SALMONELLA INFECTIONS IN WILD MAMMALS

ALESSANDRA GAFFURI

Istituto Zooprofilattico Sperimentale della Lombardia e dell’Emilia Romagna, Department of Bergamo, Bergamo, Italy

Salmonellosis, also known as paratyphoid fever, Salmonella gastroenteritis, and Salmonella septic syndrome, is a bacterial infection that causes a range of disease, from gastroenteritis and bacteraemia to a subclinical lifetime carrier state.

EPIDEMIOLOGY

Salmonellae are found throughout the world. Salmonella enterica subsp. e nterica and its serogroups A, B, C1, C2, D and E, are commonly found in warm-blooded domestic and wild animals. Salmonellosis has been reported in wild boar (Sus scrofa) and in other wild mammals, such as red foxes (Vulpes vulpes), badgers (Meles meles), hedgehogs (Erinaceus europaeus), European brown hares (L epus euro- paeus) and small rodents. It is seen less frequently in deer, which is probably the result of their feeding habits. Moreo­ver, the absence of the gall bladder, which Salmonella can colonize, also seems to eliminate them as carriers. Envi­ronmental contamination by Salmonella may differ signifi­cantly from one area to another, and wild animals may reflect the Salmonella contamination level of their habitat. Consequently, distribution of the infection and of the various serotypes can be dissimilar within the same species in different locations. In wild boar Salmonella was cultured from rectal faecal samples from 22% of the animals in Northern Portugal¹-⁸) and from caecal content in 27% of the wild boar in Northern Italy(⁹). In Spain two different serotypes were isolated from 7.5% of pooled liver, intestine and spleen samples from wild boar(¹⁰). The most frequent serotypes isolated in these investigations belonged to the S. enterica subsp. e nterica, namely: S.Worthington, S. Coeln, S. Ball and S. Typhimurium. In this species sero­logical surveys in Slovenia¹-¹¹) and in Spain(¹²) showed 47% and 7% positivity, respectively.

Salmonella infection is also described in wild carnivores. From foxes and badgers in Northern Italy S. Typhimu- rium, S.Veneziana, S. Thompson and S. Heidelberg were isolated¹-¹³). In Spain, Salmonella was detected in 7.1% of the red foxes and in 18.2% of the badgers investigated; the most frequently isolated serotypes were S. Enteritidis, S. Newport, S. Give and S. Umbilo(¹⁰). In Norway, 6.5% of red foxes showed faecal carriage of S. Typhimurium, S. Hessarek and S. Kottbus(¹⁴).

In Cornwall, UK, a 14-year study on badgers demon­strated that 7.2% of animals were infected; the most common isolated serotypes were S.Agama, S. Indiana and S. Typhimurium(¹⁵).

Although salmonellosis has been reported in wild rumi­nants, it has seldom been detected in Europe. In Denmark, a roe deer (Capreolus capreolus) was found to be positive for Salmonella, but 575 animals from the same area were found to be negative¹-¹⁶). Low prevalence is also reported in roe deer and red deer ( Cervus elaphus) in Northern Italy'-¹⁷’¹⁸)

TABLE 31.1 Salmonella isolations and seroprevalence in wild mammals.

and in Spain¹-¹⁰).

Small mammals and rodents are considered a potential source of transmission or maintenance of Salmonella. These animals reflect the contamination of their habitat, so infection prevalence can vary significantly)²⁰).

Hedgehog populations are considered high-level carriers of Salmonella in many countries. A study on the prevalence of Salmonella Typhimurium in Norwegian hedgehogs showed results varying from 0 to 40%, depending on the area sampled)²¹), whereas in Denmark and in England the hedgehog seems to be a reservoir for S. Enteriditis PT11)²²).

The data from all of these studies are summarized in Table 31.1.

As Salmonella are probably able to adapt to multiple hosts, it is likely that most wild animals, including rodents, birds and reptiles, are susceptible to infection. The prob­ability of infection is also influenced by environmental contamination and feeding habits. Adult animals generally have a more pronounced immune response and higher level of antibody production towards a variety of antigens and are therefore more resistant than young animals to Salmonella infection)¹⁸); any factor that can lower the immunological response might increase the incidence and severity of infection⁾⁹⁾.

Although animals are the major reservoir for Salmonella, environmental contamination caused by humans and live­stock has become an increasing source of infection. Sur­vival of the bacterium in the environment depends on climatic conditions; wet and shaded areas are more suitable than those exposed to the sun. Salmonella can survive for up to 4 months in pond water and in pasture soil and for more than 2 years in dried bovine manure and avian faeces. Wild avian species can be a source of infection and can also act as carriers and spread the organism in the environ­ment, as well as carnivores, which may be infected with Salmonella following the ingestion of infected prey animals.

Salmonellosis occurs more frequently during the cold months of the year, i.e. November to April in Europe)¹⁰).

A wide range of wild animals can be infected, and it is possible that all species can be accidental hosts and can potentially be reservoirs as healthy carrier-shedders.

The reservoir for Salmonella is the intestinal tract of warm- and cold-blooded animals. As Salmonella can survive in the environment for a long time, infection may occur following oral ingestion of contaminated food or water. The most likely route of transmission is faecal-oral, but infection through the mucous membranes of the con­junctivae or upper respiratory tract is also possible.

PATHOGENESIS, PATHOLOGY AND IMMUNITY

The pathogenesis and evolution of the infection depend on the virulence of the serotype, the infectious dose and the status of the host. The virulence genes of Salmonella determine their ability to overcome host defences and the innate defences of the digestive system. In fact, the alimen­tary tract is a hostile habitat for Salmonella because of the acid barrier of the stomach and the intestinal bacterial flora. Following ingestion, Salmonella rapidly colonize the intestinal tract, invade the intestinal mucosa and multiply in the regional lymphoid tissue. From the intestinal phase the infection can develop to a systemic form as the patho­gens spread from the gut-associated lymphoid tissue )GALT) into the circulatory system. The spleen and the liver are the first organs involved in the septic form; in these organs the bacteria can survive and multiply within the macrophages. Fatal bacteraemia with endotoxaemia may occur a few days after inoculation. The resistance of the host to systemic infection or a low infectious dose can result in chronic subclinical carrier status, and these animals can potentially become shedders.

The lesions associated with salmonellosis differ depend­ing on the disease form and the Salmonella serotype involved. The septicaemic form usually shows hyperaemia of the gastric mucosa, colitis, enlargement of mesenteric lymph nodes, liver and spleen and lung congestion.

A report of Salmonella Choleraesuis infection in free- ranging wild boar describes splenic hypertrophy, focal liver and kidney necrosis and haemorrhagic enteritis¹-²¹). In farmed wild boar Salmonella Choleraesuis was described to cause ulcerative ileitis, typhlitis, colitis and petechial haemorrhage on serosal surfaces; some animals showed yellow foci in the liver and enlarged spleen)⁸). In red deer with Salmonella infection, intestinal meteorism (gas in the digestive system), lung congestion and haemorrhagic petechiae on the serosa was described)¹¹), and emaciation and haemorrhagic enteritis was reported in an infected red fox found dead)¹⁶).

The histological features of the septicaemic form are those typical of Gram-negative sepsis and are mainly asso­ciated with endothelial damage. In particular, hyperplasia of reticular splenic cells and lymph nodes, generalized swelling of endothelial cells and histiocytes and fibrinoid thrombi have been reported. The macroscopic necrotic liver foci are characterized histologically by acute coagu­lative hepatocellular necrosis and reactive granulomas when there is a longer course of development. In the spleen, small necrotic foci containing many Salmonella bacteria may be present. In the enteric form, there is necrosis of cryptal and surface enterocytes, involving also the muscularis mucosa and the submucosa. Necrosis of Peyer’s patches or lymphoid atrophy are characteristic.

Once infected, the host develops a rapid immune response that can be influenced by the dose, the virulence and the antigenic characteristics of the bacteria, and by the age of the host. Very young and old animals may have a poor immunological response.

CLINICAL SIGNS AND TREATMENT

Salmonella infection may be clinically not evident, as sub- clinical carrier status is common.

Clinical infection is seldom described in free-living animals and clinical signs may depend on animal species, host status )physiological, co- infection, stress) and sero­type. The resulting wide range of visible signs depends on whether localization of the infection is enteric or septic. In the former case they are characterized by diarrhoea and weight loss, while in the latter by fever, lethargy, dyspnoea, diarrhoea, abortion and death. During a salmonellosis out­break in farmed wild boar the most frequent clinical signs were progressive weight loss, anorexia, weakness, lethargy and death after about 5 days)⁸). In red deer with peracute and acute disease, anorexia, depression and nervous clini­cal signs were described)¹¹). Experimentally infected foxes did not show any clinical signs)²¹).

Treatment, for example of wild animals such as hedge­hogs brought to rehabilitation centres, is often not recom­mended because of the zoonotic implications, and strict hygiene precautions must be followed. If undertaken, treatment consists of supportive care )fluid therapy, anti- diarrhoeal drugs) and appropriate antibiosis, preferably based on culture and sensitivity testing. Antibiotic treat­ment may, however, promote carrier status. Prognosis is usually grave, especially in cases of septicaemia, and severely affected animals should be euthanized.

DIAGNOSIS

Diagnosis is based on bacterial isolation, whereas serology is useful for population screening.

Cultures can be performed from samples collected during necropsy, and specimens should be taken where visible lesions are present, particularly from the liver, spleen, mesenteric lymph nodes, small intestine and colon. In living animals faecal samples or rectal swabs are suitable for culture. Environmental specimens, such as water, soil or waste and sewage disposal, can also be cultured.

Direct isolation on non-selective (e.g. blood agar) and weakly selective media (Gassner, MacConkey agar) can be performed in the case of septicaemic and acute infection, but standard isolation methods are based on the use of pre- enrichment, enrichment and selective plating media. International standard methods (ISO 6579:2002 annex D) involve pre-enrichment in buffered peptone water, enrichment on modified semi-solid Rappaport-Vassiliadis (MSRV) and isolation on XLD and other selective plate media. Pre-enrichment media are useful for isolating Sal­monella from environmental samples, manure, faeces and subclinical animals and for detecting Salmonella that has been damaged or stressed by inhibitory effects (heating, freezing or otherwise processing). Buffered peptone water or universal pre-enrichment broth is commonly used. Enrichment media are liquid or semi-solid agar media that enhance Salmonella growth and inhibit other bacte­ria. The most widely used media are: Muller-Kauffman broth, selenite F, selenite cystine, Rappaport-Vassiliadis broth, semi-solid Rappaport-Vassiliadis medium and semi-solid Salmonella medium. Selective plating media are inoculated with enrichment media and allow the inhibi­tion of growth of other bacteria and differentiate Salmo­nella colonies through biochemical characteristics. Brilliant green agar, Rambach agar, XLD and Hectoen enteric agar are among the wide range of specific media available. Presumptive Salmonella colonies are identified by bio­chemical tests and serotyping. Other detection methods are immunomagnetic separation, enzyme-linked immu­nosorbent assay (ELISA) and nucleic acid recognition methods; these methods are used in human foodstuff hygiene testing and are not validated for animal and envi­ronmental sources¹-¹²).

There are a number of diagnostic tests for serological screening in livestock. They are used to identify infected populations, not infected individual animals, although they can identify carrier status, when an animal may excrete the bacteria intermittently but has persistent IgG concentrations¹-¹⁸). Serological diagnosis may be affected by the sensitivity of the test and by the limited range of Sal­monella serovars or serogroups the test can detect. Some non- invasive serovars cannot be detected by serological assays; moreover cross-reaction between serovars and other bacteria may occur(¹²). Tube and microagglutination techniques, as well as competitive ELISA, can be used in all animal species, including wild mammals. Serological surveys have been performed on wild boar in Spain⁽¹²⁾ using the tube agglutination test, whereas an ELISA commercial kit was used in Slovenia(¹¹). Wild reindeer in Iceland were tested with lipopolysaccharide-coated antigens from S. Typhimurium and S. Choleraesuis and a specific anti-reindeer immunoglobulin peroxidase conjugate¹-¹⁹).

MANAGEMENT, CONTROL AND REGULATIONS

Salmonellosis is widespread in livestock, especially in the pig and poultry industries; environmental contamination from livestock and human waste and sewage is the major risk for wildlife infection. Therefore, control and manage­ment strategies are adopted to decrease the prevalence of infection in intensive animal husbandry and eliminate environmental pollution. Particular attention should be paid to biosecurity measures and rodent control, as rodents are involved in maintaining the infection in farms and in the surroundings¹-¹⁰). In the member states of the EU, a compulsory survey on the prevalence of Salmonella spp. in pigs and poultry is carried out to achieve the EU’s objec­tive to eradicate Salmonella and other food-borne zoonotic infections.

PUBLIC HEALTH CONCERN

Humans contract Salmonella mainly through the ingestion of contaminated food. As Salmonella may have a wildlife reservoir and also contaminate the environment, some outdoor activities may be responsible for human infection. Drinking contaminated water, eating snow and direct contact with wild birds and their faeces are all factors that increase the risk of infection. In some cases human out­breaks correlate to an infected wildlife population, as happened in Norway, where hedgehogs were probably responsible¹-²¹). Another possible source is game meat. Hunters should be trained to avoid contaminating meat with intestinal content or faeces. A trained person should examine the organs of the hunted animal to detect lesions consistent with diseases that may be dangerous for the consumer. The marketing and trade of game meat must observe EU legislation, in particular Reg. (EC) 853/2004 and Reg. (EC) 852/2004.

SIGNIFICANCE AND IMPLICATIONS FOR ANIMAL HEALTH

Mutual transmission of Salmonella between domestic and wild animals can occur; the most effective way to prevent spread of infection among different species populations is to eradicate the disease in livestock and adopt appropriate husbandry hygiene and management protocols. Safety measures should be taken on farms to control the presence of potential carriers such as birds, small mammals, foxes, badgers and hedgehogs. More attention should be paid to the health of domestic animals that graze on summer pastures to avoid pasture contamination. In addition, human and animal waste should be properly treated before disposal.

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