Chapter 5: Risk Assessment: How do you decide how much to worry about exposed animals?

When one animal from a population is diagnosed with CPV, the question arises: what do you do about others in the environment? Are they all likely to get sick? Will widespread quarantine or depopulation be necessary? Or is it okay to simply carry on business as usual, or somewhere in between? The answers to these questions are dependent on several factors.

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 (i.e. vaccinated eight days prior to exposure – 5 days for vaccine to be fully effective and 3 days of shedding prior to clinical signs) over five months old is at very low risk of infection. However, even incompletely vaccinated animals may survive a possible exposure.

Risk Assessment - Shelter Environment

Risk due to environmental spread is reduced if:

• The facility is not crowded.
• Dogs are housed singly or in stable pairs/groups.
• Dogs are not handled or removed from their run during cleaning (e.g. double sided kennels 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 Assessment - Individual Animal Factors

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 (sub-cutaneous) vaccine at least one week prior to exposure.
  • Have a documented history of vaccination at or after 18-20 weeks of age and within three years prior to exposure.
• Risk is greater in puppies under 5 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 or titer negative animals.
• To help with thinking about and planning for a CPV outbreak, which is nice to be able to do when not actually faced with a case of CPV, we developed the CPV Outbreak Simulator and guide.  This resource allows you to work through a real life outbreak scenario numerous times until you feel confident in your risk assessment skills. It also lets you get a sense for the shortcomings of risk analysis – every once in a while, in the simulator as in life, you will do everything right and an infected animal will slip past your radar. However, you can also clearly see how many more lives are saved through careful risk assessment than either depopulation or failure to respond at all.  For more information, view the CPV outbreak simulator.

Serology to assess individual dog risk

Serology is a very useful tool to further clarify the need for quarantine of individual dogs.^8,9  In-house serology tests can be used and have the advantage of more rapid turn-around time as compared to sending out blood samples, often within minutes.  

Note about who to test: tests should only be used on dogs without clinical signs of disease.  Healthy adult dogs (> five months old) vaccinated at least eight days prior to exposure (i.e. the day that the dog with CPV had clinical signs) can be considered low risk and titer testing is not usually necessary.

The Synbiotics TiterchekTM kit is designed to test for canine distemper virus (CDV) or canine parvovirus (CPV) antibodies in canine serum.  The results are compared to positive and negative control wells and give non-quantitative positive or negative results.  This kit is a well test; each time the test is run two additional wells must be used to run a positive and negative control.  Because of this, the test is most economical when running several tests at once. Also since this is a well test rather than a “snap” kit, ensuring the staff running these tests is sufficiently skilled is essential. There is a video demonstrating its use on YouTube

More recently, another in-house option has become available in the US, the VacciCheck ImmunoCombTM test by Biogal.  This kit provides semi-quantitative antibody titer levels for CPV and CDV (as well as Canine Adenovirus).  The kit is a "self-contained" dot ELISA titer test kit, not needing any reagent preparation.  The kit looks like a flat comb; each tooth of the comb is a test for an individual dog and includes the positive and negative controls.  Results can be scored by their shade relative to the positive on a scale from 1- 6.   Results develop for all three 3 viruses on the same comb simultaneously. The test provides results within approximately 20 minutes.  There is a video demonstrating its use on YouTube.

Adult dogs without clinical signs testing positive for protective titers are at low risk for developing CPV infection. It is reasonable to move these dogs through the shelter as usual rather than placing them in quarantine.

Interpretation of titers in dogs < five 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 is prudent to advise adopters or rescuers of their recent exposure to CPV. Continue their vaccine schedule as usual.

Puppies, especially heavily exposed puppies or puppies from high risk environments, should be snap tested before or in conjunction with titer testing to rule out early subclinical infection which could overcome maternally derived antibodies (MDA).  A positive snap test along with a positive titer, indicates subclinical infection and these dogs should be isolated.  A negative snap and negative titer result would indicate that the puppy is not currently shedding but is not protected if exposed to CPV and thus should be quarantined.  A negative snap and positive titer result indicates that the puppy is not currently shedding and is likely protected (for now) if exposed to CPV.  These puppies should be bathed and moved out to adoption or rescue as promptly as possible. 

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 2 weeks is indicated for this group if possible.

A Caveat About Risk

Remember that low risk and high risk are only that: evaluation or risk levels, not an absolute guarantee of a particular outcome. Virtually all tests have limitations, and the tests for antibody titers are no exception. Two studies estimated the sensitivity and specificity of the Synbiotics TitercheckTM at 92- 98% and 94-98% respectively. (Litster et al, Vet J 193(2): 363-366.; Gray et al, J Am Vet Med Assoc 240(9): 1084-1087.) This means that occasionally dogs without protective titers will test positive and vice versa.

For many shelters, the combination of evaluation of exposure level, clinical condition, snap and titer test results will lead to a reasonable degree of confidence of a dog's likelihood of infection, such that the risk for an adopter or rescuer in taking on a "low risk" dog would be no greater than adopting an untested dog at any other time (when the shelter was not experiencing an outbreak).

However, if a truly no-risk scenario is desired, and it is affordable, quarantine of all exposed dogs for 14 days would be the safest bet. In many shelters, this is simply untenable, and evaluation of risk levels is the best way to manage a potential outbreak with minimal loss of life or spread of disease. Adopters and rescuers should be counseled accordingly.