Zoonotic dirofilariosis is caused by Dirofilaria immitis and Dirofilaria (Nochtiella) repens. The disease is usually present in/or imported from those areas that are endemic for canine dirofilariosis. D. immitis in humans can cause lung nodules that are often confused with pulmonary neoplasia, while D. repens can localize both in the subcutaneous tissue and in the eye. Current epidemiological studies indicate that the prevalence of human infection is increasing. This is likely due to several factors including increased medical awareness and greater parasitological pressure caused by an increase both in animal reservoirs and vector density. Definitive diagnosis, particularly of lung nodules, can be challenging due to the lack of specific, non-invasive procedures. Prevention of infection in the canine host and the development of reliable serological diagnosis are both important instruments in the control of human dirofilariosis.
D. immitis is a common parasite of dogs and other canids, and in some extent of cats, in many regions of the world and human infections with D. immitis are found wherever theparasite is enzootic. In the natural hosts, D. immitis is the causative agent of canine and feline heartworm disease, inducing severe/extremely severe pathological changes in both the hosts. The parasite In Europe and parts of Africa and Asia, D.repens is a common subcutaneous parasite of dogs and, in some areas, of cats andwild carnivores as well. This filarial nematode is an important agent ofhuman subcutaneous and ocular dirofilariosis.
Human hosts are infected in the same way as the animal reservoirs, by the bite of a mosquito carrying infective larvae (L3). Humans are considered inappropriate (or accidental) hosts for zoonotic dirofilaria and nematodes do not normally reach sexual maturity. It is thought that the host's immune response eliminates most infective larvae and in those cases where development does occur, the worm dies much sooner than in its natural host. In pulmonary dirofilariosis by D. immitis, the worms are usually detained in a branch of the pulmonary artery where they produce an embolism and later a pulmonary nodule (Cordero et al., 1992; Orihel and Eberhard, 1998). Sexually mature but isolated adults (which makes reproduction impossible) have also been observed occasionally in different locations. For instance, Faust et al. (1941) reported one case of infection in an aged black woman caused by one adult male worm found in inferior vena cava and only one human case of D. immitis infection has been described where microfilariae were present in the blood (Nozais et al., 1994).
Subcutaneous dirofilariosis by D. repens appears as a small subcutaneous nodule which gradually grows during weeks or months. Its consistency is hard and elastic, and presents marked erythema. When the location is ocular, the worms are situated in the conjunctiva and can be extracted by incision (Pampiglione et al., 1995). Both pulmonary and subcutaneous lesions must be differentiated from neoplasia (Muro et al., 1999).
Current epidemiological studies indicate that human dirofilariosis is increasing in prevalence and several authors have recently described it as an emerging disease in different areas of the world. Bronshtein et al, (2003) have recently reported 11 cases of subcutaneous dirofilariasis in the Moscow area, with a strong prevalence for the male genitalia, that were diagnosed in less than 2 years time. Countries like Taiwan are currently reporting their first cases ever (Tsung and Liu, 2003). According to Raccurt (1999), 48% of human dirofilariosis cases reported in France have been diagnosed in the only the last 10 years.
However, the geographical distribution of human dirofilariosis is difficult to define, since a retrospective review of previously published clinical cases offers, more than the real distribution, the areas where the greatest interest exists regarding these zoonoses and/or where the best tools for its detection are found. Increased medical awareness has undoubtedly contributed to the numerous reports recently found. From 1965, the year in which the first case of pulmonary dirofilariosis was reported, until 1989, 165 pulmonary cases were published, most of them in three well defined areas (United States, Japan and Australia) (Vélez et al., 2001). However, in the following 13 years (1990-2003) at least 130 cases have been reported in 15 countries while 780 cases of subcutaneous/ocular dirofilariosis have been reported in 30 European, Asian and African countries (data from Pampiglione & Rivasi, 2000; Pampiglione et al., 2001; Cordonnier et al., 2002). The number of cases of human subcutaneous dirofilariosis due to D. repens reported in the literature after the first observation by Addario (1885) amounted to about 410 in 1995, 181 of which from Italy (Pampiglione et al, 2001). However, it has to be emphasized that this conditions was documented in few more than 10 reports 50 years ago while from 1990 to 1999 60 cases has been reported by Pampiglione et al (2001) and they conclude that now the infection has to be considered an emergent zoonosis at least in Italy. Medical awareness alone, however, cannot account for the sudden rise in reported cases in recent years. Other factors likely play a role in increased prevalence, like the spread of animal dirofilariosis into areas previously considered non endemic. The spreading of heartworm infection in dogs in southern Switzerland is a good example of this. In 1995, 4 dogs out of 371 stray and unwanted dogs were found positive for HW infection in southern Switzerland (canton Tessin) (Deplazes et al., 1995) and at least one imported dog was treated for HW disease (Arnold et al., 1994). In 1998, Bucklar et al.(1998) had reported both D. immitis and D. (N.) repens in 0.6% and 1.6% respectively of 479 Swiss dogs examined. One of these dogs may have acquired the infection in the Canton Tessin, suggesting an autochthonous cycle. These observations were confirmed by Genchi et al. (1998) in dogs who had never left their resident areas (mainly watchdogs kept in courtyards) in southern Switzerland. Diagnosis was performed by haematology and serology, and worms were visualized by echocardiography. In 2001, the presence of both D. immitis and D. repens in dogs in south of Switzerland was again surveyed by Petruschke at al. (2001). The observed prevalence was 6%. This is probably due to climatic changes that facilitate the introduction of competent vectors in areas in which they were not previously present, and to the lack of control measures in animal reservoirs (Genchi et al., 2001).
Recent epidemiological studies have also looked at the serological evidence of exposure to D. immitis in humans living in endemic areas. In an endemic zone of Western Spain in which D. immitis infects 33,3% of the resident dogs, authors have reported specific IgG in 5.6%, IgM in 2.6% and IgE in 12.6% of the human population (Simón et al., 1991; Muro et al., 1991; Espinoza et al., 1993). Considering that no concomitance of different classes of antibodies was observed (IgG and IgM were detected together in only one case), the seroprevalence in humans of this zone was 20.97%. Moreover, in a radiologic follow-up made on the same population, 8 cases of pulmonary dirofilariasis during 2 years were found in a human population of 50.000. This suggests that the parasitosis is likely underdiagnosed (Muro et al., 1999). In another study, human populations living in two endemic areas with different prevalences in their canine populations were considered. The areas were Pavía (Italy) (canine prevalence of D. immitis 50-80%), and Salamanca (the same area in which the previous study was conducted; known canine prevalence 33.3%). In humans, the highest IgG seroprevalence to D. immitis was found in Pavía where 32.3% of the sera analyzed were positive. Seroprevalence in Salamanca was 10%. Moreover, mean antibody levels were also significantly higher in Pavía than in Salamanca, indicating a stronger antigenic stimulus of the human immune system in the area in which the parasites are more abundant (Prieto et al., 2000). It is interesting to remember that 10 years before, the IgG seroprevalence in the endemic zone of Salamanca was only 5.6%. The increase of seroprevalence in this area can be explained by the urban development of this zone with a parallel increase of canine population and to the lack of preventive measures in dogs.
A real concern of the medical profession is the lack of reliable, non-invasive methods for the diagnosis of human dirofilariosis, especially in the case of pulmonary lesions. It is impossible to distinguish radiologically a nodule by Dirofilaria from a potentially malignant tumour. Milanez de Campos et al (1997), in a retrospective study on 24 cases of pulmonary dirofilariosis in Brazil, reported that nearly 50 % of affected patients were asymptomatic. When clinical signs are present, they are not specific for dirofilariosis (chest pain, coughing, fever, haemoptysis and dyspnoea). Definitive diagnosis is usually made with wedge biopsy, videothoracoscopy or, very rarely, by fine needle biopsy (Milanez de Campos et al, 1997). Identification of worms recovered intact from the tissues is rather easy, but identification in transverse sections can be problematic due to the variability of the morphological characteristics used for the identification at different levels of the body of the same worm (Orihel & Eberhard, 1998), and to the destruction caused by the inflammatory reaction of the hosts.
Several studies have been aimed at improving the diagnostic tools for human dirofilariasis. The polymerase chain reaction technique (PCR) is invaluable for identification of parasites when their morphological features are altered, due to its high sensitivity, even with minimal amounts of parasite DNA (Favía et al., 1996). Nevertheless, this technique does not avoid surgical intervention previous to obtaining the tissue samples or the worms themselves. Serology is an alternative to these invasive methods. In spite of the small number of worms that cause human infections there is a considerable antibody response, which can be used for diagnosis. Different antigenic complexes (Simón et al., 1991; Santamaría et al., 1995; Simón et al., 1997) and molecules derived from Dirofilaria spp have been employed in ELISA or Western blot (Phillip & Davis; 1985; Sun & Sugane, 1992; Perera et al., 1994 and 1998). Cancrini et al. (1999) reported that in eight cases of subcutaneous/ocular dirofilariasis, PCR and serological techniques correlated well.
Coming back to the title question of this presentation if the human Dirofilaria infections can be considered as a real zoonotic concern x points must be emphasized:
The expanding areas suitable for the maintenance of Dirofilaria infection both in mosquito vectors and animal reservoirs as a consequence of changing climatic conditions (global warming).
The low specificity of mosquito species involved in Dirofilaria transmission in the blood meal for their needs of survival and development. Many species have blood meals both on humans and animals.
The increasing number of reports of human infections.
In conclusion, the opinion of the authors is that human Dirofilaria infection has to be included in the list of emerging zoonoses.
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