Aerosol Transmission of Infectious Viral Diseases in Swine

Infectious Pig Diseases are a Problem for Both Producers & Consumers.

By Qiang Zhang & Amy La, University Of Manitoba

For producers, diseases can cause a loss of productivity both in mortality and growth and increase the costs associated with medical care and bio-security (Holtkamp et al. 2013).

For consumers, disease spread will cause shortages and price increases of pork products as well as food safety concerns.

Some examples of infectious pig diseases that affect Manitoban pig farms are porcine epidemic diarrhea virus (PEDV) and porcine reproductive and respiratory syndrome virus (PRRSV). Suspected cases of PEDV are investigated and monitored by the Manitoba government (Manitoba Agriculture and Resource Development 2022).

There are infectious pig diseases that are significant in other parts of the world but have not been recently introduced into Canada, including foot-and-mouth disease virus (FMDV) and African swine fever virus (ASFV), which are federally reportable diseases. The entry of these diseases into Canada would be highly damaging to the hog industry (Inspection Canada 2021).

How are infectious animal diseases spread?

There are several routes in which infectious viral diseases can be spread by or transmitted to animals:

  1. Direct contact: an animal becomes infected from being in direct contact with an infected animal.
  2. Indirect contact (fomite): an animal becomes infected from contacting a contaminated object (fomite) such as clothing, boots, vehicles or farm equipment.
  3. Oral: an animal becomes infected from eating or drinking items that are contaminated with virus.
  4. Vector: an animal becomes infected from being in contact with an infected insect, bird or pest.
  5. Airborne: viral pathogens attached on aerosols travel through the air and cause infection in an animal by being inhaled (aerosol transmission) or by landing on an object that the animal later contacts (indirect contact or fomite transmission).

Pigs standing in barn
    Jodie Aldred photo

Of the five routes of transmission, airborne transmission is least understood and most difficult to prevent.

What is an aerosol?

An aerosol is a small solid or liquid particle that stays afloat in the air for a long time because of its small size. When viruses are present in aerosols, they are considered to be airborne. Viruses that are airborne can travel over long distances in the wind from an infected farm to a downwind farm, leading to possible cases of aerosol or airborne transmission.

Standard biosecurity protocols recommended by the Canadian Swine Health Board are effective in reducing cases of direct contact, fomite, oral and vector transmission (Canadian Swine Health Board 2010).

In comparison, it is difficult to prevent the entry of airborne viruses into hog barns, so aerosol and airborne transmission can still occur even with good biosecurity measures in place on farms.

Can all viruses cause infection by aerosol or airborne transmission?

There is scientific evidence showing that FMDV and PRRSV can spread to susceptible farms from an infected farm over long distances due to aerosol or airborne transmission. For example, PRRSV can travel 9.1 km (Otake et al. 2010) and FMDV 250 km (Saskatchewan Agriculture Knowledge Centre).

Diagram showing how pathogens spread
    When airborne pathogens are released from a infected facility, there are generally three outcomes: (1) fall on the ground due to gravity (for large aerosols), (2) lose infectivity in the atmosphere, and (3) reach another facility to cause infection. -University of Manitoba photo

PEDV and ASFV are not respiratory diseases and are commonly not categorized as airborne diseases. However, airborne transmission over short distances was demonstrated for both PEDV (Gallien et al. 2018) and ASFV (Olesen et al. 2017).

Specifically, research has shown that PEDV and ASFV spread from infected pigs to susceptible pigs by airborne transmission within the same room, but there has not been enough research to conclude whether long-distance transmission of PEDV and ASFV is possible.

What farms are at most risk of aerosol or airborne transmission?

In general, farms that are located in areas with a high population of pig barns are most at risk. However, any farm that does not have additional protective measures against aerosol transmission has some level of risk of being infected by aerosol or airborne transmission.

How can I prevent transmission?

Adding an air filtration system can remove aerosols from incoming air. Research has shown that farms with air filtration systems had reduced risk of infection from PRRSV (Alonso et al. 2013).

Specifically, using mechanical air filters with an efficiency rating of at least MERV 14 or electrostatic filters with a rating of at least MERV 15 was shown to be effective.

When considering the option of using filters, one should weigh the risk of infection and the cost of air filtration systems.

There is a high capital cost to install filtration systems as well as a need to change other components of the ventilation system because air filters can cause pressure loss and reduced airflow in ventilation.

Additionally, there is a maintenance cost, as air filters may require periodic replacement. BP

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