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Distemper in dogs: blood work, PCR and the vaccine pitfall

Distemper is a viral disease that can affect the lungs, the gut, the skin and the brain. It mostly strikes unvaccinated puppies. The blood count gives clues, such as low lymphocytes, but the diagnosis rests on finding the virus with PCR, and a recent vaccine can complicate the reading.

Written by the AI DiagAnimals editorial teamReviewed by the AI DiagAnimals TeamUpdated Our editorial policy

The essentials

Distemper is caused by a virus related to measles. It destroys lymphocytes early on, so a low lymphocyte count is a common finding. Viral inclusions in blood cells confirm it when seen, but they are very rare. PCR on blood, a conjunctival swab, urine or spinal fluid is the usual test. PCR can also pick up virus from a recent live vaccine, and antibody tests are hard to read in vaccinated dogs.

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What distemper is and which dogs are at risk

Canine distemper virus is a morbillivirus, closely related to the measles virus. Its main effects are respiratory and digestive signs, a weakened immune system and damage to the brain and spinal cord [1]. More than half of infections are probably silent [2].

Most sick dogs are young and unvaccinated. In one study of 20 dogs with the nervous form, the mean age was about 12 months and only one dog was fully vaccinated; half came from shelters or the street [2]. Shelters are high-risk places: in the USA, most puppies under 6 months and 30–50% of adult dogs have little or no antibody against distemper when they arrive [7].

The virus still circulates in Europe. In Portugal, 33.8% of 637 samples from dogs suspected of distemper tested positive between 2013 and 2025, with a sharp drop after 2020, and the virus is still found in wild carnivores [9].

First signs of distemper

  • Early, general signs: fever, loss of appetite and lethargy [2].
  • Breathing: nasal and eye discharge, cough, pneumonia [2].
  • Gut: vomiting and diarrhea, which is why distemper can be confused with parvo and other gut infections [2, 4].
  • Skin: thickened, hard nose and foot pads [2].
  • Nervous system: muscle twitching (myoclonus), seizures, wobbliness or weakness. Nervous signs can start weeks after the first illness, or without any earlier signs [2, 8].

What the blood count shows

  • Low lymphocytes (lymphopenia): the virus kills lymphocytes early in the infection, which weakens immunity [1]. In dogs with the nervous form, lymphopenia and anemia were the most common findings, and all dogs with lymphopenia died soon after diagnosis [2].
  • Low white blood cells: young dogs infected in an experiment had mild fever and a low white count [14].
  • High neutrophils or anemia: possible, and the blood count can also be normal [2].
  • Distemper inclusion bodies: small clumps of virus inside red or white blood cells. They prove distemper when seen, but they appear only briefly in the acute phase and are very rarely found [3].

Blood work supports the suspicion but cannot confirm distemper on its own [2, 8]. See the white blood cell count in dogs and the CBC for dogs.

Reading the lymphocyte line

In distemper, the lymphocyte line is often the one flagged low. The example below is illustrative.

An annotated line from a lab report · Illustrative example
AbbreviationResultUnitLaboratory intervalFlag
LYMPH Lymphocytes0.310³/µL0.4–2.9L
  1. Abbreviation LYMPH or LYM: lymphocytes, the white cells the distemper virus attacks first.
  2. Result 0.3 × 10³/µL in this illustrative line: low.
  3. Unit 10³/µL (thousands per microliter) in the US; ×10⁹/L elsewhere, with the same number.
  4. Laboratory interval An illustrative dog interval; each laboratory sets its own.
  5. Flag L: below the interval. Low lymphocytes are not specific to distemper; in a young, unvaccinated dog with discharge or cough, vets look for the virus [2].

How distemper is confirmed

  • PCR (RT-PCR): the main test. It finds the virus’s genetic material. Whole blood, urine, conjunctival swabs and tonsils carry a lot of virus [4]. In one study of 53 positive dogs, blood was positive in 94.3%, urine in 83.0%, rectal swabs in 71% and conjunctival swabs in 50.9% [5]. Conjunctival swabs turned positive earlier and stayed positive longer in young dogs infected in an experiment [14].
  • Spinal fluid (CSF) in the nervous form: when nervous signs appear long after the first illness, blood and swabs are often negative. In 20 dogs with nervous signs, PCR was positive in 80% of CSF samples versus 55% of blood samples [2]. CSF analysis can show an increase in lymphocytes (lymphocytic pleocytosis), which points toward distemper [8].
  • The vaccine pitfall: sensitive PCR tests can detect the live vaccine virus. In a recently vaccinated dog, a low positive result can be vaccine or early infection, so some laboratories use tests that tell the vaccine strain from wild virus [6].
  • Antibody tests (serology): of little help for diagnosis, because vaccination or a past infection also produces antibodies [2]. In-clinic distemper antibody kits were not reliable in acutely ill dogs [7].
  • Rapid antigen tests: less sensitive than PCR; one rapid test found only 30 of 53 PCR-positive dogs [5].

Outlook

The outlook depends on the form. In 20 dogs with the nervous form, 14 died or were euthanized and 5 recovered [2]. In recently vaccinated dogs with breathing signs, 80% recovered when an antigen test was negative and 55% when it was positive [13]. Unvaccinated puppies get sicker than adult dogs [12]. Treatment is supportive hospital care [2]; ask your vet about your dog’s situation.

Related articles

White blood cell count in dogs

Lymphocytes, neutrophils and the stress pattern.

Read the article

Anemia can also appear: see anemia in dogs. For cats, see feline panleukopenia, often called feline distemper.

En français : maladie de Carré du chien.

Questions to ask your veterinarian

  1. Are the lymphocytes low, and what else in the blood count fits distemper?
  2. Which sample is best for PCR at this stage: blood, a conjunctival swab, urine or spinal fluid?
  3. Could a recent vaccine explain a positive PCR, and can the laboratory tell vaccine virus from wild virus?
  4. Is an antibody test useful for my dog, given its vaccines?
  5. How should my other dogs be protected, and are their core vaccines up to date?
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Frequently asked questions

Can adult dogs get distemper?

Yes. In a challenge study, 8 of 9 unvaccinated adult dogs became ill, though puppies were sicker [12]. Rare cases also occur in vaccinated adults, sometimes with sudden nervous signs [11].

What can be mistaken for distemper?

Its signs overlap with other respiratory and gut infections [4]. Vets also rule out parvovirus, canine adenovirus, Bordetella, salmonellosis and lead poisoning, and muscle twitching can have other nervous causes [2].

Can a vaccinated dog get distemper?

It is uncommon, but cases in vaccinated dogs have been reported [10, 11]. In one study, only 1 of 20 sick dogs was fully vaccinated [2]. Vaccinated adult dogs with antibodies resisted the virus for up to 4.4 years [12].

What are the first signs of distemper in a dog?

Fever, loss of appetite and tiredness, then eye and nose discharge, cough, and sometimes vomiting or diarrhea. Nervous signs can follow later [2].

Which vaccines protect against distemper?

Distemper is one of the three core vaccines recommended for every dog worldwide, with adenovirus and parvovirus. Puppies are vaccinated every 2 to 4 weeks until 16 weeks of age or older [7].

Sources

  1. Beineke A et al. Pathogenesis and immunopathology of systemic and nervous canine distemper. Vet Immunol Immunopathol. 2009;127(1-2):1-18. doi.org/10.1016/j.vetimm.2008.09.023
  2. Sarchahi AA et al. Detection of canine distemper virus in cerebrospinal fluid, whole blood and mucosal specimens of dogs with distemper using RT-PCR and immunochromatographic assays. Vet Med Sci. 2022;8(4):1390-1399. doi.org/10.1002/vms3.790
  3. eClinPath, Cornell University College of Veterinary Medicine. Viruses (canine distemper inclusions). eclinpath.com/hematology/infectious-agents/viruses
  4. Elia G et al. Detection of canine distemper virus in dogs by real-time RT-PCR. J Virol Methods. 2006;136(1-2):171-176. doi.org/10.1016/j.jviromet.2006.05.004
  5. Fischer CD et al. Detection and differentiation of field and vaccine strains of canine distemper virus using RT-nqPCR and RFLP analysis. J Virol Methods. 2013;194(1-2):39-45. doi.org/10.1016/j.jviromet.2013.08.002
  6. Wilkes RP et al. Real-time RT-PCR method for detection of canine distemper virus modified live vaccine shedding for differentiation from infection with wild-type strains. J Vet Diagn Invest. 2014;26(1):27-34. doi.org/10.1177/1040638713517232
  7. Squires RA et al. 2024 guidelines for the vaccination of dogs and cats (WSAVA Vaccination Guidelines Group). J Small Anim Pract. 2024;65(5):277-316. doi.org/10.1111/jsap.13718
  8. Amude AM et al. Clinicopathological findings in dogs with distemper encephalomyelitis presented without characteristic signs of the disease. Res Vet Sci. 2007;82(3):416-422. doi.org/10.1016/j.rvsc.2006.08.008
  9. Lopes R et al. Molecular detection of canine distemper virus in Portugal: what explains the post-2020 decline? A retrospective RT-qPCR study. Viruses. 2026;18(7):734. doi.org/10.3390/v18070734
  10. Martella V et al. Canine distemper virus. Vet Clin North Am Small Anim Pract. 2008;38(4):787-797. doi.org/10.1016/j.cvsm.2008.02.007
  11. Galán A et al. Uncommon acute neurologic presentation of canine distemper in 4 adult dogs. Can Vet J. 2014;55(4):373-378. pubmed.ncbi.nlm.nih.gov/24688139
  12. Jensen WA et al. Use of serologic tests to predict resistance to canine distemper virus-induced disease in vaccinated dogs. J Vet Diagn Invest. 2015;27(5):576-580. doi.org/10.1177/1040638715602291
  13. Kapil S, Neel T. Canine distemper virus antigen detection in external epithelia of recently vaccinated, sick dogs by fluorescence microscopy is a valuable prognostic indicator. J Clin Microbiol. 2015;53(2):687-691. doi.org/10.1128/JCM.02741-14
  14. Kim D et al. Comparison of tissue and fluid samples for the early detection of canine distemper virus in experimentally infected dogs. J Vet Med Sci. 2006;68(8):877-879. doi.org/10.1292/jvms.68.877

Laboratory methods and reference intervals vary. This article explains; it does not diagnose. For a decision about your pet, use the original report and talk to your veterinarian. Our editorial policy.

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