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usually aborted spontancously if it is infected during the first trimester; if infeclion occurs later in the pregnancy the fetus may be stillborri or the newbom baby acutely ill. In neonatal listeriosis two distinct clinical syndromes are usually present. An early-onset syndrome. which is primarily septi-cemic, is associated with Iow birthweight, and is characterized by increased neonatal mortality rates (30%) and a higher frequency of reported obstetric complications. The late-onset or meningitic form of the disease occurs in infants of normal birthweight and is characterized by a Iow death ratę (10%) and no obstetric complications.⁴²

CDC has now classified listeriosis as a reporta-ble disease; LCDC is trying to follow suit. Avail-able data indicate that L. monocytogenes is infre-quently identified as a human pathogen. Most infections are probably asymptomatic and develop in vaginal, cervical or intestinal tissues.⁶ The inri-dence ratę is increasing worldwide,^ł,*^4,’^4S possibly because of the increased awareness of the disease, the frequency of organ transplantation, the increasing number of cancer patients and the increasing size of the elderly population.

Although there is no generał agreement on the best therapy for listeriosis, ampicillin, with or without an aminoglycoside, is still recommended.⁴⁶ Trimethoprim-sulfamethoxazole was found to be effective in controlling meningitis due to L. mon-ocytogenes

Immune response

Protection against listeriosis dearly seems to rely on cellular rather than humoral immunity.⁴¹ In mice rcsistance against infection is regulated genet-ically and can be divided into three phases.⁴⁹ The first phase depends on the presence of fixed or resident macrophages in the tissue. Studies have shown that the macrophages will destroy about 90% of the organisms that are present initially; the remaining bacteria grow logarithmically within susceptible macrophages in the liver and the spleen, maximal numbers being reached 48 to 72 hours after the initial exposure.⁵⁰

The second phase of resistance in mice in-volves the accumulation of monocyte-derived in-flammatory macrophages. There is a prompt influx of these cells, which are able to control the rapid bacterial growth. Susceptible mice have a shortage of these macrophages at the site of infection. The inflammatory response seems to be triggered by the listerial resistance gene.⁵¹

The third phase of resistance results in the elimination of the organism and depends on the accumulation of immunologically activated macrophages.⁵⁷-^5J

Serologie tests

Although a number of tests have been des-CMAJ, VOL 138, MARCH 1. 1988

cribed for the serologie diagnosis of listeriosis, the agglutination assay remains the standard method of detecting antibodies to L. monocytogenes.⁵** The use of serologie tests for diagnosing Lis* teria infections seems to be of limited value, however. Newboms and immunocompromised people may not show an increased antibody titre. L monocytogenes antigens will cross-react with the antigens of other gram-positive bacteria⁵⁷*⁵¹ (most notably Staphylococcus aureus and Strepto-cocciis faecalis); therefore, even patients with no previous exposure to Listeria may have a substan-tial Listeria agglutination titre. There appears to be no change in the antibody production from IgG to IgM during the course of infection; therefore, IgM cannot be used as an indicator of recent infection.⁵⁶

Detection of Listeria in foods

There is no acceptable method being used to isolate Listeria from foods, although several prom-ising new methods have been described.^{39 59}*⁶² The cold enrichment procedurę,⁶³ in which samples are kept at 4°C and subcultured weekly for up to 6 months, still appears to be the most sensitive method. However, a morę rapid and sensitive test must be developed to detect Listeria in foods.

Surveillance in Canada

Listeriosis is a notifiable disease in only two provinces, neither of which has to report the cases to LCDC. However, contact between federal and provincial laboratories is eonstant, so that record-ing of confirmed cases is probably good. Less severe illness possibly goes undiagnosed, and early termination of pregnancy without fetal sepsis, or unsuspected cases ending in stillbirth, do occur. The difficulty in identification and the lack of specificity of the serologie tests have resulted in an incomplete epidemiologie picture of listeriosis in Canada.

A surveillance program for L. monocytogenes infection in Canada is being planned by the Department of National Health and Welfare to determine the pubtic health importance of food contamination. A specific objective is to identify infected patients and their surrounding Controls by variables that may lead to an elaboration of impor-tant links between infection and specific foods. A reference laboratory will be established to assist with identification of the organism and with sub-typing (serotyping, phage typing and isoenzyme typing) for epidemiologie purposes.

Overview

Even though the source of most Listeria infections is unknown, the evidence of foodbome transmission in humans is now quite convincing.

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