(Vernon, CA)  --  U.S. agriculture officials say a company in Vernon, California is recalling more than 20-thousand pounds of frozen pizza after word of possible listeria contamination.  The pizzas were sold at select Walmart stores in boxes labeled as "Marketside Extra Large Supreme Pizza" with lot code 20547.  The pizzas were sent by RBR Meat Company to distribution centers in California, Nevada, Utah, and Washington.  Listeriosis is typically caused by the consumption of contaminated food, and is especially dangerous for older people and pregnant women.