Harmful Animals
The harmful animals are those that cause injury to plants and domestic animals. Human beings are affected directly or through bites or stings or by transmission of various kinds of pathogens. The nature of harmful insects ranges from simple nuisance value of cockroaches to spreading of epidemic diseases, such as malaria, filariasis by mosquitos. For the convenience of our study the harmful animals are grouped under the following categories namely disease causing organisms, poisonous animals, fouling organisms and pests.Disease causing organisms - Vectors Some insects are injurious to man as vectors of human diseases. Through the ages millions of people have died of dieases transmitted by insects. There are a number of insect-borne diseases, and they may be transmitted in different ways.
Housefly - Musca domestica House flies are cosmopolitan in distribution. They are closely associated with humans and thrive best where people are careless in the disposal of wastes. Adult flies are non-parastic. They feed on all kinds of decaying and decomposing matter. It is an important mechanical vector in the transmission of diseases like typhoid (Salmonella typhosa - a bactericum), dysentery (Entamoeba histolytica) and cholera (Vibrio sps.) The housefly cause diseases through food contaminations. Control : Housefly control is normlly done in 3 different ways, namely sanitary, mechanical and chemical methods. Populations of houseflies can be controlled by proper disposal of manure, garbage, sewage, food waste, human excreta and other organic materials. Mechanical practices such as screening, using of traps or sticky paper or baits can be valuable in excluding houseflies. Insecticides may be used against larvae. Spraying with 2% malathion 1% chlordane or lindane, 0.5% tremephos are effective
Sand flies - Phlebotomus papatasi These flies are 4 mm long. Only the female possess piercing-sucking mouth parts and are haematophagous. The males are non parasitic, feeding on moisture. They are small slender insects with hairy bodies. Through biting this fly transmits the disease called kala-azar. The causative oraganisms is Leishmania, a parasitic protozoan. During the day time the flies remain hiding. At night they come out to feed. The sand fly attacks during night times. The insect sucks the parasite from an infected person, along with blood. In side the body of the fly, the parasite undergoes changes. When an infected fly bites man, the parasites pass into the blood and fresh infection is effected. The parasites mostly concentrate in the capillaries of spleen, liver and bone marrow. The disease is characterized by the symptoms like anaemia and emaciation.
Control : Spraying of 5 % DDT / BHC easily kills the flies. The pyrethrum ointment used on exposed part of the body works as a repellent
Rat fleas - Xenopsylla cheopis The insect parasite, Xenopsylla cheopis is commonly known as the Asiatic rat flea. Both male and female fleas take in the bacillus pasteurella pestis from infected rats during feeding. This rat - flea is responsible for the transmission of plague from man to man, or from rat to man. When this bacterium is introduced into the skin, the lymph glands become inflammed. This is known as bubonic plague. Frequently, the bacilli become established in the victims blood. The condition is then referred to as septicemic plague. If the victim’s lungs become involved, it is referred to as pneumonic Plague When the rat flea sucks the blood of man or a rat infected with plague, the bacilli enter into its stomach and grow there into large numbers. The flea thus heavily laden with the bacilli, may bite a healthy man and introduce the bacilli into the wound and cause infection. The bacilli are deposited by the flea on the skin along with the faeces. The bite of the flea causes scratchings and the bacilli are introduced into the blood when the skin is scratched.
The human louse - Pediculus humanus:
Louse is a blood sucking ectoparasite of man. It is cosmopolitan in distribution .
The human louse is a major vector for three important human diseases, relapsing fever, typhus and trench fever
| Disease - | Parasite |
| Relapsing fever | Borrelia sp |
| Typhus | Rickettsia sp |
| Trench fever | Rickettsia sp |
Mosquitoes : Anopheles, Culex, Aedes sp. Mosquitoes are cosmopolitan in distribution. They are nocturnal in habit and are found in abundance in damp, marshy lands near stagnant water. Only female mosquitoes are adapted to suck the blood of human beings and function as carrier of viral, protozoan and nematode diseases.
Culex mosquitoes serve as the vectors for filariasis or elephantiasis. This disease is caused by the nematode parasite, Wuchereria bancrofti. It is commonly known as filarial worm. It is found in the lymphatic vessels and lymph glands of man. The female worms give birth to living embryos known as microfilariae. The microfilariae normally circulate at night(10 to 2 am) in the peripheral blood. At that time they are ingested by the mosquito along with blood, the mosquito is not just a mechanical carrier of the parasite. Developmental changes take place in the body of the parasite. When the infected mosquito next bites another person, the larvae penetrate the superficial skin to find their way into the lymphatic vessels, and attain sexual maturity. In severe infection the adults cause blocking of lymphatic system which results in the enlargement of legs, arms, scrotum, and mammary glands. It is known as elephantiasis. The Anopheles mosquito transmits plasmodium, a causative protozoan for malaria (Refer : Plasmodium) Another type of mosquito, Aedes transmits yellow fever through a virus.
Poisonous Organisms
Free living organisms have developed some device to protect themselves against predators. These protective devices ranges from the simple stinging cells of Physalia to the massive poison glands of the snakes. The list of poisonous organisms is exhaustive. A few of the important poisonous organisms are mentioned here.
Physalia These are marine coelenterates. They are notorious for the painful sting they can inflict on unsuspecting swimmers who accidently brush against them. They attack using stinging cells on their trailing tentacles. Their powerful stings cause painful local inflammation and can even be fatal.
Scorpion. In scorpion the sting is attached to the posterior part of the last segment. It consists of bulbous base and a sharp curved barb that injects the venom. The venom is produced by a pair of oval glands. The scorpion raises the posterior abdomen over the body making it curved forward. A stabbing motion is used in stinging. The venom of most scorpions is sufficiently toxic to kill a vertibrate. The venom of the scorpion Androctonus is equivalent in toxicity to cobra venom. The neurotoxic venom of scorpions is very painful and may cause paralysis of the respiratory muscles or cardiac failure in fatal cases. Anti venoms are available for these species
Centipede : Centipedes are distributed throughout the world. They live in soil and humus and beneath stones. The largest centipede is the tropical American Scolopendra gigantea which may reach 26 cm in length. They have a large pair of poison claws sometimes called maxillipeds. Each claw bears a terminal pointed fang. The venom although painful is not sufficiently toxic to be lethal to man even to small children. However S. gigantea has been known to cause human death.
Honey bees and Wasps : In worker honey bee (undeveloped females) the poisonous sting is situated at the hind end of the body. It is a pointed structure provided with minute hooks or barbs at its free end. On stinging the tip of sting gets detached. Hence a bee can sting only once.
Unlike the bee the wasp is able to withdraw its sting from the wound. Hence it can sting again. In wasp the sting is a modified ovipositor and once it has penetrated the skin of the victim poison is injected as in a hypodermic syringe. The wasp’s poison is a histamine. The sting by honey bees and wasps lead to pain and inflammation.
Poisonous fishes More than 700 species of fishes have poison glands. Venom in fishes is of two kinds. One kind of venom is produced by specialized glands which may occur in various parts of the body. In the second, the flesh itself may secrete some toxic substance and the fish becomes poisonous and inedible. There are several poisonous cartilaginous fishes. The poison glands are usually associated with a spine or sting as in the case of sting ray. In the sting ray(Trygon), the poison glands lie along a lateral groove on each side of the spine on its tail. The spine causes pain and numbness in the flesh of victim. The large Barracuda of Cuba and other tropical islands have poisonous flesh, which when eaten cause pain in joints and extremities, nausea, vomiting and general trembling.
The Puffer fish, (Tetrodon) is considered to be world’s most dangerous fish. Its ovaries, intestine, kidneys, skin and eyes contain a neurotoxin called Tetradoxin. This toxin has no antidote. It is several times deadlier than cyanide. In a dilute form, tetradoxin is used as a pain killer for victims of neuralgia, arthritis and rheumatism
Poisonous snakes : Indian poisonous snakes are the cobras, the kraits, the vipers and the sea snakes. These can be distinguisted from the non poisonous by the tail, the arrangement and size of scales, plates and shields found over the body.
Cobra : It is well known all over India. When provoked it raises its head and expand the skin of the neck region in the form of a characteristic hood. The hood may bear a spectacle mark. Such cobras are called two ringed or spectacled forms. In others there is a oval spot surrounded by an ellipse. These are known as the one ringed or monocled variety. They are found in Bengal. In still others there is no mark on the hood. Only two species of cobra are found in India. They are Naja naja (Indian cobra) and Ophiophagus hannah (king cobra).
Krait : These are common poisonous snakes of India. There are two common Indian kraits. They are the common krait (Bungarus coeruleus) and the banded krait (B. fasciatus).
Vipers : There are two classes of vipers. Some have a distinct pit on the sides of the head between the nostril and the eye in the region called ‘lore’. These are called pit vipers. The other one is the pitless viper.Vipers are viviparous in nature. The vipers have movable upper jaw, so that the fangs when not in use can be folded backwards. It gets erected with the opening of the mouth while inflicting injury. It produces a loud hissing sound by expelling air through nostrils. Pitless Viper - Vipera russellir (Russell’s viper) Echis carinata (The little Indian viper).
Pit viper - Trimeresurus sp
Sea Snakes : Sea snakes can always be distinguished from other snakes by their laterally compressed tails. This is an adaptation to their life in the sea. All sea snakes are highly poisonous.
Eg. Hydrophis sp Enhydrina sp
Poison Apparatus of a Snake The poisonous snake possesses a poison apparatus comprising of a
pair of poison glands, a pair of poison ducts and a pair of fangs. The poison glands are situated on either side of the upper jaw below and behind eyes. They are specialized salivary glands. A duct carries the venom secreted from each gland to the fang. A fang is meant for injecting the venom into the body of the prey. Fangs are specialized teeth of the upper jaw which are tubular or grooved.
Biting mechanism in Cobra Cobra is not an aggressive snake. When disturbed, it attempts to escape. When the snake attacks, the mouth opens by lowering the lower jaw. This makes the fangs to be erect to penetrate the muscles of the victim. When the mouth is closed the poison glands are pressed. The venom thus reaches the fangs and is injected into the body of the victim. This whole process takes place in no time.
Snake Venom There are two types of snake venoms. One type acts mainly on the nervous system (neurotoxic). It affects the optic nerves (causing blindness) or the phrenic nerve of the diaphragm (causing paralysis of respiration). The other type is haemolytic. It breaks down the red blood corpuscles and blood vessels and produces extensive extravasation of blood into the tissue spaces
