4-year-old female River Otter (Lutra canadensis)Found dead on display at the local zoo. The body was found under water.

Gross Description:  

The referring veterinarian performed the necropsy and reported pulmonary congestion, multifocal areas of pallor on the surface of the liver, and thickened myocardium.

Histopathologic Description:

Liver sections show unencapsulated, moderately well-defined inflammatory foci characterized by infiltrates of eosinophils, lymphocytes, macrophages and multinucleate giant cells which affect approximately 10% of the tissue. Within these inflammatory foci are occasional curvilinear nematode parasites with discernable nuclear columns consistent with microfilarial forms. Smaller, primarily eosinophilic foci are randomly present within the parenchyma as well. Periportal zones contain increased numbers of mononuclear cells.

Morphologic Diagnosis:  

Hepatitis, eosinophilic and granulomatous, multifocal, marked with microfilaria.


Dirofilaria immitis

Contributor Comment:  

Dirofilaria immitis infection has been previously reported in the River otter (Lutra canadensis), however it is not known if the species serves as a definitive host for the parasite8. This is in contrast to the Eurasian otter (Lutra lutra) which appears to be able to support the filarial infection3. The gross necropsy in this case was performed by the referring veterinarian who did not observe adult worms, However, numerous microfilaria were observed histologically within pulmonary and myocardial blood vessels. Eosinophilic granulomatous hepatitis has been reported in River otters previously, therefore the lesion may be a manifestation of the infection in this species6. The parasite was not speciated, however is consistent with the genus Dirofilaria. 

Specific syndromes associated with heartworm disease include asymptomatic infection, glomerulonephritis, allergic pneumonitis, eosinophilic granulomatosis, pulmonary embolization, congestive heart failure, caval syndrome and aberrant migration1. Changes in this case to correlate best with eosinophilic granulomatosis, however the organ most severely affected was the liver, not the lung. Microfilaria lodged in the sinusoids causing a striking inflammatory response. While microfilaria were clearly present in the circulation, similar foci of inflammation were not observed in lung, kidney or heart, suggesting that the liver may be predilection site for this infection in otters.

JPC Diagnosis:  

1. Liver: Hepatitis, granulomatous, eosinophilic, multifocal to coalescing, moderate, with hepatocellular degeneration and microfilaria, River otter (Lutra canadensis), carnivore.
2. Liver, hepatocytes: Vacuolar change, glycogen-type, diffuse, mild.

Conference Comment:  

It is not possible to determine the genus and species of an organism through examination of microfilaria only in tissue cross sections. Body length, the shape of the head and tail, presence or absence of a sheath, and curvature of the body are all used to aid in identification, and even then that may not be sufficient to classify even to the genus level.9 Obtaining an entire microfilarial organism from a blood or tissue specimen, examination through antigen or antibody tests on serum samples from infected animals or PCR on microfilaria may all aid in identification. Even with the presence of adult worms, their specific speciation may be difficult to impossible if the parasite has not been well described and characterized.7

There are numerous species of Dirofilaria that primarily infect particular host species. The most well known cause of microfilariasis of animals in North America is Dirofuilaria immitis, which is primarily found in the dog.4 With the exception of D. immitis, most species of Dirofilaria adults are found within the subcutaneous tissues.7 Morbidity and mortality associated with D. immitis infections have been reported in mustelids, canids, felids, otariids, and other domestic and nondomestic carnivores6, including the river otter.8 D. lutrae infections have only been reported in North American river otters, with adults occurring in subcutaneous spaces and very rarely in the cardiopulmonary vasculature.6,7 Adult forms of D. repens are found within the subcutaneous tissues of canines, other carnivores and occasionally humans.4 D. tenuis is primarily found in raccoons of the southern USA, and D. striata within bobcats. 

Characteristic histopathologic lesions in the lungs of dogs with dirofilariasis include villous endarteritis with luminal occlusion caused by villous intimal proliferation and medial hypertrophy.2 Nitric oxide (NO) production has been implicated in the inflammatory response during filarial infections.5 NO production can be induced by a recombinant Wolbachia surface protein.5 Wolbachia is an intracellular endosymbiont bacteria that resides within some filarial organisms, including Dirofilaria immitis.4

In a recent Journal of Wildlife Diseases article, an unidentified filarial organism was isolated from wild populations of the black-footed ferret.9 Although this unidentified microfilaria elicited a positive reaction to ELISAs for D. immitis, it only shared approximately 76% molecular identity with D. immitis, while sharing approximately 97% identity with Acanthocheilonema viteae.9

Morphologic characteristics of some microfilariae of onchocercids adapted from Wisely et. al.9
Filarial speciesLength mSheathHost
Dirofilaria immitis233-322noDog
Dirofilaria striata230-371noBobcat
Dipetalonema reconditum270-290noDog
Loaina uniformis285yesEastern cottontail
Dirofilariaeformia pulmoni213-288noEastern gray squirrel
Brugia beaveri300yesRaccoon
Brugia lepori210-330yesSwamp rabbit
Mansonella llewellyni290 ± 5noRaccoon
Monsonella interstitium 250 ± 5noEastern gray squirrel
Molinema arbuta280-297noNorth American porcupine
Acanthocheilonema reconditum269-283noDog
Acanthocheilonema mephitis186-218noStriped skunk


1. Atkins C: Canine heartworm disease. In: Textbook of Veterinary Internal Medicine, eds. Ettinger SJ, Feldman EC, 6th ed., pp. 1118-1137. Elsevier Saunders, St. Louis, Missouri, 2005
2. Kawabata A, Nakagaki K, Yoshida M, Shirota K: Histopathological comparison of pulmonary artery lesions between Raccoon Dogs (Nyctereutes procyonoides) and domestic dogs experimentally infected with Dirofilaria immitis. J Vet Med Sci 70:301-303, 2008
3. Matsuda K, Baek B, Lim C: Eurasian Otter (Lutra lutra), a definitive host for Dirofilaria immitis. J of Zoo and Wildl Med 34:200-201, 2003
4. Maxie MG, Robinson WF: Cardiovascular system. In: Jubb, Kennedy, and Palmers Pathology of Domestic Animals, ed. Maxie MG, 5th ed., vol. 3, pp. 87-89. Elsevier Limited, St. Louis, MO, 2007
5. Morch³n R, Bazzocchi C, L³pez-Belmonte J, Mart+�-�n-Pancho JR, Kramer LH, Grandi G, Sim³n F: iNOs expression is stimulated by the major surface protein (rWSP) from Wolbachia bacterial endosymbiont of Dirofilaria immitis following subcutaneous injection in mice. Parasitol Int 56:71-75, 2007
6. Neiffer DL, Klein EC, Calle PP, Linn M, Terrell SP, Walker RL, Todd D, Vice CC, Marks SK: Mortality associated with melarsomine dihydrochloride administration in two North American River Otters (Lutra canadensis) and a Red panda (Ailurus fulgens fulgens). J of Zoo and Wildl Med 33:242-248, 2002
7. Orihel TC, Eberhard ML: Zoonotic filariasis. Clin Microbiol Rev 11:366-381, 1998
8. Snyder DE, Hamir AN, Nettles VF and Rupprecht CE: Dirofilaria immitis in a River Otter (Lutra canadensis) from Louisiana. J of Wildl Dis 25:629, 1989
9. Wisely SM, Howard J, Williams SA, Bain O, Santymire RM, Bardsley KD, Williams ES: An unidentified filarial species and its impact on fitness in wild populations of the black-footed ferret (Mustela nigripes). J Wildl Dis 44:53-64, 2008

A virtual slide is not available for this case.

Fig. 1-1 Liver

Fig. 1-2 Liver

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