MAF to Fund Salmonella Brandenburg Research

4 September 1998

The Ministry of Agriculture and Forestry will provide up to $20,000 for research into Salmonella Brandenburg.

The aim of the project is to determine the source of infection in Otago and Southland and the manner by which the disease is spreading in Otago, Southland and Canterbury.

There will be three main components to the research:

  • Questionnaire

A questionnaire will be used to obtain information from farmers to help determine the source of the disease and the means by which it spread.

The questionnaire has been circulated by Invermay and Lincoln Animal Health Laboratories to veterinary practitioners involved in sheep farming in Otago, Southland and Canterbury. Funding will be required to analyse the results of this questionnaire.

  • Study of the role of black-backed gulls in the spread

To determine the possible role of black-backed gulls in the spread of salmonella it is proposed to examine black-backed gulls for Salmonella Brandenburg on an affected farm and a non-affected farm from each of the following areas: Darfield, Ashburton, Milton, Winton and Otautau areas.

The birds will be poisoned using alphachloralose, a relatively humane poison used in control operations by Department of Conservation. Black-backed gulls are known scavengers and are not a protected species.

Ten birds from each farm will be post-mortemed and the hind gut sampled and cultured for salmonella.

  • Study of the role of carrier ewes

It is known that animals which have survive salmonella can become carriers, with the bacteria remaining in the gut for up to several years, without causing any clinical signs in the animal itself. These animals can pass the bacteria in their faeces and thereby spread it to other animals.

The same farms as above will be used in a study to determine the role of carrier ewes in spread of infection. Twenty ewes from the affected mobs will be identified and their faeces sampled. Samples will also be taken from 20 ewes from the non-affected farms. These samples will be cultured for salmonella and at least one of these cultures will be identified by ESR Communicable Disease Group.

At weaning the same ewes will again be faecal-sampled for salmonella culture.

All salmonella cultures will be sent to Dr Stan Fenwick at the Institute of Veterinary, Animal and Biomedical Science (IVABS), Massey University for typing. This will provide accurate identification of the organisms which will help in the understanding of the likely spread of the infection. IVABS is contributing resources for the work.

Background

Salmonellosis in sheep has been around for many years causing occasional outbreaks of severe diarrhoea and deaths, but until now the type involved has usually been Salmonella Hindmarsh or Salmonella Typhimurium. Until last year, Salmonella Brandenburg had caused only sporadic outbreaks of disease in domestic animals. A review of Laboratory Network files showed that, since 1990, 44 isolates of Salmonella Brandenburg have been recorded. Between 1990 and 1996 there were sporadic cases in dogs, cattle, sheep, goats, pigs, birds and deer. Most of these cases were in the North Island One case was recorded in sheep in Canterbury in 1996.

In 1997 the disease profile changed. Thirty-one isolations of Salmonella Brandenburg were recorded, 27 of them from an epidemic of sheep abortion which involved 17 farms in mid Canterbury and in which a significant proportion of ewes died within a few days of aborting. One 1997 outbreak was in Southland, and again ewes died after aborting.

This year, since late July the disease has occurred in clusters of adjacent farms in the Winton and Milton areas, and it seems to spread radially from infected farms. In the Otautau area the clustering effect is not so evident. Occasional sporadic cases have been recorded outside these areas.

There have been heavy losses on some farms, with up to 1 % of ewes aborting daily and up to 10% of ewes affected in total. On average about 3 to 4 % of ewes abort and without treatment, a third to half subsequently die. There are reports that during an outbreak, ewes with twins are more likely to be affected and at lambing some ewes which have been clinically normal produce one dead and one live lamb.

Affected ewes become very dull and fevered and occasionally they develop severe diarrhoea. The disease has been recorded only in late pregnancy ewes and twin-bearing ewes seem most at risk.

The Southland farm which experienced an outbreak last year is surrounded by affected farms this year, but so far there have been no cases on the farm. In Canterbury outbreaks have again occurred in the area in which outbreaks occurred last year. On one farm which experienced an outbreak last year, the disease has affected only 2-year-old ewes this year. These reports imply that immunity has persisted, at least in older ewes, since last year’s outbreaks.

It is likely that Salmonella Brandenburg is like other types of salmonella in that the reservoir for infection is the intestinal tract of animals and surviving animals may become carriers (subclinical excreters). Salmonellae can survive for 9 months or more in the environment in sites such as moist soil, water, faecal particles and animal feeds.

The source of Salmonella Brandenburg infection in the current outbreaks is not known, but it seems likely that carrier sheep are involved in spreading infection. The increased traffic in ewes from drought-affected Canterbury to Otago and Southland for wintering may have been significant. The relatively mild winter in the south may have favoured survival of the organism in the environment. Mechanical vectors such as seagulls may have contributed by spreading infection from farm to farm, and from paddock to paddock.

Vaccination in the face of an outbreak using the standard Salmonella vaccine has been practised on some farms because Brandenburg shares certain O antigens with Typhimurium. However, practitioners have reported mixed success. On some farms the daily rate of abortions slowed and fewer ewe deaths occurred from 7 days after vaccination, while on other farms the abortions continued as before.

It has been shown that in late pregnancy the antigenic response of ewes to novel antigens is muted, although they may mount an anamnestic response to previously encountered antigens. It may be that ewes which have previously been vaccinated against Salmonella Typhimurium and Salmonella Hindmarsh will be protected relatively quickly if re-vaccinated in the face of a Brandenburg outbreak. Data from the current outbreaks are being collected to test this hypothesis.

Salmonella Brandenburg is a known cause of human salmonellosis, and the number of cases reported annually has been steadily increasing since 1994.

Associated with the recent Otago and Southland outbreaks there have been at least four cases in farmers with affected flocks. Articles written by laboratory staff and veterinary practitioners have been published in local newspapers and practitioner newsletters, warning farmers of the risks and urging them to avoid accidental ingestion of the bacteria by adopting good standards of hygiene when dealing with sheep that might be infected.

Media inquiries:

Debbie Gee, Director, MAF Communications 04-474 4258
Dr Marjorie Orr, Invermay Animal Health Laboratory 03-489 3809
Dr Stan Fenwick, Massey University 06-356 9099 ext 7659

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