In 1974, when the US Food and Drug Administration (FDA) approved selenium as a feed supplement, inorganic selenium, primarily as sodium selenite, became the traditional source for dietary supplemental selenium for poultry and livestock (Leeson and Summers, 1991).
That decision in 1974 was based on cost of the selenium supplements and lack of information on selenomethionine, and it was ironic because the commonly used plant- and animal-based feed ingredients contain selenium almost exclusively as organic compounds such as the naturally occurring selenoamino acids (Burk and Hill, 1976; Levander, 1986; Cai et al., 1995).
Without a doubt, the use of inorganic selenium supplements in feeds has improved the performance of all classes of commercial poultry, but modern high-yielding poultry have higher metabolic rates and different nutritional needs compared with poultry from 35 years ago, thereby signaling a need to reassess the nutrient requirements.
Inorganic selenium has some problems associated with its use. Among those problems are the minimal levels of selenium in meat proteins and the potential for toxicity if too high a dietary level of inorganic selenium is provided to chickens. Sodium selenite has a documented pro-oxidant influence in all animals tested (Spallholz, 1997; Terada et al., 1999). Thus, a need to revisit organic selenium as a feed supplement for poultry is apparent.
After many years of laboratory and field research, a source for natural organic selenium (Sel-Plex®, Alltech, Inc.), was approved for use in the poultry industry by the US FDA (Federal Register, 2000 and 2002). Sel-Plex® provides a cocktail of selenium compounds (Kelly and Power, 1995), but selenomethionine in the selenium-enriched yeast cellular protein component is the primary form of selenium in Sel-Plex®.
The organic selenium profile in Sel-Plex® is similar to the organic selenium profile in plants and grains (Kelly and Power, 1995). The organic selenium in Sel-Plex® is readily available and will be absorbed actively (Mahan, 1995) from the intestine via the Na+-dependent methionine transport system (Spencer and Blau, 1962) while sodium selenite is absorbed passively by diffusion from the intestinal tract (Schrauzer, 2000).
Selenium influence on host:virus interaction
There are more than 13,000 scientific studies demonstrating increased susceptibility to infection in association with malnourishment in both humans and lower vertebrate animals. Most of those studies have focused on poor diets and how the host immune system has been negatively affected. In comparison, little is known about how the malnourished host may affect a virus and how that virus then interacts with the host. There is one ultramicro trace element, selenium, that seems to play a major role in infections due to RNA viruses (Combs, 2001; Field et al., 2002; Lyons et al., 2003; Beck et al., 2004).
Research into the influence of malnutrition on host-viral interactions was initiated by Melinda A. Beck and colleagues at the University of North Carolina at Chapel Hill when she discovered the emergence of new viral variants in a selenium-deficient model (Beck et al., 1994; 1995; 1998; 2003). Beck’s group found that mice deficient in selenium were more susceptible to coxsackievirus B3. Those mice infected with a normally harmless strain of coxsackievirus developed myocarditis, because in the selenium-deficient mice, the avirulent virus had mutated to a virulent form. The re-isolated virus was then shown to be virulent by inducing myocarditis in normal mice.
The viral genome mutation from avirulent to virulent was shown to be mediated by increased oxidative stress due to selenium deficiency. Beck and colleagues also demonstrated that even influenza virus is influenced by host selenium status (Beck et al., 2001; Nelson et al., 2001; Beck et al., 2004). Using the low pathogenic influenza A/ Bangkok/1/79 (H3N2) virus strain that produces a mild pneumatitis in mice, Beck et al. (2001) found much more severe pathology in selenium-deficient mice than in mice fed a selenium-adequate diet. Part of the increased level of pathology was due to increased proinflammatory cytokine production in lungs of selenium-deficient mice compared with selenium-adequate mice.
Following Beck’s pioneering work, numerous other studies have revealed similar relationships involving host selenium deficiency and virulence of RNA viruses. Selenium is very important in the maintenance of health status in HIV-infected patients (Foster, 2003). It has been reported that selenium inhibits HIV replication (Look et al., 1997) and reactivation by hydrogen peroxide (Sappey et al., 1994).
Selenium also plays a protective role in patients with hepatitis B and C infections preventing progression to cirrhosis and liver cancer (Yu et al., 1997; 1999). Measles virus infected children given a 12-month nutritional supplement with selenium had significantly lower incidences of diarrhea, fever, and acute lower respiratory infection (Juyal et al., 2004). Additionally, Broome et al. (2004) reported that patients given a live attenuated polio vaccine along with selenium supplementation had improved immune functions and stopped shedding the polio vaccine virus more quickly, and a higher rate of selenium supplementation (100 μg/day) was better than a lower rate (50 μg/day).
Influence of selenium on avian virus:host interaction
To date there has been little information linking selenium with the responses of poultry species to viral infections. One study with chicks was identified in which supplementation of vitamin E and selenium to diets already adequate in these nutrients was assessed (Panda and Rao, 1994). Compared with unsupplemented groups, birds fed the vitamin E and selenium supplement had enhanced immune function, including higher geometric mean titers in the tube agglutination test against infectious bursal disease virus (IBDV) and higher numbers of rosette–forming cells in the peripheral blood.
Newcastle Disease virus (NDV) infections also appear to be affected by vitamin E and selenium supplementation. Bassiouni et al. (1990) observed improved resistance against NDV, increased antibody titers, and improved weight gain associated with selenium and vitamin E supplementation. Swain et al. (2000) have shown that broiler chicken responses to NDV vaccine were enhanced by selenium and vitamin E supplements greater than NRC requirements.
Additionally, Marin et al. (2003) reported increased antibody titers against IBDV and NDV, increased serum protein levels, and prevention of bursal damage in aflatoxin-B1-exposed chickens when the chickens were given dietary supplements of selenium and vitamin E. Singh et al. (2006) reported a synergistic effect between vitamin E and selenium in chickens given a NDV vaccine. NDV-vaccinated chickens had higher hemagglutination inhibition titers when given the combination of selenium and vitamin E along with higher circulating levels of immunoglobulins and circulatory immune complexes.
Reovirus in chickens
In the poultry world, there are numerous economically important RNA viruses. Among these are the reoviruses, which are species-specific and have a broad range of virulence in chickens (Robertson et al., 1984). Reoviruses can infect joints and tendons, the respiratory tract, and the intestinal tract. Recently, there have been numerous field cases of enteric reovirus infections leading to malabsorption syndrome, moderate to severe weight gain depression, and increased mortality.
Malabsorption syndrome was investigated by Jensen and colleagues (1991) who found that both selenium and vitamin E were effective in reducing mortality. In fact, Swick (1995) has recommended that selenium be supplemented to the highest allowable dietary level to reduce reovirus-related mortality associated with malabsorption syndrome. The higher metabolic rates of the modern high meat-yielding broiler chicken might contribute to the severity of RNA virus infection because the RNA viruses depend on high levels of oxidative stress to facilitate replication.
Thus, it is important to investigate the role selenium might play in RNA virus infection. This work was conducted to examine the influence of dietary selenium on intestinal morphology of broiler chickens challenged with an enteric avian reovirus (ARV).
MATERIAL AND METHODS
Cobb broiler chicks were hatched from eggs produced by breeders that had been maintained on isocaloric torula yeast diets that provided 1) no selenium (less than 0.02 ppm), 2) Sel-Plex® as the source of organic selenium (0.3 ppm Se), or 3) sodium selenite (0.3 ppm Se). Hatchling chicks were placed in pens in heated brooder batteries and were given the same diet that had been fed to their parents. Body weights were determined at 14 and 21 days of age, and at those ages tissues were collected for other analyses related to selenium and reovirus infection.
On the day of hatch, 30 chicks in each of the three dietary treatments were placed into either control or ARV-infected groups (total of 180 chicks) in heated metal growing batteries in separate isolation rooms. Chicks in the ARV-infected groups were given an oral gavage of 0.5 mL of the reovirus ARV-CU98 (104.2 plaque-forming units/chick), and control chicks were given the medium only. At 21 days of age, the chicks used in this report were killed by carbon dioxide asphyxiation and intestinal tracts were dissected to determine total and segmental weight and length. Tissues were also collected for histomorphometry.
An individual bird was regarded as the experimental unit, and there were five birds per dietary selenium and virus treatment group analyzed. A total of 30 chickens were used in this phase of the overall study. After dissection, the duodenum and ileal segments were collected and fixed in Carnoy’s solution. Tissues were processed for histology and stained with hematoxylin-eosin.
A computerized microscope-based image analyzer (Southern Micro Instruments, Atlanta, GA) was used to determine the histomorphometric parameters for villus height, villus width at its mid-height, villus perimeter length, crypt depth, external muscle layer thickness, and villus height:crypt depth ratio (Fan et al., 1997). The criterion for selection of histological sections for examination was based on the presence of an intact lamina propria, and villi were chosen that were perpendicularly sectioned through the midline axis.
All data from this completely randomized experimental design were analyzed using the general linear models procedure of SAS (SAS Institute, 1996). Significant differences were determined by analysis of variance and were separated by Least Significant Difference at P≤0.05. For the histomorphometric calculations, an average of ten measurements for each parameter from each bird were statistically analyzed as a one-way analysis of variance.
Fisher’s Least Significant Difference was used to test differences between means only when the analysis of variance indicated significance at P≤0.05 (Motulsky, 2005).
Body weight was depressed significantly by reovirus infection at both 14 days (Control = 514 g; Infected = 362 g) and 21 days (Control = 962 g; Infected = 724 g) postchallenge, but there were no differences in body weights for the three selenium treatments within control and infected groups.
It was of interest however, that weight gain from 14 to 21 days in organic selenium-fed chicks was somewhat greater than that in the no selenium and sodium selenite groups in both control (no selenium = 466 g, Sel-Plex® = 476 g; selenite = 460 g) and infected (no selenium = 355 g, Sel-Plex® = 388 g, selenite = 342 g) groups. The concentration of plasma protein was reduced significantly by reovirus infection at 14 days post-challenge, but Sel-Plex® supplementation had maintained plasma protein at a higher concentration compared with no selenium and selenite feeding at both 14 and 21 days post-challenge.
Reovirus infection increased duodenal and jejunal segment relative wet weight, but reovirus infection did not affect ileum relative wet weight and decreased large intestine wet weight (Table 1).
A selenium effect was seen in the relative weights of the duodenum with organic selenium decreasing relative wet weight in control birds and increasing the relative weight in the virus-challenged birds and the selenite-fed diet, causing a decrease in relative weight in the infected birds. This caused a significant diet × virus interaction for duodenum relative weight compared with the whole of the GIT. On a live weight basis (Table 2), the whole GIT, duodenum, jejunum, and ileum showed increases in relative weight due to infection. The large intestine relative weights were not affected by virus but Sel-Plex® supplementation caused a consistent increase in relative weight.
The heart also showed an increase in relative weight due to selenium treatment. Overall, reovirus-infected birds exhibited longer and heavier gastrointestinal tracts. Reovirus infection did not affect length of the intestines (data not shown). However, there were statistically significant diet effects on average gastrointestinal lengths, with torula yeast increasing length by almost 9% compared with the two selenium source diets (P<0.05).
Table 1. Selenium and reovirus influence on duodenum, jejunum, ileum and large intestine relative weights (%).
In conclusion, it can be stated that dietary selenium supplementation has improved performance of broiler chickens ever since it was introduced as an approved feed ingredient in 1974. Nevertheless, the modern broiler, which is a faster growing and significantly larger bird than the broiler of 1974, is at risk for distress from increased oxidative stress due to its higher rate of muscle growth and metabolism.
It has been demonstrated that it is possible to improve the performance of the modern broiler with a simple replacement of inorganic selenium with organic selenium (Sel-Plex®), which allows for development of an improved redox status compared with sodium selenite (Edens, 2001).
This study demonstrated that the use of Sel-Plex® also has a major influence on intestinal morphology that facilitates the potential for improved nutrient assimilation for digested food. This is especially important for chickens that might have been exposed to enteric viruses, such as the enteric reovirus that has a limited time of residence but long lasting effects on the performance of the bird.
As the bird begins to recover from the viral infection in the gastrointestinal tract around 14 days after virus challenge (Rebel et al., 2004), it is important to resume rapid and efficient assimilation of nutrients.
These results suggest that Sel-Plex®, more so than sodium selenite, has that potential at least in reovirus-infected chickens. These observations suggest that the improvements in performance of broiler chickens fed Sel-Plex® can be partially explained by improved integrity of the intestinal tract and potentially an improvement in the immune status as well.
Authors: F.W. EDENS1, S. BURGOS1, J. READ-SNYDER1, A. CANTOR2 and A. PESCATORE2
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