Pig producers have been abruptly reminded of the risk of novel disease challenges with recent identification of pseudorabies antibodies in a commercial Iowa farm, and the rapid spread of New World screwworm (NWS) into multiple states and multiple species in the southern United States.
While the pseudorabies response is largely complete, the absence of a viable sterile fly program ensures that NWS will be with us at the domestic level for quite some time. As discussions surrounding NWS continue to evolve, producers should shift their focus from where the parasite may spread to how their operations would detect and respond to a suspected case.
Preparedness is not solely about preventing introduction of a pathogen. It is also about recognizing the disease early enough to minimize the consequences.
Understanding the Pathogenesis
NWS (Cochliomyia hominivorax) is an obligate parasite whose larvae consume living tissue. This characteristic distinguishes it from normal fly activity around scratches and wounds.
Following NWS infestation on fresh wounds or mucosal surfaces, the larvae penetrate viable tissue and create rapidly expanding lesions that may become complicated by secondary bacterial infections. Clinical progression can be swift, making early recognition essential.
The invasion of the larvae into healthy tissue is what differentiates this disease – these deep, necrotic lesions expand quickly and cause severe compromise to infected animals. We generally think of flies as pests, but if untreated, the NWS fly becomes a killer.
Surveillance Should Focus on Wounds
Unfortunately, livestock have numerous potential sites for infestation, including umbilical cords, processing incisions, injection sites, fresh ear tags, scratches, shoulder sores and vulva tears. As such, surveillance protocols should extend beyond identifying clinically ill pigs and include holistic evaluation of wound prevention and healing.
Rather than asking whether a wound exists, producers should ask whether their facility is minimizing wounds to the extent possible and ensure that existing lesions are healing as expected.
Differential Diagnosis in the Field
Veterinarians evaluating suspicious lesions should consider bacterial wound infections, traumatic injuries, abscesses and NWS as differential diagnoses.
Wounds that continue to enlarge despite appropriate care, exhibit foul odor or contain larvae within viable tissue, however, warrant immediate investigation and reporting through appropriate regulatory channels.
The blunt reality is that vets need to be concerned about any lesions that aren’t healing appropriately. We need to follow the adage “see something, say something” by complying with requirements that vets immediately communicate lesions of concern to their USDA Animal and Plant Health Inspection Service (APHIS) Area Veterinarian in Charge, as well as their local State Animal Health Official.
Continuity of Business Planning
Preparedness also includes maintaining operational capacity.
Enhanced movement requirements, traceability expectations and regulatory oversight may accompany disease response activities. Producers participating in preparedness initiatives and maintaining current documentation will be positioned to navigate these requirements more effectively.
Producers in designated NWS-infected zones should be prepared to permit feed movements, trash pickup, rendering activities and other actions that create transmission risk. Producers not familiar with the APHIS Emergency Management Response System (EMRS) should take a moment to familiarize themselves. The EMRS Customer Permit Gateway document provides some excellent background information on this program, which will be used to facilitate permitting necessary activities as needed.
Looking Ahead
“Hope for the best, plan for the worst.” From a veterinary perspective, NWS reinforces an enduring principle of swine health management: Careful observation remains one of the most valuable diagnostic tools we have.
A wound that does not follow the expected course of healing may represent more than an isolated injury. Recognizing that possibility early and responding appropriately protects not only the individual operation, but the broader U.S. swine industry.
Careful observation isn’t just valuable for identifying potential lesions – take it step further and identify the potential root cause of future lesions in your operation. Do you have any gating that needs repaired? Any rough edges on feeders? How is the body condition on your lactating sows – good enough to consistently prevent shoulder sores in weaned sows?
It’s cliche, but an ounce of prevention is worth a pound of cure when it comes to NWS, and by being proactive we can help minimize the risk of infection on our farms.
