How plant extracts can improve the immune responses of PRRS-infected pigs

Thursday, April 3, 2014

U.S. research suggests that plant extracts can have a positive effect on growth performance, disease resistance and immune responses of weaned pigs experimentally infected with porcine reproductive and respiratory syndrome virus

by JANICE MURPHY

Porcine reproductive and respiratory syndrome (PRRS) is one of the most economically significant diseases in the swine industry. The clinical signs of PRRS in weaned pigs include fever, lethargy, respiratory distress and poor performance.

The PRRS virus targets immune cells, such as alveolar macrophages, monocytes and dendritic cells, resulting in a complex interaction with the immune system. This can impede the immune system's response and increase the risk for secondary infections in pigs by other invading pathogens.

The swine industry has exhibited a growing interest in the health benefits of plant extracts. These secondary plant metabolites exhibit antimicrobial, anti-inflammatory or antiviral effects which may have beneficial impacts on pig performance and health. Previous research has shown that extracts from capsicum (peppers), garlic and turmeric were effective in reducing diarrhea and decreasing the adverse effects of E. coli infection by producing an anti-inflammatory effect in pigs. However, it is not clear whether these plant extracts have beneficial effects on the immune system of pigs suffering from viral infections.

A study was conducted at the University of Illinois at Urbana-Champaign to evaluate the effects of these three plant extracts on growth performance, disease resistance and immune responses of weaned pigs experimentally infected with porcine reproductive and respiratory syndrome virus (PRRSV).

A total of 64 21-day-old weaned pigs (7.8 ± 0.3 kg BW), tested to be free of PRRSV, were randomly allotted to one of eight treatments in a 2 × 4 factorial arrangement in a randomized complete block design. In the PRRSV challenge group, all 32 pigs were inoculated intranasally with two millilitres of a high-virulence strain of PRRSV (Purdue isolate P-129V) containing 105 per cent tissue culture infective dose. In the unchallenged group, pigs were inoculated with three millilitres phosphate buffered saline (PBS) as the sham control.

The four dietary treatments were:

•    The complex nursery basal diet (CON)
•    CON with 10 mg/kg capsicum oleoresin (CAP)
•    CON with 10 mg/kg garlic botanical (GAR)
•    CON with 10 mg/kg turmeric oleoresin (TUR)

The experimental diets were formulated to meet or exceed National Research Council (1998) requirements of weaned pigs and were provided ad libitum to pigs throughout the experimental period. The control diet did not contain any growth promoters, such as spray-dried plasma, antibiotics or zinc oxide (Table 1).

Pigs were housed in 64 individual pens, four in each of 16 disease containment chambers, in a special facility for 28 days – 14 days before and 14 days after the inoculation (day 0).

Blood was collected on days 0, 7 and 14 post-inoculation to measure the total and differential white blood cells (WBC), and serum was collected to measure viral load, PRRSV antibody titer, tumour necrosis factor-alpha (TNF-α), IL-1β, C-reactive protein (CRP) and haptoglobin (Hp). TNF-α and IL-1β are pro-inflammatory cytokines, which are usually released rapidly following an infection, which in turn trigger the production of the acute phase proteins, such as CRP and Hp, in the liver.

In the unchallenged group, all piglets were PRRSV-negative throughout the experiment. The PRRSV challenge significantly decreased ADG, ADFI and G:F from day 0 to 14 (Table 2). Feeding TUR improved G:F of the PRRSV-infected pigs over CON during the same timeframe.

On day 2 post-inoculation, the PRRSV-infected pigs showed the first signs of lethargy and anorexia. However, they did not exhibit any respiratory symptoms, such as coughing. The PRRSV infection significantly increased rectal temperature on days 7, 9, and 11 post-inoculation compared with the sham group. A significant interaction between PRRSV infection and dietary treatment was observed on day 4: the PRRSV-infected pigs fed the treatment diets had lower rectal temperature than those fed the CON, suggesting that feeding the plant extracts delayed the onset of fever caused by the virus. All pigs in the sham group remained clinically healthy throughout the study.

Lung lesions were evident in the PRRSV-infected pigs, whereas the PRRSV-negative pigs remained lesion-free. In addition, all PRRSV-infected pigs exhibited interstitial pneumonia as well as accumulations of necrotic debris and mixed inflammatory cells in the alveolar spaces of the lungs.

On day 7, PRRSV caused a significant decrease in the numbers of WBC and neutrophils, but the levels increased significantly on day 14, suggesting the PRRSV-infected pigs managed to fight back with a weak immune response. Feeding GAR significantly increased B cells and CD8+ T cells of PRRSV-infected pigs compared with the CON (Table 3). Even though the PRRSV challenge significantly increased serum viral load, TNF-α and IL-1β on day 7, and serum viral load, CRP, and Hp on day 14, PRRSV-infected pigs fed plant extracts were able to significantly reverse this effect (Table 4).

The results of this experiment confirmed earlier research that feeding plant extracts to weaned pigs enhanced the pigs' immune response – in this case to a PRRSV challenge – and may help combat the negative impacts of infection by reducing viral load, pro-inflammatory cytokine, acute phase proteins, and by improving feed efficiency.

The three plant extracts tested in this study showed different effects on performance and immunity. Feeding capsicum and garlic had a bigger impact on the appearance of fever in infected pigs than feeding turmeric, while pigs consuming turmeric had relatively lower viral load and higher feed efficiency than the other two. Although all three plant extracts helped regulate the immune responses of PRRSV-infected pigs, the garlic treatment affected the adaptive immunity of infected pigs as evidenced by the greater number of B cells and CD8+ T cells.

Based on these results, it is clear that supplementation with plant extracts is effective in reducing the adverse effects of PRRSV by improving the immune responses in pigs. In order to precisely identify the mechanisms involved and the different effects of individual plant extracts, further studies are warranted. BP

Source: Y. Liu, T. M. Che, M. Song, J. J. Lee, J. A. S. Almeida, D. Bravo, W. G. Van Alstine, and J. E. Pettigrew. 2013. Dietary plant extracts improve immune responses and growth efficiency of pigs experimentally infected with porcine reproductive and respiratory syndrome virus. J Anim Sci. 2013 Dec; 91(12):5668-79. doi: 10.2527/jas.2013-6495.

Janice Murphy is a former Ontario agriculture ministry swine nutritionist who now lives and works in Prince Edward Island.