Physiological reactions to "Toxocara" infection depend on the host’s immune response and the parasitic load. Most cases of "Toxocara" infection are asymptomatic, especially in adults. When symptoms do occur, they are the result of migration of second stage "Toxocara" larvae through the body.

Covert toxocariasis is the least serious of the three syndromes and is believed to be due to chronic exposure. Signs and symptoms of covert toxocariasis are coughing, fever, abdominal pain, headaches, and changes in behavior and ability to sleep. Upon medical examination, wheezing, hepatomegaly, and lymphadenitis are often noted.

High parasitic loads or repeated infection can lead to visceral larva migrans (VLM). VLM is primarily diagnosed in young children, because they are more prone to exposure and ingestion of infective eggs. "Toxocara" infection commonly resolves itself within weeks, but chronic eosinophilia may result. In VLM, larvae migration incites inflammation of internal organs and sometimes the central nervous system. Symptoms depend on the organ(s) affected. Patients can present with pallor, fatigue, weight loss, anorexia, fever, headache, rash, cough, asthma, chest tightness, increased irritability, abdominal pain, nausea, and vomiting. Sometimes the subcutaneous migration tracks of the larvae can be seen. Patients are commonly diagnosed with pneumonia, bronchospasms, chronic pulmonary inflammation, hypereosinophilia, hepatomegaly, hypergammaglobulinaemia (IgM, IgG, and IgE classes), leucocytosis, and elevated anti-A and –B isohaemagglutinins. Severe cases have occurred in people who are hypersensitive to allergens; in rare cases, epilepsy, inflammation of the heart, pleural effusion, respiratory failure, and death have resulted from VLM.

Ocular larva migrans (OLM) is rare compared with VLM. A light "Toxocara" burden is thought to induce a low immune response, allowing a larva to enter the host’s eye. Although there have been cases of concurrent OLM and VLM, these are extremely exceptional. OLM often occurs in just one eye and from a single larva migrating into and encysting within the orbit. Loss of vision occurs over days or weeks. Other signs and symptoms are red eye, white pupil, fixed pupil, retinal fibrosis, retinal detachment, inflammation of the eye tissues, retinal granulomas, and strabismus. Ocular granulomas resulting from OLM are frequently misdiagnosed as retinoblastomas. "Toxocara" damage in the eye is permanent and can result in blindness.

A case study published in 2008 supported the hypothesis that eosinophilic cellulitis may also be caused by infection with "Toxocara". In this study, the adult patient presented with eosinophilic cellulitis, hepatosplenomegaly, anemia, and a positive ELISA for "T. cani"s.

The incubation period for "Toxocara canis" and "cati" eggs depends on temperature and humidity. "T. canis" females, specifically, are capable of producing up to 200,000 eggs a day that require 2-6 weeks minimum up to a couple months before full development into the infectious stage. Under ideal summer conditions, eggs can mature to the infective stage after two weeks outside of a host. Provided sufficient oxygen and moisture availability, "Toxocara" eggs can remain infectious for years, as their resistant outer shell enables the protection from most environmental threats.However, as identified in a case study presented within the journal of helminthology, the second stage of larvae development poses strict vulnerabilities to certain environmental elements. High temperatures and low moisture levels will quickly degrade the larvae during this stage of growth.

A few days after ingestion epigastric pain, fever, vomiting, and loss of appetite resulting from migration of larvae through intestinal wall to the abdominal cavity will appear in the patient. Migration in the subcutaneous tissues (under the skin) causes intermittent, migratory, painful, pruritic swellings (cutaneous larva migrans). Patches of edema appear after the above symptoms clear and are usually found on the abdomen. These lesions vary in size and can be accompanied by pruritus, rash, and stabbing pain. Swellings may last for 1 to 4 weeks in a given area and then reappear in a different location. Migration to other tissues (visceral larva migrans), can result in cough, hematuria, ocular (eye) involvement, meningitis, encephalitis and eosinophilia. Eosinophilic myeloencephalitis may also result from invasion of the central nervous system by the larvae.

Although tapeworms in the intestine usually cause no symptoms, some people experience upper abdominal discomfort, diarrhea, and loss of appetite. Anemia may develop in people with the fish tapeworm. Infection is generally recognized when the infected person passes segments of proglottids in the stool (which look like white worms), especially if a segment is moving.

Rarely, worms may cause obstruction of the intestine, and very rarely, T. solium larvae can migrate to the brain causing severe headaches, seizures and other neurological problems. Neurocysticercosis can progress for years before the patient displays symptoms.

In at least one case, cancer cells from a tapeworm spread to the human host in an immunocompromised man, producing swelling, obstructions, and other conventional symptoms of human-originated cancer.

How myiasis affects the human body depends on where the larvae are located. Larvae may infect dead, necrotic (prematurely dying) or living tissue in various sites: the skin, eyes, ears, stomach and intestinal tract, or in genitourinary sites. They may invade open wounds and lesions or unbroken skin. Some enter the body through the nose or ears. Larvae or eggs can reach the stomach or intestines if they are swallowed with food and cause gastric or intestinal myiasis.

Several different presentations of myiasis and their symptoms:

The following are commonly used terms when referring to gnathostomiasis

- Gnathostoma

- Larva migrans profundus

- Nodular migratory eosinophilic panniculitis

- Physaloptera

- Spiruroid larva migrans

- Wandering swelling

- Yangtze edema

Nosocomial myiasis is myiasis acquired in a hospital setting. It is quite frequent, as patients with open wounds or sores can be infested if flies are present. To prevent nosocomial myiasis, hospital rooms must be kept free of flies.

The most common symptom is coughing and other typical symptoms are wheezing and weight loss. These symptoms are caused by larvae that reside in the lungs where immunity develops and the accumulation of mucus cause blockage of the airway into the lungs.

Tapeworm infection is the infestation of the digestive tract by a species of parasitic flatworm (known as a cestode), called tapeworms. Live tapeworm larvae grouped in cysts (coenuri)are sometimes ingested by consuming undercooked meat. Once inside the digestive tract, a larva can grow into a very large adult tapeworm. Additionally, many tapeworm larvae cause symptoms in an intermediate host. For example, cysticercosis is a disease involving larval tapeworms in the human body.

Visceral larva migrans (VLM) is a condition in humans caused by the migratory larvae of certain nematodes, humans being a dead-end host, and was first reported in 1952. Nematodes causing such zoonotic infections are "Baylisascaris procyonis", "Toxocara canis", "Toxocara cati", and "Ascaris suum". These nematodes can infect but not mature in humans and after migrating through the intestinal wall, travel with the blood stream to various organs where they cause inflammation and damage. Affected organs can include the liver, heart (causing myocarditis) and the CNS (causing dysfunction, seizures, and coma). A special variant is ocular larva migrans where usually "T. canis" larvae travel to the eye.

Only a few roundworm eggs are necessary to cause larva migrans in the human child or adult. However, visceral larva migrans seems to affect children aged 1–4 more often while ocular larva migrans more frequently affects children aged 7–8. Between 4.6% and 23% of U.S. children have been infected with the dog roundworm egg. This number is much higher in other parts of the world, such as Colombia, where up to 81% of children have been infected.

Cutaneous larva migrans is a condition where nematodes such as "Ancylostoma braziliense" migrate to the skin.

A list of causative agents of larva migrans syndromes is not agreed upon and varies with the author.

Because the larvae are in an abnormal host, they do not mature to adults but instead migrate

through the skin until killed by the host's inflammatory response. This migration causes

local intense itching and a red serpiginous lesion. Treatment with a single dose of oral ivermectin results in cure rates of 94–100%.

If an animal is suspected of lungworm infection, there are many ways to detect this parasitic infection such as performing one or more of the following techniques: a complete medical history including lung auscultation (stethoscope examination), doing a chest xray, fecal examination for detection of ova or larvae, examination of respiratory secretions for ova or larvae, and/or a complete blood count (CBC) to check for signs of increase in eosinophils

The infection is usually contracted by persons walking barefoot over

contaminated soil. In penetrating the skin, the larvae may cause an allergic

reaction. It is from the itchy patch at the site of entry that the early

infection gets its nickname "ground itch". Once larvae have broken through the skin,

they enter the bloodstream and are carried to the lungs (unlike ascarids,

however, hookworms do not usually cause pneumonia). The larvae migrate from

the lungs up the windpipe to be swallowed and carried back down to the

intestine. If humans come into contact with larvae of the dog hookworm or the

cat hookworm, or of certain other hookworms that do not infect humans, the

larvae may penetrate the skin. Sometimes, the larvae are unable to complete their

migratory cycle in humans. Instead, the larvae migrate just below the skin

producing snake-like markings. This is referred to as a creeping eruption or

cutaneous larva migrans.

When adult worms attach to the villi of the small intestine, they suck on the host's blood, which may cause abdominal pain, diarrhea, cramps, and weight loss that can lead to anorexia. Heavy infections can lead to the development of iron deficiency and hypochromic microcytic anemia. This form of anemia in children can give rise to physical and mental retardation. Infection caused by cutaneous larvae migrans, a skin disease in humans, is characterized by skin ruptures and severe itching.

Necatoriasis is the condition of infection by "Necator" hookworms, such as "Necator americanus". This hookworm infection is a type of helminthiasis (infection) which is a type of neglected tropical disease.

Helminths (), also commonly known as parasitic worms, are large multicellular organisms, which can generally be seen with the naked eye when they are mature. They are often referred to as intestinal worms even though not all helminths reside in the intestines. For example, schistosomes are not intestinal worms, but rather reside in blood vessels. The word helminth comes from Greek "hélmins", a kind of worm.

There is no consensus on the taxonomy of helminths. It is simply a commonly used term to describe certain worms with some similarities. These are flatworms (platyhelminthes), namely cestodes (tapeworms) and trematodes (flukes), and roundworms or nemathelminths (nematodes) – both of these are parasitic worm types – and the annelida, which are not parasitic or at the most ectoparasites like the leeches.

Helminths are worm-like organisms living in and feeding on living hosts. They receive nourishment and protection while disrupting their hosts' nutrient absorption. This can cause weakness and disease of the host. Those helminths that live inside the digestive tract are called intestinal parasites. They can live inside humans and other animals. In their adult form, helminths cannot multiply in humans. Helminths are able to survive in their mammalian hosts for many years due to their ability to manipulate the immune response by secreting immunomodulatory products. All helminths produce eggs (also called ova) for reproduction. These eggs have a strong shell that protects them against a range of environmental conditions. The eggs can therefore survive in the environment, outside their hosts, for many months or years.

Many, but not all, of the worms referred to as helminths belong to the group of intestinal parasites. An infection by a helminth is known as helminthiasis, helminth infection or intestinal worm infection. There is a naming convention which applies to all helminths: the ending "-asis" (or in veterinary science: "-osis") is added at the end of the name of the worm to denote the infection with that particular worm. For example, "Ascaris" is the name of a type of helminth, and ascariasis is the name of the infectious disease caused by that helminth.

There is no real consensus on the taxonomy (or groupings) of the helminths, particularly with the nematodes. The term "helminth" contains a number of phyla, many of which are completely unrelated. However, for practical considerations the term is still used nowadays to describe four groups with superficial similarities, the phyla Annelida, Platyhelminths, Nematoda and Acanthocephala.

There is in fact no helminth classification; it is an "artificial" term.

The most important helminths in the sanitation field are the human parasites, which is why most people relate the term helminth to them, where they are classified as nemathelminthes (nematodes) and platyhelminthes, depending on whether they possess a round or flat-shaped body respectively. The latter are further divided into cestodes and trematodes depending on whether or not they have a segmented body.

Ringworm (dermatophytosis) is actually caused by various fungi and not by a parasitic worm.

The infection causes a red, intensely pruritic (itchy) eruption. The itching can become very painful and if scratched may allow a secondary bacterial infection to develop. Cutaneous larva migrans usually heals spontaneously over weeks to months and has been known to last as long as one year. However, the severity of the symptoms usually causes those infected to seek medical treatment before spontaneous resolution occurs. Following proper treatment, migration of the larvae within the skin is halted and relief of the associated itching can occur in less than 48 hours (reported for thiabendazole).

This is separate from the similar cutaneous larva currens which is caused by "Strongyloides". Larva currens is also a cause of migratory pruritic eruptions but is marked by 1) migratory speed on the order of inches per hour 2) perianal involvement due to autoinfection from stool and 3) a wide band of urticaria.

Porocephaliasis is a condition associated with species in the closely related genera "Porocephalus" and "Armillifer". (The term "pentastomiasis" encompasses all diseases of Pentastomida, which includes Porocephaliasis and Linguatulosis.)

Porocephaliasis is associated with contact with snakes. (This is in contrast with Linguatulosis, which is associated with contact with dogs or wolves.)

It has been reported from Africa, Malaysia and the Middle East. Its occurrence has been rare in Europe and North America where it has been found in immigrants and travelers.

A canine vector-borne disease (CVBD) is one of "a group of globally distributed and rapidly spreading illnesses that are caused by a range of pathogens transmitted by arthropods including ticks, fleas, mosquitoes and phlebotomine sandflies." CVBDs are important in the fields of veterinary medicine, animal welfare, and public health. Some CVBDs are of zoonotic concern.

Many CVBD infect humans as well as companion animals. Some CVBD are fatal; most can only be controlled, not cured. Therefore, infection should be avoided by preventing arthropod vectors from feeding on the blood of their preferred hosts. While it is well known that arthropods transmit bacteria and protozoa during blood feeds, viruses are also becoming recognized as another group of transmitted pathogens of both animals and humans.

Some "canine vector-borne pathogens of major zoonotic concern" are distributed worldwide, while others are localized by continent. Listed by vector, some such pathogens and their associated diseases are the following:

- Phlebotomine sandflies (Psychodidae): "Leishmania amazonensis", "L. colombiensis", and "L. infantum" cause visceral leishmaniasis (see also canine leishmaniasis). "L. braziliensis" causes mucocutaneous leishmaniasis. "L. tropica" causes cutaneous leishmaniasis. "L. peruviana" and "L. major" cause localized cutaneous leishmaniasis.

- Triatomine bugs (Reduviidae): "Trypanosoma cruzi" causes trypanosomiasis (Chagas disease).

- Ticks (Ixodidae): "Babesia canis" subspecies ("Babesia canis canis", "B. canis vogeli", "B. canis rossi", and "B. canis gibsoni" cause babesiosis. "Ehrlichia canis" and "E. chaffeensis" cause monocytic ehrlichiosis. "Anaplasma phagocytophilum" causes granulocytic anaplasmosis. "Borrelia burgdorferi" causes Lyme disease. "Rickettsia rickettsii" causes Rocky Mountain spotted fever. "Rickettsia conorii" causes Mediterranean spotted fever.

- Mosquitoes (Culicidae): "Dirofilaria immitis" and "D. repens" cause dirofilariasis.

It is usually asymptomatic unless the complication and infection is severe. But in some recorded cases, symptoms include nasopharyngitis accompanied by pain, itching of throat and ears. Coughing, hemoptysis and vomiting are verifiable indications as well as sneezing, bleeding, dyspnea, and inflammation.

It is prevalent in parts of Africa and Asia where eating snake meat is common. In Africa it has also been associated with groups who use the snake as a totem. Unlike linguatuliasis, humans are only ever an accidental intermediate host for "Armillifer", i.e. the larvae establish themselves in the visceral organs causing human visceral pentastomiasis, but adults do not occur in the human respiratory system. After a while the larvae die within the host and sometimes calcify, leaving characteristic crescent-shaped structures seen in X-ray. In extreme cases a heavy parasite burden can have serious medical consequences and can even be fatal.

Hookworm eggs are shed in the infected dog (or other animal) feces to the ground and beach sand, where they then develop over a period of 1–2 weeks into the infectious larval form (filariform larvae). The filariform larvae can burrow through intact skin that comes into contact with soil or sand that is contaminated with feces. Although they are able to infect the deeper tissues of animals (through to the lungs and then the intestinal tract), humans are incidental hosts and the larvae are only able to penetrate the epidermis of the skin and thus create the typical wormlike burrows visible underneath the skin. These parasites apparently lack the collagenase enzymes required to penetrate through the basement membrane deeper into the dermal layers of the skin.

Eating raw or semi-cooked infected liver or lymph nodes infected with nymphal L. serrata causes severe symptoms in the human nasopharynx. Submaxillary and cervical lymph nodes sometimes enlarge and the neck is swollen. Complications include abscesses in the auditory canals, facial paralysis, and enlarged tonsils producing asphyxiation. These symptoms are well recognized as a disease call “halzoun syndrome” in Lebanon and nearby countries.

In Egypt, infected camels and buffalo may also be a source of infection for dogs, which are companions of man in desert and semi-desert areas where grazing is a major profession, and in villages, where dogs are also common. Infected dogs, in turn, are a source of infection to man who may be an intermediate host.

The symptoms of leishmaniasis are skin sores which erupt weeks to months after the person is bitten by infected sand flies.

Leishmaniasis may be divided into the following types:

- Cutaneous leishmaniasis is the most common form, which causes an open sore at the bite sites, which heals in a few months to a year and half, leaving an unpleasant-looking scar. Diffuse cutaneous leishmaniasis produces widespread skin lesions which resemble leprosy, and may not heal on its own.

- Mucocutaneous leishmaniasis causes both skin and mucosal ulcers with damage primarily of the nose and mouth.

- Visceral leishmaniasis or "kala-azar" ('black fever') is the most serious form, and is potentially fatal if untreated. Other consequences, which can occur a few months to years after infection, include fever, damage to the spleen and liver, and anemia.

Leishmaniasis is considered one of the classic causes of a markedly enlarged (and therefore palpable) spleen; the organ, which is not normally felt during examination of the abdomen, may even become larger than the liver in severe cases.