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Roundworms are large intestinal nematodes and among the most prevalent parasites on Earth. Adult worms live in the small intestine and can reach 15–35 cm in length. Eggs are shed in feces and can survive in soil for years. In pets, Toxocara species are the most common intestinal parasites and can occasionally infect humans (visceral larva migrans).

Tapeworms are flat, ribbon-like segmented worms that attach to the intestinal wall using suckers and hooks. They have no digestive system and absorb nutrients directly through their skin. Adult worms shed egg-filled segments (proglottids) that look like grains of rice. Some species (Echinococcus) can form dangerous cysts in organs.

Giardia is a microscopic flagellated protozoan that colonizes the small intestine. It exists in two forms: the active trophozoite (pear-shaped, with two nuclei resembling a face) and the hardy cyst form that can survive in cold water for months. It is a leading cause of waterborne diarrheal illness worldwide.

Toxoplasma gondii is an obligate intracellular parasite that can infect virtually any warm-blooded animal. Cats are the only definitive host where the parasite reproduces sexually. In most healthy humans, infection causes no symptoms. However, it poses serious risks to pregnant women (congenital toxoplasmosis) and immunocompromised individuals.

Lice are tiny wingless insects that live on the skin and hair of their host. They are highly host-specific — human lice cannot survive on pets and vice versa. Human head lice (Pediculus humanus capitis) are the most common, especially in school-age children. They feed on blood and lay eggs (nits) cemented to hair shafts close to the scalp.

Fleas are wingless, laterally compressed insects that are extraordinary jumpers — they can leap 150 times their own body length. The cat flea (Ctenocephalides felis) infests both cats and dogs. Only 5% of the flea population lives on the pet — 95% (eggs, larvae, pupae) are in the environment. A single female can lay 2,000 eggs in her lifetime.

Ticks are blood-feeding arachnids (related to spiders and mites) that are important vectors of numerous serious diseases. They cannot jump or fly — they 'quest' by holding onto vegetation with their back legs and reaching out with their front legs to grab passing hosts. Hard ticks (Ixodidae) feed for days; soft ticks (Argasidae) feed quickly.

Hookworms are small (1–1.5 cm) but voracious blood-feeders. They attach to the intestinal wall with cutting plates or teeth and feed continuously, secreting anticoagulants to keep blood flowing. A single Ancylostoma duodenale can consume 0.2 mL of blood per day. Heavy infections cause severe iron-deficiency anemia, particularly dangerous in puppies, kittens, and children.

Scabies is caused by microscopic burrowing mites that tunnel into the outer layer of skin (stratum corneum) to lay eggs. The intense itching is an allergic reaction to the mites, their eggs, and feces. In dogs, the same genus causes sarcoptic mange — a severe, highly contagious skin disease. Human and dog strains can cross-infect but do not establish permanent infestations in the non-primary host.

Whipworms are named for their distinctive shape — thin at the front (like a whip handle) and thicker at the rear. The thin end embeds into the intestinal wall, causing inflammation and bleeding. Eggs are extremely hardy and can survive in soil for years. Dogs are commonly infected with Trichuris vulpis, which does not infect humans.

Bedbugs are small, flat, wingless insects that feed exclusively on blood, primarily at night. They do not transmit disease but cause significant skin reactions, sleep disruption, and psychological distress. Infestations spread rapidly through luggage, clothing, and used furniture. Modern populations show widespread resistance to many pesticides, making eradication challenging.

Despite its name, ringworm is not caused by a worm — it is a fungal infection of the skin, hair, and nails caused by dermatophytes. These fungi feed on keratin, the protein found in skin, hair, and nails. The infection produces the characteristic ring-shaped, scaly rash. It is highly contagious and can spread between humans, pets, and soil. Cats — especially kittens — are a major reservoir and common source of human infection.

Mosquitoes are blood-feeding insects and the world's deadliest animal vectors of disease. Female mosquitoes require a blood meal to produce eggs and in doing so transmit parasites, viruses, and bacteria. Key parasitic diseases they transmit include malaria (Plasmodium spp.), heartworm in dogs and cats (Dirofilaria immitis), and canine and feline filariasis. They also vector viral diseases such as dengue, Zika, West Nile, and yellow fever.

Leeches are blood-sucking annelid worms that attach to a host using suckers at both ends of their body. They secrete hirudin, a powerful anticoagulant, to keep blood flowing during feeding. Aquatic leeches are found in freshwater lakes, rivers, and swamps; terrestrial leeches inhabit moist forest floors in tropical regions. While a single leech bite is rarely dangerous, heavy infestations can cause significant blood loss, especially in small animals.

Pinworms are small, thin white worms about the size of a staple that infect the large intestine and rectum. They are the most common worm infection in the United States and are highly contagious, spreading easily among children in schools and households. The infection is rarely serious but causes intense anal itching, particularly at night when female worms migrate to the perianal area to lay eggs.

Liver flukes are parasitic flatworms that infect the bile ducts and liver of their hosts. Fasciola hepatica is the most widespread species, found on every continent except Antarctica. Infection occurs by eating aquatic plants (especially watercress) contaminated with encysted larvae (metacercariae). While primarily a disease of livestock (sheep, cattle), it is an increasingly recognized human and pet pathogen. Chronic infection can cause serious liver damage and bile duct obstruction.

Cryptosporidium is a microscopic protozoan parasite that infects the small intestine, causing cryptosporidiosis — a diarrheal illness that can be severe or life-threatening in immunocompromised individuals. It is transmitted primarily through contaminated water (including treated swimming pools and drinking water) and is highly resistant to standard chlorine disinfection. Young children, the elderly, and people with weakened immune systems are most at risk.

Babesia are microscopic protozoan parasites that invade and destroy red blood cells, causing a malaria-like illness called babesiosis. Transmitted by the same black-legged tick (Ixodes scapularis) that spreads Lyme disease, co-infection is common. Dogs are affected by different Babesia species than humans but can suffer severe hemolytic anemia. Babesiosis can be life-threatening in elderly, immunocompromised, or asplenic individuals.

Strongyloides stercoralis is a unique intestinal nematode capable of autoinfection — larvae can re-infect the same host without leaving the body, allowing infection to persist for decades. In immunocompromised individuals, this can lead to hyperinfection syndrome, a life-threatening condition where massive numbers of larvae disseminate throughout the body. Dogs are also susceptible to a related species (S. stercoralis or S. canis).

Demodex are microscopic cigar-shaped mites that live in hair follicles and sebaceous glands. In small numbers they are considered normal skin flora in humans and most dogs. Demodicosis (demodectic mange) occurs when the immune system fails to keep mite populations in check — most commonly in puppies, elderly dogs, or immunocompromised individuals. Unlike sarcoptic mange, demodectic mange is generally not contagious between animals.

Chagas disease is caused by the protozoan parasite Trypanosoma cruzi, transmitted by triatomine bugs (kissing bugs) that defecate near their bite wound. The parasite enters through the bite wound, eyes, or mucous membranes. Chagas has two phases: an acute phase (often mild or asymptomatic) and a chronic phase that can cause severe heart disease and digestive complications decades later. Dogs are important reservoir hosts and can develop cardiac disease similar to humans.

Blastocystis is a microscopic single-celled organism found in the intestines of humans and many animals. Its role as a pathogen is debated — many infected individuals have no symptoms, while others experience significant gastrointestinal distress. It is increasingly linked to irritable bowel syndrome (IBS) and gut microbiome disruption. Multiple subtypes (ST1–ST17) exist, with some more associated with disease than others.

Anisakis is a parasitic nematode (roundworm) found in the flesh of many marine fish and squid. Humans become accidental hosts by eating raw or undercooked seafood — sushi, sashimi, ceviche, and lightly cured fish are common sources. The larvae cannot complete their lifecycle in humans but burrow into the stomach or intestinal wall, causing a painful condition called anisakiasis. Marine mammals such as dolphins and seals are the definitive hosts.