Epidermal and Animal Protein Allergens

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Booklet: Epidermals & other animal proteins
Allergy - Which allergens? Phadia AB, 2005.


The non-ingested allergens produced by mammals and birds comprise a large and complex group. Incidence of sensitisation appears to be increasing, and this is partly attributable to three factors. One is the longstanding but growing practice of keeping pets. Western ways are winning out, with record levels of Dog and Cat ownership, and with companion animals living within homes, sleeping on furniture or even with humans on their beds, and being handled and otherwise interacted with quite closely and frequently. With birth rates decreasing almost worldwide, pets help fill in for relationships that are rarer than previously: relationships with children, with younger siblings, and with grandchildren. For this reason and others, a pet allergy can be a particularly distressing one. In purely medical and practical terms, the treatment of choice is obvious: removal of the animal from the home. In emotional and social terms, things can be much more difficult, and this makes it crucial to begin with a correct diagnosis and a thorough understanding of the mechanisms of the allergy.

The second reason for the growth in allergy in this area is the increasing concentration and scale of agriculture and food manufacture. On the traditional farm of previous generations a variety of foodstuffs – meat, plant and dairy – were produced seasonally, usually limiting the exposure of farm workers to any one substance. With massive and monotonous exposure of workers to allergenic substances, it is not surprising that occupational allergy has risen dramatically. One of the predominant risks is allergy to epidermal and other animal proteins. Another important source of occupational animal allergies is laboratories. Rats, Mice, Guinea pigs and other animals are kept in large numbers for large-scale, statistically meaningful research, and workers who feed and handle them and clean their cages receive a high dose of their proteins.

The third important factor is growth in pest populations. Pigeons, Mice and Rats have found a solid ecological niche in the human urban environment, where they have ready food supplies and few predators. Rodents and some birds also infest agricultural environments, which are especially rich in food. Fabulous-sounding accounts are publicised of millions of pests living in certain districts. Modern sanitation and other hygiene measures have limited the infectious diseases pests and their parasites previously spread, but the problem of the presence of the animals and exposure to their various proteins remains. Animal pests are small enough to nest practically anywhere, and they live by foraging excursions, spreading their proteins efficiently wherever food might be found – which is almost everywhere. Rats are the chief offenders: they can pass through very tiny holes, and they can gnaw through concrete. They are also the most difficult to exterminate. As a complicating factor, certain strains of Rats, Mice and Pigeons are also kept as pets.

Animal allergen exposure is by several routes. In the case of birds, it is usually by their dried dung that becomes airborne as particulates, or by substances shed from their feathers. Mammals tend to shed allergens when proteins flake away from the outer layer of skin or epithelium; through serum albumin that is found in their saliva; or through dander, which comprises particles from their hair or fur. But these chemically distinct substances are very often combined, especially in the case of animals like Cats and rodents that wash themselves by licking, spreading saliva on both skin and fur. The urine of some mammals like Rats is a significant allergen and can be found either on the animal or in its environment. Bovine serum albumin occurs in meat and milk and so can act as a food allergen. It is also a substance very frequently employed in laboratories, for testing and for culturing cells and bacteria.

Common symptoms of bird allergy are asthma, allergic rhinitis and allergic conjunctivitis. A more severe syndrome, which can even result in death, is extrinsic allergic alveolitis (also known as hypersensitivity pneumonitis, Bird Fancier's Lung and Farmer's Lung), an inflammation of the lung parenchyma in the terminal bronchioles and alveoli. Bird-Egg Syndrome is characterised by cross-reactivity between extrinsic bird proteins and ingested egg. Symptoms include rhinitis, urticaria, angioedema, and gastro-intestinal problems. Where exposure to bird proteins is intense, as in chicken farming and the feather processing industry, occupational allergies can occur.

Also where mammals are concerned, asthma, allergic rhinitis and allergic conjunctivitis are the most common symptoms. Dermatological symptoms can occur from exposure to some mammals (e.g., Cows), and angioedema and dyspnoea have also been reported after exposure to some (e.g., Deer). In farming, laboratories, and the fur industry, occupational allergies have been reported. Laboratory workers may experience anaphylactic reactions after bites, scratches and needle pricks (1-3). Swine Breeder's Lung, a form of extrinsic allergic alveolitus, is in rare cases associated with pig farming (4-5).

 

Cross-reactivity
Within this group of allergens, crossreactivity must be viewed in the context of enormous genetic diversity. The animals in question have nothing universally in common but the fact that they are warmblooded, terrestrial vertebrates, and in some cases their evolutionary divergence goes back more than a hundred million years. Nevertheless, broad cross-reactivity can be demonstrated.

Sub-groups such as the birds naturally display cross-reactive attributes. For example, Budgerigar and Chicken feather and egg yolk extracts have common epitopes, which may be important in the cross-sensitisation necessary for Bird-Egg Syndrome (6-8). There is an association between Horse, Cow and Deer (9-11), and between Ferret and Mink (12). Allergies to Dog and to Cat have been frequently linked (13-16), and the link has been extended to Fox and Mink.(17) In contrast, the Guinea pig and the Siberian Hamster appear distinctly un-cross-reactive (18-19).

But as indicated above, immunological relationships are not limited to the species among which they might be more or less expected. Cow's hair and Cow's milk represent one instance of cross-reactivity between a respiratory allergen and a food allergen (20-22). Cat and ingested Pork have a special immunological connection (23). Studies have demonstrated the potential for cross-reactivity to albumins from animals as different as Dog, Cat, Mouse, Chicken, Rat and even human (24-25). The lipocalin family of proteins is common to practically all respiratory animal allergens, including Horse, Cow, Dog, Mouse and even Cockroach, and also occurs in the betalactoalbumin of Cow's milk: a certain degree of cross-reactivity can therefore be expected (26).

References:

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    2. Wolfle T, Bush RK. The Science and Pervasiveness of Laboratory Animal Allergy ILAR 2001;42(1):1-3
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    4. Schlegel V, Liebetrau G, Pohl WD. Swine breeder's lung – a form of exogenous allergic alveolitis. [German] Z Erkr Atmungsorgane 1990;174(2):143-8
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    7. Mandallaz MM, de Weck AL, Dahinden CA. Bird-egg syndrome. Cross-reactivity between bird antigens and egg-yolk livetins in IgEmediated hypersensitivity. Int Arch Allergy Appl Immunol 1988;87(2):143-50.
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