Health: Listeriosis - rare in pigs, becoming more frequent in humans

Tuesday, December 2, 2008

The outbreak at Maple Leaf's Toronto plant made headlines. What is listeriosis, how common is it and how can it be avoided?

by S. ERNEST SANFORD

The listeriosis outbreak traced back to contaminated processed meats from a Maple Leaf processing plant in Toronto jumped on to national and international headlines in mid-August.

At the time of writing, more than 50 people had become ill with 19 deaths confirmed, mostly in Ontario, but also in British Columbia, Alberta and Saskatchewan. Almost all deaths have been in the elderly (over 70 years old), many in nursing homes or hospitals. All evidence to date indicates the outbreak is linked to contamination in a meat slicer in the Maple Leaf processing plant.

What does listeria do to pigs and people?
Listeriosis is caused by a bacterium, Listeria monocytogenes, an intracellular aerobic organism which lives in the tonsils of pigs and other animals and is shed in feces, where it contaminates soil and various crops. L. monocytogenes is widespread in nature. It lives very well in the environment and actually multiplies and thrives in cold environments like refrigerators.

Listeriosis is not commonly diagnosed as a primary disease in swine. Over the last decade, however, it has become increasingly diagnosed as a food-borne disease of people, frequently resulting from eating cheeses and deli meats, including hot dogs, cold cuts, processed luncheon meats such as salami or bologna, refrigerated or frozen ready-to-eat foods and from drinking unpasteurized (raw) milk or cream.

Listeriosis in pigs
As a pathologist in the diagnostic lab, I have diagnosed cases of listeriosis in aborted or neonatal piglets perhaps once every other year or so. More precisely, I probably saw about one case a year in the 1970s and early '80s and then maybe one case every other year in the late '80s and '90s.

Obviously my encounter with listeriosis in pigs was very rare, but quite distinctive when I did see it. It appeared in the form of occasional aborted fetuses or several newborn pigs in one or more litters which died and, on postmortem, proved to have multiple tiny abscesses scattered throughout the liver. Bacteriological cultures would confirm L. monocytogenes in the liver and other tissues. 

It was much more common for me to diagnose listeriosis in chinchillas, often as outbreaks on chinchilla ranches with many deaths and multiple, large, clearly visible abscesses scattered throughout the livers.

It was not uncommon in dairy cows and occasionally in sheep also. Affected cows and sheep would circle continuously to one side, called "circling disease," because of Listeria infection in the brain.

Listeria infection in pigs may occasionally cause nervous signs, but it's more likely to result in weakborn pigs and sometimes abortion. By far, however, the more common condition is for pigs to be infected (carriers) with the listeria bacterium but show no clinical signs.

The number of cases in pigs has not increased in recent years, but the number of documented cases of illness and death in humans tracked back to eating processed luncheon meats, cold cuts, raw vegetables and cheeses has increased dramatically.

Public health/ zoonotic concerns
Unlike the situation in pigs, listeriosis has become an increasingly serious public health problem for humans in recent years. It is caused by people eating foods contaminated with the L. monocytogenes bacterium, especially refrigerated or frozen ready-to-eat foods.

Most infections in healthy people probably go undetected or present as people with "flu-like" symptoms of fever, muscle aches and sometimes nausea. Headaches, stiff neck, loss of balance and convulsions occur if infection spreads to the brain. Mild, transient diarrhea sometimes occurs, but infections in pregnant women can result in premature deliveries, abortions, stillbirths or infections in newborn babies.

Others at increased risk from L. monocytogenes include persons with cancer, diabetes, kidney disease, AIDS or those taking immunosuppressive medications (for example,. organ transplant recipients). Healthy persons under 65 are rarely affected.

Between 25 and 51 cases of listeriosis in humans are recorded annually in Canada and about 2,500 serious illnesses and 500 deaths are recorded annually in the United States. The Canadian Food Inspection Agency, the federal agency responsible for food inspections and keeping our food supplies safe, was investigating three more unrelated listeriosis outbreaks in different parts of Canada right on the heels of the ongoing Maple Leaf outbreak. 

Food recalls have increased in numbers in tandem with the increase in the numbers of human cases of listeriosis. In 2007, nearly three million pounds of food were recalled in the United States because of Listeria contamination.

Treatment in pigs
Listeria bacteria are sensitive to penicillin and ampicillin. Listeriosis is so rare as a clinical problem in pigs, however, that treatment is seldom necessary. In rare outbreak situations, it is necessary to identify sources and contributing factors to the infection and reduce or eliminate them.

Treatment and control in people
Most Listeria infections in people are asymptomatic and thus treatment would not be necessary. In immunosuppressed individuals, young children and pregnant women with clinical symptoms, antibiotics are administered to prevent adverse consequences. Even with prompt treatment, however, some infections may result in death. 

Proper cooking of raw pork and other meats kills the bacterium. Perishable and ready-to-eat foods should be consumed as soon as possible. Raw vegetables must be washed thoroughly before eating. Likewise, hands and utensils used in food preparation (knives, cutting boards) must also be washed thoroughly after handling uncooked foods.  Unpasteurized (raw) milk or foods made from unpasteurized milk, such as many soft cheeses, should be avoided. Persons in high risk groups should avoid eating luncheon or deli meats unless they have been reheated until steaming. BP

S. Ernest Sanford, DVM, Dip. Path., Diplomate ACVP, is a swine specialist with Boehringer Ingelheim Vetmedica (Canada) in Burlington. Email: ernest.sanford@boehringer-ingelheim.com