ECOLOGY OF PARASITES: Relationship Between the Parasites, Environment and Host
Lavanya K.V1. and Mohan, H.V2.
1Assistant Professor (OPG), Institute of Animal Health and Veterinary Biologicals (IAH&VB), Hebbal, Bengaluru-560024, Email ID: lavanyakv616@gmail.com
2Assistant Professor, Department of Veterinary Public Health and Epidemiology, Veterinary College, Bengaluru-560024, Email ID: mohanhv@gmail.com
Key words: Ecology, Epidemiology, Environment, Parasites, Parasitism, Host
Every living organism hosts a variety of commensals and parasites for varying lengths of time or their entire lives. These commensals and parasites have both positive and negative impacts on the hosts (causing disease or death). Because of this, understanding the ecology of parasites is crucial for comprehending the parasitism and epidemiology of parasitic diseases, which in turn aids in their prevention and control. Unfortunately, this subject has not got enough attention. Ecology is the scientific study of how organisms (hosts and parasites) interact with their environments. Parasites and their hosts are favoured in their association initially by ecological segregation due to environmental and behavioural factors, but when the contact is established, a multiplicity of host stimuli and parasite response determine the establishment of parasitism. Parasites can be found in the environment as free-living larvae as well as in the host, where they can develop and reproduce. The conditions present in the hosts and the environment are very important for their survival and disease transmission.
Environment as a niche: Favourable temperature, adequate moisture and sufficient oxygen are important to the free living phases of the life cycle and regulate the levels and periodicity of infection, in addition to the grazing behaviour and species susceptibility of the hosts. In addition, defence against the deadly consequences of freezing, direct solar heat, and desiccation. Parasites can only survive, advance to the reproductive stage, and spread to other hosts provided conditions in the internal and external surroundings are met to a sufficient degree. The physical environment provides triggers for certain species to
- i) Hatching of the eggs of many trematodes and nematodes
- ii) Direction of movement, such as cercariae swimming to or away from light,
iii) Periodic activity
- iv) Molting of first and second life stages
Host as a niche: The host will provide microhabitats for the parasites to invade, occupy, and reproduce. The majority of endoparasites passes through or settles in the alimentary canal of the host, which has a critical environment. The digestion, physiology, nutrition, immunity and allied factors in the host influence excystation, evagination and exsheathment and establishment and maintenance of endoparasites in their specific locations.
Host immunity is an important regulatory constraint on the spread and perpetuation of infection within the host community. The greater the proportion of immunity within the population, the lower will be the potential for disease transmission. Acquired immunity can cause second and later infections to be eliminated with no overt signs of disease.
Co-evolution of host and parasite: Continuing mutual counter-adaption between the host and parasite results in parallel evolution, so that associations develop within which lineages coevolve. Adaptations by hosts to avoid infection by the parasites result in selection pressure on the parasite to circumvent them, which is favourable to the parasite. This can result in the extinction of a species or lead to a change in the population through the selection of resistant species or strains.
Host parasite relationships: comprise the adaptation of the parasites to ensure infection, survival, and reproduction on the one hand, while immunological manifestations cause delayed or arrested development, extending the prepatent period, lowered fecundity, reduced longevity, and even morphological changes. For instance, in H.contortus the fecundity is high, resistance of free living stages is somewhat poor, host range is narrow, the development of host resistance is weak, and vulnerability to anthelmintics is high; on the contrary, in T. colubriformis, egg laying is low, resistance of free living larvae is considerable, host range is wide, host resistance is pronounced, and vulnerability to drugs is low. When there are unfavourable conditions in the host and environment, the developmental or larval stages of a parasite undergo hypobiosis. It results in the arrested, retarded, inhibited, or suppressed state of the developing stages, delaying attainment of the adult or reproductive conditions.
The modes of infection such as ingestion, skin penetration or transplacental migration influence parasite accessibility to the host and immune forces within the host. Mosquitoes, biting and non-biting flies, ox warbles, bot flies, culicoides, ticks, mites, lice and fleas to lesser extent, all generate severe morbidity and mortality among livestock. The epizootics being dependent on the prevalence and abundance of particular species, their feeding preferences, life history patterns, longevity, flight range, response to climatic fluctuation and biocides. The number of parasites within an individual host is therefore controlled by the rate at which new infections arrive and the rate at which established parasites die.
Effect of parasitism on the individual host: The effect of parasitism on the individual host may be injurious or defensive. Injurious effects show a wide range of severity, leading to manifested disease and often resulting in the death of the host. The mechanisms of injury are mechanical, chemical, inflammatory, and the introduction of other pathogens. It is obvious that not every infected animal is diseased, the host’s physiological or immunological factors may counteract the parasite’s development. Young animals, which are highly susceptible, acquire infections early in life, while older animals evince a degree of resistance or immunity acquired from an experience of infection or naturally.
Limiting factors for the growth and distribution of parasites in the ecosystem
The growth and distribution of parasites are influenced by environmental factors inside and outside the bodies of parasites and their hosts.
1) Temperature and climate: climatic factors and temperature are of major importance in parasitism. Eg., Hymenolepis nana can sustain high temperatures, but they consequently changed their morphology and their physiological reactions to a marked degree. The Haemonchus contortus third infective stage larvae are in the grass blades during the early morning and evening. Low humidity accompanied by either low or high temperatures inhibits vertical migration. Radiation, biogenic salts, currents, pressures, soil, the transparency of water, and airborne atmospheric gases also play a role in parasite ecology.
2) Seasonal variations: Whenever the environment inside and outside the host is best suited for the dissemination of the parasite, the infective stages readily locate and invade the host. Examples like worm eggs that accumulate in the winter will hatch at once in the warm weather of March–July, and in the drought season, many worm larvae invade the hosts. In terms of ectoparasitic arthropods, overly dry and wet ground would be limiting for fleas, but not for the other ectoparasites.
3) Periodicity: Periodicity is a parasite behaviour trait that varies between parasites. For example, the number of amoebic cysts in the faeces of infected animals or humans will regularly fluctuate. The malarial parasites are much more infectious to mosquitoes at night than during the day, because exflagellation of the microgametocytes occurs more readily at night. Similarly, microfilariae may remain in deeper vessels and during the night they come to the body surface, indicating some relationship between the behaviour of the young worms and mosquito visits at night.
4) Nutrition: The nutrition of the host is essential to the survival of the parasites, and even many parasites make partial compensation for this loss. The partially oxidised substances produced by the parasites are frequently used by the host. Malnutrition due to parasitic infection is typically the result of either a severe infection or both. Deficiency in the host’s diet generally has the effect of rendering the host more vulnerable to parasitic infections. Examples are the accumulation of lice during the winter, the anaemia caused by ticks, and the intestinal environment becoming more favourable to the parasites in animals with poor nutrition.
5) Stock Management: Certain management practises will create conditions for the availability of helminth infective stages. A high density of animal stocking will increase the contamination level. Similarly, many pasture management practises have direct or indirect effects on arthropod populations. The parturition rate in a flock may also influence the likelihood of parasitic infection.
6) Steroid therapy: In many animal diseases, steroids are widely used as a therapy, which makes animals susceptible to parasitism. For example, the excretion of cysts will be prolonged following the administration of steroids in Toxoplasma gondii infected cats, and egg production will also be increased by nematodes following steroid treatment. Therefore, pasture contamination will increase.
7) Absence of acquired immunity: acquired immunity is critical for developing strong resistance to reinfection; if it is lacking due to a lack of infection exposure, animals become more susceptible.
8) Longevity of infective stages: the free-living stages of most parasites will survive in the environment or in intermediate hosts for prolonged periods in temperate zones and in parts of the subtropics. Therefore, reinfection can occur in batches for young ones.
9) The influence of genetic factors: The genetic factors in the host will determine susceptibility or resistance to some species of parasites. For example, some breeds of sheep are more susceptible to H.contortus, the abomosal nematode. Bos indicus breeds of cattle are more resistant to ticks and other haematophagus insects than Bos taurus breeds. The N’dama breed of cattle in West Africa is known to be tolerant to trypanosomosis.
Conclusion
The concept of parasite ecology can be summarised by stating that temperature, humidity, rainfall, and many other climatic conditions will influence parasite survival and, in turn, cause infection. When the parasite enters the host under favourable conditions, host immunity, nutrition, genetic factors, etc. will influence the establishment of infections. If the interaction or interdependence is disrupted due to adverse climate change, the infection rate will drastically increase.
