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Veterinary Parasitology 152 (2008) 167–170

Short communication

Intraarticular Dirofilaria immitis microfilariae

in two dogs

Steven Hodges a,*, Mark Rishniw b

a Michigan Veterinary Specialists, 29080 Inkster Road, Southfield, MI 48034, United Statesb Department of Biomedical Sciences, Box 11 Veterinary Research Tower, College of Veterinary Medicine,

Cornell University, Ithaca, NY 14583, United States

Received 28 September 2007; received in revised form 4 November 2007; accepted 8 November 2007

Abstract

Dirofilaria immitis microfilariae were found in the synovial fluid of two dogs. One dog had clinical and cytological evidence of

polyarthritis at the time of presentation. The second dog presented with severe effusion in a single joint and was later diagnosed with

synovial sarcoma of the affected joint. These patients were not protected with heartworm prophylaxis and lived in heartworm

endemic areas. Though there is documentation of D. immitis microfilaria in the synovial fluid of several clinically normal research

dogs with cytologically normal synovial fluid, to our knowledge these are the first documented cases of intraarticular microfilaria in

a dog with cytologically confirmed polyarthritis. Based on these unique cases, D. immitis infection should be considered a

differential diagnosis in patients with polyarthropathies. Interpretive caution must be used when intraarticular microfilaria are

present, as concurrent etiologies may also be present.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Dirofilaria; Microfilaria; Intraarticular; Polyarthritis; Dog

1. Introduction

The clinical presentation of dogs with Dirofilaria

immitis infection is typically associated with complica-

tions due to the anatomic location of adult heartworms

in the right pulmonary artery and, to a lesser degree, the

right ventricle. Extra-cardiopulmonary affects second-

ary to D. immitis infection have been well documented

in dogs with canine heartworm disease. The aberrant

migration of adult worms into numerous tissues appears

throughout the veterinary literature. In such cases,

clinical signs not classically associated with canine

* Corresponding author at: Oklahoma Veterinary Specialists, 515 W

Main Street, Jenks, OK 74037, United States. Tel.: +1 918 299 4900;

fax: +1 918 299 6366.

E-mail address: okvetspec@sbcglobal.net (S. Hodges).

0304-4017/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.vetpar.2007.11.018

heartworm disease have been reported (Otto, 1974;

Blass et al., 1989; Hribernik et al., 1989; Elkins and

Berkenblit, 1990; Frank et al., 1997; Healey et al.,

2003). A thorough search of the literature revealed

minimal references to the aberrant migration of D.

immitis microfilaria, specifically intraarticular migra-

tion and associated clinical signs.

Weinberger et al. (1979) studied the effect of trauma

on the canine joint in a group of fifteen dogs from the

southern United States. Microfilaria was discovered in

the synovial fluid of one of the dogs during postmortem

examination. The dog appeared clinically well and had

grossly normal appearing joints. Synovial fluid analysis

revealed a normal cell count with no evidence of

inflammation. Systemic microfilaremia was diagnosed

with the Knott test and the acid phosphate test was used

to identify the microfilaria as D. immitis in origin. The

S. Hodges, M. Rishniw / Veterinary Parasitology 152 (2008) 167–170168

Fig. 1.1. D. immitis microfilaria discovered during cytologic exam-

ination of synovial fluid obtained from the right stifle.

significance of the intraarticular microfilaria was

unknown.

2. Case report

2.1. Case 1

An approximately 3-year-old, intact male mixed

breed dog presented to Michigan Veterinary Specialists

as a rescue from the Gulf Coast of the United States

following the 2005 Hurricane Katrina. Physical

examination did not reveal any significant abnormal-

ities. A complete blood count (CBC), biochemical

profile, urinalysis, fecal floatation and smear and

heartworm antigen test were performed. Significant

findings included hyperglobulinemia (4.1 g/dL, range

1.6–3.6 g/dL), a slight monocytosis (930/mL, range 0–

840/mL), hookworm (Ancylostoma spp.) eggs on fecal

floatation and a positive heartworm antigen test. A

Knott’s test was performed and was positive for

microfilaria.

Treatment was administered for intestinal and

ectoparasites. Because of suspected very high worm

burdens, a treatment protocol for D. immitis was

recommended by the American Heartworm Society

specifically for heartworm positive dogs rescued from

Hurricane Katrina (American Heartworm Society,

2007). Microfilaricidal treatment consisted of oral

ivermectin (Heartguard1) administered at a dose of

6 mg/kg (preventative dose) on days 1 and 14 and every

30 days thereafter. On day 28, a one-time dose of 50 mg/

kg of ivermectin was administered orally. Two months

following the initial dose of ivermectin, adulticide

therapy was initiated using the two-step, three-dose

melarsamine (Immidicide1) protocol.

Four days after the initial treatment with ivermectin,

the dog developed a mild fever (39.3 8C) and lameness

which initially responded to treatment with a non-

steroidal anti-inflammatory (NSAID) (deracoxib (Dera-

maxx1) 2.2 mg/kg daily). Despite continued NSAID

therapy, the dog developed an intermittent, shifting limb

lameness and low-grade fever. Joint palpation revealed

mild pain, however, no joint effusion was appreciated.

Serology for Ehrlichia canis, Borrelia burgdorferi and

Rickettsia rickettsii were negative. Discontinuation of

NSAID therapy resulted in worsening of fever,

progressive lameness and anorexia within 36 h. Because

of persistent lameness and low-grade fever, arthrocent-

esis was performed.

Synovial fluid was obtained from the left radio-

carpal joint and left and right stifles. Cytological

evaluation revealed low numbers of microfilariae

(Fig.1.1) with mild mononuclear inflammation in the

right stifle and low numbers of microfilariae with mixed

inflammation consisting of macrophages, small lym-

phocytes and non-degenerate neutrophils in the left

stifle. Neither sample had red blood cells or evidence of

hemosiderin in macrophages that would be suggestive

of contamination or previous intraarticular hemorrhage.

Synovial fluid from the left carpus was within normal

limits with moderate peripheral blood contamination

and no microfilaria seen.

Molecular identification of the microfilariae

observed in the synovial fluid smear was performed

as previously described, with minor modifications

(Rishniw et al., 2005). Briefly, 75 mL lysis buffer

(25 mM NaOH, 0.2 mM disodium EDTA, pH 12) was

applied to an air-dried DiffQuick-stained smear of the

joint fluid with four microfilariae visible on the slide.

After suspending the contents of the smear in the lysis

buffer, the solution was aspirated and transferred to a

1.5 mL Eppendorf tube, heated at 95 8C for 20 min and

immediately mixed with 75 mL of neutralization buffer

(40 mM Tris–HCl, pH 5) on ice (Truett et al., 2000).

Five microliters of the resultant solution were used for

the PCR genotyping reaction as previously described,

using DIDR-F1 and DIDR-R1 primer pairs. Genotyping

confirmed the microfilariae to be D. immitis in origin.

After the arthrocentesis was performed, NSAID

therapy was reinstated and the fever and lameness

improved. Five days after the first dose of melarsamine

the dog developed a mild cough and hemoptysis. The

NSAID was discontinued and treatment initiated with

prednisone 1 mg/kg PO q12 h for suspected pneumo-

nitis. The dog was released to a rescue organization and

medical care was continued through the primary

S. Hodges, M. Rishniw / Veterinary Parasitology 152 (2008) 167–170 169

veterinarian. The dog completed the remainder of

adulticide treatment without incidence and is currently

only receiving prophylactic heartworm prevention. The

dog has shown no signs of lameness as of the date of this

manuscript.

2.2. Case 2

On 20 June 2006, a 7-year-old, castrated male

German Shepherd Dog was referred to Northwest

Indiana Veterinary Emergency Specialty Hospital

(NIVESH) for the evaluation of left rear leg lameness

of several weeks duration. Initial physical examination

findings included a grade III/IV left rear leg lameness

with profound swelling of the left stifle joint. There was

no cranial drawer instability palpated. Radiographs of

the left stifle revealed severe joint effusion but no

radiographically apparent degenerative joint disease.

The patella appeared to be elevated from the distal

femur, separated by what appeared to be a soft tissue

density.

Arthrocentesis of the left stifle was performed and

75 mL of a mildly translucent and viscous fluid was

removed from the joint. Cytology of the synovial fluid

revealed minimal cellularity with the primary

nucleated cell population consisting of monocytic

type cells with minimal activation as well as rarely

seen lymphoid cells. Minimal amounts of peripheral

blood were present. A single microfilaria was seen in

one sample. The patient was treated with an NSAID

and returned to the primary veterinarian for further

diagnostic testing and treatment for suspected canine

heartworm disease.

A heartworm antigen test was positive and a Knott’s

test confirmed microfilaremia. Adulticide therapy was

initiated using the two-step, three-dose melarsamine

(Immidicide1) protocol. One month following the

completion of heartworm treatment, a microfilaricidal

dose (1.9 mg/kg) of milbemycin (Interceptor1) was

given and a negative Knott’s test was obtained 2 weeks

later. Left rear leg lameness persisted after the

completion of treatment for heartworm disease.

The patient returned to NIVESH for evaluation of

persistent non-weight-bearing left rear leg lameness.

The left stifle was very swollen and painful on palpation

and muscle atrophy of the leg was present. Attempts at

arthrocentesis were unsuccessful and radiographs

revealed bony lysis of the medial femoral condyle.

Amputation of the left pelvic limb was performed and

the left stifle joint was submitted for histopatholgy.

Histopathologic examination of the joint revealed a

synovial sarcoma.

3. Discussion

Although canine heartworm disease is common in

many parts of the world, polyarthritis secondary to

intraarticular microfilaria has not previously been

described. Intraarticular microfilariae have been

reported previously but inflammation associated with

the joint and clinical signs of polyarthritis were not

present in those dogs (Weinberger et al., 1979). One of

the two cases reported here had concurrent intraarticular

microfilaria and inflammation in two different joints,

neither of which had significant blood contamination.

Furthermore, the clinical signs associated with arthritis

resolved after completion of treatment for D. immitis

infection.

The dog in the second case had a synovial cell

sarcoma of the left stifle where the intraarticular

microfilaria was discovered. It is feasible that the

sarcoma was present at the time of arthrocentesis and

the integrity of the joint was compromised, allowing

migration of microfilaria into the joint. It is also feasible

that the sarcoma was responsible for the inflammatory

component of the synovial fluid.

The method by which microfilariae entered the joints

in the dog from the first case is unknown. In cases where

adult worms have been found in aberrant locations, it

has been hypothesized that immature stages may

occlude small vessels or capillaries resulting in rupture

of the vessel and release of the larva into the aberrant

location (Otto, 1974). There the larva matures into an

adult worm where it may later cause clinical signs not

typically associated with heartworm disease.

Filarial arthropathies have been described in humans

(Waltzing and Bloch-Michel, 1971; Dorfmann and de

Seze, 1972; Jaffres et al., 1983; Kohem et al., 1994;

Gallagher et al., 2002). In most cases, treatment of the

filariasis led to resolution of the arthropathy. The exact

mechanism whereby arthritis is induced in humans with

filarial diseases is unknown. Proposed hypotheses

include local release of proteases by the worms which

might directly damage the synovial tissue (Petralanda

et al., 1986) or intraarticular deposition of immune

complexes. Immune complexes containing filarial

antigens, however, are yet to be detected in human

synovial tissue (Dissanayake et al., 1982).

The presence of intraarticular microfilaria in the dog

in Case 1, and the resolution of clinical signs after

completion of appropriate heartworm treatment, sug-

gests that canine heartworm disease should be a

differential diagnosis in patients presenting with signs

of polyarthritis. However, as demonstrated by the dog in

Case 2, careful evaluation for other causes of lameness

S. Hodges, M. Rishniw / Veterinary Parasitology 152 (2008) 167–170170

or polyarthritis should be pursued if symptoms do not

resolve after appropriate heartworm treatment.

Acknowledgements

The authors thank Dr. Elizabeth Jay, ACVP, Antech

Diagnostics, for the synovial fluid analysis, Dr. Bradley

Coolman, ACVS, Northeast Indiana Veterinary Emer-

gency and Specialty Hospital, for the submission of case

number 2 and Tracy Stokol, BVSc, PhD, DACVP (clin

path), Dept Pop Med and Diag Sci, Cornell, CVM for

obtaining the photographs of the microfilaria.

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