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Occult dirofilariasis in a dog.


Canine heartworm disease caused by the filarial nematode, Dirofilaria immitis is a major potentially life-threatening disease of dogs with worldwide distribution and global significance. Besides, its veterinary importance, it also has zoonotic potential in many geographical regions. Microfilariosis is a widespread problem among dogs in India (Gogoi, 2000). The usual definitive host of D. immitis is the domestic dog, although cats, wolves, coyotes, foxes, muskerats and sea-lions may act as suitable hosts and reservoir of the disease. Mosquitoes of the genera Aedes, Anopheles and Culex are all suitable intermediate hosts and vectors. Some species of fleas, lice and ticks may also act as vectors. Occult heartworm infection in canines is defined as the presence of adult Dirofilaria immitis in dogs without concurrent circulatory microfilariae (Rawlings et al., 1982; Knight, 1977; Wong et al., 1973; Otto, 1978). An occult infection may be the outcome of prepatent infection, unisexual adult infections, worm sterility following therapy and immune-mediated sterile infection. This is encountered in about 5-25% of the infected dogs, paving the way to false-negative diagnosis (Rawlings loc. cit; Knight loc. cit Wong loc. cit and Otto loc. cit). 20-30% of canine infections and the majority of the infected cats are negative for microfialriae (Rawlings and MacCall, 1982; Rawlings and Calvert, 1995). The disease may either be associated with microfialriae in the patient's blood, a patent infection, or it may be occult. A patent infection may be overlooked because of inadequate volume of blood examined, concentration technique not used and diurnal and seasonal periodicity which reduces the microfialriae to very low levels (Stogdale, 1984).

History and Observations

A non-descript bitch aged 81/2 years weighing 15 kg was presented with anamnesis revealing a recent whelping by the bitch (her pups apparently in good health status) and there was complaint of persistent coughing, dullness, depression, anorexia and going down in condition. Taking cues from the anamnesis as revealed by the owner, a detailed physico-clinical examination was undertaken which defined few remarkable problems viz. cardiovascular abnormality (polypnoea, variable pulse strength), increased frequency of micturition with intermittent incontinence along with persistent isoasthenuria, congested mucus membrane, cachexia, mild diarrhoea, off-feed condition, ocular discharge, prolonged capillary refill time (CRT) coupled with hyperthermia. Cardiac examination was indicative of marked arrhythmia (80-82 beats/minute), moderate polypnoea (50-55 breaths/minute) with variable pulse strength. Complete blood count was unremarkable in terms of mild leucocytosis and thrombocytopenia. An alteration in serum biochemical profile was registered viz. mild elevation in alanine aminotransferase, alkaline phosphatase and creatinine kinase (136 U/L, 150 U/L and 130U/L, respectively). A mild degree of proteinuria was encountered. Serial modified Knott's tests were undertaken at different interval over next few days but came out negative for microfilariae.


Taking recent whelping into consideration in the light of various clinico-haemato-biochemical results, the bitch was treated symptomatically. The patient was put under antibiotic umbrella by administration of Inj. ceftriaxone @ 5 mg/kg Bwt. (I/V) for five days, Inj. Meloxicam @ 0.5 mg/kg Bwt. (I/M) for three days. This was further coupled with administration of Inj. dexamethasone and Inj. chlorphenaramine maleate @ 0.5 mg/kg Bwt. (I/M) for three days. Besides, a mild fluid therapy was initiated bearing in mind the critical illness and hypovolemic status of the patient with the solo motto to restore haemodynamic and renal function, thereby, improving cardiac input and tissue perfusion and correcting metabolic acidosis. The patient was advocated prompt rest under cage-confinement or strict exercise restriction in the light of the fact that an aggravated status may be precipitated by exercise.

This registered a transient improvement of the degree of suffering which gradually aggravated. Further attempts to stabilize the bitch proved futile and the patient succumbed to its sufferings during the supportive therapy, paving the way to final diagnosis upon subsequent post-mortem examination. An appreciable number of heartworms were noticed in pulmonary artery, vena cava and right ventricle of the heart appearing as spaghetti (Fig. 1 and 2). An extensively echhymosed kidney with marked nephritis, too, was retrieved (Fig. 3). Grossly the pulmonary arteries were found to be thickened, enlarged and tortuous, with roughened endothelial surfaces. Anaemia, too, was evident. Cardiac hypertrophy was evident upon PM Examination on account of obstruction of the right ventricle and pulmonary arteries with adult worms. An important finding was the globose or rounded appearance of the heart from dilation of the right atrium.





In the present clinical case, the patient may have, naturally, acquired the infection wherein adult dogs reveal high circulating microfilariae count culminating into occult infections when it gets protracted over a few months (Rawlings loc.cit). The severity of the lesions and hence clinical ramifications are related to the relative no. of worms (ranging from one to over 250), the duration of infection and the host and parasite interaction (Atkins, 2005). In asymptomatic dogs, pre-patent and unisexual infections are the most common cause of occult infection (Rawlings loc.cit and Knight loc.cit) which is in contrast to the present case which was fairly symptomatic. Another possibility may be a transplacental transmission of the microfilariae. New-borne pups, born to heavily infected bitches may receive the microfilariae transplacentally which in turn eliminate the microfilariae by producing the antibodies, making them sensitized to the microfilariae (Wong loc.cit). Host-produced antibody against the microfilariae chiefly accounts for an occult heartworm infection which results in immune-mediated death of the microfilariae (Rawlings loc.cit; Knight loc.cit and Wong loc.cit). Modified Knott's tests which are recommended as the most reliable blood tests for detecting the presence of microfilariae were negative in the present case. Moreover, the case was dealt with in winter season during which the microfilariaemia is reduced (Knight loc.cit). Microfilariae exhibiting diurnal and seasonal periodicity, being most numerous in the blood stream during the early morning and late evening, makes multiple sampling quite imperative as it significantly increases the likelihood of diagnosis (Acevedo et al., 1981 and Knight loc.cit). The clinical manifestations of canine dirofilariasis vary from absence of any symptoms to a wide range of symptoms (Gogoi, 2002; Zislin, 1981 and Hatkin, 1985). Proteinuria encountered in this case may have been precipated by the antigen-antibody complexes which are formed in response to heartworm antigens, which in turn culminates into glomerulonephritis in the heartworm infected dogs (Grauer, 2003). As evident by the post-mortem examination, the present case may be a case of caval syndrome wherein death frequently ensues within 24-72 hours due to cardiogenic shock, complicated by anaemia, metabolic acidosis and DIC in case the treatment is either not undertaken or delayed. Eosinophilic pneumonitis is most often reported in true occult heartworm disease, when immune-mediated destruction of microfilariae in the pulmonary microcirculation produces amicrofilariaemia, which incites an inflammatory reaction/eosinophilic pneumonitis (Calvert et al., 1985).


The present piece of clinical paper puts an insight into an occult dirofilarial infection in a non-descript bitch. In the light of various findings, it can be concluded that an occult dirofilarial infection still poses a challenge to field veterinarians. Indirect FAT and ELISA (now available as kits) which detect most prepatent infections, as well as both patent and occult adult heartworm infections accurately and precisely, are practically not feasible under field conditions. Hence, the diagnostic modalities underlying the definitive diagnosis of dirofilariasis is application of Indirect FAT and ELISA in conjunction with routine modified Knott's tests (multiple sampling), haematology, urinalysis, serum biochemical profile and thoracic radiography. Radically, heartworm infection is completely preventable with macrolide prophylaxis and recommended at the beginning of 6-8 weeks of age, precluding any antigenic testing (Merck Veterinary Manual, 9th Edn, 2005).


Acevedo, R.A., Theis, J.H., Kraus, J.F. and Longhurst, W.M. (1981). Combination of filtration and histochemical stain for detection and differentiation of Dirofilaria immitis and Dipetalonema reconditum in the dog. Am. J. Vet. Res. 42: 537- 540.

Atkins, C. (2005). Canine Heartworm Disease. In: Textbook of Veterinary Internal Medicine. Ettinger, S.J. and Feldman, E.C. Vol.2, 6th Edn., Elsevier Saunders, U.S.A., pp. 1119.

Calvert, C.A. and Losonsky, J.M. (1985). Occult disease associated allergic pneumonitits. J. Am. Vet. Med. Assoc. 186: 1097.

Gogoi, A.R. (2002). Filarids of animals of animals in India. J. Vet. Parasitol. 16 : 131.

Grauer, G.F. (2003). Pathogenesis of heartworm-induced glomerulonephritis. In : Proc. Amer. Heartworm Assoc. Society 01. Batavia, III., American Heartworm Society (In press).

Hatkin, J. and McGrath, J.G. (1985). An unusual manifestation of dirofilariasis. Vet. Med. 80: 52-54.

Knight, D.H. (1977). Heartworm heart disease. Adv. Vet. Sci. Comp. Med. 21 : 107-49.

Merck Veterinary Manual. (2005). Heartworm Disease (Dirofilariasis). 9th Edn. Published by Merck & CO. Inc. Whitehouse Station, N.J., U.S.A. pp. 105-106.

Otto, G.F. (1978). The significance of microfialraemia in the diagnosis of heartworm infection. In: Proceedings of the Heartworm Symposium'77. Bonner Springs: Veterinary Medicine Publishing Company. p. 22-30.

Rawlings, C.A.; Dawe, D.I.; McCall, J.W.; Keith, J.C. and Prestwood, A.K. (1982). Four types of occult dirofilaria immitis infection in dogs. J. Am. Vet. Med. Assoc. 180: 1323-26.

Rawlings, C.A. and Calver, C.A. (1995). Heartworm disease. In: E.S.J. Ettinger ESJ and Feldman, E.C. (4th Edn.): Textbook of Veterinary Internal Medicine, W.B. Saunders, Philadelphia, p. 1046-1068.

Stogdale, L. (1984). Testing for occult heartworm infection. Can. Vet. J. 25: 171-74.

Wong, M.M.; Suter, P.F.; Rhode, E.A. and Guest, M.F. (1973). Dirofilaria without circulating microfilaria: a problem in diagnosis. J. Am. Vet. Med. Assoc. 163: 133-39.

Zislin, A.A. (1981). Asymptomatic dirofialriasis and sudden death in a dog. Canine Practice 81 :51-54.

Hemant Kumar (1), Dipti Kiran (2)

District Veterinary Hospital

O/o-The Deputy Director of Veterinary Services

Jashpur--496331 (Chhattisgarh)

(1.) Veterinary Assistant Surgeon

(2.) Post Graduate Scholar, Dept. of Vety. Pathology, Ranchi Veterinary College, Ranchi
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Title Annotation:Short Communication
Author:Kumar, Hemant; Kiran, Dipti
Publication:Intas Polivet
Article Type:Report
Geographic Code:9INDI
Date:Jan 1, 2012
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