exposure, ensuring compliance is essential. It is recom- mended that educational programs be implemented for physicians and patients as to the most effective therapeutic strategies leading to optimal outcomes (Wang et al 2004). Ownership of pets Pet ownership was found to markedly increase the risk of sensitization to pets (Al-Mousawi et al 2004). The prevalence of asthma, rhinitis, and skin allergy was significantly more common in families with animals than in those without (Bener et al 2004). The secretary proteins from a large number of animals carry or contain powerful allergens capable of causing severe hypersensitivity reactions. Cats and dogs are the main culprits particularly if they are allowed to enter and remain in the bedroom. Allergic rhinitis and asthma are common clinical mani- festations of allergies to animals. Therefore, removal of the pet (cat or dog) from the home is still recommended for sensitized patients with ongoing allergic diseases. The major cat allergen (Fel d 1) is a glycoprotein that is transported in the air by particles smaller than 2.5 µm (Luczynska et al 1990), and these particles can remain airborne for long periods. They are also adherent, contaminating an entire environment for weeks or months after cessation of allergen exposure (Wood et al). A typical example is that allergens adherent to the clothing of people who have cats at home can trigger allergic reactions in sensitized individuals. It makes avoidance and prevention by sensitized individuals much more difficult, requiring public cooperation and awareness. It has been suggested in almost all clinical guidelines for sensitized patients with allergic rhinitis and/or asthma that pets (ie, cats and dogs) should be removed and to also carefully vacuum clean all carpets, mattresses, and upholstered furniture. During the last few years, data of several epidemiologic studies have challenged the conventional understanding for pet avoidance in sensitized patients. Thus, the role of pet keeping during infancy for the development of allergy and asthma remains controversial. It was found that early exposure to a furry animal at home would lead to tolerance and reduced asthma at school age (Hesselmar et al 1999). A recent cohort study in Sweden with 1228 infants who were born over a 1-year period was investigated by questionnaires, and 817 of the children were skin prick tested both at age 1 and 4 years (Sandin et al 2004). The results showed that pet keeping during the first year of life was not associated with an increased risk of atopy at 4 years of age, although a positive skin prick test to cat was more common at 1 year. Therapeutics and Clinical Risk Management 2005:1(2) Allergic rhinitis These findings may even suggest that dog keeping during the first year of life might provide some protection from pollen allergy and late-onset wheezing and increase the risk of early-onset transient wheezing in children with heredity for asthma. A wide range of allergens have been associated with allergic diseases. Total allergen avoidance appears to be effective (eg, symptom-free patients outside the pollen season or occupational exposure). However, patients are often sensitized to many allergens and the degree of exposure varies within indoor and outdoor environments. Moreover, the magnitude of reduction of allergen load needed to reduce symptoms is still unclear. However, it is clear that allergen exposure leads to symptoms. Thus, avoidance measures are still recommended but more research is strongly needed (Bousquet et al 2001). Others The clinical expression of allergic disease has been reported in relation to other factors, such as changes in lifestyle, modification in diet, geographic variations, climate, socioeconomic conditions, family structure or history, infant feeding, excessive allergen exposure especially during early life, and cigarette smoking. Conclusion An increasing worldwide prevalence of allergic diseases such as allergic rhinitis and asthma has been observed over the last decades. The reasons for the increase of prevalence of these diseases are still incompletely understood. Many epidemiological studies have shown that environmental exposure to bacterial products (ie, endotoxin) may have a crucial role in the development of tolerance to ubiquitous allergens found in natural environments. Thus, the hygiene hypothesis becomes particularly attractive, since it is closely linked to modernization of lifestyles and improvement in living conditions. The future challenge is to understand the complex interplay between epidemiology and molecular genetics that have eventually caused sensitization and clinical manifestation of allergic diseases. It will have important implications for the development of new therapeutic strategies and prevention of allergic diseases. References Al-Mousawi MS, Lovel H, Behbehani N, et al. 2004. Asthma and sensitization in a community with low indoor allergen levels and low pet-keeping frequency. J Allergy Clin Immunol, 114:1389–94. 121