As crops are cultivated to feed the world's population, secernenteans become more numerous as they feed on agricul tural plants. Parasitic secernenteans cause yield losses and they may join with other soil-living organisms such as bacteria, fungi, and viruses to advance disease development in plants. They also can cause loss of nutrients and water into the plant, thus increasing the plant's susceptibility to other dangers. Secernenteans, when infecting humans, can cause various diseases and, in some circumstances, death to the human host. When infected animals are used as food or kept as pets for humans, these parasites can cause and transmit various diseases and, often, death to the hosts. On the other hand, free-living forms of secernenteans can be good indication of biodiversity, and important to the health and survival of humans. They help to cycle carbon and nitrogen and to breakdown organic matter in the soil environment.
1. Cod worm (Phocanema decipiens); 2. Barber's pole worm (Haemonchus contortus); 3. African river blindness nematode (Onchocerca volvulus); 4. Dog hookworm (Ancylostoma caninum); 5. Rat lungworm (Angiostrongylus cantonensis); 6. Threadworm (Strongyloides stercoralis); 7. Canine heartworm (Dirofilaria immitis); 8. Citrus spine nematode (Criconema civellae); 9. Maw-worm (Ascaris lumbricoides). (Illustration by John Mega-han)
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