Fowl Typhoid in Laying Hens: Description of the disease and Protection Conferred by a Live Salmonella Enteritidis Vaccine

Introduction.

Salmonella Gallinarum with the biotypes Gallinarum and Pullorum causes two septicaemic diseases in poultry: Fowl Typhoid and Pullorum Disease. The former Salmonella Pullorum serovar is not recognised anymore as such. Both biotypes are differentiated by a few biochemical and molecular tests. This bacterium is highly adapted to produce a septicaemic disease in birds. In general all other animals’ species are refractory to the disease. Mammals may be infected without showing any illness. In contrast to zoonotic Salmonella serovars, Salmonella Gallinarum do not cause any gastroenteric disease in humans transmitted throughout the food chain. Many animals, including men, may become asymptomatic carriers. Vectors such as rodents, flies, darkling beetles and red mites are very important reservoirs and sources of infection. Red mites may introduce Salmonella Gallinarum directly into the blood stream when they bite the hen.

Fowl Typhoid has been officially eradicated from North America, Oceania, Japan and many European countries. Despite these countries are reported as being “free” of Fowl Typhoid infection, many wild avian species can harbour Salmonella Gallinarum. Fowl Typhoid subsists as an endemic infection in some countries from Central and South America, Africa, Middle East, and several CIS and Asian countries. In some commercial poultry operations Salmonella Gallinarum is found but remains officially underreported, or in fact, the infection may be really unknown when outbreaks take place in backyard flocks.

Birds become chronic carriers passing the disease to their offspring through eggs. Approximately 1/3 of the eggs are contaminated. Cross contamination in the hatchery is crucial for spreading the disease. Horizontal transmission usually takes place orally although sometimes occurs by respiratory route when dry manure is manipulated in the farm and the hen inspires the dust. Environmental contamination (e.g. feed, water and litter) and cannibalism are significant factors triggering the infection. It has to be taken into account that Salmonella Gallinarum is able to survive in favourable environment or inside red mites during several months.

Pathogenesis of Typhoid Salmonella.

Salmonella is able to pass through the intestinal wall following three alternative routes: (i) Salmonella may cross M cells of the Peyer's patches located near the entrance of caecum, (ii) through diffuse lymphatic tissue of the gut; (iii) alternatively Salmonella may enter by the apical pole of intestinal cells and once inside the cytoplasm produces a "Salmonella containing vacuole". Inside this vacuole Salmonella Gallinarum replicates and the bacterial cells are transported to the basolateral side of the cell in where they are liberated in the lamina propria. The third way of entrance is through a dendritic cell. This dendritic cell emits pseudopods between epithelial cells and capture Salmonella that are located in the lumen.

In the lamina propria Salmonella is engulfed by macrophages. In this sub-epithelial location Salmonella causes macrophage apoptosis, process that triggers the inflammation cascade that attracts more phagocytes. Invasion through the brush border and the T junctions of the intercellular space do not cause any damage and therefore goes unnoticed to the immune system. In addition the lack of flagella of Salmonella Gallinarum provides a hiding advantage, as presence of flagellar proteins (which are highly antigenic) stimulate the immune response.

At difference to other intracellular pathogens that multiply free in the cytoplasm Salmonella induces the formation of intracellular vacuoles. These vacuoles mature in 1 hour. After 3 hours latency Salmonella multiplies inside the vacuole. Inside the vacuole Salmonella is protected from the action of antibodies, lysozymes and from antibiotics that are unable of any intracellular action.

Macrophages carry Salmonella Gallinarum inside the "Salmonella containing vacuole" and disseminate Salmonella Gallinarum systemically. Infected macrophages enter by diapedesis into blood vessels and are dragged by the blood stream to the reticulum endothelial and reproductive tissues.

Enterohepatic cycle.

Once Salmonella reached the liver following systemic infection, colonisation of the gall bladder and bile ducts of the liver takes place, in where bacteria replicates extracellularly in the lumen and actively invade the gall bladder epithelium. Afterwards bacteria replicate inside hepatic epithelial cells, into Salmonella-containing vacuoles but, at this stage of infection, do not translocate to the lamina propria and mucosa. This intracellular infection leads to a local inflammatory response mediated by heterophils with subsequent tissue damage and epithelial desquamation, leading to a massive release of Salmonella cells and hepatic cellular debris into the lumen. A massive release of cells and Salmonella clogged the bile ducts and liberates bile into hepatic tissues, which acquire a green colour. At this stage, high numbers of salmonelas are secreted together with the bile into the caudal end of the duodenum by the common bile duct. Only then, Salmonella Gallinarum populated the intestine, is excreted in high number and contaminates the environment. The endogenous infection occurs around the fifth day post-infection. This interval of time is called pre-patent period, which is basically the time elapsed since the bird is orally infected until the appearance of Fowl Typhoid symptoms.

In typhoid infections the invasion and septicaemia occurs immediately after oral invasion and intestinal colonisation occurs at a second stage as result of an endogenous infection, when the bird eliminates salmonellas into the intestinal lumen via the common bile duct. In contrast, with paratyphoid infections the intestinal colonization occurs immediately after oral infection and invasion and septicaemia happens at a second stage when Salmonella have previously colonised the gut.

The events in the liver lead to pathognomonic lesions easily recognized during the necropsy. Adult birds suffering acute Fowl Typhoid usually have a swollen, friable, often bile stained liver due to the destruction of the epithelial cells of the gallbladder and the bile ducts, which leads to a severe stasis of bile. After a fortnight the infection becomes chronic with development of round whitish necrotic foci that are included into the hepatic parenchyma.

Mixed typhoid and paratyphoid infections.

After many studies carried out in mice it is known that Salmonella growth results in an increase in the number of infected cells with low bacterial numbers in most cells. As Salmonella triggers apoptosis the liberated salmonellas are able to invade neighbouring cells in order to grow the necrotic foci as more and more inflammatory cells are attracted to the site. Some of the liberated Salmonella invade the blood stream and develop new necrotic foci. An experiment carried out in mice using 2 different marked strains of Salmonella Typhimurium described very well the mechanism of the necrotic foci development. When the same mouse is inoculated with 2 different marked strains of Salmonella Typhimurium, it was found that each inflammatory focus is composed by only one strain, whereas none inflammatory focus is composed by two strains. This experiment demonstrated that two different strains of Salmonella may invade the same tissues in an independent way and also explains how mixed infections simultaneously occur in the same animal.

Recent research showed that Salmonella Gallinarum together with paratyphoid serovars, including Salmonella Enteritidis, may concurrently infect the same farm and even the same chicken. Molecular techniques allowed the detection of Salmonella Enteritidis in samples that have been taken from Fowl Typhoid diseased laying hens. This may be a common phenomenon that is not normally detected when only standard bacteriology is carried out. In fact, when a meticulous study is performed, and a number of different samples from the same farm are taken during a long period of time, mixed infections with different serovars are commonly detected. For instance, in a survey carried out in laying hen farms, Salmonella Enteritidis was isolated together with many other different serovars, including up to 5 different serovars in the same farm. As typhoid and paratyphoid may occur together in the same farm, in countries with Fowl Typhoid it is necessary to simultaneously protect the chickens against both related serovars: S. Gallinarum and S. Enteritidis.

Both Salmonella Gallinarum and Salmonella Enteritidis colonised eggs.

Salmonella Gallinarum and Salmonella Enteritidis are two phylogenetically related bacteria that recognise a common ancestor bacterium. Both serovars are clonally related and share many common pathogenic factors that allows invasion and egg colonisation. They share the adhesion fimbria SEF14, common D1 O antigens, the same mechanism of infection and intracellular multiplication, and the same lymphokines that allow cross protection among them and the same Salmonella Plasmid Virulence operon. Because they share pathogenic factors, both bacteria have a tendency to invade the reproductive tissues and are able to colonise the hen’s genital tract.

Salmonella Gallinarum often causes multiple misshapen ovary follicles, ceasing the production of eggs. On the contrary, the hen infected with Salmonella Enteritidis usually maintains a normal production, but with contaminated eggs.

Control and eradication are essential.

Breeder poultry flocks usually are free from infection due to strict Government official control plans. In contrast laying hens, particularly in multiple age farms, are more often contaminated because Salmonella persist in the environment or in carrier chickens; hence, new batches of Salmonella Gallinarum free 1 day-old birds may acquire the infection in the farm. It has to be considered that all surviving birds remain infected for life and the disease will persist permanently in the farm unless all animals are eliminated and the complete farming area shall undergo an appropriate period of fallowing after being emptied and, where appropriate, cleansed and disinfected together with treatment with rodenticides, insecticides and acaridicides.

According to the susceptibility of the bird and its immunity the disease may be unapparent or produce variable mortality, ranging from 0 to 100%. High mortality may be triggered by any stress factor as for instance any requirement of high productivity such as an egg production peak, intense reproductive activity or forced molting. High economic costs are due to the disposal of dead birds, the constant loss due to culling, the closure of hatcheries and the increase feed and veterinary costs. In addition there are important economic costs due to commercial limitations such as loss of sanitary status, either for the affected poultry industry as for the country when export products are involved.

Eradication programmes are very costly and indefectibly require Government support. The control of the infection has to be aimed not only to the breeder farms but also to the laying hens and broilers as well. The lack of Government support for the instauration of control plans in laying hen farms is the main cause of the endemic situation that is maintained in many countries. To calculate the cost of eradication it should be considered the cost of elimination and replacement of infected flocks and decontamination of premises. Once the disease is eradicated the costs related to a permanent surveillance aimed to avoid re-infection should be considered. Indirect costs related to training and educations also need to be taken into account.

Diagnostic and monitoring also play an important role.

In contrast to zoonotic Salmonella, serology is used to detect infected flocks and estimate the prevalence of Fowl Typhoid infection within a flock. The rapid whole blood plate agglutination test can identify reactors in the farm because agglutination antibodies appear from 3 to more than 10 days after infection. This test is used in eradications programs only for chickens but is unreliable in turkeys and ducks due to false reactors. Since the extended infections of Salmonella Enteritidis a high percentage of false reactors are usually detected; therefore serology only provides a presumptive diagnosis and requires confirmation by bacteriology or molecular tests before deciding to eliminate positive reactors.

Vaccine based on a metabolic drift mutant of Salmonella Enteritidis.

Metabolic drift mutans have minus mutations in essential enzymes and metabolic compartments. The alterations of metabolic pathways lead to a longer generation time and to a corresponding reduction in virulence. This vaccine strain is very safe because it has 3 independent cromosomal mutations, which prevents any reversion to virulence. The attenuation still ensures invasiveness as well as in vivo propagation during enough time to produce immunity before the vaccine strain is eliminated from the chicken. Therefore this strain elicits mucosal and cellular immunity.

Because the vaccine can be administered at the 1st day of life, at this age the vaccine strain colonisation is very high and mucosal immunity is intensely produced together with competitive exclusion. At this time the vaccine strain constitutes up to a 10% of the intestinal chicken flora; after a maximum of 21 day the vaccine strains is not excreted anymore.

This vaccine strain is unable to survive in the environment avoiding any the risk of possible horizontal transmission. In addition, this live vaccine strain can be easily differentiated from wild type strains by simple antibiotic sensibility tests. During several experiments carried out with the vaccine strain it has been demonstrated no reversion to virulence or transmission through eggs. Also, it has been demonstrated that this vaccine reduces faecal excretion of Salmonella Enteritidis, leading in turn to less contaminated eggs after oviposition. Likewise, less infected organs means less infection in the reproductive tract, which decreases the production of infected eggs before oviposition. Less contaminated eggs means, of course, fewer chances for an outbreak to occur in humans.

Studies with AviPro® Salmonella Vac E against Fowl Typhoid.

These studies were carried out at INTA Balcarce, Argentina. The laying hens used in these trials belonged to the Lohmann Classic layer line. The chickens were colour-sexed at the moment of hatching. Salmonella free chickens were reared in complete isolation from the 1st day of life under strict isolation and high biosecurity measures. All chickens were caged from the 1st day of life and were individually identified with a metallic wing tag. Vaccinated and non-vaccinated chickens were separately reared. The challenges were carried out in a separate building and after the infection hens were kept within isolators.

Vaccinations. According to the dose recommended by the manufacturer, 0.5 mL containing 100 to 500 million salmonellas per chicken were orally administered by gavage of into the crop. A subcutaneous route was also experimentally tested injecting the same oral dose behind the neck. Chickens were vaccinated at the first day of life and at the 6th, 16th and 30th week of life.
Challenge strain. Salmonella Gallinarum INTA 91 was used. The virulence was enhanced by subcutaneous inoculations in 18-week-old cocks.
Challenge dose. The 50 percent lethal dose was calculated in pre-trials. One Lethal dose was set as 0.5 mL containing 20 thousand salmonellas per bird. This lethal dose was orally administered by gavage into the crop. Vaccinated and non-vaccinated laying hens were challenged at 28 and 52 weeks of age.

Optimum protection was observed when the hens were challenged at week 28.

Experimental design. (figure 1) Each experimental group was composed by 17 hens. The group identified as 3-O was given 3 oral doses. The group identified as 2-O-S was given 2 oral doses and the last dose was administered subcutaneously. A further control group remained non-vaccinated. The 3 groups were challenged at week 28. This challenge was done 12 weeks after the last vaccine dose. All birds were killed 21 days after challenge. (Figure 1).

Fowl Typhoid in Laying Hens: Description of the disease and Protection Conferred by a Live Salmonella Enteritidis Vaccine - Image 1

Figure 1: Illustration of the challenge model. Chacana, P. A. and Terzolo H. R. AVIAN DISEASES 50:280–283, 2006.

Shedding of the vaccine strain. After the first vaccination at 1st day of life, the vaccine strain could be recovered from all cloacal swabs up to the tenth day post-vaccination. Thereafter all faecal samples were consistently negative. In contrast after the vaccination boosters given at weeks 6, 16 and 30 of age the vaccine strain could not be isolated anymore. (Figure 2).

Fowl Typhoid in Laying Hens: Description of the disease and Protection Conferred by a Live Salmonella Enteritidis Vaccine - Image 2

Figure 2: Shedding of S. Enteritidis vaccine strain after vaccination at 1st day of age AviPro® Salmonella Vac E. Chacana and Terzolo, AVIAN DISEASES 50:280–283, 2006.

Reduction of Salmonella Gallinarum faecal excretion. When 3 oral doses were administered at the 1st day of life and on weeks 6 and 16, the faecal excretion was reduced from 100% in the hens from the non-vaccinated control group to 20% in the hens of the vaccinated group. When the 3rd dose was administered by subcutaneous route the vaccinated group reduced the faecal excretion to 10%.

Protection against mortality. All except one (16) non-vaccinated hens died whereas all except one (16) orally vaccinated hens survived. No mortality was registered in the group that received 2 oral doses and 1 subcutaneous dose. (Figure 3).

Fowl Typhoid in Laying Hens: Description of the disease and Protection Conferred by a Live Salmonella Enteritidis Vaccine - Image 3

References: 3-O (red): Three oral doses; 2-O-S (blue): Two oral doses and one subcutaneous dose; Controls (green): Unvaccinated hens.

Figure 3: Mortality rate 12 weeks after the 3rd dose of AviPro® Salmonella Vac E. Chacana and Terzolo. AVIAN DISEASES 50:280–283, 2006.

Protection against disease. All diseased hens manifested anorexia, somnolence and depression, but none of them had diarrhea. As a rule Salmonella Gallinarum was isolated from the organs of all dead hens. In contrast, Salmonella Gallinarum could not be recovered from any of the hens that remained alive until their sacrifice days 21 post-challenge.

No protection was observed when the hens were challenged at week 52.

Further studies showed that no protection was afforded when challenges were carried out either at 22 or 36 weeks after the last vaccine dose. It was demonstrated that protection is related with the time elapsed between the last vaccine dose and the challenge. Considering the above, a booster vaccination every 12 weeks is strongly recommended.

Conclusions.

Fowl Typhoid generates important economic losses for the global poultry industry. Salmonella Enteritidis is able to cross immunise against Salmonella Gallinarum. Repeated vaccination protects against mortality, organ colonisation and reduces the faecal excretion rate avoiding spread of salmonelas in the environment. Protection depends on the time elapsed after the last booster vaccination; hence oral revaccinations in drinking water each 3 months is highly recommended. The vaccine strain Rif12/Sm24/Ssq can be used to design strategies to simultaneously prevent both Typhoid and Paratyphoid infections. If Fowl Typhoid and Salmonella Enteritidis cause infections or if Fowl Typhoid is eradicated but Salmonella Enteritidis is present, vaccination with a live S. Enteritidis vaccine is recommended. Vaccination alone is not enough to control salmonellosis; therefore they should be conceived as part of an integrated concept which also includes hygiene, strict biosecurity measures, diagnostic and monitoring, nutritional management and with good farming practices.

Bibliography.

1. Audisio, M.C., Terzolo, H. R. Virulence analyses of a Salmonella gallinarum strain by oral inoculation of 20-day-old conventional chickens. Avian Diseases 46:186-191. 2002.

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3. Chacana, P.A., Terzolo H.R. (2006). Protection conferred by a live Salmonella Enteritidis vaccine against fowl typhoid in laying hens. Avian Diseases 50: 280-283.

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8. Pulido-Landínez, M., Sánchez-Ingunza, R., Guard, J., do Nascimento, V. P. (2014). Presence of Salmonella Enteritidis and Salmonella Gallinarum in commercial laying hens diagnosed with fowl typhoid disease in Colombia. Avian Diseases, 58:165-70.

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