Tools for managing shelter outbreaks of canine parvovirus (Proceedings)

Introduction and significance in a shelter

Canine parvovirus (CPV) poses a serious threat in animal shelters, as a double stranded DNA virus that has rapid spread and the ability to persist in the environment for months to years. Vaccination is the cornerstone to keeping this virus at bay. Although parvoviruses evolve, they tend to be antigenically stable so vaccination confers reliable protection even against newer strains such as CPV 2c. The incubation period is relatively short so quarantine of exposed animals may be a realistic possibility along with risk assessment based on antibody titers and PCR.

Who can be infected?

While puppies are most likely to suffer severe disease and death, any unvaccinated dog, of any age, can become infected with CPV. Some strains of CPV currently circulating in the U.S. can infect domestic cats as well as dogs

Disease course and transmission of parvovirus

Parvovirus is shed in all body fluids of acutely infected dogs, and is frighteningly easy to transmit via contaminated fomites such as hands, feet, equipment or surfaces. The incubation period is usually 4-6 days. Length of incubation can be affected by the dose to which the animal is exposed, and prolonged incubation periods of up to 14 days have been informally reported in naturally exposed shelter dogs. To be on the safe side, it is prudent to quarantine dogs for at least two weeks after a known exposure. Complicating control is the fact that viral shedding can occur for 2-3 days prior to development of clinical signs, as early as 3 days after exposure. In a busy shelter where early signs could be missed, the period of undetected shedding may be even longer. Take this into account when evaluating risk of exposed dogs – assume that the patient was contagious for 3-4 days prior to diagnosis, and consider risk. Although no carrier state exists for CPV, viral shedding can continue for up to 2 weeks post-recovery. A negative parvo ELISA test in a dog that initially tested positive is suggestive that virus is no longer being shed in significant quantities.

Vaccination

Vaccination is the cornerstone of parvovirus prevention in shelters and communities. In the absence of maternal antibody interference, a single modified live vaccine can confer protection within 3-5 days. Research to date has found that currently available vaccines protect against all known strains of parvovirus, including CPV-2c. All dogs and puppies > 4 weeks of age should be vaccinated at the time of shelter admission (or ideally, at least a week before), including those who are injured or mildly ill. Revaccinate puppies every two weeks until 18 weeks of age as long as they remain in the shelter, and consider revaccinating adult dogs at least once, 2 weeks after the first vaccine or after adoption. For pregnant dogs, the risk of parvo infection must be balanced against the risk of vaccine-induced abortion. Because maternal antibody can interfere with vaccination in puppies under 16 weeks, they also need to be mechanically protected from exposure and ideally, removed from the shelter to foster care, adoption or rescue as quickly as possible.

Clinical signs

The most common signs of parvovirus infection include fever, anorexia, lethargy, dehydration, vomiting and diarrhea, which may or may not be hemorrhagic. Leukopenia is common though not universal. Co-infection with other pathogens, a common circumstance in shelters, can lead to more severe disease.

Recognition and diagnosis

Diagnosis of CPV is fortunately reasonably straightforward in most cases. While not perfect, in-house fecal ELISA tests are reportedly quite specific and sensitive even for recently emerged strains. A recent study found that the Idexx SNAP test detected 80%, 78% and 77% of parvovirus 2a, 2b, and 2c respectively. As with any test, false results are possible. Negative results will occur later in the course of disease when virus is bound by antibody or no longer being shed; this should not be interpreted to contradict earlier positive results. Weak false positives may also reportedly occur due to recent vaccination. However, this is likely uncommon, particularly with the Idexx brand SNAP test. In general, positive results should be taken seriously even in recently vaccinated dogs.

Of course, in all cases history, signalment and clinical signs should be considered along with test results. With much at stake, confirmatory diagnostic testing should be performed, especially if the result does not fit the rest of the clinical picture. Other accessible in-shelter diagnostic tools include blood smear/CBC looking for leukopenia and, if a dog dies or is euthanized, in-house necropsy for characteristic segmental enteritis. Fecal samples can be submitted to a laboratory for PCR with rapid turn-around time; this method is sensitive to detect parvoviral infection but also more likely to detect vaccine virus in recently vaccinated dogs. PCR analysis is the only method to distinguish between the various strains of parvovirus; however, this has minimal clinical relevance as the approach to prevention and treatment is identical regardless of strain. Histopathology and immunohistochemistry on a necropsy specimen is the gold standard for diagnosis, and should be performed in atypical outbreaks if any dog dies or is euthanized (e.g. apparent infections in well-vaccinated animals; persistent outbreak in the face of good control measures).

Risk assessment: how do you decide how much to worry about exposed animals?

Not all exposed dogs will become infected. Due to varying levels of maternal antibody, it is not even uncommon for only some members of a litter to develop disease. The risk of infection depends on the animal's individual immune and vaccination status, the overall cleanliness of the environment and the level of proximity between the exposed and infected animal. The most important factor in disease risk is vaccination: a "fully" vaccinated animal over four months old is at very low risk of infection. However, even incompletely vaccinated animals may survive a possible exposure.

Risk due to environmental spread is reduced if :

     • The facility is not overcrowded

     • Dogs are housed singly or in stable pairs/groups and are not handled or removed from their run during cleaning (e.g. double sided runs used correctly)

     • Animal housing areas are steel, sealed concrete, or other non-porous, non-scratched surface and can be effectively cleaned and disinfected

     • A proven parvocidal disinfect is used daily to disinfect all animal housing areas, including transport vehicles, exam surfaces and common rooms

     • Separate tools and equipment are used for each area of the shelter

     • Animals are handled with hand washing or change of gloves between individuals

     • Clinical signs appeared within a few days of shelter intake (and therefore the animal was more likely exposed in the community versus in the shelter)

Risk due to animal immune status is reduced if:

     • All animals are vaccinated immediately upon intake

     • Risk is very low in animals > 4-5 months old that are either:

     • Vaccinated with an MLV SC vaccine at least one week prior to exposure

          o Have a documented history of vaccination at or after 16 weeks of age and within three years prior to exposure

          o Risk is greater in puppies under 4 months old even if vaccinated (due to maternal antibody interference)

     • Risk is greater in animals vaccinated less than a week before exposure

     • Risk is greatest in closely exposed, unvaccinated animals

If a single case occurs in an area where all animals have been vaccinated and environmental spread risk is deemed low based on the above-listed factors, quarantine or depopulation may not be necessary. If spread is observed or few of the above precautions are in place, the whole ward or even the whole shelter may need to be considered at risk/exposed. An intermediate response is to assess risk for immediately adjacent kennels only – this is logical only if exposure is likely much lower in dogs elsewhere in the ward.

Serology to assess individual dog risk

Serology is a useful tool to further clarify the need for quarantine of individual dogs. Titer testing should be performed using a validated test or laboratory. Some laboratories can report quantitative results within 24 hours of sample submission, making this a reasonably efficient decision-making tool. The Synbiotics in-house TiterchekTM kit can also be used and has the advantage of more rapid turn-around time. Because this is a well-type kit rather than a "snap" kit, make sure staff running the test are sufficiently skilled. Asymptomatic adult dogs testing positive for protective titers (1:80 at a laboratory or positive on the Synbiotics TiterChek kit) are at low risk for developing parvo infection. It is reasonable to move these dogs through the shelter as usual rather than placing them in quarantine. Interpretation of titers in dogs < 5 months of age is a little less clear-cut, as positive titers may reflect either an active immune response or waning maternal antibody. Puppies testing positive are likely low risk but this is less certain than with adults, and immunity may rapidly wane. These puppies are relatively safe to move to adoption or rescue, but should leave the shelter quickly if possible, and it may be prudent to advise adopters or rescuers of their recent exposure. All dogs, of any age, testing negative for protective titers at the time of exposure must be considered high risk; however, many of these dogs will not develop infection. Quarantine for two weeks is indicated for this group.

Disinfection: how do you get rid of it?

As noted above, parvovirus can remain viable for months and years, especially in a dark, moist environment. Effective choices include 5% sodium hypochlorite (household bleach) freshly mixed and applied to a clean surface at ½ cup per gallon (1:32 dilution). Products in the same family as bleach that have also been found effective include calcium hypochlorite (e.g. Wysiwash®) and sodium dichloroisocyanurate (e.g. Bruclean®); like bleach, these have no detergent properties and must be applied to a pre-cleaned surface. Other proven products include potassium peroxymonosulfate (e.g. Trifectant®) and accelerated hydrogen peroxide (e.g. Virox®, Accel®), which both reportedly have greater detergent properties and better activity in the face of organic matter compared to bleach and related products. Independent studies have repeatedly shown that quaternary ammonium disinfectants (e.g. Triple Two®, Rocal®) do not reliably kill parvovirus, in spite of repeated reformulation and label claims of efficacy.

There is no benefit to a waiting period prior to re-use of a kennel after parvo decontamination; either mechanical cleaning and disinfection was effective, or it wasn't. Waiting a day or even a couple of weeks will not result in a significant further decrease in contamination. To be on the safe side, kennels should be completely cleaned, disinfected, and dried at least twice before re-use, however this can happen in a short period of time if the area or kennel is needed urgently. For areas such as yards and homes where disinfection is not an option, careful and repeated mechanical cleaning can be effective if applied diligently. Yards can be flushed with water and dried, and a disinfectant with reasonable efficacy in the face of organic matter can be sprayed on the area using a pesticide applicator or hose-end sprayer. If repeated cases occur after thorough efforts at cleaning, close the area to youngsters and unvaccinated animals for at least 6-12 months (and in general do not use grass or dirt areas for juvenile animals in a shelter). In the meantime, maximize exposure to sunlight and drying of the environment.

The bottom line

Although it may be impossible to exclude the canine parvovirus completely from an open population, having a well though out vaccine, housing and cleaning program in place before a problem occurs can go a long way towards minimizing spread. If the worst happens and an outbreak occurs, rapid and accurate diagnosis, coupled with effective risk assessment and quarantine, will limit damage and get things quickly back to normal.