Phylum Nematoda (nematodes, round worms, threadworms, Aschelminths)

Large, important phylum -at least 20,000 described species, many economically or medically important Ranks #4 in species number after Arthropoda,Mollusca, and Chordata, but many are undescribed maybe a million species in all.

Most are very small, < 2mm But some are nearly 1m long Abundant (100’s in a cupful of soil) Diverse

Flatworm vs. Roundworm Acelomate vs. pseudocoelomate –No body cavity vs false cavity Don’t shed skin vs. Shed skin One hole digestive tract vs. two hole digestive tract

Living & Eating Patterns Ecologically widespread: free-living marine, freshwater, terrestrial (interstitial), many parasitic on plants and animals. Herbivorous, carnivorous, saprphagous

Nematode body plan Usually small- most less than a millimeter, a few pencil-sized. cylindrical body, tapered at both ends muscular pharynx to suck in liquid food

Nematode body plan

Nematode Movement Hydrostatic skeleton, longitudinal muscle only longitudinal muscles only- characteristic sinusoidal undulating motion –More like a snake than an earthworm Longitudinal muscles that pull against that pressure to provide a spiral lashing motion –For burrowing and penetrating host tissues

Nematode Reproduction No asexual budding or fragmentation most are dioecious, use internal fertilization Sexual dimorphism males have terminal cloaca and copulatory spicules (Picture at right) female genital pore at midbody

Cuticle Syncitial epidermis secretes outer cuticle of collagen which is tough and flexible. Cuticle is molted (shed), usually four times during the life of a nematode as it grows, before reaching the adult stage. No cilia A molted cuticle and lack of cilia are features shared with arthropods

Importance of Nematoda Many are important parasites of humans and domestic animals Others are important agricultural pests Some are used as biological control agents One is widely used for research on development and developmental genetics

Some human-parasitic nematodes Ascariasis: Ascaris lumbrioides Pinworms: Enterobius vermicularis Whipworms: Trichuris trichiura Hookworms: Necator, Ancylostoma Trichinosis: Trichinella spiralis River blindness: Onchocerca volvulus Guinea worm: Dracunculus medinensis Heartworm: Dirofilaria immitis Filarial worms: Wucheria, Loa, Brugia

Ascaris lumbricoides Most common nematode parasite of humans 1 billion people infected world wide (1/6 of all people on earth!). Most common in tropical and subtropical regions, and areas with inadequate sanitation. Occurs in rural areas of the southeastern United States.

Ascaris life cycle Adults (up to 30 cm long) live in human intestine. Each female produces approximately ~200,000 eggs per day – these pass in feces and are transmitted by fecal contamination of water or food. Eggs are very long-lived and persistent. When eggs are ingested the larvae (filariae) penetrate the gut, enter blood vessels, migrate via the circulatory system to the lungs, then are coughed up and swallowed

Symptoms of Ascariasis: A few adult worms usually cause no symptoms. Large numbers can cause abdominal pain and intestinal obstruction. Migrating adult worms can occlude the bile tract, causing jaundice and malnutrition. During the lung phase of larval migration, lung damage can occur. Don’t eat the eggs.

Trichinella spiralis parasite of man and other mammals- causes trichinosis Adults in the intestine- small size, not pathogenic. Females give live birth to ~1500 larvae over 3-4 weeks Each is about 0.1 mm long. larvae cause disease – they penetrate lymphatics and blood vessels, ride circulation to various tissues Larvae form coiled cysts in muscle, brain. Adults develop if the cyst is eaten- the cysts remain infective for several years

Common in pigs & rats- pigs eat rats rats eat pig scraps from butchering poorly cooked venison or pork sausage is usual cause of infection in North America Also common in marine mammals. Eskimos are a high risk group if they eat raw blubber. Trichinella infection was once common; however, now it is relatively rare. Average of ~50 cases in the US per year reported to CDC.

Transmitted by biting insects Adults live in tissues, blood vessels, or lymphatic ducts of vertebrate host Produce live young (filariae) that invade tissues and fluids Wucheria bancrofti rarely causes a condition called elephantiasis by blocking lymphatic return and causing chronic edema and fibrosis Onchocerca volvulus causes “river blindness” affects 30 million people Dirofilaria- dog & cat heartworm