This is the third of a series of articles dealing with the pathogenic bacterium Escherichia coli (abbreviated E. coli) within the context of leafy vegetable crops in California. The purpose of this article is to provide information on the identification, biology, and control of rodents. The presumed role of rodents in the spread of E. coli is also discussed.
Since the outbreak of E. coli 0157:H7 on spinach from California's Central Coast in September 2006, there have been a number of measures proposed and implemented to help reduce the risk of microbial contamination in leafy green vegetables. Unfortunately, there is a lack of science-based information supporting many of these practices.
The presumed role of animals in the microbial contamination outbreaks has caused companies to require growers to exclude animals from fields. As a result, there have been efforts made to control rodents and other species by using traps, baits, and fences.
Terry Salmon, Extension vertebrate pest specialist, gave a presentation on rodent control at the spring California Lettuce Research Board Meeting at Harris Ranch. He discussed the biology of rodents and issues that should be kept in mind when considering rodent control around leafy green vegetable fields. A brief summary of his talk is presented here.
Common rodent species
Voles and mice: Voles have stocky bodies with short legs and tails, and ears that are partially hidden. They live underground and feed above ground. There are a number of mice species with the most common ones in our area being the deer mouse and house mouse.
Voles like dense vegetation and can be discouraged by vegetation management; it is thought that a 15-30 foot wide area cleared of vegetation may deter them. However, deer mice are not readily discouraged by such open areas. Voles and mice can be controlled with rodenticide baits, but you need to know the target animal to make the baiting program effective.
Ground squirrels: California ground squirrels hibernate in the winter and breed once a year in the spring. They have eight to nine young per litter which live underground for six weeks before emerging. They eat green vegetation in the spring and seeds in the summer. Ground squirrels use vision in order to deal with threats from predators, and therefore need open areas in order to detect predators. Therefore, removing vegetation may create more favorable conditions for this species.
Rodents can be controlled by baits such as anticoagulants and zinc phosphide. Zinc phosphide bait is an acute toxicant and is registered for crop use only in alfalfa and artichokes. Anticoagulant baits must be fed upon for a few days, and death occurs in 5-6 days. These baits are often placed in bait stations to keep them from being accessible to birds and other non-target animals.
Broadcasting baits such as zinc phosphide and anticoagulants are acceptable if stated on the label. These baits are deposited on seeds and are not attractive to squirrels in the spring due to their preference for green feed at this time of year. The key to controlling ground squirrels is controlling the burrow system since new squirrels will invade the old systems.
The USDA Food Safety Guidelines recommend that rodent pest “infestations” be controlled; however, it is unclear as to what constitutes an infestation. In general, using rodenticides indiscriminately and extensively is ill-advised due to the risk for developing resistance in the rodent population.
Other measures for controlling rodents include trapping. Exclusion fences may be effective for mice, but are not effective for ground squirrels.
A brief review of pathogenic E. coli and animal involvement is helpful before discussing the presumed role of rodents in foodborne pathogen concerns. Extensive research has made it clear that cattle are by far the most common, prevalent, and important reservoir of E. coli pathogens such as the O157:H7 strain.
Other ruminants such as sheep and goats can also carry these pathogens. Regarding non-ruminant animals as carriers of pathogenic E. coli, however, documented cases occur but are uncommon. The list includes domesticated animals such as cats, chickens, dogs, horses, pigs, and turkeys. Wild animal carriers include deer, feral pigs, Norwegian rats, rabbits, and wild birds (geese, gulls, pigeons, sparrows, and starlings).
In many of these studies, the domesticated and wild animal carriers have been associated with dairy or beef cattle facilities; it is apparent that animals such as dogs, pigs, deer, rodents, and birds have been feeding on cattle feces or otherwise exposed to E. coli from cattle. When rats have tested positive for pathogenic E. coli, all the collected animals were living in and around cattle operations.
In many of these studies, researchers question whether the non-cattle animals play a significant role in the persistence and spread of pathogenic E. coli. The issue of rodent control for food safety purposes needs to be discussed in light of the research information available. To our knowledge, voles, mice, ground squirrels, and other rodents in coastal California agricultural fields have not been found to harbor pathogenic E. coli.
Based on the studies mentioned above, it appears unlikely that these rodents will be found to be a common or important source of O157:H7 and other pathogenic strains. Therefore, unless future research findings indicate otherwise, it is hard to justify extensive trapping, baiting, fencing, and vegetation clearing for the specific purpose of reducing animal vectoring of E. coli O157:H7.
Controlling rodents around vegetable production fields must be carefully thought through due to the complex biology of the species involved. Creating open areas may discourage voles, but not mice and ground squirrels. The widespread use of baits could in the long-run create resistance in the rodent species. Careful observation of rodent pest issues should be conducted and control efforts should be based on need.
Additional research is necessary to determine if rodents in coastal agricultural areas are even involved in harboring or carrying pathogens such as E. coli O157:H7.