Canine distemper virus (CDV) remains a source of significant illness and fatality in shelters across the United States in locations where access to resources and veterinary care including CDV vaccine are limited. With the support of Maddie’s Fund®, a national family foundation established by Dave and Cheryl Duffield to revolutionize the status and well-being of companion animals, UW-Madison Shelter Medicine Program (SMP) helps to manage many CDV outbreaks across the United States each year. Using information gathered from 98 animal welfare organizations, veterinarians from the SMP and collaborators from the Wisconsin Veterinary Diagnostic Laboratory (WVDL) have authored a manuscript entitled “Prolonged persistence of canine distemper virus RNA, and virus isolation in naturally infected shelter dogs” which has been published by PLOS ONE (https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0280186). This publication provides a new perspective on managing outbreaks and individual dogs recovering from CDV that will save lives by dramatically simplifying identification of the end of the infectious period, shortening animal care days in quarantine, and conserving resources by limiting the need for extended diagnostic testing.
Background: RT-PCR was introduced as a sensitive and specific alternative to fluorescent antibody detection of CDV antigen (FA) that could be used to detect the presence of viral RNA even when the time of exposure is unknown or the dog has passed the first few weeks of illness [1–3]. RT-PCR has been used as a diagnostic tool for management of individual cases and outbreaks for most of this century. Over the years, while using RT-PCR testing for management of CDV outbreaks in shelters, a reoccurring problem has emerged. Sometimes, dogs who have recovered from clinical signs continue to test positive on RT-PCR for weeks to many months. It has been unclear if these dogs are still infectious for CDV. Holding them separated from other dogs until they tested negative on two consecutive RT-PCR tests often seemed to be the safest course of action based on the information available. But holding dogs in shelters separated or in isolation for these long durations of detection of viral RNA on RT-PCR has not been without negative consequences. Reducing separation periods as well as reducing the number of diagnostic tests needed to release dogs would have a dramatic and positive impact on disease management, animal health, welfare, and lifesaving.
New Information: Our new paper describes CDV testing data from May of 2014 to June of 2020. A total of 5,920 canine records were reviewed. Of those, 1,393 dogs tested positive at least once. Within the positive group there were 425 dogs that tested positive more than once (dogs were tested between 2-27 times), the range of a positive test in this group was from 3 to 324 days. The median duration of a positive test was 34 days and 25% of the dogs continued to test positive beyond day 62. The data indicates that it is not uncommon for dogs to test positive on RT-PCR for prolonged periods.
For six of the dogs who tested positive for prolonged periods, both RT-PCR and virus culture data was collected. By pairing virus culture with RT-PCR, we were able to determine that shedding of infectious virus occurs in the days surrounding the peak of viral RNA shedding but does not persist once the peak has passed. Our report corroborates with data from natural infections in shelter dogs the findings of a 2015 publication where Sehata and colleagues demonstrated that in an experimental setting detection of live CDV occurred in samples two days before, during, and two days after the peak of viral load shedding as detected by RT-PCR.
We have examples of this same phenomenon in human medicine, some even highlighted in the recent COVID-19 pandemic. Human measles virus (the parent virus of CDV) and SARS-CoV-2 both can demonstrate long durations of viral RNA excretion with RT-PCR remaining positive despite a clear understanding that infectious potential only lasts a short time. The quarantine times for these viruses are short and there is no requirement to have a repeat RT-PCR test with a negative result to be considered non-infectious [5,6].
Conclusions: Dogs recovering from CDV stop being infectious, posing no risk to other dogs, after the peak of viral RNA shedding has passed even if they continue to test positive on RT-PCR for prolonged periods. Once the peak of viral RNA shedding has passed and serial RT-PCR tests show steady decline in viral load, dogs may be moved forward for live release outcomes even if they still test positive for CDV on RT-PCR. This strategy for determining the end of the infectious period will reduce holding times by weeks to months. The UW-SMP is here to provide guidance on implementation of this new strategy. Please contact firstname.lastname@example.org for assistance.
 Frisk AL, König M, Moritz A, Baumgärtner W. Detection of Canine Distemper Virus Nucleoprotein RNA by Reverse Transcription-PCR Using Serum, Whole Blood, and Cerebrospinal Fluid from Dogs with Distemper. J Clin Microbiol 1999;37:3634–43.
 Elia G, Decaro N, Martella V, Cirone F, Lucente MS, Lorusso E, et al. Detection of canine distemper virus in dogs by real-time RT-PCR. J Virol Methods 2006;136:171–6. https://doi.org/10.1016/j.jviromet.2006.05.004.
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 SEHATA G, SATO H, ITO T, IMAIZUMI Y, NORO T, OISHI E. Use of quantitative real-time RT-PCR to investigate the correlation between viremia and viral shedding of canine distemper virus, and infection outcomes in experimentally infected dogs. J Vet Med Sci 2015;77:851–5. https://doi.org/10.1292/jvms.14-0066.
 Griffin DE, Lin W-H, Pan C-H. Measles virus, immune control and persistence. FEMS Microbiol Rev 2012;36:649–62. https://doi.org/10.1111/j.1574-6976.2012.00330.x.
 Widders A, Broom A, Broom J. SARS-CoV-2: The viral shedding vs infectivity dilemma. Infect Dis Health 2020;25:210–5. https://doi.org/10.1016/j.idh.2020.05.002.
Maddie’s Fund® is a family foundation established in 1994 by Dave and Cheryl Duffield and is the fulfillment of a promise to an inspirational dog, Maddie. She provided them much joy for over ten years and continues to inspire them today.
The Foundation has awarded over $265 million in grants toward increased community lifesaving, pioneering shelter medicine education and establishing foster care as a standard across the U.S.
Maddie’s Fund proudly offers the industry a national voice, important funding opportunities for bold ideas, learning resources and access to collaborate and share innovative solutions. The Foundation invests its resources in a commitment to keeping pets and people together, creating a safety net of care for animals in need and operating within a culture of inclusiveness and humility. #ThanksToMaddie.