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Heartworm Disease

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Diagnosis

The earliest time point that heartworm antigen and microfilariae can be detected is about 5 and 6.5 months post infection, respectively. Depending on the sensitivity of the particular heartworm antigen test, antigenemia may precede, but sometimes also lags behind the appearance of microfilariae by a few weeks.

Whether screening a population of asymptomatic dogs or seeking verification of a suspected heartworm infection, antigen testing is the most sensitive diagnostic method. The current generation of heartworm antigen tests identify most "occult" (microfilaria negative) infections, consisting of at least one mature female worm, and are nearly 100% specific. Microfilaria testing is complementary and may be done in tandem with antigen testing to specifically determine whether this life-cycle stage is also present in dogs that showed positive antigen testing.

Antigen tests

ELISA and immunochromatographic test systems are available for detecting circulating heartworm antigen. Each testing format has proved to be clinically useful. Differences in sensitivity exist but these are statistically insignificant. False negative results also can occur erratically with any one test. In such a case unexpected negative results can be reconciled by retesting with a different test. Specificity is consistently very high.

The amount of antigen in the circulation system bears a direct but imprecise relationship to the number of mature female heartworms. A graded test reaction can be recognized by ELISA test systems, but quantitative results are not displayed by immunochromatographic tests. Quantitative analysis of antigen results is highly speculative and requires correlation with other relevant information. For example, radiographic evidence of advanced pulmonary arterial disease typical of chronic heartworm disease and a low or absent antigenemia is consistent with the aftermath of a previous infection that has been cleared, either naturally or by treatment.

If the validity of a weakly positive result is in doubt, verification may be achieved by repeating the test and if still ambiguous, by independent confirmation by some other means, such as a second antigen test format, concentration tests for microfilariae, thoracic radiography to detect signs of heartworm disease or ultrasonographic visualization of worms. Upon request, most test manufacturers will analyze ambiguous samples in their own laboratories. If there has not been much potential for exposure, it is recommended to confirm all positive antigen tests in asymptomatic dogs prior to any adulticide therapy.

False-negative test results occur most commonly when infections are light, female worms are still immature, only male worms are present and/or the test kit (for test kits requiring refrigeration) or sample has not been warmed to room temperature. Antigen test results should be interpreted carefully, taking other relevant clinical information into consideration. However, in general, it is better to trust rather than reject antigen test results, unless that interpretation is contradicted strongly by independent clinical evidence or circumstances influencing the probability of infection.

Microfilaria tests

Most microfilaremic dogs can be detected by microscopical examination of fresh blood for cell movement created by the motility of the microfilariae. A stationary rather than a migratory pattern of movement is indicative of a Dirofilaria species. Movement beneath the buffy coat in a microhematocrit tube also may be visible microscopically. However, these are insensitive methods for examining blood in which low numbers of microfilariae (50-100/ml) are present. Therefore, it should not be assumed that no microfilariae are present until at least 1.0 ml of blood has been examined using a concentration technique (modified Knott test or filtration test). The modified Knott test is the preferred method for observing morphology and measuring microfilarial body dimensions to differentiate D. immitis from non-pathogenic filarial species such as Acanthocheilonema (formerly Dipetalonema) reconditum. Although screening may be based entirely on antigen testing, antigen-positive dogs should also be tested for microfilariae, because a microfilaremia validates the serologic results and identifies the patient as a reservoir of infection.

For the modified Knott test in short 1 ml EDTA blood plus 9 ml 2% formalin are mixed and centrifuged. The supernatant is discarded. To the remaining sample methylene blue (0.1%) or methylene green (0.2%) is added and the sediment examined microscopically for microfilariae (see Table 1).

Table 1: Details on the differential diagnosis of microfilariae in the blood of dogs (after Deplazes, 2006)

Criteria

D. immitis

D. repens

Acanthocheilonema reconditum

Dipetalonema dracunculoides*

sheath

missing

missing

missing

missing

approx. length (µm) in stained preparation

205-2831

260-3081

213-2402

246-2583

mean

<2701

>2701

not specified

2523

width (µm)4

5.0-6.5

6.0-8.0

4.0-5.0

5.0-6.0

front end

conical

blunt

blunt

conical

posterior end

straight

hook-shaped bent

hook-shaped bent (only 30-40%)

straight

evidence of acid phosphatase5

at excretion and anal porus

at anal porus

diffusely distributed

one spot each on the inner body and the anal porus; circle around excretion porus6

molecularbiol. differentiation

PCR

PCR

PCR

PCR

1Bucklar et al. (1998); 2Whiteley (1988); 3Olmeda-Garcia and Rodriguez-Rodriguez (1994); 4Ducos de Lahitte (1990); 5method: Chalifoux and Hunt (1971); 6Ortega-Mora et al. (1989)

*syn. Acanthocheilonema dracunculoides

Radiography

Radiography provides the most objective method of assessing the severity of heartworm cardiopulmonary disease. There are few radiographic changes with mild infection. Typical signs of heartworm vascular disease are enlarged, tortuous and often truncated peripheral intralobar and interlobar branches of the pulmonary arteries, particularly in the diaphragmatic lobes. These findings are accompanied by variable degrees of pulmonary parenchymal disease. The earliest and most subtle pulmonary arterial changes are found in the dorso-caudal wedge of the diaphragmatic lung lobes. As the severity of infection and chronicity of disease progress, the pulmonary arterial signs are seen in successively larger branches, and in the worst cases, eventually the right heart also enlarges.

Echocardiography

The body wall of adult heartworms is highly echogenic and produces distinctive, short parallel-sided images with the appearance of "equal signs" where the imaging plane cuts across loops of the parasite. Echocardiography can provide definitive evidence of heartworm infection, as well as an assessment of cardiac anatomic and functional consequences of the disease. However, the method is not efficient of making this diagnosis, particularly in lightly infected dogs, since the worms often are limited to the peripheral branches of the pulmonary arteries beyond the echographic field of view. When heartworms are numerous, they are more likely to be present in the main pulmonary artery, right and proximal left interlobar branches, or within the right side of the heart where they can be imaged easily. In dogs with hemoglobinuria, visualization of heartworms in the orifice of the tricuspid valve provides conclusive confirmation of the caval syndrome.

Further information

  • Current Canine Guidelines for the Diagnosis, Prevention and Management of Heartworm (Dirofilaria immitis) Infection in Dogs (revised January, 2012), Executive Board of the American Heartworm Society (AHS)
  • Bucklar H, Scheu U, Mossi R, et al.: [Is dirofilariasis in dogs spreading in south Switzerland?] Schweiz Arch Tierheilkd. 1998, 140, 255-60 [in German]
  • Chalifoux L, Hunt RD: Histochemical differentiation of Dirofilaria immitis and Dipetalonema reconditum. J Am Vet Med Assoc. 1971, 5, 601-5
  • Ducos de Lahitte J: Epidemiologie des filarioses en France. Pratiq Med Chirurg Anim Compag. 1990, 25, 305-10
  • Deplazes P: [Helminthoses of dogs and cats.] In: Schnieder T (ed.): Veterinärmedizinische Parasitologie. 6th edn., 2006, Parey in MVS, Stuttgart, pp 444-520 [in German]
  • Olmeda-Garcia AS, Rodriguez-Rodriguez JA: Stage-specific development of a filarial nematode (Dipetalonema dracunculoides) in vector ticks. J Helminthol. 1994, 68, 231-5
  • Ortega-Mora LM, Gomez-Bautista M, Rojo-Vazquez FA: The acid phosphatase activity and morphological characteristics of Dipetalonema dracunculoides (Cobbold, 1870) microfilariae. Vet Parasitol. 1989, 33, 187-90
  • Whiteley HE: Your diagnostic protocol for Dirofilaria immitis infection in dogs. Vet Med. 1988, 83, 328-45

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