The Hidden Link: How Ascarid Infection Weakens Pigs’ Defenses Against Salmonella: By Dr. Brad DeWolf from Southwest Ontario Veterinary Services

Salmonella spp. continues to be the most common bacterial organism identified in food borne illness in Canada. There were 4,826 notifications provided in 2022, representing 39% of all isolates reported to National Enteric Surveillance Program (NESP). Salmonella Enteritidis (1,840 isolates; 38%), S. Typhimurium (319 isolates; 7%) and S. ssp I 4,[5],12:i:- monophasic  (265 isolates; 5%) represented 50% of all Salmonella serotypes identified. Salmonella infection can result in Salmonella organisms residing, for months, in white blood cells associated with the intestinal tract. Stress can allow pigs to start shedding increased Salmonella. Pigs are an important reservoir for the zoonotic salmonellosis, where pigs can serve as asymptomatic carriers. It is no wonder that the critical control points for Salmonella control in pigs are an important component of the Canadian Pork Excellence food safety program. Ascaris suum (roundworms) is a common intestinal parasite. Ascaris continues to be one of the most common causes of partial trim in pork production due to “white spots” on livers. Ascaris infection can be associated with increased cost of production where hygiene of penning is a challenge. These German researchers set up an experimental infection model in order to study the impact of an ongoing Ascaris infection on the immune response to Salmonella in pigs. Pigs were infected with Ascaris first and then Salmonella and then immune function and bacterial shedding were evaluated.

The researchers found the following:

• There were higher Salmonella bacterial burdens in experimentally Ascarid / Salmonella coinfected pigs compared to pigs infected with Salmonella alone.
• The impaired control of Salmonella in the coinfected pigs was associated with repressed interferon gamma responses in the small intestine and with the alternative activation of gut macrophages evident in elevated CD206 expression.
• Ascaris single and coinfection were associated with a rise of CD4-CD8α+FoxP3+ Treg (immune modulating) in the lymph nodes draining the small intestine and liver.
• Macrophages ( white blood cells) from coinfected pigs showed enhanced susceptibility to Salmonella infection and tumor necrosis factor alpha production by myeloid cells was repressed in pigs coinfected with Ascaris.

Take Home Messages:

• The researchers conclude that acute Ascaris infection modulates different immune effector functions with important consequences for the control of tissue-invasive coinfecting pathogens such as Salmonella. They observed that a type 2/regulatory immune response that was induced during an Ascaris infection correlates with increased susceptibility of pigs to the concurrent bacterial infection. ( basically Ascaris response is redirecting the type of white blood cells to the types that are less effective for Salmonella control)
• Pigs experimentally infected with Ascarids and then Salmonella (coinfected pig) have higher Salmonella burdens compared to pigs infected with Salmonella alone. Both infections are widespread in pig production and the prevalence of Salmonella is high in endemic regions of human Ascariasis, indicating that this is a clinically meaningful coinfection.
• This research highlights that achieving control of any one clinical disease should take into consideration the control of other concurrent conditions / infection in order to “tip the balance” towards a healthy outcome.