Selective breeding in this species has produced the albino Laboratory Rat, widely used for medical and other research purposes.

House Rats comprise two species, Rattus norvegicus, the Brown or Norway Rat, and Rattus Rattus, the Black or Alexandrine Rat, which also originated in Asia, spread worldwide by ships, and is a well-known disease carrier, but has been largely displaced in cooler regions by the Brown Rat. They are roughly similar in appearance and habits. (The Brown Rat is larger, but with a shorter tail and smaller ears, while the dark-grey Black Rat is a better climber.)

Besides the House Rats, the genus Rattus contains several hundred wild-living species. In addition, many other members of the order Rodentia are called Rats: e.g., the Bandicoot Rat, the Wood or Pack Rat, the Rice Rat, the Muskrat, and the Kangaroo Rat. See also Mouse.

Direct or indirect contact with animal allergens frequently causes sensitisation.

Animal allergens are major components of house and laboratory dust.

Unexpected exposure

Most exposure to Brown Rats is unexpected. These secretive, mainly nocturnal animals pass unseen in more or less regular scavenging journeys over a variety of surfaces with which humans have daily contact. Their skin flakes, urine, feces, and saliva are left behind. They have also been known to bite sleepers.

Allergen Exposure

Geographical distribution

This species of Rat is not a native of Norway, as its name suggests. The species originated in Asia, reached Europe early in the eighteenth century and arrived in North America about 1775 on ships from England. Its distribution is now worldwide.

Many consider this Rat to be the greatest mammal pest of all time. It has caused more deaths than all the wars in history. It harbours lice and fleas and has been the source of bubonic plague, typhus, trichina, tularemia, infectious jaundice and other serious diseases. These Rats are usually a contributing factor of first importance in the spread of pandemics during war. They also cause considerable depletion and pollution of human food stores, and damage to buildings and their contents from destructive chewing of wiring, pipes, and walls. But despite human efforts to exterminate Rats, the House Rat population is probably equal to the human population.

The Brown Rat grows up to 25cm long excluding the naked, scaly tail, and sometimes weighs more than half a kilogram. It is commonly brown with whitish underparts and pink ears, feet, and tail. It breeds, and therefore aggressively forages, all year round.

Environment

As small, intelligent, bold, prolifically breeding omnivores, nesting in practically any sort of disused cavity or burrowing in the ground, and adept at swimming, jumping and climbing, Brown Rats are highly adaptable and live in a great variety of environments. They may hide in huge numbers in and around human dwellings, especially in cities, towns and their surroundings, There, they live principally in basements, on the ground floor, in subways, and in burrows under sidewalks or outbuildings. They are also frequently found in cultivated fields, grain storage facilities, livestock housing and garbage dumps. Basically, they are at home wherever there is a food source and sufficient cover from predators, and this includes some unexpected places like the salt marshes of the US Atlantic coast, where edible flotsam is washed up on the beaches.

Especially because of the numbers of Rats used in laboratories, allergy to Rats is an important occupational health problem

Rat dust is a complex allergenic source and contains allergens from Rats' urine, epithelium and saliva (1).

Allergens characterised to date include:

• Rat n 1, a 17 kDa protein (1-4)
   
• Rat n 1.01
   
• Rat n 1.02, a major allergen, a 17 kd protein (also called alpha 2u-globulin)
   
• Rat n 1A (previously known as Ag4), a 20-21 kDa protein, and Rat n 1B (previously known as Ag13), a 16-17 kDa sized protein, are both variants of the same protein. These allergens are found in hair, dander, urine and saliva (4-6).
   
• Rat n 1A is a prealbumin. Rat n 1B is an alpha-2-eu-globulin and primarily a male Rat allergen (6).
   
• Rat n 1.01 and Rat n 1.02 are lipocalins (lipocalin-pheromone binding proteins) (7).

Rat urine has been identified as a major source of the allergens in laboratory animal allergy. The age and sex of the Rat can influence the allergenic composition of the urine (1, 8). Male rodents excrete higher levels of urinary allergens than female rodents (9).

Rodents have permanent proteinuria, and thus the allergen is constantly present in their urine. They spray urine on their surroundings, where the proteins dry up and become airborne on dust particles.

Hair and epithelial fragments also carry allergenic molecules, which are primarily derived from urine and saliva. Most of the allergenic components of urine and saliva have also been detected in the fur extract. Some of the minor allergens are those antigens which appear to be unique to urine, saliva or the skin, suggesting that sensitisation to Rats can result from exposure to allergenic material from all 3 of these sources. Significant concentrations of airborne rodent allergens have been measured in both laboratories and apartments in inner cities (10-14).

Through the study of Rat-allergic patients, at least 23 allergens have been identified in Rat fur. Allergens of molecular weights of 55, 51, 19, and 17 kDa were isolated and determined to be "major" allergens. Other allergens of 74, 67 (probably albumin) and 21.5 (diffuse) kDa molecular weights were also isolated.

Salivary allergens were 17 in number, with "major" allergens of the sizes 21.5, 19.5, 19, 18, and 17.5 kDa. Many Rat-allergic subjects had serum-specific IgE to the 67 kDa (56%) and 43 kDa (64%) allergens. The most important salivary allergens have molecular weights of less than 22 kDa. Fur is the most probable source of the high-molecular-weight allergens found in Rat room dust. There was considerable variation among the Rat-allergic individuals in the binding of IgE to the separated fur and saliva allergens (8).

In serum, 75 kDa and 68 kDa serum protein allergens have been isolated, the former probably serum albumin and the latter probably transferrin. These proteins are also present in Rat urine (15). The prevalence of specific IgE in Rat-allergic patients to the 68 kDa (albumin) allergen is between 24% and 28.9% (16-18).

Prealbumin and alpha(2)-euglobulin (as these were previously termed), detected in Rat urine, are highly homologous and have now been identified as alpha(2)-globulin species. The "prealbumin" fraction corresponds to alpha(2u)-globulin originating from the salivary gland, and the "alpha(2)-euglobulin" fraction has been shown to be identical to the major urinary protein (MUP) or alpha(2u)-globulin. The two major protein fractions of Rat urine thus appear to constitute different forms of the same parent protein, alpha(2u)-globulin. These allergens are found mainly in adult male Rats. Rat n 1B is produced in the liver, where it is androgen-dependant, and in the salivary, mammary and other exocrine glands, where its production is not androgen dependant (7).

The number of Rats, Rats' bedding, cage design, and stock density influence the level of aeroallergen concentration and exposure (19). Rat allergen can also be carried on clothes or by wind to distant sites, with traces of Rat urinary aeroallergens measured in tea rooms inside and near offices outside the animal housing (20-21).

The highest airborne Mouse allergen levels have been measured during manual emptying of cages, during changing of cages on an unventilated table, and during handling of male animals on an unventilated table. Using ventilated cage-changing wagons has been shown to reduce the allergen exposure level from 77 to 17 ng/m3 (22). Similar results can be expected with Rats. Airborne Rat allergens are particles ranging from 1 to 20 micrometres in size, and can remain airborne for 60 minutes or more after disturbance. Rat allergen exposure levels less than 0.7 microg/m3 appear not to be associated with an increased risk of occupational asthma (23). More intense exposure to airborne Rat n 1 and endotoxin occurs not only during cleaning, but also during feeding tasks, probably because the allergens become airborne during the disturbance (24).

The 17 kDa dust allergen has immunological identity with Rat n 1 and is a suitable marker protein for the quantification of airborne Rat allergen (1).

In IgE immunoblot inhibition studies and histamine release tests, it has been demonstrated that patients who react to Dog albumin exhibit IgE reactivity with purified albumins from Cat, Mouse, Chicken, and Rat. Significant sequence homologies have been demonstrated with albumins from different species: Human: 82.6%, Pig: 81.8%, Cattle: 77.3%, Sheep: 78.8%, Mouse: 75.8%, and Rat: 76.2% (25).

Practically all respiratory animal allergens, including Rat, characterised at the molecular level belong to the lipocalin family of proteins. Examples are the major allergens of Horse, Cow, Dog, Mouse and Cockroach as well as beta-lactoglobulin of Cow's milk (26). A certain degree of cross-reactivity is thus possible.

IgE mediated reactions

Rat allergens found in dust, urine, epithelium and saliva are a frequent cause of asthma, allergic rhinitis and allergic conjunctivitis, mainly in laboratory workers but also in ordinary individuals (27-29).

There is a strong association between work-related symptoms and specific sensitisation (30) . Workers exposed to laboratory animals are at risk of developing asthma, rhinitis, angioedema, conjunctivitis, and urticaria. Between 10% and 33% of scientists and technicians handling small animals will develop laboratory animal allergy symptoms within 3 years of employment. Many of them will have severe symptoms requiring a change of occupation (1, 31-33).

In workers exposed to Rats, Rat urinary allergen sensitisation risk increased with increasing exposure intensity. Workers who were atopic had a clearly elevated sensitisation risk at low allergen exposure levels (34). In a cross-sectional study performed on 540 workers at 8 facilities to quantify the exposure-response relationship for allergy to Rats, no clear exposure-response relationship was observed. However, in the group of workers with less than 4 years of working experience with laboratory animals, the prevalence rate of sensitisation to Rat allergens was clearly associated with exposure levels. The exposure-response relationship was steepest for workers with atopy-associated risk factors, i.e., self-reported allergy or sensitisation to Cats or Dogs, or elevated total serum IgE. The prevalence rates of sensitisation to Rat allergens for these workers were about 15, 9.5, and 7.3 times higher in the high-, medium-, and low-exposure group, respectively, compared with the internal reference group (35).

A large epidemiological study of 5,000 laboratory workers reported symptoms in 26% exposed to Mice, 25% to Rats, 31% to Guinea Pigs, 30% to Rabbits, 26% to Hamsters, 25% to Dogs, 30% to Cats and 24% to Monkeys (36).

Two hundred and sixty-three United Arab Emirates nationals with a respiratory disease suspected of being of allergic origin were submitted to skin- and serum-specific IgE measurement. Of these individuals, 8.3% were sensitised to Cat fur, 4.9% to Goat hair, and 0.7% to Rat hair and Mouse hair (37).

Importantly, children of parents exposed to Mice, Rats and Hamsters in an occupational setting, e.g., a laboratory, were shown to be more likely to have allergic symptoms, and to have significantly more positive skin-prick tests against allergens from the hair of laboratory animals, compared to children of non-exposed parents (38).

In a study evaluating the risk of laboratory animal allergy among research staff working in laboratories separate from the animal confinement area, 20% of the subjects had serum-specific IgE >0.35 kU/l to Rat urinary allergens and/or Mouse urinary allergens, and 32% had experienced animal work-related symptoms, although 90% of aeroallergen samples from the laboratories in question were below the detection limit. More than 4 years of exposure significantly increased laboratory animal sensitisation and symptoms. Working mainly with male rodents resulted in increased risk for sensitisation and for symptoms (9).

The suitability of radioallergosorbent test (RAST) inhibition to quantify occupational exposure to Rat urinary aeroallergen (RUA) has been assessed. The authors conclude that, in view of the complexity of Rat allergens, RAST inhibition is an appropriate method for the quantification of occupational exposure to Rats (39).

Anaphylaxis following a Rat bite in a laboratory setting has been described and is clearly an occupational hazard (33).