Lymphatic Filariasis LF

Lymphatic filariasis is regarded as the second most common global arthropod-borne infectious disease, with an estimated burden of 128 million infected people distributed over 78 endemic countries (see Figure 1.7) and an estimation of 1.3 billion people at risk from developing new active LF infection annually.20 Like malaria, the predominance of LF infections are found in humid tropical areas of Asia, Sub-Saharan Africa, the western Pacific and scattered areas of the Americas. Although not fatal, LF is considered a leading cause of infirmity, permanent disability and chronic morbidity, often resulting in a societal stigma of disfigured victims.

This disease is caused by macroscopic nematode pathogens, of which Wuchereria bancrofti is responsible for 90% of human LF infections. The remaining 10% are due to two species of the genus Brugia (B. malayi and B. timori) and occur only in Asia. There are three variants of W. bancrofti recognized on periodicity patterns of circulating microfilaria (mf) found in the peripheral blood of humans, namely, the nocturnally periodic (NP), the nocturnal subperiodic (NSP) and the diurnal subperiodic (DSP) forms. Periodicity is based on the prevailing circadian distribution of mf in the peripheral blood, e.g., the nocturnally periodic (NP) form presents the majority of mf by night (peak periodicity 22.00-03.00 hrs). The primary vectors of the NP filariae are nocturnally active mosquitoes, such as Anopheles species in rural areas and

Mapa Mundo Con Espacio Geografico
Figure 1.7 Lymphatic filariasis distribution around the world (Photo courtesy of Elsevier).3

Culex quinquefasciatus in urban settings. The NP variant is responsible for the vast majority of infections occurring worldwide in patchy foci distributed along the tropical and subtropical belt. The subperiodic microfilariae are strongly correlated to a transmission by vectors of the genera Aedes and Ochlerotatus, which are diurnally active species.

More than 70 species of mosquitoes within six different genera are known vectors of W. bancrofti, including Anopheles (43 spp.), Aedes/Ochlerotatus/ Downsiomyia (approximately 20 spp.), Culex (6 spp.), and Mansonia (3 spp.).3 Among the anophelines, 36 species are capable of both malaria and LF transmission, 26 of which are regarded as major LF vector species. Bancroftian filariasis and Brugia malayi are unique among the vector-borne parasitic diseases in that larval development can take place in several genera of mosquitoes. Three main zones of LF transmission are recognized herein: (1) West Africa, Southeast Asia (rural areas), New Guinea Island, Vanuatu and Solomon Islands, where Anopheles mosquitoes are the principal vectors; (2) East Africa, Middle East (Egypt, Yemen), Southeast Asia (urban zones), and the Latin American region (e.g. Haiti) where the infection is transmitted mainly by the Cx. quinquefasciatus and Cx. pipiens group; and (3) the south-western Pacific islands (including French Polynesia) and limited areas of Southeast Asia where Aedine (Aedes, Ochlerotatus, Downsiomyia spp.) vectors play a dominant role.

Brugian filariae are predominately found in rural locations and are vectored by Anopheles and Mansonia species for B. malayi, and by An. barbirostris for B. timori. Brugia malayi occurs from scattered areas of India (south and north-east) and Sri Lanka, to Southeast Asia and the Philippines. Brugia timori is restricted to a small group of islands of the Lesser Sunda Archipelago, primarily Timor and Flores.

The cycle starts with the absorption of mf by the female mosquito during the blood meal. They reach and cross the abdominal midgut (stomach) wall into the hemocoele to migrate to the insect's thoracic flight muscles to begin development. Microfilariae do not reproduce in the vector, but rather each worm completes two intermediate larval stages (L1 and L2) moults to become a third-stage (L3) infective parasite. The L3 eventually breaks free from the flight muscles into the hemocoele and ultimately ends up in the insect's head lodged in or near the labium of the proboscis. As for the malaria parasite, the filarial development within the mosquito takes approximately 10-14 days and is also temperature dependent. When the mosquito takes a blood meal, the 1.21.6 mm long L3 infective larvae will break free through the cuticle or emerge from the tip (labellum) of the labium onto the skin. In contrast to malaria parasites, filarial nematodes are not inoculated but deposited on the skin and they must actively enter the host body via an open portal (e.g., the mosquito bite wound or a nearby break in the skin). High ambient humidity and skin moisture favour successful transmission. After entering the vertebrate host, the L3 is transported via the lymphatic vessels to the lymph nodes to begin development into mature adult worms (0.2 mm wide, up to 10 cm long). Contrary to Plasmodium, the mosquito acts as the intermediate host and humans serve as the definitive host for Wuchereria and Brugia species. It is thought that microfilariae survive and circulate freely in the blood of the human host for many months, possibly longer, while awaiting an opportunity of being picked up by mosquitoes.

Depending on transmission intensity, the LF infection is usually acquired early in childhood, although a period of ten to 20 years of exposure may be required before presenting the characteristic morbid manifestation called "elephantiasis", visible at adolescence and adulthood. Although the chronic physical phase of the disease afflicts only a small percentage of those infected, in its most apparent forms, LF morbidity can result in temporary or permanent infirmity which is often the painful and gross enlargement of the legs and arms, the genitals, vulva and mammary glands. Additionally, adult worms and mf can also cause internal damage and disease to other organs such as kidneys and lungs. The psychological and social stigmas associated with the disease are immense and it has a major social and economic impact in countries where 1050% of men and up to 10% of women can be adversely affected due to permanent damage to the lymphatic system.21

Control: Since 1997, the Global Programme to Eliminate Lymphatic Filar-iasis (GPELF) has been directed to people living in at-risk communities by providing once annual oral treatment, using a two-drug combination, either albendazole + ivermectin, or albendazole + diethylcarbamazine (DEC) for the elimination of microfilariae (mf) in the blood and disruption of the adult female reproductive capacity.20 It is expected that reducing the number of mf in humans will lower vector infection through their bite and therefore stop further transmissions. However, this doesn't occur with Aedes in Pacific Islands where Aedes become more infected when few mf are available (''facilitation phenomena'') and strong vector control is thus also needed in such situations. It is generally assumed that LF elimination, in areas where Anopheles species are transmitting NP strains of W. bancrofti, will be relatively easy to achieve. Implementing synchronous and multifaceted strategies, with MDA and comprehensive vector control as central components, can stop both filarial and malaria transmission. As such, integrated control strategies targeting both diseases in areas sharing the same Anopheles vector species are highly recommended as the most cost-effective approach.

1.2.4.3 Main Tropical Arboviruses

Arthropod-borne-virus, more commonly called Arbovirus, is a virus transmitted to a vertebrate by hematophagous arthropods which constitute the biological vector. The arboviruses include different diseases for their symptomatology and especially their epidemiology (see Table 1.2). Their precise diagnosis is delicate and requires recourse to biological examinations in specialized laboratories. Approximately 110 viruses are pathogenic for humans, 40 of them are also the cause of identified animal diseases.

The tropism of the viruses explains the principal clinical symptoms observed. All the arboviruses present a certain neurotropism. Three general clinical pictures

Main Topics in Entomology: Insects as Disease Vectors Table 1.2 Classification of arboviruses.

Family

Genus

Togaviridae Alphavirus (28 viruses, including Chikungunya, O'nyong-nyong,

Ross River, Sindbis, Mayaro, equine encephalitis) Flaviviridae Flavivirus (68 viruses, including yellow fever, dengue, Japanese encephalitis, West Nile, Kyasianur Forest disease, Omsk hemorrhagic fever) Bunyaviridae Bunyavirus (138 viruses, including Bunyamwera)

Phlebovirus (43 viruses, including Rift Valley fever) Nairovirus (24 viruses, including Crimean-Congo hemorrhagic fever) + 41 unclassified viruses Reoviridae Orbivirus (69 viruses); Coltivirus (2 viruses); + 6 unclassified viruses

Rhabdoviridae Vesiculoviris (18 viruses); Lyssavirus (16 viruses); + 36 unclassified can be observed: (1) Acute febrile syndromes ("dengue-like"): alphavirus (Chikungunya, O'nyong-nyong, Ross River, Sindbis and Mayaro); flavivirus (dengue and West Nile); bunyavirus (Bwamba, Bunyamwera and Tataguine); phlebovirus (Rift Valley fever); (2) Encephalitic syndromes: alphavirus (equine encephalitis); flavivirus (Japanese encephalitis and West Nile); (3) Hemorrhagic syndromes: flavivirus (dengue hemorrhagic fever, yellow fever, Kyasianur Forest, Omsk hemorrhagic fever); phlebovirus (Rift Valley fever); nairovirus (Crimean-Congo hemorrhagic fever).

The arboviruses affect vertebrates and arthropods; this is referred to as "horizontal transmission". In certain cases, a "vertical transmission" may occur, as in the case of transovarian or trans-stadial transmission, i.e. the viruses pass through the genital tractus and preimaginal stages and a new adult generation is infected. The arthropod remains infective all its life. Any climatic change is likely to involve important effects on the foci of the arbovirosis.

General arbovirosis prevention is difficult but actions can be taken such as: (1) monitoring the epidemiologic foci, e.g. human, vertebrate and vectors, to prevent any outbreak; (2) surveillance of the wild vertebrate hosts (illusory) or domestic ones (limited effect); (3) control of the wild vectors (impossible) or domestic or peri-domestic vectors (possible but difficult due to diversity of man-made breeding sites); (4) protection of the receptive human population (strongly recommended) based upon skin repellents or treated clothes, even mosquito nets, while vaccines are available only against yellow fever and Japanese encephalitis.

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