Host Behaviour Manipulation By Helminths Biology Essay

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What is a helminth? Helminths are also known as parasitic worms; Cestodes, Trematodes and Nematodes. They are a division of eukaryotic parasites that live inside and feed off their hosts. They receive nourishment and protection from their host whilst causing weakness and disease. They can be found inside both humans and animals. Offspring of these parasitic worms can be passed on through mosquitoes, faeces, contaminated water and poorly cooked meat and fish. The most common are the roundworm and the pin worm and is estimated to infect over 40 million Americans. [1]

An explanation of the termĀ 'Host behaviour manipulation' and why this process is undertaken

Host behaviour manipulation is caused when the parasite alters the behaviour of its host in ways that are ultimately beneficial to the parasite or its offspring. [2] Increasing the parasite's chances of survival, transmission & reproductive success. Host behaviours can be altered by photo-taxis, locomotion, behavioural fevers, foraging behaviour, reproduction and a variety of social interactions, to name only a few [2, 3, 4, 5] Effects of parasites on hosts behaviour can be direct, for example by manipulating the nervous system, or indirect by manipulating the immune or endocrine system or the metabolism of the host [2, 3, 6]

How easy / difficult is it to prove that a Parasite is actually manipulating behaviour.

It is very difficult to prove whether hosts are deliberately manipulated by parasites or whether it is accidental. An example of this is the Gordian worm, genus Mermis and the affects it has on the behaviour of grasshoppers and crickets. It is not a common fact that land animals randomly jump into water, but once the parasite reaches maturity, infected crickets seeks out water and obligingly commit suicide by drowning [2]. This could be accidental in the fact that the grasshopper or cricket drowns but in actual fact it is deliberate as the mature parasite is aquatic and needs to emerge in water.

What is the purpose for pathogens (if any) of manipulating the host

Parasite control of the host behaviour, life history and physiology

Parasites control the host behaviour, life history and physiology to maximise their own survival, fitness and reproductive success. A form of control is suppressing the host's immune response, parasitic castration was once regarded as just a side effect of parasitism but is now regarded by some as a diversion of host resources to benefit parasites.

Parasitic Castration

An example of parasitic castration caused by nematode worms is when the Mermithid nematode needs the adult host (mayfly) to remain near water so that the parasite can emerge and mate. It is only the female mayflies that remain near water, the males may disperse to land. But parasitized males become feminised causing the genital claspers to not develop and making the eyes appear female like. Parasitized males also begin to remain near water after mating and even behave as if laying eggs. [7] Feminisitation

Manipulation of host's resource allocation process

An indirect form of castration is when the parasite manipulates the host's resource allocation process. An example of this is Hymenolepis diminuta (a species of tape worm) and the definitive host being a rat and the intermediate host being a flour beetle. The Tapeworm produces 2 molecules that slow down the host's egg yolk production and accumulation; this causes more fat to accumulate in the host's fat body (storage organ) which is a more easily utilisable food source for the parasite. The Tapeworm infects the definitive host (the rat) when beetle is eaten by rat, uninfected worms can defend themselves with secretions from a repugnatorial gland, in infected worms the gland is non functional, thereby making beetles easier to capture and consume.

Changes in behaviour of parasitized animals / humans

Examples of host behaviour manipulation: Dicrocoelium (Liver fluke) The cercaria encyst in a mollusc host encases cysts in a mucus layer and excretes them, ants then eat the mucus balls, the cercaria migrates to the ants head, encysts in suboesophageal ganglion and causes a change in the hosts behaviour. Infected ants, untypically climb up a plants stem and attaches themselves with their jaws in an exposed position e.g on leaves. The Infected ant moves up the stems around evening time and in mid-day heat the ants move down stem, returning later in day, grazing mammals then eat the plant along with the ant. [3] This shows an example of changing the host's behaviour to suicidal behaviour.

Leucochloridium (Parasitic worm); The snail is the intermediate host, during the dispersing stage, sporocyst containing many cercariae resemble caterpillars, during the day they migrate into the snail's thin walled tentacles which they distend. The sporocysts also pulsate making the snail even more detectable to its prey, infected snails also move into more exposed locations such as leaf surfaces which increase the risk of detection, at night the sporocysts migrate back into the snail's body. [8] This shows how the host's behaviour changes causing exposure to predators.

Dracunculus medinensis(Guinea Worm). By the time the female partly emerges from the host's (human) skin, the juvenile worms are present in her uterus. Unlike the mother worm, the juveniles elicit an inflammatory response in the host - painful skin blisters. The host seeks out water to relieve the pain allowing the juveniles to enter the water in search of copepod. [9] The host behavioural change caused by the parasite here is moving the host towards the parasite dispersing site.

Host physiology: Plasmodium

Plasmodium is a unicellular parasite that causes the disease Malaria. There are more than 100 different species of Plasmodium, yet only four types are known to infect humans including, Plasmodium falciparum, vivax, malariae, and ovale. While each type has a distinct appearance under the microscope, they each can cause a different pattern of symptoms. Plasmodium falciparum is the major cause of death in Africa, while Plasmodium vivax is the most geographically widespread of the species and the cause of most malaria cases diagnosed in the United States. Plasmodium malariae infections produce typical malaria symptoms that persist in the blood for very long periods, sometimes without ever producing symptoms. Plasmodium ovale is rare, and is isolated to West Africa. [10]

Plasmodium's life cycle is dependent on the insect host (for example, the Anopheles mosquito) and the carrier host (humans) for its propagation. The Plasmodium parasite undergoes sexual reproduction in the insect host, this is done by uniting two sex cells, producing sporozoites. The infected mosquito then bites the host and feeds on human blood which provides nourishment for her eggs and causes the sporozoites to enter the bloodstream of the human host. The parasite heads straight to the human liver cell, there it is protected from the immune system. [10]

Physiological alterations are resulted in a malaria-infected erythrocyte which involves the structure and function of the erythrocyte membrane. Novel parasite-induced permeation pathways (NPP) are formed (in some cases) with an increase in the activity of specific transporters within the red blood cells (RBC). The NPP are thought to have evolved to provide the parasite with the appropriate nutrients, explaining the increased permeability of many solutes. However, the true nature of the NPP remains an enigma. The properties of the transporters and channels on a normal RBC differ dramatically from that of a malaria-infected RBC. The dynamics of the membranes, including how the fats that makeup the membrane are deposited are altered. The increase in transport of solutes is bidirectional and is a function of the developmental stage of the parasite. The alterations in erythrocyte membrane are proportional to the maturation of the parasite.

What the benefits are to the parasite (if any) and the affects that will be had on the host.

Intermediate hosts

Aswell as definitive hosts there are also intermediate hosts, therefore resulting in host behaviour manipulation at an intermediate level. An example of this is Polymorphus worms. The Polymorphus worm has the Gammarus amphipod (shrimp) as an intermediate host. The Polymorphus species causes behavioural changes in the shrimp, such as the depth at which it adopts in water, leading to the host (shrimp) to be more likely eaten by the definitive host (in this case muskrats and ducks) The parasite of diving ducks causes shrimp to avoid water surfaces, whereas the parasite of surface-feeding ducks and muskrats causes the shrimp to stay close to the surface.

Larval stages of parasitic helminths use intermediate hosts not only as a source of nutrients and protection against environmental hazards, but also as vehicles taking the parasites to their definitive host. [11] Many helminths are capable of manipulating the behaviour or colouration of their intermediate hosts in ways that render them more susceptible to predation by the parasites definitive host. [12, 13, 14]

Host behaviour and change in appearance

Host behaviour and appearance - nematode parasite of ants - Infected Cephalotes atratus ants are normally black, develop a bright red abdomen (gaster) and tend to hold it in an elevated position - an alarm posture in ants. The ant also become sluggish and the gaster is easily broken off, making it easier for birds to pluck, the birds usually do not eat the ants as they have heavily armoured and defended by distasteful chemicals, birds are not hosts - they are simply a means of dispersing the parasite.

The impact of parasite manipulation and predator foraging behaviour on predator - prey communities

Parasites are known to directly affect their hosts at both the individual and population level. Trophically transmitted parasites may manipulate the behaviour of intermediate hosts, fundamentally altering the pattern of contact between these individuals and their predators. Though manipulations do not directly affect the persistence of the predator and prey populations they can greatly alter the quantitative dynamics of the community, potentially resulting in high mplitude oscillations in abundance. The precise impact of host manipulation depends greatly on the predator's functional response, which describes the predator's foraging efficiency under changing prey availabilities. Even if the parasite is rarely observed within the prey population, such manipulatins extend beyond the direct impact on the intermediate host to affect the foraging success of the predator, with profound implications for the structure and stability of the predator - prey community. [15]

Conclusion - summary of the story

Insect eggs lack the ability to defend themselves, including physiologically so many parasitoids attack the host eggs.

Hookworm nematodes tear the hosts intestinal tissues, releases molecules that interfere with the hosts coagulation 'cascade'.

Wolbachia bacteria: master manipulators of host reproduction

Attachment mechanisms

Parasitic worms offspring are passed on through contaminated water, faeces, mosquitoes, through poorly cooked meat, wild fish. Worm eggs/larvae or adult can enter a human body through the mouth nose anus or skin and most species attaching themselves to the intestinal tract.

Resistant body covering

Parasitic worms within the body's digestive systems are protected by a strong keratin layer so the worms can resist the digestive enzymes within the hosts body.

Disruption / suppression of the host's immune response

Disruption / suppression of the hosts immune response, the viruses suppress the hosts physiological defences, wolbachia in filarial nematode worm suppresses mammal host's immune system so benefiting the nematode.

Anticoagulant action

Antigenic variation

Antigenic variation in helminths = The term refers to the mechanism by which a parasite alters its surface proteins in order to evade the hosts immune response. For organisms that target long lived hosts, repeatedly infect a single host and are easily transmitted. Antigenic variation can occur through 3 broadly defined genetic processes; gene mutation, recombination and switching, in all cases antigenic variation results in pathogens that are immunologically distinct from the parental strains. (Look at page 62 of handout for examples)