Rodent infestation and subsequent allergen exposure can occur in a wide range of environments, including homes, schools, hospitals, stores, restaurants, and animal laboratory facilities.1 The amount of rodent allergen exposure in a particular environment depends on numerous factors, the most important of which is the presence of rodents. Other factors include reservoirs and ingress of allergens from other locations.
The health effects of rodent exposure start with sensitization, which leads to sensitivity (ie, allergy) and then to morbidity if exposure continues. Exposure to rodent allergens, particularly to levels above 1.6 μg/g of dust, is associated with an increased risk of developing rodent-specific IgE. For that reason, a recommendation is made to reduce rodent allergen exposure as much as is feasible.
It is important to recognize that exposure to a specific concentration of rodent allergen should not be interpreted to represent a rigid clinical threshold. Allergen levels associated with an increased risk of sensitization or disease may instead represent artifacts related to specific characteristics of a study population and to the distribution of allergen levels in their environments and to their prevalence of sensitization or disease. Reported thresholds may also represent purely statistical phenomena, which should not be assumed to be biologically relevant. Study thresholds also tend to ignore the underlying shape of the exposure-response relationship, presuming that there is a steady, low-level risk up until a specific allergen concentration at which the risk increases to a higher level and remains at that higher level. It is possible that a more linear or even sigmoidal type of relationship exists between exposure and morbidity, which could mean that any amount of exposure might potentially be harmful.
Once sensitization has occurred, continued exposure is associated with a risk of developing disease. If a sensitized person develops asthma or rhinitis, further exposure increases their risk of morbidity. With further exposure to rodent allergens, individuals who are sensitized will tend to develop symptoms, such as wheezing and rhinorrhea. To minimize this risk, exposure reduction is recommended for sensitized individuals. Individuals who are sensitized and who already have developed a respiratory disease are strongly advised to avoid further exposure. It is possible to develop tolerance to rodent allergens. Rodent-tolerant individuals may develop rodent-specific IgG4, which is believed to serve as a blocking antibody. Because this mechanism is still under investigation, no recommendation is made for the routine measurement of rodent-specific IgG4.
The clinical evaluation of rodent allergy begins by asking patients about their rodent exposure along with questions to determine whether there is a relationship between exposure to rodents and clinical symptoms. Animal laboratory workers can usually provide a reasonably accurate history of whether rodent exposure triggers their symptoms; however, homeowners are often incapable of providing an equally accurate history. That is because the extent of exposure to rodents is not always clear unless the home has an obvious infestation. Establishment of a relationship between rodent exposure and symptoms is further complicated because homes with increased concentrations of rodent allergens are likely to have other allergens of similar importance that also can serve as triggers of symptoms. This makes it difficult for a homeowner to report an exclusively rodent-related symptom history.
Patients with suspected exposure to rodents, including all urban dwellers and families living in rural areas, should be evaluated for the presence of specific IgE either with skin prick testing or specific IgE tests. The decision to perform this test should be guided by knowledge of where rodents are likely to be present and whether there is a history of rodent exposure in the patient’s home or workplace. This provides an estimate of the risk that rodent exposure is associated with the risk of developing symptoms. Tests for the presence of rodent sensitization have been studied most extensively in animal laboratory workers. The performance characteristics of skin prick test and measurement of specific IgE for mouse allergy are summarized in Table 1. Immunotherapy for rodent allergy has not been adequately studied, so no recommendations are provided regarding administration of rodent-specific immunotherapy.
The problem with obtaining high-level evidence to demonstrate the efficacy of rodent avoidance measures is that most interventions are nonspecific. Measures that remove rodent allergens also remove other allergens at the same time, making it difficult to prove that removal of the rodent allergen itself is responsible for any clinical benefit that is found. Environmental reduction of rodent allergens can most effectively be accomplished by complete removal of the animals. Unlike the situation with furry animals, homeowners generally are willing to comply with recommendations for rodent removal. Obviously, eradication of a rodent infestation is more challenging from a logistical standpoint than removal of a pet. Exposure assessment and reduction are best attained using integrated pest management (IPM).
IPM begins with identification and removal of facilitative factors (food, water, shelter) that allow populations of rodents to inhabit an environment, thus reducing its carrying capacity. It also includes blocking pathways of rodent ingress by sealing openings. The rodents themselves can be removed with use of bait traps, rodenticides, and, in some cases, introduction of rodent predators, such as cats. Measurement of rodent allergens in settled dust is of uncertain clinical utility because analytic tests are not standardized and clinically relevant exposure thresholds are not well defined. The most potentially helpful use of rodent dust analysis is to evaluate the effectiveness of an intervention by comparing samples from before and after the intervention.
Laboratory animal handlers require special consideration. Sensitization and sensitivity (ie, development of allergy symptoms with exposure to an allergen in a sensitized person) progress rapidly in predisposed individuals who work in such facilities, which is why monitoring of employees for sensitization is recommended. Specific techniques to limit animal allergen exposure in such facilities also are recommended.