MORBIDITY: +

MORTALITY: ++

ETIOLOGY: BACTERIAL

Listeriosis is caused by Listeria monocytogenes, a gram-positive, motile, very resistant coccobacillus, or oval-shaped rod. It can grow in a wide range of temperatures, from 39° to 111° F. Because it tolerates cooler temperatures, listeriosis is seen more often in temperate and cooler climates. The normal reservoirs of L. monocytogenes are soil and the gastrointestinal tracts of mammals. The organism can be killed by pasteurization or cooking, but it can grow in food stored in a refrigerator, in food with a high salt content, and in an acid environment.

HOSTS

L. monocytogenes has been found in mammals, birds, fish, insects, crustaceans, water, milk, cheese, deli meats, hot dogs, sewage, silage, feces, genital secretions, nasal discharges, and soil. In humans, groups that are most affected are pregnant women, newborns, people with compromised immune systems, elderly people, and those taking steroids. These groups are considered high-risk groups. In animals, adult ruminants seem to be most commonly affected. Healthy people and animals rarely get sick if infected with L. monocytogenes but can pass the organism in their feces.

TRANSMISSION

L. monocytogenes is spread from animal to animal via the fecal-oral route. An infected animal passes L. monocytogenes in its feces, which contaminates vegetation. A susceptible host eats the vegetation and becomes infected. Animals also get infected by eating contaminated silage and hay or by drinking contaminated water. People usually become infected with L. monocytogenes by eating food or drinking water that has been contaminated with feces that contain the organism. Some ready-to-eat foods become infected after they have been heat processed (Figure 24).

FIGURE 24 Listeriosis.

LISTERIOSIS IN ANIMALS

The most frequently seen form of listeriosis in animals is encephalitis, seen in adult ruminants and, rarely, in adult pigs. Sheep and goats are most susceptible to L. monocytogenes infection. The disease is one of winter and spring, because this is a time when animals are confined and fed silage or hay. If either the silage or hay has been contaminated with L. monocytogenes, it could be a source of infection to the animals.

The organism enters the body through abrasions in the mouth and travels to the brain, where it causes unilateral encephalitis. The incubation period is about 10 days. The signs exhibited by an infected animal include depression, fever, and incoordination. Infected animals will segregate themselves into corners and frequently are seen leaning against a fence or building. An infected animal may start circling in one direction. The direction is determined by the side of the brain affected (Figure 25). If the facial nerve is affected, it will be unilateral and will be seen as a drooping ear, drooping eyelid, dilated nostril, drooping lower lip, tongue protrusion, excessive salivation, and difficulty eating. As the disease progresses, the animal becomes progressively paralyzed, will fall down on its side and paddle its legs as if running, enter into a coma, and eventually die. The mortality rate can be as high as 70% in sheep. Cattle seem to be a bit more tolerant, but the mortality rate can still reach 50%.

FIGURE 25 Listeriosis in a sheep showing unilateral facial paralysis

(From Quinn PJ et al: Clinical veterinary microbiology, London, 1994, Mosby.)

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