In a two-year study spanning from October 2010 through July 2012, the FDA Center for Veterinary Medicine (CVM) screened over 1,000 samples of pet food for bacteria that can cause foodborne illnesses.1 (The illnesses are called “foodborne” because the bacteria are carried, or “borne,” in or on contaminated food.) The study showed that, compared to other types of pet food tested, raw pet food was more likely to be contaminated with disease-causing bacteria.
Raw pet food was not included in the first year of the study. In the second year, CVM expanded the study to include 196 samples of commercially available raw dog and cat food. The center bought a variety of raw pet food online from different manufacturers and had the products shipped directly to six participating laboratories.2 The raw pet food products were usually frozen in tube-like packages and made from ground meat or sausage.
The participating laboratories analyzed the raw pet food for harmful bacteria, including Salmonella and Listeria monocytogenes. In past projects, CVM had monitored dog and cat food for the presence of Salmonella. But before this study, the center “had not investigated the occurrence of Listeria in pet food,” said Renate Reimschuessel, a veterinarian at CVM’s Office of Research and one of the study’s principal investigators. Dr. Reimschuessel further noted that “quite a large percentage of the raw foods for pets we tested were positive for the pathogen Listeria monocytogenes.” (Pathogens are disease-causing germs, like some bacteria. Not all bacteria are harmful pathogens, though. Some bacteria are helpful to people and animals, such as those that live in the intestines and contribute to a healthy gut.)
Of the 196 raw pet food samples analyzed, 15 were positive for Salmonella and 32 were positive for L. monocytogenes (see Table 1).
|Type of Pet Food Sample||No. samples tested||No. positive for Salmonella||No. positive for L. monocytogenes|
|Raw pet food||196||15||32|
|Dry exotic pet fooda||190||0||0|
|Semi-moist dog foodc||120||0||0|
|Semi-moist cat foodc||120||0||0|
|Dry dog foodd||120||0||0|
|Dry cat foodd||120||1||0|
a Non-cat and non-dog food, such as dry pellets for hamsters, gerbils, rabbits, amphibians, and birds.
Note: CVM did not collect or test canned and wet pet food samples in this study.
Based on the study’s results, CVM is concerned about the public health risk of raw pet food diets. As Dr. Reimschuessel explained, the study “identified a potential health risk for the pets eating the raw food, and for the owners handling the product.” Owners who feed their pet a raw diet may have a higher risk of getting infected with Salmonella and Listeria monocytogenes.
Because raw pet food is more likely than other types of pet food to contain Salmonella and Listeria monocytogenes, the single best thing you can do to prevent infection is to not feed your pet a raw diet. However, the FDA Center for Veterinary Medicine is aware that some people prefer to feed their pets this type of diet.
If you choose to feed raw pet food to your pet, be aware that you can infect yourself with Salmonella or L. monocytogenes by spreading the bacteria from the contaminated food to your mouth. For instance, you may accidentally ingest the bacteria if you touch your mouth while preparing the raw food or after handling a contaminated utensil. If you get Salmonella or L. monocytogenes on your hands or clothing, you can also spread the bacteria to other people, objects, and surfaces.
Here are some tips to prevent infection with Salmonella and L. monocytogenes:
- Thoroughly wash your hands with soap and water (for at least 20 seconds) after handling raw pet food, and after touching surfaces or objects that have come in contact with the raw food. Potential contaminated surfaces include countertops and the inside of refrigerators and microwaves. Potential contaminated objects include kitchen utensils, feeding bowls, and cutting boards.
- Thoroughly clean and disinfect all surfaces and objects that come in contact with raw pet food. First wash with hot soapy water and then follow with a disinfectant. A solution of 1 tablespoon bleach to 1 quart (4 cups) water is an effective disinfectant. For a larger supply of the disinfectant solution, add ¼ cup bleach to 1 gallon (16 cups) water. You can also run items through the dishwasher after each use to clean and disinfect them.
The Difference between Cleaning and Disinfecting
Cleaning removes germs (like bacteria) and dirt from surfaces and objects. Cleaning works by using soap (or detergent) and water to physically remove germs and dirt. This process doesn’t necessarily kill germs, but by removing them, cleaning lowers the number of germs and the risk of spreading infection.
Disinfecting kills germs on surfaces and objects. Disinfecting works by using chemicals, such as bleach, to kill germs. This process doesn’t necessarily clean dirty surfaces and objects or remove germs, but by killing germs after cleaning, disinfecting can further lower the risk of spreading infection.
Source: How to Clean and Disinfect Schools to Help Slow the Spread of Flu – Centers for Disease Control and Prevention
- Freeze raw meat and poultry products until you are ready to use them, and thaw them in your refrigerator or microwave, not on your countertop or in your sink.
- Carefully handle raw and frozen meat and poultry products. Don’t rinse raw meat, poultry, fish, and seafood. Bacteria in the raw juices can splash and spread to other food and surfaces.
- Keep raw food separate from other food.
- Immediately cover and refrigerate what your pet doesn’t eat, or throw the leftovers out safely.
- If you’re using raw ingredients to make your own cooked pet food, be sure to cook all food to a proper internal temperature as measured by a food thermometer. Thorough cooking kills Salmonella, L. monocytogenes, and other harmful foodborne bacteria.
- Don’t kiss your pet around its mouth, and don’t let your pet lick your face. This is especially important after your pet has just finished eating raw food.
- Thoroughly wash your hands after touching or being licked by your pet. If your pet gives you a “kiss,” be sure to also wash your face.
No matter what type of pet food you feed your pet, you should always follow these safe handling instructions.
Salmonella bacteria cause the foodborne illness salmonellosis. Named after Daniel E. Salmon, a veterinarian who spent his career studying animal diseases for the U.S. Department of Agriculture, Salmonella bacteria have been known to cause illness for over 100 years.
Each year in the U.S., about 42,000 laboratory-confirmed cases of salmonellosis in people are reported to the Centers for Disease Control and Prevention (CDC). Because many milder cases are not diagnosed or reported, CDC estimates that more than 1.2 million cases of salmonellosis in people occur annually in the U.S. CDC also estimates that about 400 people die each year from the disease.
A common source of Salmonella infection is contaminated food, such as:
- Raw or undercooked meat and poultry products;
- Raw or undercooked eggs and egg products;
- Raw or unpasteurized milk and other dairy products; and
- Raw fruits and vegetables.
Symptoms start within 12 hours to three days after a person eats contaminated food. People can also ingest Salmonella by handling contaminated food and then transferring the bacteria from their hands to their mouths. Most people recover from salmonellosis in four to seven days without treatment. Children under 5, pregnant woman, the elderly, and people with weakened immune systems (such as people with cancer or other diseases) are at higher risk for salmonellosis and may develop more severe symptoms.
Symptoms of salmonellosis in people include:
- Diarrhea (which may be bloody);
- Vomiting; and
- Stomach pain.
Animals, especially cattle, chickens, rodents, reptiles, and amphibians, can naturally carry Salmonella in their intestines and show no signs of illness. People can get salmonellosis from handling these animals and then transferring the bacteria from their hands to their mouths.
Symptoms of salmonellosis in dogs and cats include:
- Diarrhea (which may be bloody);
- Loss of appetite; and
- Decreased activity level.
FDA has a zero-tolerance policy for Salmonella in pet food, meaning the agency intends to take action against any commercially-made pet food found to be contaminated with the harmful bacteria.
You’ve probably heard of Salmonella and are familiar with the symptoms of salmonellosis, but you may not have heard much about the lesser known foodborne illness listeriosis. Listeriosis is caused by the bacteria Listeria monocytogenes and is a leading cause of hospitalization and death due to foodborne illness, especially in industrialized countries.
Compared to other foodborne illnesses, listeriosis is rare but very serious with a high mortality rate of 20 to 30 percent. Over 90 percent of people with listeriosis are hospitalized. Each year in the U.S., CDC estimates that about 1,600 people get sick with listeriosis, and of these, about 260 die. The European Union has similar numbers: in 2009, there were 1,645 reported cases of listeriosis, with an estimated 270 deaths (Wieczorek et al).
Three scientists, led by E.G.D. Murray, first isolated L. monocytogenes in 1926 from the livers of sick rabbits and guinea pigs.3 But the bacteria weren’t identified as a major cause of foodborne illness in people until the 1980s when several large outbreaks occurred.
L. monocytogenes is widespread in the environment, especially in soil and water. Animals, particularly cattle, can carry the bacteria without appearing sick.
L. monocytogenes have several characteristics that allow them to survive a long time in both food and food processing factories—they can live in both acidic and salty conditions, and unlike most bacteria, they can grow and multiply at low temperatures. This last characteristic makes the bacteria a potential problem even in properly refrigerated food. One study also found a relatively high percentage of frozen raw beef products contaminated with L. monocytogenes (Pao and Ettinger). This finding highlights the ability of the bacteria to thrive in cold environments.
People become infected with L. monocytogenes by eating contaminated food or by handling contaminated food and then transferring the bacteria from their hands to their mouths. Babies can become infected in utero or at birth if their mothers ate contaminated food during pregnancy.
The bacteria can contaminate a variety of food, such as:
- Raw meat;
- Ready-to-eat processed meat such as hot dogs and deli meat (both factory-sealed packages and products sold at deli counters);
- Raw vegetables;
- Refrigerated pates;
- Ready-to-eat smoked seafood and raw seafood;
- Prepared or stored salads, including coleslaw and fresh fruit salad;
- Soft cheeses made with unpasteurized milk; and
- Unpasteurized milk and milk products.
Pasteurization and cooking kill L. monocytogenes. However, in some ready-to-eat food, such as hot dogs and deli meats, contamination may occur after the food is cooked in the factory but before it’s packaged.
Listeriosis occurs almost exclusively in pregnant women, newborns, the elderly, and people with weakened immune systems. People with HIV/AIDS are about 300 times more likely to get the disease than people with normal immune systems. Healthy children and adults occasionally get listeriosis, but rarely become seriously ill.
L. monocytogenes can invade many places in the body, including the:
- Membranes surrounding the brain and spinal cord (called the “meninges”);
- Gastrointestinal tract; and
Symptoms of listeriosis vary depending on the body site, or sites, affected. The length of time between when a person ingests the bacteria and first shows symptoms—the incubation period—also varies depending on which body sites are affected. On average, the incubation period for listeriosis is three weeks. This is long compared to other, more common foodborne illnesses like salmonellosis.
Pregnant women are about 20 times more likely to get listeriosis than other healthy adults. Scientists don’t know why pregnant women are so susceptible to the disease. It usually affects pregnant women who are healthy and don’t have other risk factors.
The most common, and sometimes only, sign of listeriosis in pregnant women is fever. They often have a flu-like illness with non-specific symptoms, such as fatigue and muscle aches. The symptoms are often temporary and go away on their own. Some pregnant women show no symptoms.
While listeriosis in the mother is mild, infection in the fetus and newborn can be severe. The disease causes miscarriage, stillbirth, premature birth, and life-threatening infection of the newborn.
Newborns suffer the most serious consequences of listeriosis. They have either early- or late-onset disease depending on when their symptoms first appear (see Table 2).
|Early-onset listeriosis||Late-onset listeriosis|
|Baby||Usually premature||Usually full-term and healthy|
|Mother||Recent flu-like illness before delivery||No signs of illness before delivery|
|Source of infection||In utero from L. monocytogenes bacteria crossing the placenta from mother to baby||Often unclear—the baby is possibly infected at birth from contact with the mother’s birth canal or gastrointestinal tract (maternal feces), or after birth from the environment|
|Develops||0 to 7 days after birth (average is 36 hours)||5 to 30 days after birth (average is 14 days)|
|Commonly Causes||Blood infection and pneumonia||Meningitis|
|Severity||Up to one-third of newborns die despite adequate treatment with antibiotics||Better prognosis than early-onset disease|
|Sources: Jacobson; Mylonakis et al; Poulsen and Czuprynski; and the Victorian Department of Health (Australia)|
Symptoms of listeriosis in people who aren’t pregnant include fever, muscle aches, headache, stiff neck, confusion, loss of balance, and convulsions.
Although L. monocytogenes can infect many animal species, dogs and cats rarely get listeriosis and they usually don’t show signs of disease. One reference mentions only six reported cases in dogs from 1947 to 2000, and the dogs showed a wide range of symptoms (Läikkö et al).
Listeriosis is mainly seen in ruminants, such as cattle, goats, and sheep. Sheep are particularly sensitive to the bacteria. Ruminants most commonly show severe neurologic symptoms, such as loss of balance, circling, and unusual body spasms. Fever, loss of appetite, and decreased activity level are also usually seen. Some ruminant herds have had large numbers of late-term pregnancy losses. Newborn ruminants typically develop a blood infection. Listeriosis in ruminants most often occurs in the winter and early spring and is likely caused by the animals eating spoiled silage.
Listeriosis is more common in rabbits and rodents than dogs and cats. Rabbits and chinchillas (South American rodents) are particularly sensitive to the bacteria. The disease usually causes a blood infection. Pregnancy loss and uterine inflammation are also common, especially in chinchillas. These reproductive problems are often associated with gastrointestinal symptoms, such as diarrhea or constipation. Large outbreaks of listeriosis have been seen in captive rabbits and rodents. The source of infection in these outbreaks was thought to be contaminated food.
- Goulet V, King LA, Vaillant V, et al. What is the incubation period for listeriosis? BMC Infect Dis 2013;13:11-17.
- Hoelzer K, Pouillot R, Dennis S. Animal models of listeriosis: a comparative review of the current state of the art and lessons learned. Vet Res 2012;43:18-44.
- Jackson KA, Iwamoto M, Swerdlow D. Pregnancy-associated listeriosis. Epidemiol Infect 2010;138:1503-1509.
- Jacobson L. Listeriosis. Pediatr Rev 2008;29:410-411.
- Läikkö T, Båverud V, Danielsson-Tham M-L, et al. Canine tonsillitis associated with Listeria monocytogenes. Vet Rec 2004;154:732.
- Mylonakis E, Paliou M, Hohmann EL, et al. Listeriosis during pregnancy: a case series and review of 222 cases. Medicine (Baltimore) 2002;81:260-269.
- Pao S, Ettinger MR. Comparison of the microbial quality of ground beef and ground beef patties from Internet and local retail markets. J Food Prot 2009;72:1722-1726.
- Poulsen KP, Czuprynski, CJ. Pathogenesis of listeriosis during pregnancy. Anim Health Res Rev 2013;14:30-39.
- Poulsen KP, Faith NG, Steinberg H, et al. Bacterial load and inflammation in fetal tissues is not dependent on IL-17a or IL-22 in 10-14 day pregnant mice infected with Listeria monocytogenes. Microb Pathog 2013;56:47-52.
- Schroeder H, van Rensburg, IB. Generalised Listeria monocytogenes infection in a dog. J S Afr Vet Assoc 1993;64:133-136.
- Wieczorek K, Dmowska K, Osek J. Characterization and antimicrobial resistance of Listeria monocytogenes isolated from retail beef meat in Poland. Foodborne Pathog Dis 2012;9:681-685.
- Centers for Disease Control and Prevention
- National Enteric Disease Surveillance: The Listeria Initiative
- U.S. Department of Agriculture Food Safety and Inspection Service, Food Safety Fact Sheets for Foodborne Illness and Disease
- New York State Department of Health Fact Sheet on Listeriosis (Listeria infection)
- Communicable Disease Prevention and Control Unit, Victorian Department of Health (Australia), Guidelines for the control of infectious diseases, Listeriosis
1 Nemser S, Reimschuessel, R. Food Emergency Response Network (FERN) Microbiology Cooperative Agreement Program (MCAP), FDA Center for Veterinary Medicine (CVM) Special Project: Pet food testing for selected microbial organisms. Final Report 2010-2012. The study was conducted by FDA CVM’s Veterinary Laboratory Investigation and Response Network (Vet-LIRN), in collaboration with FERN MCAP laboratories. The journal citation is Nemser S, Doran T, et al. Investigation of Listeria, Salmonella, and Toxigenic Escherichia coli in Various Pet Foods. Foodborne Pathog Dis 2014;11:706-709.
2 Florida Department of Agriculture and Consumer Services; Michigan Department of Agriculture; Minnesota Department of Agriculture; North Carolina Department of Agriculture; Ohio Department of Agriculture; and Washington Department of Agriculture.
3 The bacteria were originally called Bacterium monocytogenes, but in 1940, the genus name was changed to Listeria.