intended use.

69.(2) Without limiting the generality of subsection (1):

a. meat products must be:

treated and handled in a manner that ensure that they are not and do not become contaminated; and

kept or stored under conditions that avoid contamination; and

b. food processing operations must be performed in a timely manner that prevents the microbial contamination of meat products."

You may also want to read about the sampling and testing procedures (see section 12 of the National Meat and Poultry Regulations).

In general, food industries established standards for their incoming supplies that are equal or above what the Regulations demand. It is in the interest of manufacturers to have adequate specifications for each of their raw materials. We cannot comment on what Maple Leaf is doing in this regard as most of the standards and procedures are proprietary.

Brodie Fenlon, globeandmail.com: I'm afraid we're out of time. Thank you so much to Ms. Corso and the Food Safety Network at University of Guelph in Ontario. We couldn't get to all the questions, so if you still have a pressing concern or question, please contact the Network directly at (1-866-503-7638) and or by e-mail .