e222 Sus s 1

Summary

Sus s 1, the serum albumin of Sus scrofa (domestic pig), is a respiratory and food minor allergen present in dander, meat, milk, and saliva. Sus s 1, a member of the serum albumin family, displays cross-reactivity mainly among mammalian homologues. Sus s 1, a heat-labile protein, is denatured during food processing.    

Epidemiology

Worldwide distribution

Contact with S. scrofa occurs mainly during food ingestion, food preparation, and in occupational settings, however it has also been listed as an emerging unconventional pet [1, 2]. Of note, domestic pig, also called “swine”, and wild boar belong to the same S. scrofa species, with the latter being hunted and in some cases farmed [3].

Environmental Characteristics

Source and tissue

Sus s 1, a serum albumin, is an extracellular protein and the most abundant protein component of mammalian blood. It is also present in dander, milk, and saliva  [1]. 

Risk factors

Serum albumin sensitization may occur through inhalation or food ingestion, usually in association with a major allergen of the primary sensitizer [1]. However, serum albumins including Sus s 1 are heat-labile and therefore do not resist thermal treatment [1].

In the occupational setting of swine farming, IgE-mediated hypersensitivity reactions are infrequent, despite high levels of exposure to swine allergens [4].  

Clinical Relevance

Sensitization to serum albumins as food allergens may manifest as clinical reactions of variable intensity, from mild to severe [1].

The clinical relevance of serum albumins as a respiratory allergen is usually difficult to assess given its frequent association with major allergens from the same primary sensitizer and the infrequent finding of monosensitization to serum albumin, however, case reports of severe asthma caused by serum albumins from cat (Fel d 2) or horse (Equ c 3) are available [1].

Cross-reactive molecules

Sus s 1 IgE cross-reactivity, potentially extending to any serum albumin homologue, has been documented for Fel d 2 (cat serum albumin) and Gal d 5 (chicken serum albumin or alpha-livetin) [1, 5]. The degree of cross-reactivity between individual serum albumin may vary [5].

At the clinical level, Sus s 1 cross-reactivity may be relevant for cross-reactivity between pork (S. scrofa meat) and serum albumins from cat (Fel d 2, “pork-cat syndrome, which can also be caused by other cross-reactive allergens), cow (Bos d 6, bovine serum albumin present in cow’s milk and meat) and more generally meat and milk from other species [1, 5].

Molecular Aspects

Biochemistry

Sus s 1 is a large protein of 60 kDa (607 aminoacids), acidic, found as a monomer [1, 6]. Similar to other serum albumin family members, Sus s 1 contributes to the osmotic pressure, divalent ion transport (zinc, calcium, and magnesium), and binds numerous drugs [1, 6]. Sus s 1 allergenicity is heat-labile despite the presence of multiple disulfide bridges [1].

Isoforms, epitopes, antibodies

As of June 13, 2022, only one isoallergen of Sus s 1 has been included in the World Health Organization (WHO) and International Union of Immunological Societies (IUIS) Allergen Nomenclature [7].

Cross-reactivity due to structural similarity

IgE cross-reactivity among serum albumins is mainly explained by their aminoacid sequence identity, and partly by the highly conserved protein structure [1]. Clinically relevant IgE cross-reactivity has indeed been described for Sus s 1 and chicken serum albumin Gal d 5 sharing only 42% of aminoacid sequence identity [1, 5].

Diagnostic Relevance

Serum albumins including Sus s 1 are markers of cross-reactivity in most patients, with a very limited number of reports of monosensitization [1]. 

Cross-Reactivity

Sus s 1 is one of the allergen components involved in the pork-cat syndrome due to cross-reactivity with cat serum albumin Fel d 2 [1, 5]. The pork-cat syndrome denotes an allergic reaction to raw or underdone pork meat in a cat-allergic patient. Of note, clinically relevant cross-reactivity between Sus s 1 and the chicken serum albumin Gal d 5 (alpha-livetin) has been documented [5].

Sus s 1 cross-reactivity with other mammals’ serum albumins may contribute to meat and milk hypersensitivity reactions [1].

Exposure

The main route of exposure to Sus s 1 is through ingestion of pork meat. 

Compiled By

Author: Joana Vitte

Reviewer: Dr. Christian Fischer

Last reviewed: June 2022.