Summary

Swine (Sus scrofa) urine proteins are inhalant allergens that cause respiratory symptoms in sensitized individuals. In addition to widespread use as production animals, pigs can be found as household pets. As such swine urine protein allergens are an important cause of allergic respiratory disease, particularly in occupational disease among farmworkers, swine production workers, and swine veterinarians.

Epidemiology

Worldwide distribution

Respiratory diseases are commonly seen in workers in swine production facilities. Studies have shown up to 50% of workers in swine production facilities have bronchitis, organic dust toxic syndrome, hyperreactive airway disease, chronic mucous membrane irritation, or other respiratory problems. High frequencies of respiratory symptoms and airway obstruction are also seen in swine veterinarians with work-related symptoms including rhinitis symptoms (60.3%), cough and chest tightness (55.2%), and wheezing (35.1%).

Risk factors

Exposure to swine urine occurs in work settings, such as farms, veterinary clinics and production facilities, or in the home. A survey of European animal farmers found that pig farmers were at highest risk of developing work-related respiratory symptoms, versus farmers working with cattle, poultry, and sheep.

A history of atopic disease predisposes individuals to sensitization; studies have shown that atopic people are more at risk of developing asthma from exposure to environmental antigens than non-atopic persons.

Proximity to swine confinement facilities is a risk factor for respiratory symptoms; prevalence of wheezing among adolescents was 5% higher at schools located within 3 miles of confined swine feeding, compared with those located beyond 3 miles, and 24% higher at schools in which swine odor was discernible twice per month, compared with schools with no discernible odor.

Route Of Exposure

Main

Inhalation.

Clinical Relevance

Exposure to swine urine allergens is a significant cause of IgE-mediated occupational allergy and asthma in both exposed and naïve workers in swine production.

In a review of 14 epidemiologic studies from four countries (n=2,786), cough and phlegm were the most common symptoms (range, 12‒55%), with frequent reports of tightness of chest and wheezing (12‒33%), and episodes of toxic dust syndrome. Acute symptoms directly associated with work were between 1.5‒2x more prevalent than chronic symptoms, however, while IgE antibodies to swine house allergens were common, no relation to symptoms could be found.

A recent systematic evidence update reported the prevalence and incidence of respiratory symptoms in subjects working with swine. In a range of cross-sectional studies, exposure to swine was associated with increased risk of asthma and wheeze (odds ratio [OR]=3.4, 95% CI 1.6‒7.0), rhinoconjunctivitis (OR=1.5, 95% CI 1.2‒1.9), and chronic bronchitis (range OR: 3.2 to 4.3).

A population study of animal farmers in Denmark, Germany, Switzerland, and Spain found a significant dose-response relationship for pig farmers (n=885) between daily hours worked inside animal houses and respiratory symptoms. In this study, farmers keeping only pigs had significantly higher ORs for work-related shortness of breath (OR=1.50, 95% CI 1.18‒1.92), cough without phlegm (OR=1.78, 95% CI 1.43‒2.21), wheezing (OR=1.54, 95% CI 1.16‒2.05), nasal irritation (OR=1.48, 95% CI 1.17-1.88) and flu-like illness (OR=1.37, 95% CI 1.05-1.78) than cattle farmers, and the highest risk of all work-related symptoms of all farmers evaluated except cough with phlegm. However, pig farmers had a lower OR of nasal allergies compared to other farmers.

A case study of four female workers at intensive swine production facilities in Canada reported that each developed acute onset of wheezing and cough suggestive of asthma within weeks of commencing full-time employment. None of the workers were aware of preexisting asthma, allergy or hay fever, and all reported improvements when exposure to swine ceased. However, two individuals remained mildly symptomatic up to 4–5 months after exposure.

One case study reported a highly atopic female (aged 21) with eczema who developed severe itching and shortness of breath within 5 minutes of entering an agricultural pig house. The patient’s eczema settled 2 hours after leaving the building. A skin prick test with 1.0 g/L pig urine extract elicited a 3 mm skin weal, and inhalation of a nebulized dose of pig urine (0.0015 g) for one minute caused a decline in forced expiratory volume of 70% (from 2.3 to 0.7 L). Additionally, a radioallergosorbent test (RAST) demonstrated positive specific IgE antibody to pig urine.

Conversely, a population-based cross-sectional study in Italy suggested that swine breeding did not increase, and might decrease, the risk of pollen sensitization and allergic disease. In this study of native-born swine breeders (n=78) and the general population (‘non-breeders’, n=82), breeders had lower prevalences of respiratory allergy (12.8% vs 31.1%, respectively, p=0.002), asthma (6.4% vs 15.8%, p=0.059), rhinitis (16.7% vs 51.2%, p<0.001), persistent cough (5.1% vs 15.9%, p=0.028), and sensitization to grass (7.7% vs 25.6%, p=0.002).

Cross-Reactivity

There are limited data available regarding the cross-reactivity of swine urine allergens with those of other species.

An early study identified a jointly-recognized cat and pig serum albumin. Inhibition experiments using sera of 10 patients with specific IgE to cat dander and pork showed that the spectrum of IgE reactivity to cat serum albumin completely contained IgE reactivity to porcine serum albumin, which the authors suggested meant that sensitization to cat was the primary event.