zoonoses and communicable diseases common to ... - PAHO/WHO

LISTERIOSIS 175There is no uniform criterion regarding when to reject foods according to thedegree of contamination by L. monocytogenes. Several countries (France, US)require that there be no contamination at all, while others (Canada, Germany) havea certain tolerance. It is impossible to ensure the total absence of Listeria spp. in allfoods (Dehaumont, 1992).In California (USA), a six-month study was conducted on the prevalence ofListeria spp. in environmental samples from 156 milk-processing plants. Listeriaspp. was isolated from 75 (12.6%) of the 597 environmental samples. Half of theisolates were identified as L. monocytogenes. Of the 156 plants, 46 gave positivesamples for Listeria spp. and 19.9% of these isolates were identified as L. monocytogenes(Charlton et al., 1990).Role of Animals in the Epidemiology of the Disease: The epidemiology of sporadiclisteriosis is still not well known. Most researchers consider it a disease commonto man and animals and not a zoonosis per se. It is likely that animals contributeto maintenance of listeria in general in nature and especially to its distribution.Studies conducted in recent years suggest that man and animals can contract theinfection from many sources. Most cases in man occur in urban areas, where thereis little contact with animals. Nonetheless, animals may be the source of the infection.In one case, infection was confirmed in a woman who drank raw milk; the sameserotype of Listeria was isolated from the raw milk and from the woman’s prematuretwins. The etiologic agent was isolated from 16% of cows that had listerialabortions. The previously described outbreaks caused by milk, meat, or vegetablescontaminated by manure from listeria-infected animals demonstrate that animalsmay be an important source of infection.There are indisputable cases of direct transmission of the infection from animalsto man. A cattleman assisted during the delivery of a cow, inserting his arms in theuterus. Within the next 24 hours, a rash appeared on his hands and one arm anddeveloped into pustules. He later experienced fever, chills, and generalized pain. Thesame phage type was isolated from the cow’s vagina and from the cattleman’s pustules(Cain and McCann, 1986). The veterinary profession is particularly at risk ofcontracting cutaneous listeriosis. Many veterinarians have become ill after attendingcows that aborted, fetuses, or newborns, or after conducting autopsies of septicemicanimals. The most frequent lesion is a papular exanthema (Owen et al., 1960;Nieman and Lorber, 1980; Hird, 1987). Contact with sick birds may also causehuman infection (Gray and Killinger, 1966).Diagnosis: Diagnosis can be made only through isolation of the causal agent. Ifthe sample is obtained from usually sterile sites, such as blood, cerebrospinal fluid,amniotic fluid, or biopsy material, seeding can be done directly in blood agar, withincubation at 35°C for a week and daily checks. Listeria can be isolated from anyorgan in septicemic fetuses.In sheep, goats, or cattle with encephalitis, samples of the medulla oblongatashould be cultured. In septicemic fowl, rodents, or neonatal ruminants, blood orinternal organs should be cultured. The “cold enrichment” method is used especiallyin epidemiological investigations and is indicated for culturing highly contaminatedspecimens. However, this method has no diagnostic value for clinical cases becauseof the time it takes, since treatment with antibiotics (preferably ampicillin) shouldbegin as soon as possible to be effective.