What You Don't See: Asymptomatic (Subclinical) Carriers As Sources of Infectious Disease

While the term “asymptomatic carrier” has been making headlines recently with regard to the SARS-CoV2 virus that causes COVID-19, the concept is actually nothing new to epidemiologists and other researchers who study a variety of diseases in humans and animals. Asymptomatic carriers play critical roles in the transmission of infectious diseases in humans, horses, and other species, but they largely go undetected, thereby hampering control efforts.

Asymptomatic carriers, also known as subclinical or silent shedders, are individuals infected with a pathogen that do not show clinical signs or symptoms of disease. Although they appear healthy, they can incubate and shed the organism into the environment in nasal secretions, respiratory droplets, feces, etc., presenting potential sources of infection for others. Some pathogens remain viable in the environment for extended periods after being shed by horses or other animals. Direct evidence of asymptomatic carriers is understandably scarce, which complicates how health professionals measure the effectiveness of disease mitigation responses.

It is important for horse owners to understand the potential roles of asymptomatic carriers in their biosecurity plans. Horses that appear healthy can shed contagious pathogens and endanger the health of other animals. Asymptomatic individuals can have the pathogen but not show signs of disease for many reasons. They may already have immunity to that particular pathogen or are not genetically susceptible. Stress, caused by transportation, illness, intense exercise, hospitalization, foaling, weaning, etc., can activate pathogen shedding by asymptomatic carriers, sometimes leading to disease outbreaks.

Research on horses that are asymptomatic carriers of strangles shows that once a horse recovers, it can be a source of infection for at least 6 weeks after clinical signs have resolved - the equine equivalent of Typhoid Mary. Up to 10 percent of horses that recover from an outbreak can become long-term silent shedders for months to years, acting as reservoirs that can perpetuate the disease. Horses can also be carriers during the incubation period without any clinical signs.

During outbreaks of strangles in large herds, identifying and treating carriers is essential to eliminate long-term sources of the disease.

Asymptomatic carriers play significant roles in other equine infectious disease outbreaks as well. These include respiratory diseases such as those caused by equine herpesvirus, equine coronavirus, and equine influenza virus, gastrointestinal illnesses such as salmonellosis, and venereal diseases such as contagious equine metritis. For some diseases, such as equine infectious anemia, carriers pose such a significant threat to others that they are euthanized or restricted to permanent quarantine.

Reliable detection of asymptomatic carriers is important as equine populations worldwide become increasingly more mobile. A study that utilized the databases of the World Organization for Animal Health (OIE) and international surveillance reports noted that 88 percent of incidences of introduction of pathogens into importing countries involved infected horses that did not show any clinical signs at the time they were imported.

Simple biosecurity measures can help reduce the risk of disease from asymptomatic carriers. These include isolating new arrivals and testing as needed (in consultation with your veterinarian), regularly vaccinating resident horses, and avoiding the use of shared equipment between horses.