FrançaisHeartworm in dogs and who is at risk
Heartworm larvae enter the dog through a mosquito bite. They grow into adult worms at about day 120, and their offspring, called microfilariae, appear in the blood as early as 6 months but usually by 7 to 9 months after infection [1].
Heartworm is endemic in all 48 contiguous US states [1]. Among dogs tested nationally, the share with a positive antigen test rose from 1.11% in 2013 to 1.28% in 2016. Mississippi had the highest rate (8.37% in 2013, 7.53% in 2016) and Texas reached 3.19% in 2016 [2]. Prevalence increased in much of the country between 2012 and 2018 [3].
The two heartworm tests
- Antigen test: detects a protein secreted mainly by adult female worms. It is nearly 100% specific, so false positives are rare [1]. A negative result is best written as “no antigen detected”, because early or light infections can be missed [1]. The color of a positive spot cannot tell how many worms are present [1].
- Low worm burdens: in an older study of dogs with 1 to 10 worms, sensitivity ranged from 52% to 76% depending on the kit [4]. Current in-clinic kits did much better against necropsy: at least 97.5% overall, and 96–100% in dogs with 1 to 5 female worms [5].
- Male worms only: no verified test reliably detects an infection with only male worms [1].
- Microfilaria test (modified Knott test): a blood sample is mixed with formalin, spun, and the sediment stained and examined under the microscope [1]. Heartworm microfilariae measure 295–325 µm, which helps tell them from a harmless look-alike, Acanthocheilonema reconditum (250–288 µm) [1]. In high-prevalence areas, about 20% of infected dogs have no microfilariae in the blood, so the microfilaria test does not replace the antigen test [1].
In-clinic combination tests such as the 4Dx check for heartworm antigen together with antibodies to several tick-borne infections. See tick-borne diseases in dogs.
When to test, and why a positive is confirmed
- Not too early: antigen can be detected about 5 months after infection and microfilariae about 6 months after. A reasonable interval is 7 months, so there is no need to antigen-test a puppy under 7 months old [1].
- On preventives: antigen may stay undetectable until about 9 months after infection [1].
- Every year: all dogs over 7 months old should be screened yearly with both an antigen test and a microfilaria test [1].
- After a gap in prevention or a change of product: test, then test again 6 months later [1].
- Confirm a positive: every positive antigen test should be confirmed before treatment, either by finding microfilariae or with a different type of antigen test [1].
- “Blocked antigen”: immune complexes can hide antigen and cause a false negative. Heating the sample before testing can release it, but this is not recommended for routine screening. It is considered when results do not fit, for example microfilariae seen but no antigen detected [1, 6]. In southeastern US shelter dogs, blocked antigen was found in 5.2–7.7% of dogs [1].
What the blood count and urine show
| Test | Possible finding | What it means |
|---|---|---|
| White blood cells | High eosinophils | Heartworm is a common cause of eosinophilia in dogs [7], but in dogs with worms in the heart it was found in only 20% [8]. A normal count does not rule heartworm out. |
| Platelets | Low platelets | Below 175,000/µL in 52% of dogs with worms in the heart [8]. |
| Red blood cells | Anemia | Found in 59% of dogs with worms in the heart [8]. |
| Urine protein (UPC) | Protein in the urine | UPC above 0.5 in 19.1% of infected dogs and borderline in 17%; higher with heavy worm burdens and with microfilariae [9]. In dogs with worms in the heart, 93% had protein in the urine [8]. |
| Urine color | Dark urine, hemoglobin in the urine | A sign of red cells bursting in caval syndrome, an emergency [1, 8]. |
See eosinophils in dogs and cats, low platelets in dogs and the CBC for dogs.
Reading the UPC line
Vets often check the urine protein in a heartworm-positive dog. Here is an illustrative example line.
- Abbreviation UPC, sometimes UP:UC or UPCR: urine protein divided by urine creatinine.
- Result 0.9 in this illustrative line: more protein than expected passes into the urine.
- Unit A ratio, with no unit.
- Laboratory interval In a study of heartworm-positive dogs, above 0.5 counted as proteinuria and 0.2–0.5 as borderline [9].
- Flag H: above the interval. In heartworm, proteinuria was more common with heavy worm burdens [9]. See protein in urine.
Disease classes and outlook
Caval syndrome happens when worms block blood flow through the heart. It is usually fatal within 2 days if the worms are not removed surgically without delay [1]. It has been reported in 13% to 20% of heartworm-positive dogs [8].
| Class | Typical signs |
|---|---|
| 1, mild | No signs, or a cough |
| 2, moderate | Cough, tiring easily on exercise, abnormal lung sounds |
| 3, severe | Also difficult breathing, enlarged liver, fainting, fluid in the abdomen; can be fatal |
| 4, caval syndrome | Sudden severe lethargy and weakness, with hemoglobin in the blood and urine |
Related articles
En français : la dirofilariose du chien.
Questions to ask your veterinarian
- Did the test check for both antigen and microfilariae?
- Was my dog tested long enough after possible exposure?
- How will a positive result be confirmed?
- Should we check the blood count and the urine protein (UPC)?
- Which disease class is my dog in?
Get your dog’s heartworm test and blood work explained together.
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Frequently asked questions
Can you test a dog for heartworms?
Yes. A small blood sample is used for an antigen test and a microfilaria test. Both are advised once a year for every dog over 7 months old [1].
What is a 4DX test?
An in-clinic blood test that checks for heartworm antigen together with antibodies to several tick-borne infections. Its heartworm part is an antigen test, with the same limits: it can miss very early, light or male-only infections [1]. See tick-borne diseases in dogs.
Can a dog test negative and still have heartworms?
Yes. The test may be done too soon after infection, the worm burden may be low or male-only, or antigen may be blocked by immune complexes [1, 6].
How long will a dog live with heartworms?
It depends on the disease class. A dog in class 1 may show no signs, while caval syndrome is usually fatal within 2 days without prompt surgical removal of the worms [1]. Your vet will judge the class from the signs and tests.
Does a darker positive line mean more worms?
No. The color intensity of a positive antigen test cannot reliably tell the number of worms [1].
Sources
- American Heartworm Society. Canine guidelines for the prevention, diagnosis, and management of heartworm (Dirofilaria immitis) infection in dogs, revised 2024. heartwormsociety.org/veterinary-resources/american-heartworm-society-guidelines
- Drake J, Wiseman S. Increasing incidence of Dirofilaria immitis in dogs in USA with focus on the southeast region 2013-2016. Parasit Vectors. 2018;11(1):39. doi.org/10.1186/s13071-018-2631-0
- Self SW et al. Regional and local temporal trends in the prevalence of canine heartworm infection in the contiguous United States: 2012-2018. Parasit Vectors. 2019;12(1):380. doi.org/10.1186/s13071-019-3633-2
- Courtney CH, Zeng Q. Comparison of heartworm antigen test kit performance in dogs having low heartworm burdens. Vet Parasitol. 2001;96(4):317-322. doi.org/10.1016/s0304-4017(01)00374-0
- Henry LG et al. Comparison of six commercial antigen kits for detection of Dirofilaria immitis infections in canines with necropsy-confirmed heartworm status. Vet Parasitol. 2018;254:178-182. doi.org/10.1016/j.vetpar.2018.02.037
- Little S et al. Prime detection of Dirofilaria immitis: understanding the influence of blocked antigen on heartworm test performance. Parasit Vectors. 2018;11(1):186. doi.org/10.1186/s13071-018-2736-5
- eClinPath, Cornell University College of Veterinary Medicine. Leukocytes. eclinpath.com/hematology/leukogram-changes/leukocytes
- Romano AE et al. Intracardiac heartworms in dogs: clinical and echocardiographic characteristics in 72 cases (2010-2019). J Vet Intern Med. 2021;35(1):88-97. doi.org/10.1111/jvim.15985
- Carretón E et al. Evaluation of serum biomarkers and proteinuria for the early detection of renal damage in dogs with heartworm. Vet Parasitol. 2020;283:109144. doi.org/10.1016/j.vetpar.2020.109144
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.