Filariasis is a group of chronic diseases caused by nematodes from the Filariaidae group. They are characterized by the existence in the human body of both mature forms of the parasite and larvae called microfilariae. This group of diseases includes lymphatic filariasis, onchocercosis, loaosis, infections with Mansonella sp., As well as zoonotic filariosis, and their common feature is disease transmission by insects of the order of Diptera.
Lymphatic Filariasis
Occurrence
Lymphatic filamentous diseases are widespread in many tropical and subtropical countries of Africa, Asia, South America, the Caribbean and Oceania. Everywhere there is the most common of the philarios – vusherriosis, caused by Wuchereria bancrofti. Brugia malai filariasis occurs in Indonesia, Malaysia, the Philippines, India, southern China, Korea and Japan, and Bruges timori-induced filariosis is confined to Timor and its neighboring Indonesian islands.
The transmitters of lymphatic filariasis are mosquitoes of the genus:
- Culex,
- Anopheles,
- Aedes,
- Mansonia.
These mosquitoes do microfilaria along with the blood of an infected person. The mosquito’s body undergoes a series of transformations leading to the formation of the third larval stage, which passes into the human body during the mosquito bite. Microfilariae exhibit the diurnal periodicity of occurrence in human blood, synchronized with the circadian rhythm of the activity of carrier insects.
Most lymphatic filariasis are active at night, with peak microfilariae activity in the blood from 9.00:2.00 p.m. to around XNUMX:XNUMX a.m. The main vector are mosquitoes of the genus Anopheles in rural areas and the genus Culex in cities. Only on the islands of the Eastern Pacific, where the Aedes mosquitoes are active during the day, there is a variable periodicity of W. bancrofti microfilariaemia, usually manifested by their highest presence in the host’s blood in the afternoon.
Man is the only natural definitive host for worms that cause lymphatic filariasis, although some monkeys can be infected experimentally. Endemic outbreaks of filariasis tend to be small and the risk of infection in locations distant from each other by only a few or several kilometers may vary significantly
The causes of lymphatic filariasis
Lymphatic filamentous diseases result from infection:
- Wuchereria bancrofti,
- Brugia malai,
- Brugia timori.
Mature female Wuchereria bancrofti are 80-100 mm long and about 0,25 mm thick, while the males are about 40 mm long. Mature Bruges malai and Bruges timori are half the size of Wuchereria bancrofti.
Symptoms of filariasis
The occurrence of clinical symptoms of filariosis is related to the impact on the host’s organism of mature forms of the parasite, microfilariae, the host’s immune response to circulating filarial antigens and bacterial superinfection.
In lymphatic filariasis, mature individuals residing in the lymph vessels and sinuses of the lymph nodes are the main factor causing disease symptoms. They cause:
- widening of lymphatic vessels,
- thickening of the lymphatic wall,
- slowing down the lymph flow in these vessels.
Dead or dying worms cause the biggest inflammatory changes in the lymphatic system. Inflammatory infiltrates appear, consisting of multinucleated cells and histiocytes, damage to the endothelium of lymphatic vessels, narrowing of the lumen, damage to the valves, which ultimately leads to complete obliteration of these vessels. Frequent bacterial inflammation of the lymphatic vessels and adjacent tissues causes paroxysmal fever and contributes to further destruction of the lymphatic vessels.
The host immune response to worm antigens causes production eosinophilic granulomas. This complex mechanism leads to:
- lymph stagnation,
- the formation of puffiness,
- the formation of hard infiltrates in the subcutaneous tissue,
- the formation of elephantiasis.
The generalized inflammatory reaction to circulating allergy-type microfilariae, combined with the activation of eosinophils, plays a leading role in tropical lung damage pulmonary eosinophilia.
It is not known why only a few infected people develop this pathology. In recent years, much research has focused on the role of Wolbachia in the pathogenesis of infection. This bacterium is the endosymbionite of worms that cause lymphatic filariasis – without its presence, they cannot produce microfilariae. Antimicrobial therapy to eradicate this organism has been shown to reduce symptoms in some cases, but the exact role of this bacterium in the pathogenesis of human lymphatic filariasis requires further research.
Clinical symptoms of filariasis
Lymphatic filariasis is characterized by a wide spectrum of symptoms – from asymptomatic infections to elephantiasis of the limbs and testicular hydrocele. Important! Most people who are infected have no symptoms.
Some of them remain asymptomatic for the rest of their lives, while others develop them after several months or many years without symptoms. Among people without obvious clinical symptoms, we distinguish two groups of patients:
- more are those with microfilariae in the blood,
- in a less numerous group, microfilariae are not detected, and the only evidence of infection is the presence of the circulating antigen of the parasite in the blood.
The acute form of filariasis manifests itself:
- fever,
- inflammation of the nodes and lymph vessels.
It occurs at the earliest after a few months’ stay in endemic areas, it lasts 3-15 days, it can repeat even several times a year, although in some patients it occurs only a few times in life. Inflammation most often affects one lymph node and the lymph vessels distal to it, and is mainly located on the lower limbs, less often on the upper limbs. The affected lymph node is swollen and tender, sometimes redness and increased warmth of the area are observed. Lymphatic vessels are hard and inflamed. Sometimes it is accompanied by inflammation of the surrounding veins with thrombosis.
Sometimes, especially in the case of infections with Bruges sp., Abscesses develop, sometimes with fistulas and skin ulcerations. Repeated episodes of inflammation lead to obstruction of the lymphatic system. The result is initially lymphoedema, reversible after lifting the limb or massage, and then elephantiasis – a permanent increase in the volume of the affected limb with hardening of the skin and proliferation of connective tissue. In these cases, papillary and nodular changes often occur in the skin of the affected limb.
In the case of Wuchereria bancroftiIn addition to the lesions on the limbs similar to the infection with Bruges sp., the genital organs are often affected. Then, swelling of the epididymides, testes and scrotum appears, initially soft and with a tendency to periodically decrease, then hard and constant. Lymph and tissue fluid accumulate between the testicle and the testicular sheath, sometimes in very large amounts, giving the appearance of a testicular hydrocele.
Some patients, especially those infected with Wuchereria bancrofti, gradually develop the chronic form of the disease without acute symptoms. Chyluria is a rare symptom of filariasis. Increased pressure in the perirenal lymphatic vessels sometimes leads to rupture and the passage of lymph into the urinary system.
A separate group of symptoms related to lymphatic filariasis is tropical pulmonary eosinophilia. It manifests itself:
- dry, unproductive cough
- shortness of breath
- inflammatory infiltrates in the lungs,
- significantly increased number of eosinophils in peripheral blood and bronchial lavage.
Rare clinical symptoms of filariasis are: arthritis, glomerulonephritis and polymyositis. These symptoms are believed to be related to the involvement of immune complexes in the pathogenesis of the disease.
Filariasis – diagnosis
It is important in the diagnosis of lymphatic filariasis detection of circulating microfilariae in the blood. Blood should be drawn at the time when microfilariae is most likely to be detected, which is in most cases at night.
In areas where, in addition to night periodicity, there is a daily or variable periodicity, parasitological testing should be performed with at least two blood samples taken 12 hours apart. Thin blood smears stained with the Giemsa method are made. In order to increase the sensitivity of the test, compaction methods are used, the most common of which is the Knott method.
A more sensitive diagnostic method of filariasis, useful especially in tropical pulmonary eosinophilia, is detection of the circulating antigen of the parasite. It is detected by enzyme immunoassay and immunochromatographic tests. Serological tests, which are usually based on tests with Dirofilaria immitis antigens, are of limited use, useful only for confirming the filariotic nature of chronic, long-lasting lesions such as elephantiasis of the limbs or chronic genital lesions. However, these tests show cross-reactivity for all phylliosis and are almost always positive in residents of endemic regions. In recent years, the PCR method has been introduced to the diagnostics of filariasis, although its clear advantage over traditional tests has not been demonstrated.
On the other hand, mature forms of Wuchereria bancrofti and Brugia sp. Can be visualized in dilated lymphatic vessels using ultrasonography. This is especially true of the genital localization of lesions.
Differentiation of filarioses
Acute forms of filariasis require differentiation from infections associated with lymphadenopathy, such as:
- tuberculosis,
- leprosy,
- sarcoidosis,
- bartonellosis.
Lymphadenitis secondary to skin or wound infections should also be considered. A similar clinical picture can also be seen in thrombophlebitis. Chronic lymphoedema followed by elephantiasis may be caused by postoperative damage to the lymphatic system, cancer, circulatory failure or renal failure, congenital abnormalities of the lymphatic system (Milroy’s disease). The clinical picture of elephantiasis may also be caused by the post-thrombotic syndrome and the reaction to foreign bodies, e.g. silica dust.
Tropical pulmonary eosinophilia should be differentiated from:
- bronchial asthma and other helminthiasis in which the larvae pass through the lungs (ascariasis or nematode),
- with Churg-Strauss syndrome,
- Wegener’s granulomatosis,
- less often with other collagen diseases.
Treatment of lymphatic filariasis
The drug of choice in the treatment of lymphatic filariasis is dietylkarbamazyna (DEC). This drug has a strong parasiticidal activity against microfilariae, and sometimes it is also active against mature forms of the parasite. When used at a dose of 6 mg / kg bw / day for 14 days, it reduces microfilariae lasting up to several months. Treatment with diethylcarbamazine should be started at a low dose of 50 mg once a day, the next day the dose should be increased to 150 mg / day in 3 divided doses, on day 3 300 mg / day in 3 divided doses, and the full dose can be taken from day 4. . Such management reduces the risk of an increased allergic reaction to disintegrating microfilariae.
A drug with a similar effect, mainly used in mass programs aimed at eradication of filariasis, is ivermectin administered as a single dose of 150 mg / kg body weight, once a year. It is also effective albendazole at a dose of 2 times 400 mg for 10 days.
In mass therapeutic programs, the beneficial effect is also achieved by single administration of ivermectin and albendazole at a dose of 400 mg. Recently, the use of antibiotics has been proposed to eradicate Wolbachia bacteria which are essential for the production of microfilariae by mature females.
Few clinical trials have shown the effectiveness of administering doxycycline in a dose of 2 times 100 mg for 4-6 weeks in reducing microfilaraemia and clinical symptoms. Limb elephantiasis and genital lesions are usually irreversible conditions and no clinical improvement is achieved with antiparasitic treatment.
Improvement or inhibition of disease progression can be achieved by:
- hygiene,
- antibacterial treatment of superinfections,
- physiotherapy,
- exercises to improve the circulation of the lymph.
Surgical removal of the testicular hydrocele is sometimes used.
What is the prognosis of filariasis?
In acute forms of the disease, the prognosis is good, although the patient may require annual treatment for many years. The prognosis for recovery in chronic forms is poor. Elephantiasis-type lesions, as well as changes in male sexual organs, are irreversible and lead to significant disability. Lymphatic filariasis is rarely the cause of death.
Source: J. Cianciara, J. Juszczyk, Infectious and parasitic diseases; Czelej Publishing House