Listeriosis

D J SCHNEIDER

GENERAL INTRODUCTION: REGULAR, NON-SPORING, GRAM-POSITIVE RODS

Introduction

Listeriosis, caused by Listeria monocytogenes, is characterized in some livestock species by three distinct syndromes of meningoencephalitis, abortion or stillbirth, and neonatal septicaemia.^{6, 33} Mastitis and keratoconjunctivitis have also been recorded.^{1, 3} While meningoencephalitis occurs most commonly in ruminants, septicaemia is more frequently found in monogastric animals and young ruminants. The disease is rare in humans and, with the exception of its cutaneous form, is often fatal.^{1, 6, 34}

Listeria monocytogenes is widely distributed in nature, but the disease is usually sporadic and its prevalence low.^{24, 32, 34} Listeriosis particularly affects sheep, goats and cattle. Although the first isolation of L. monocytogenes was from a South African gerbil in 1927,²⁶ the disease has only been reported in goats,⁹ sheep²⁴ and chinchillas⁸ in this country. The disease is not of significant economic importance in southern Africa.

Aetiology

Listeria monocytogenes are Gram-positive, non-acid-fast, non-spore-forming, short, rod-shaped bacteria. Five serotypes, designated 1 to 5, and a number of subtypes have been identified.^{6, 23, 25, 34} Serotype 5 (now called Listeria ivanovii²⁹) is commonly isolated abroad from cases of listerial abortion in sheep, but is of low pathogenicity.^{6, 7}

The optimal growth temperature of the organism is between 30 and 37 °C but some growth may occur in the range of 4 to 44 °C. It propagates best at neutral or slightly alkaline pH and is usually killed at a pH below 5,0. The organism is readily cultured on ordinary media under aerobic conditions, but growth is facilitated by an atmosphere containing 10 per cent carbon dioxide.^{5, 14}

Epidemiology

Listeriosis is found throughout the world. It is, however, more common in countries and regions with a cold, temperate temperate climate, such as New Zealand, certain parts of Australia, North America, Europe, and the UK, than in those with a tropical or subtropical climate. In some countries the prevalence of listeriosis has increased during recent years.^{1, 22, 23} Clinical disease occurs particularly during late winter and early spring.^{4, 6, 21, 22, 32}

Sheep, goats and cattle are the livestock species most frequently affected. The morbidity is usually less than 10 per cent, but the mortality rate in those suffering from meningoencephalitis is often 100 per cent.^{6, 31}

Listeria monocytogenes is widely distributed in nature and has been isolated from a wide variety of healthy and diseased mammals and bird species, as well as from soil, water, sewage, mud and silage.^{5, 18, 32} It may also be found on raw vegetables and fruit. A large proportion of healthy sheep and goats are subclinical carriers of L. monocytogenes, excreting organisms in their faeces and milk when stressed.¹⁵ The organism may withstand low temperature pasteurization of milk, and may be found in dairy products in which it constitutes a danger to human health. It tolerates freezing.³⁰

Listeriosis is often associated with the feeding of large quantities of poor quality silage with a pH in excess of ^{5,5.16, 31, 32, 34} Spoilage and a high pH of silage resulting from poor preparation and maintenance result in a growth of the bacteria, particularly in the top and side layers of the silage.^{10, 13, 23} It is also suspected that, apart from nutritional stress, reduced resistance of the animal because of adverse environmental factors may predispose to disease. ^{24, 31, 34} In sheep, cases of listerial meningoencephalitis may occur 30 days after their initial introduction to silage feeding.²³

Encephalitis has been produced experimentally by the inoculation of L. monocytogenes into the dental pulp⁴ and lips of sheep and goats;² in these cases the incubation periods ranged between 14 and 28 days.

The higher prevalence of listerial meningoencephalitis in certain age groups of sheep has been linked to the changing of teeth in early spring.⁴ It is suspected that ingested L. monocytogenes reach the fine dental terminals of the trigeminal nerve and so cause an ascending neuritis and meningoencephalitis.

Pathogenesis

Listeria monocytogenes is an intracellular organism.¹⁵ In the meningoencephalitis syndrome, these organisms are usually present only in the brain.

Although the pathway by which bacteria reach the brain is unclear, it is thought that they probably gain entrance to the body through wounds in the mucosa of the oral cavity. They then reach the brain stem via branches of the trigeminal nerve. The unilateral occurrence of micro-abscesses and the perivascular infiltration of lymphocytes, which are largely restricted to the brain stem (pons and medulla oblongata), are supportive of this view.¹⁹

Following ingestion, organisms may also penetrate the intestinal mucosa to produce a clinically inapparent infection, a bacteraemia with localization of bacteria in various organs, or a fatal septicaemia. In pregnant animals, organisms localize particularly in the uterus. Infection early in pregnancy usually causes abortion, while in late pregnancy it results in stillbirth or in lambs which develop fatal septicaemia soon after birth.⁶

Clinical signs

The syndromes of meningoencephalitis, abortion and septicaemia seldom occur together in an outbreak.²³

In livestock, meningoencephalitis occurs most commonly in ruminants. In goats, sheep and calves...