The most spectacular symptom of lymphatic filariasis is elephantiasis, a stage 3 lymphedema with thickening of the skin and underlying tissues. This was the first mosquito-borne disease to be discovered. Elephantiasis results when the parasites lodge in the lymphatic system and cause blockages to the flow of lymph. Infections usually begin in childhood.

The skin condition the disease causes is called "elephantiasis tropica" (also known as "elephantiasis arabum").

Elephantiasis mainly affects the lower extremities; the ears, mucous membranes, and amputation stumps are affected less frequently. However, various species of filarial worms tend to affect different parts of the body: "Wuchereria bancrofti" can affect the arms, breasts, legs, scrotum, and vulva (causing hydrocele formation), while "Brugia timori" rarely affects the genitals. Those who develop the chronic stages of elephantiasis are usually amicrofilaraemic and often have adverse immunological reactions to the microfilariae as well as the adult worms.

The subcutaneous worms present with skin rashes, urticarial papules, and arthritis, as well as hyper- and hypopigmentation macules. "Onchocerca volvulus" manifests itself in the eyes, causing "river blindness" (onchocerciasis), one of the leading causes of blindness in the world.

Serous cavity filariasis presents with symptoms similar to subcutaneous filariasis, in addition to abdominal pain, because these worms are also deep-tissue dwellers.

Elephantiasis leads to marked swelling of the lower half of the body.

Filariasis such as loiasis most often consists of asymptomatic microfilaremia. Some patients can develop lymphatic dysfunction causing lymphedema. Episodic angioedema (Calabar swellings) in the arms and legs, caused by immune reactions are common. Calabar swellings are 3–10 cm in surface non erythematous and not pitting. When chronic, they can form cyst-like enlargements of the connective tissue around the sheaths of muscle tendons, becoming very painful when moved. The swellings may last for 1–3 days, and may be accompanied by localized urticaria (skin eruptions) and pruritus (itching). They reappear at referent locations at irregular time intervals. Subconjunctival migration of an adult worm to the eyes can also occur frequently, and this is the reason Loa loa is also called the "African eye worm." The passage over the eyeball can be sensed, but it usually takes less than 15 min. Gender incidence of eyeworms have approximately the same frequency, but it tends to increase with age. Eosinophilia is often prominent in filarial infections. Dead worms may cause chronic abscesses, which may lead to the formation of granulomatous reactions and fibrosis.

In the human host, "Loa loa" larvae migrate to the subcutaneous tissue where they mature to adult worms in approximately one year, but sometimes up to four years. Adult worms migrate in the subcutaneous tissues at a speed less than 1 cm/min, mating and producing more microfilariae. The adult worms can live up to 17 years in the human host.

Lymphadenitis, the swelling of the lymph nodes, is a commonly recognized symptom of many diseases. An early manifestation of filariasis, lymphadenitis more frequently occurs in the inguinal area during "B. malayi" infection and can occur before the worms mature.

"B. malayi" is one of the causative agents of lymphatic filariasis, a condition marked by infection and swelling of the lymphatic system. The disease is primarily caused by the presence of worms in the lymphatic vessels and the resulting host response. Signs of infection are typically consistent with those seen in bancroftian filariasis—fever, lymphadenitis, lymphangitis, lymphedema, and secondary bacterial infection—with a few exceptions.

Filariasis is a parasitic disease caused by an infection with roundworms of the Filarioidea type. These are spread by blood-feeding black flies and mosquitoes. This disease belongs to the group of diseases called helminthiases.

Eight known filarial nematodes use humans as their definitive hosts. These are divided into three groups according to the niche they occupy in the body:

- Lymphatic filariasis is caused by the worms "Wuchereria bancrofti", "Brugia malayi", and "Brugia timori". These worms occupy the lymphatic system, including the lymph nodes; in chronic cases, these worms lead to the syndrome of "elephantiasis".

- Subcutaneous filariasis is caused by "Loa loa" (the eye worm), "Mansonella streptocerca", and "Onchocerca volvulus". These worms occupy the subcutaneous layer of the skin, in the fat layer. "L. loa" causes "Loa loa" filariasis, while "O. volvulus" causes river blindness.

- Serous cavity filariasis is caused by the worms "Mansonella perstans" and "Mansonella ozzardi", which occupy the serous cavity of the abdomen. "Dirofilaria immitis", or the dog heartworm rarely infects humans.

The adult worms, which usually stay in one tissue, release early larval forms known as microfilariae into the host's bloodstream. These circulating microfilariae can be taken up with a blood meal by the arthropod vector; in the vector, they develop into infective larvae that can be transmitted to a new host.

Individuals infected by filarial worms may be described as either "microfilaraemic" or "amicrofilaraemic", depending on whether microfilariae can be found in their peripheral blood. Filariasis is diagnosed in microfilaraemic cases primarily through direct observation of microfilariae in the peripheral blood. Occult filariasis is diagnosed in amicrofilaraemic cases based on clinical observations and, in some cases, by finding a circulating antigen in the blood.

Lymphatic filariasis, also known as elephantiasis, is a human disease caused by parasitic worms known as filarial worms. Most cases of the disease have no symptoms. Some people, however, develop a syndrome called elephantiasis, which is marked by severe swelling in the arms, legs, breasts, or genitals. The skin may become thicker as well, and the condition may become painful. The changes to the body may harm the affected person's social and economic situation.

The worms are spread by the bites of infected mosquitoes. Three types of worms are known to cause the disease: "Wuchereria bancrofti", "Brugia malayi", and "Brugia timori", with "Wuchereria bancrofti" being the most common. These worms damage the lymphatic system. The disease is diagnosed by microscopic examination of blood collected during the night. The blood is typically examined as a smear after being stained with Giemsa stain. Testing the blood for antibodies against the disease may also permit diagnosis. Other roundworms from the same family are responsible for river blindness.

Prevention can be achieved by treating entire groups in which the disease exists, known as mass deworming. This is done every year for about six years, in an effort to rid a population of the disease entirely. Medications used include antiparasitics such as albendazole with ivermectin, or albendazole with diethylcarbamazine. The medications do not kill the adult worms but prevent further spread of the disease until the worms die on their own. Efforts to prevent mosquito bites are also recommended, including reducing the number of mosquitoes and promoting the use of bed nets.

In 2015 about 38.5 million people were infected. About 950 million people are at risk of the disease in 54 countries. It is most common in tropical Africa and Asia. Lymphatic filariasis is classified as a neglected tropical diseases and one of the four main worm infections. The disease results in economic losses of many billions of dollars a year.

"Loa loa" filariasis is a skin and eye disease caused by the nematode worm "Loa loa". Humans contract this disease through the bite of a deer fly or mango fly ("Chrysops" spp), the vectors for "Loa loa". The adult "Loa loa" filarial worm migrates throughout the subcutaneous tissues of humans, occasionally crossing into subconjunctival tissues of the eye where it can be easily observed. "Loa loa" does not normally affect one's vision but can be painful when moving about the eyeball or across the bridge of the nose. The disease can cause red itchy swellings below the skin called "Calabar swellings". The disease is treated with the drug diethylcarbamazine (DEC), and when appropriate, surgical methods may be employed to remove adult worms from the conjunctiva.

Wuchereria bancrofti is a human parasitic roundworm that is the major cause of lymphatic filariasis. It is one of the three parasitic worms, together with "Brugia malayi" and "B. timori", that infect the lymphatic system to cause lymphatic filariasis. These filarial worms are spread by a variety of mosquito vector species. "W. bancrofti" is the most prevalent of the three and affects over 120 million people, primarily in Central Africa and the Nile delta, South and Central America, the tropical regions of Asia including southern China, and the Pacific islands. If left untreated, the infection can develop into a chronic disease called elephantiasis. In rare conditions it also causes tropical eosinophilia, an asthmatic disease. There is no commercially available vaccine however high rates of cure have been achieved with various anti-filarial regimens and lymphatic filariasis is the target of the WHO Global Program to Eliminate Lymphatic Filariasis with the aim to eradicate the disease as a public health problem by 2020.

Adult worms remain in subcutaneous nodules, limiting access to the host's immune system. Microfilariae, in contrast, are able to induce intense inflammatory responses, especially upon their death. "Wolbachia" species have been found to be endosymbionts of "O. volvulus" adults and microfilariae, and are thought to be the driving force behind most of "O. volvulus" morbidity. Dying microfilariae have been recently discovered to release "Wolbachia" surface protein that activates TLR2 and TLR4, triggering innate immune responses and producing the inflammation and its associated morbidity. The severity of illness is directly proportional to the number of infected microfilariae and the power of the resultant inflammatory response.

Skin involvement typically consists of intense itching, swelling, and inflammation. A grading system has been developed to categorize the degree of skin involvement:

- Acute papular onchodermatitis – scattered pruritic papules

- Chronic papular onchodermatitis – larger papules, resulting in hyperpigmentation

- Lichenified onchodermatitis – hyperpigmented papules and plaques, with edema, lymphadenopathy, pruritus and common secondary bacterial infections

- Skin atrophy – loss of elasticity, the skin resembles tissue paper, 'lizard skin' appearance

- Depigmentation – 'leopard skin' appearance, usually on anterior lower leg

- Glaucoma effect – eyes malfunction, begin to see shadows or nothing

Ocular involvement provides the common name associated with onchocerciasis, river blindness, and may involve any part of the eye from conjunctiva and cornea to uvea and posterior segment, including the retina and optic nerve. The microfilariae migrate to the surface of the cornea. Punctate keratitis occurs in the infected area. This clears up as the inflammation subsides. However, if the infection is chronic, sclerosing keratitis can occur, making the affected area become opaque. Over time, the entire cornea may become opaque, thus leading to blindness. Some evidence suggests the effect on the cornea is caused by an immune response to bacteria present in the worms.

The skin is itchy, with severe rashes permanently damaging patches of skin.

The most spectacular symptom of lymphatic filariasis is elephantiasis – edema with thickening of the skin and underlying tissues—which was the first disease discovered to be transmitted by mosquito bites. Elephantiasis results when the parasites lodge in the lymphatic system.

Elephantiasis affects mainly the lower extremities, while the ears, mucous membranes, and amputation stumps are affected less frequently. However, different species of filarial worms tend to affect different parts of the body; "Wuchereria bancrofti" can affect the legs, arms, vulva, breasts, and scrotum (causing hydrocele formation), while "Brugia timori" rarely affects the genitals. Those who develop the chronic stages of elephantiasis are usually free from microfilariae (amicrofilaraemic), and often have adverse immunological reactions to the microfilariae, as well as the adult worms.

The subcutaneous worms present with rashes, urticarial papules, and arthritis, as well as hyper- and hypopigmentation macules. "Onchocerca volvulus" manifests itself in the eyes, causing "river blindness" (onchocerciasis), one of the leading causes of blindness in the world. Serous cavity filariasis presents with symptoms similar to subcutaneous filariasis, in addition to abdominal pain, because these worms are also deep-tissue dwellers.

Infections by "Mansonella perstans", while often asymptomatic, can be associated with angioedema, pruritus, fever, headaches, arthralgias, and neurologic manifestations. "Mansonella streptocerca" can manifest on the skin via pruritus, papular eruptions and pigmentation changes. "Mansonella ozzardi" can cause symptoms that include arthralgias, headaches, fever, pulmonary symptoms, adenopathy, hepatomegaly, and pruritus. Eosinophilia is often prominent in all cases of Mansonelliasis. "M. perstans" can also present with Calabar-like swellings, hives, and a condition known as Kampala, or Ugandan eye worm. This occurs when adult M. perstans invades the conjunctiva or periorbital connective tissues in the eye. "M. perstans" can also present with hydrocele in South America. However, it is often hard to distinguish between the symptoms of Mansonelliasis and other nematode infections endemic to the same areas.

The signs and symptoms of helminthiasis depend on a number of factors including: the site of the infestation within the body; the type of worm involved; the number of worms and their volume; the type of damage the infesting worms cause; and, the immunological response of the body. Where the burden of parasites in the body is light, there may be no symptoms.

Certain worms may cause particular constellations of symptoms. For instance, taeniasis can lead to seizures due to neurocysticercosis.

The pathogenesis of "W. bancrofti" infection is dependent on the immune system and inflammatory responses of the host. After infection, the worms will mature within 6–8 months, male and female worms will mate and then release the microfilariae. These microfilariae can be released for up to ten years.

1. The asymptomatic phase usually consists of high microfilaremia infection, and individuals show no symptoms of being infected. This occurs due to cytokine IL-4 suppressing the activity of TH1 cells in the immune system. This can occur for years until the inflammatory reaction rises again.

2. In the inflammatory (acute) phase, the antigens from the female adult worms elicit inflammatory responses. The worms in the lymph channels disrupt the flow of the lymph, causing lymphedema. The individual will exhibit fever, chills, skin infections, painful lymph nodes, and tender skin of the lymphedematous extremity. These symptoms often lessen after five to seven days. Other symptoms that may occur include orchitis, an inflammation of the testes, which is accompanied by painful, immediate enlargement and epididymitis (inflammation of the spermatic cord).

3. The obstructive (chronic) phase is marked by lymph varices, lymph scrotum, hydrocele, chyluria (lymph in urine), and elephantiasis. Microfilariae are not normally present in this phase. A key feature of this phase is scar formation from affected tissue areas. Other features include thickening of the skin and elephantiasis, which develops gradually with the attack of the lymphatic system. Elephantiasis affects men mainly in the legs, arms, and scrotum. In women, the legs, arms, and breasts are affected.

This is an unusual form of epidemic epilepsy associated with onchocerciasis. This syndrome was first described in Tanzania by Louise Jilek-Aall, a Norwegian psychiatric doctor in Tanzanian practice, during the 1960s. It occurs most commonly in Uganda and South Sudan. It manifests itself in previously healthy 5–15-year-old children, is often triggered by eating or low temperatures and is accompanied by cognitive impairment. Seizures occur frequently and may be difficult to control. The electroencephalogram is abnormal but cerebrospinal fluid (CSF) and magnetic resonance imaging (MRI) are normal or show non-specific changes. If there are abnormalities on the MRI they are usually present in the hippocampus. Polymerase chain reaction testing of the CSF does not show the presence of the parasite.

In extreme cases of intestinal infestation, the mass and volume of the worms may cause the outer layers of the intestinal wall, such as the muscular layer, to tear. This may lead to peritonitis, volvulus, and gangrene of the intestine.

Brugia timori is a human filarial parasitic nematode (roundworm) which causes the disease "Timor filariasis." While this disease was first described in 1965, the identity of "Brugia timori" as the causative agent was not known until 1977. In that same year, "Anopheles barbirostris" was shown to be its primary vector. There is no known animal reservoir host.

"B. timori" microfilariae have nuclei that extend to the tip of the tail, which is also characteristic of "B. malayi" but not "W. bancrofti". "B. timori" microfilariae are slightly larger than "B. malayi" microfilariae.

Examination of blood samples will allow identification of microfilariae of "M. perstans", and "M. ozzardi" based. This diagnosis can be made on the basis of the morphology of the nuclei distribution in the tails of the microfilariae. The blood sample can be a thick smear, stained with Giemsa or hematoxylin and eosin. For increased sensitivity, concentration techniques can be used. These include centrifugation of the blood sample lyzed in 2% formalin (Knott's technique), or filtration through a Nucleopore membrane.

Examination of skin snips will identify microfilariae of "Onchocerca volvulus" and "M. streptocerca". Skin snips can be obtained using a corneal-scleral punch, or more simply a scalpel and needle. It is important that the sample be allowed to incubate for 30 minutes to 2 hours in saline or culture medium and then examined. This allows for the microfilariae that would have been in the tissue to migrate to the liquid phase of the specimen. Additionally, to differentiate the skin-dwelling filariae "M. streptocerca" and "Onchocerca volvulus", a nested polymerase chain reaction (PCR) assay was developed using small amounts of parasite material present in skin biopsies.

Generally speaking, acanthocheilonemiasis does not show initial symptoms. However, if symptoms do arise, it is typically in individuals who are visiting highly infected areas rather than natives to those areas. A major common laboratory finding is an increase in specialized white blood cells, which is called eosinophilia.

Other symptoms include itchy skin, neurological symptoms, abdominal and chest pain, muscle pain, and swelling underneath the skin. If there are abnormally high levels of white blood cells, then a physical examination will most likely find an enlarged spleen or liver.

In certain scenarios, nematodes may physically lodge into the chest or abdomen, resulting in an inflammation. Diagnosis of this condition usually occurs via a blood smear examination under light microscopy.

Acanthocheilonemiasis is a rare tropical infectious disease caused by a parasite known as "Acanthocheilonema perstans". It can cause skin rashes, abdominal and chest pains, muscle and joint pains, neurological disorders and skin lumps. It is mainly found in Africa. The parasite is transmitted through the bite of small flies. Studies show that there are elevated levels of white blood cells.

Acanthocheilonemiasis belongs to a group of parasitic diseases known as filarial disease (nematode), all of which are classified as Neglected Tropical Diseases. Filarial disease results when microfilariae, which are nematode larvae, reach the lymphatic system; microfilariae reside in the serous cavities of humans. They have a five-stage life cycle that includes birth to thousands of live microfilariae within the host (i.e. human body), and then translocation via blood meal to the dermis layer of the skin. It is here that microfilariae cause major symptoms, which are edema and thickening of the skin and underlying connective tissues. It can also cause skin rashes, abdominal and chest pains, muscle (myalgia) and joint pains, neurological disorders and skin lumps. In addition, it causes spleen and liver enlargement, which is called hepatosplenomegaly. Studies show elevated levels of leukocytes, or white blood cells, which is referred to as eosinophilia. It is mainly found in Africa. The parasite is transmitted through the bite of small flies ("A. coliroides").

Neglected tropical diseases (NTDs) are a diverse group of tropical infections which are especially common in low-income populations in developing regions of Africa, Asia, and the Americas. They are caused by a variety of pathogens such as viruses, bacteria, protozoa and helminths. These diseases are contrasted with the big three diseases (HIV/AIDS, tuberculosis, and malaria), which generally receive greater treatment and research funding. In sub-Saharan Africa, the effect of these diseases as a group is comparable to malaria and tuberculosis. NTD co-infection can also make HIV/AIDS and tuberculosis more deadly.

In some cases, the treatments are relatively inexpensive. For example, the treatment for schistosomiasis is US$0.20 per child per year. Nevertheless, in 2010 it was estimated that control of neglected diseases would require funding of between US$2 billion and US$3 billion over the subsequent five to seven years. Some pharmaceutical companies have committed to donating all the drug therapies required, and mass drug administration (for example mass deworming) has been successfully accomplished in several countries. However, preventive measures are often more accessible in the developed world, but not universally available in poorer areas.

Within developed countries, neglected tropical diseases affect the very poorest in society. In the United States, there are up to 1.46 million families including 2.8 million children living on less than two dollars a day. In countries such as these, the burdens of neglected tropical diseases are often overshadowed by other public health issues. However, many of the same issues put populations at risk in developed as developing nations. For example, from poverty stem problems such as lack of adequate housing, thus exposing individuals to the vectors of these diseases.

Twenty neglected tropical diseases are prioritized by the World Health Organization (WHO), though other organizations define NTDs differently. Chromoblastomycosis and other deep mycoses, scabies and other ectoparasites and snakebite envenoming were added to the list in 2017. These diseases are common in 149 countries, affecting more than 1.4 billion people (including more than 500 million children) and costing developing economies billions of dollars every year. They resulted in 142,000 deaths in 2013—down from 204,000 deaths in 1990. Of these 20, two were targeted for eradication (dracunculiasis (guinea-worm disease) by 2015 and yaws by 2020), and four for elimination (blinding trachoma, human African trypanosomiasis, leprosy and lymphatic filariasis by 2020).

Lymphatic filariasis is also known as elephantiasis. There are approximately 120 million individuals infected and 40 million with deformities. Approximately two-thirds of cases are in Southwest Asia and one-third in Africa. Lymphatic filariasis is rarely fatal. Lymphatic filariasis has lifelong implications, such as lymphoedema of the limbs, genital disease, and painful recurrent attacks. Most people are asymptomatic, but have lymphatic damage. Up to 40 percent of infected individuals have kidney damage. It is a vector-borne disease, caused by nematode worms that are transmitted by mosquitoes.

It can be treated with cost-effective antihelminthic treatments, and washing skin can slow or even reverse damage. It is diagnosed with a finger-prick blood test.

A persistent or recurrent cough that gets aggravated at night, weakness, weight loss and a low fever raises the possible diagnosis of this disease. Some children with this disease may also have enlarged lymph nodes in the neck and elsewhere. Others may cough up a little blood and may also have a wheeze.

Tropical diseases are diseases that are prevalent in or unique to tropical and subtropical regions. The diseases are less prevalent in temperate climates, due in part to the occurrence of a cold season, which controls the insect population by forcing hibernation. However, many were present in northern Europe and northern America in the 17th and 18th centuries before modern understanding of disease causation. The initial impetus for tropical medicine was to protect the health of colonialists, notably in India under the British Raj. Insects such as mosquitoes and flies are by far the most common disease carrier, or vector. These insects may carry a parasite, bacterium or virus that is infectious to humans and animals. Most often disease is transmitted by an insect "bite", which causes transmission of the infectious agent through subcutaneous blood exchange. Vaccines are not available for most of the diseases listed here, and many do not have cures.

Human exploration of tropical rainforests, deforestation, rising immigration and increased international air travel and other tourism to tropical regions has led to an increased incidence of such diseases.

The signs and symptoms of malaria typically begin 8–25 days following infection; however, symptoms may occur later in those who have taken antimalarial medications as prevention. Initial manifestations of the disease—common to all malaria species—are similar to flu-like symptoms, and can resemble other conditions such as sepsis, gastroenteritis, and viral diseases. The presentation may include headache, fever, shivering, joint pain, vomiting, hemolytic anemia, jaundice, hemoglobin in the urine, retinal damage, and convulsions.

The classic symptom of malaria is paroxysm—a cyclical occurrence of sudden coldness followed by shivering and then fever and sweating, occurring every two days (tertian fever) in "P. vivax" and "P. ovale" infections, and every three days (quartan fever) for "P. malariae". "P. falciparum" infection can cause recurrent fever every 36–48 hours, or a less pronounced and almost continuous fever.

Severe malaria is usually caused by "P. falciparum" (often referred to as falciparum malaria). Symptoms of falciparum malaria arise 9–30 days after infection. Individuals with cerebral malaria frequently exhibit neurological symptoms, including abnormal posturing, nystagmus, conjugate gaze palsy (failure of the eyes to turn together in the same direction), opisthotonus, seizures, or coma.