Subcutaneous Dirofilariosis

Subcutaneous dirofliariosis is caused by the filarial nematode Dirofilaria repens, a close relative of Dirofilaria immitis, the so called heartworm. Like D. immitis, D. repens is transmitted through a mosquito bite.

Dirofilaria repens is an emergent zoonotic pathogen, although infection is not transferred directly from dog to man but by mosquito vectors. Infection in humans usually results in a subcutaneous nodule in the upper part of the body, close to the eyes as well as under the conjunctiva. Less often inner organs can be affected including the lungs. Human cases are reported from more than 30 countries. In Europe the highest numbers of cases are reported from Italy, France, Greece and the Ukraine.

Pathogens

Dirofilaria repens (Nematoda: Filaroidea, Onchocercidae) is a whitish and filariform worm (male 5-7 cm, female 6-17 cm) localized as adult mainly in the subcutaneous connective tissue of the dog. Other animals affected are the red fox (Vulpes vulpes) and more rarely the cat. The microfilariae are found in the peripheral blood, waiting to be picked up by a mosquito. The main representative of the genus Dirofilaria is Dirofilaria immitis, commonly called heartworm and infectious agent of heartworm disease.

Epidemiology

Subcutaneous dirofilariosis occurs in the Old World and, here in particular, throughout the Mediterranean sub-region, South Asia and sub-Saharan Africa. In Europe, the dog is the main reservoir host. European regions, for which the disease is endemic, include Spain, France, Italy, the Balkan peninsula, Slovakia, Romania, Ukraine and Russia, but meanwhile there are also cases reported from Portugal, Germany, Switzerland, Austria, Latvia etc. (see CVBD Occurrence Map Europe).

In Europe, the known vectors of Dirofilaria repens are mosquito species of the genera Anopheles, Aedes, Culex and Coquillettidia, with Culex pipiens pipiens and Aedes albopictus implicated as the main vectors in southern Europe.

Transmission

Similar to Dirofilaria immitis, Dirofilaria repens third stage larvae (L3) are transmitted to the dog during blood feeding of a mosquito (mainly Anopheles and Aedes, but also Culex species). Inside the vertebrate host, the larvae develop and mature to the adult form, which then produces its offspring, the so-called microfilariae. Mosquitoes become infected with microfilariae while taking a blood meal from an infected animal. Finally, the nematode larvae develop inside the ectoparasite to L3, which can then enter the next mammal. The developmental process is temperature-dependent and takes about 8 to 20 days at temperatures above 22°C. At 18°C, the development needs 28 days. In the mammalian host, the L3 migrate to the subcutaneous tissue and develop further.

In dogs, the prepatent period is 189–239 days (Webber & Hawking, 1955), although in a more recent study the first microfilariae were found in the bloodstream on day 164 after infection (Petry et al., 2015). Dirofilaria repens nematodes may live up to ten years (on average two to four years) and females potentially produce microfilariae throughout their lifespan.

Pathogenesis

After the transmission of Dirofilaria repens third stage larvae (L3) during feeding of an infected female mosquito, the L3 migrate to the subcutaneous or connective tissues of the mammalian host and undergo two additional moults (from L3 to L4 and to preadult worms), finally maturing into adults. Usually, no inflammatory reaction or connective capsules are surrounding the living parasite, which can be seen moving actively under the connective serous layers. Although canine D. repens infections very often go asymptomatically, several non-specific dermal alterations have been reported such as skin nodules, pruritus, thinning, itching and asthenia.

 

Diagnosis

Adult worms

As Dirofilaria repens is mainly located in subcutaneous tissues, small, painless nodules with varying location may be investigated surgically. The samples can be preserved in 5% formaldehyde solution or alcohol.

Adult worms are long and whitish, without any grossly obvious identifying features. Males are about 5-7 cm and females are about 10-17 cm in length. Under magnification male worms can be observed to possess a series of small papillae arranged in a "v" shape and two unequal spicules at their caudal end.

 

Microfilariae

For the detection of microfilariae, blood samples should be prevented from coagulation using EDTA or heparin. These samples need to be processed in order to remove other cellular components. This can be performed using the modified Knott’s technique (see also under Heartworm Disease). Microfilariae are 207-360 µm long and 5-8 µm wide. There are overlaps with the sizes of other microfilariae, so confirmation of identity is a specialist skill.

 

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

Criteria

Dirofilaria immitis

Dirofilaria repens

Acanthocheilonema reconditum

Dipetalonema dracunculoides*

Dipetalonema (Cercopithifilaria) grassi**

Sheath

missing

missing

 missing

 missing

present, smooth and delicate7

Approx. length (µm) in stained preparation

205-2831

260-3081

213-2402

246-2583

>5507 (660)8

Mean

<2701

>2701

not specified

2523

not specified

Width (µm)4

5.0-6.5

6.0-8.0

4.0-5.0

5.0-6.0

5-15

Front end

conical

blunt

blunt

conical

blunt

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); 7Euzeby (1981); 8Tarello (2004)

*syn. Acanthocheilonema dracunculoides

**syn. Acanthocheilonema or Cercopithifilaria grassii; more frequently found on dermal tissue


In contrast to commercially available test systems for the detection of circulating antigens of Dirofilaria immitis, no similar specific serological tests are available for D. repens. But molecular methods, such as multiplex PCRs, testing parts of adult specimens, microfilariae (in whole blood or on filter paper), or larval stages in the mosquito vectors have been developed.

Clinical Signs

Most of the infected dogs are asymptomatic. A small, painless, subcutaneous nodule caused by the presence of an adult worm may be found during examination. Other causes of subcutaneous swelling should be considered for differential diagnosis. Location of the swelling may vary over time as the worms can move through the subcutaneous tissue. Apart from an asymptomatic course of infection, a number of non-specific dermal alterations have also been reported such as skin nodules, pruritus, thinning, itching and asthenia. Furthermore lesions may also appear as circular hairless areas with lichenification, hyperpigmentation and erythematous and scaling margins. Lesions may also be found in the lumbosacral, perianal and scrotal regions, as well as in the connective tissue between muscles during surgery.

Treatment & Prevention

Adult worms that are located as small, painless, subcutaneous nodules are removed surgically. Treatment in form of anthelmintic therapy of canine dirofilariosis is indicated for dogs suffering from clinical signs, such as dermal swelling, subcutaneous nodules and pruritus. Therapy of positive dogs is also indicated to reduce the risk of zoonotic transmission. A long term treatment (six monthly doses) with a combination of imidacloprid/moxidectin spot-on provided efficacy against adult stages of Dirofilaria repens and eliminated microfilariae. Additionally, spot-on treatments containing moxidectin provided preventive efficiency.

The risk of infection can be also minimised by applying topical drugs which repel and kill the transmitting mosquitoes. Especially for regions, which harbour not only mosquitoes but also sand flies and ticks, it may be more suitable to use broad-spectrum ectoparasiticides for prevention of pathogen transmission.

References

Transmission

Webber W, Hawking F: Experimental maintenance of Dirofilaria repens and D. immitis in dogs. Exp Parasitol. 1955, 4, 143-64

Petry G, Genchi M, Schmidt H, et al.: Evaluation of the adulticidal efficacy of imidacloprid 10%/moxidectin 2.5% (w/v) spot-on (Advocate®, Advantage® Multi) against Dirofilaria repens in experimentally infected dogs. Parasitol Res. 2015, 114, Suppl 1, S131-44

 

Diagnosis

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

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]

Ducos de Lahitte J: Epidemiologie des filarioses en France. Pratiq Med Chirurg Anim Compag. 1990, 25, 305-10

Euzeby J: Diagnostic Expérimental des Helminthoses animales. 1981, ed. „Informations Techniques des Services Vétérinaires“, Ministére de l’Agriculture, Paris, France

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

Tarello W: Identification and treatment of Dipetalonema grassii microfilariae in a cat from central Italy. Vet Rec. 2004, 155, 565-6

Whiteley HE: Your diagnostic protocol for Dirofilaria immitis infection in dogs. Vet Med. 1988, 83, 328-45

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