The Most Prevalent Form Of Life On Earth Biology Essay


Parasitism is one of the most prevalent, in fact, probably the most prevalent form of life on Earth. It is proposed that more than half of living species are involved in symbiotic interactions (Askew 1971) which are of great medical and economical importance. For instance, one of the most serious diseases responsible for a high rate of demises across human populations, namely malaria, is the result of parasites. Further, the economic loss, caused by parasitized plants and farm animals, reaches enormous amount of money each year. It is evident that the parasites, the most abundant living organisms, are also considered as one of the biggest threat nowadays. For this reason, I give an overview of the parasites concentrating on nematode family and infected by it Equidae species.

Family Equidae is the largest and one of best distributed group of modern perrisodactyls including horses, Equus ferus caballus, donkeys, Equus africanus asinus, and zebras and their hybrids. They are prized for aspects including ecological and economical importance. For these reasons, it is important to carry on regular surveys, which help in establishment of their general health, well-being, performance, and development.

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Figure : Head end of horse strongylid showing leaf crown; downloaded from species, except commercial use, are unfortunately a subject to parasite infections. The most dangerous and highly reported types of parasite occurring in the organs and tissues of gastrointestinal track of Equidae are endoparasitic nematodes of the family Strongylidae, common name strongylids, with at least 66 species recorded (Kuzmina et al. 2007). These worms have a well-established buccal capsule, and a mouth collar including two leaf-crowns (Fig. 1). Commonly studied and ecologically important two subfamilies of strongylids (Lichtenfels, 1980) are Strongylinae (common name strongylins), generally big or medium- size worms with a round buccal capsule; and Cyathostominae (common name cyathostomins), generally small to medium-size worms with a cylindrically shaped buccal capsule (Lichtenfels et al. 2008).

Infections caused by strongylids are usually severe with a high degree of weakness, poor growth, weight loss, colic, diarrhoea, allergy, anaemia, also death (Getachew et al. 2010; Pereira and Vianna, 2006; Slivinska, 2009), where in even the lightest cases the consequences may affect development, performance and reproduction of animals (Ogbourne, 1978). For that reason, it is crucial to familiarize with the life cycles of these parasites in order to establish the effective anthelmintic control programs. Successful surveys, prevention and control programs are crucial because they disrupt the life cycle of parasite and help in establishment of anthelmintic efficacy.

The life cycle of Strongylinae (common species Strongylus vulgaris, S. equinus and S. edentatus) begins in the large intestine where adult worms lay eggs, which are passed in the faeces to the environment (e.g. pasture vegetation), Fig.2. These moult, develop into second- and third- stage larvae to eventually develop into infective third-, fourth- and fifth- stage larvae, which are ingested by an animal when it consumes contaminated grass or water. These larvae are resistant to cold temperature thanks to a protective sheath, hence, can persist in colder environment for longer.

Once the larvae are in the body they migrate through its various parts. S. vulgaris, the bloodworm, travel inside the wall of the arteries that lead to the small and large intestines causing blood clots, which may be a reason of scar tissue formation in arteries. Then the larvae travel to the large intestine where they mature and begin the new cycle by producing eggs.

The life cycle of S. edentatus and S. equinus is similar, however, their larvae migrate through the liver, which may cause its damage. After they migrate to the lumen of the large intestine, maturation is completed.

Figure : Life cycle of S. vulgaris nematodes; downloaded from

The life cycle of Cyathostominae is the same as Strongylinae life cycle, except the larvae do not exceed the intestine wall but they are encysted in the large colon wall, Fig. 3. They are able to emerge from the gut wall causing clinical disease known as ‘larval cyathostominosis’, which leads to oedema, diarrhoea, pyrexia, colic, and death in more than half of the cases (Love et al. 1999).

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Figure : Life cycle of Cyathostominae nematodes; downloaded from

In general, cyathostomins are considered as the most abundant and prevalent internal nematodes across Equidae species nowadays (Anjos et al. 2006; Chapman, 2003; Kaplan, 2002; Kuzmina et al. 2007; Pereira and Vianna, 2006; Silva et al. 1999; Slivinska, 2006, 2009) but importance or pathogenosity of individual Cyathostominae species is still unknown (Love et al. 1999).

According to published literature, there are three common Cyathostominae species recorded worldwide: C. longibursatum, C. nassatus and S. catinatum (Silva et al. 1999; Anjos et al. 2006; Kuzmina et al. 2007; Bucknell et al. 1995). Nonetheless, there are possibilities that less abundant species are underestimated because of small sample size and the fact that species occurring in low numbers may be easily overlooked (Chapman, 2003). For this reason, and to obtain best possible ways of parasite control, scientists started to concentrate more on cyathostomins research.

Consequently, there have been numerous research performed on communities of strongylid populations in different intestine compartments of Equidae in order to establish pattern of their abundance, prevalence and intensity. So far, the studies concentrated on caecum (Souto-Maior et al. 1999), dorsal colon (Anjos and Rodrigues, 2003) and ventral colon (Anjos and Rodrigues, 2006), Fig.4. Related to these organs internal parasitic communities are influenced by several factors including body size of the host, immunity response, diet, host group size and climate (Feliu et al. 1997; Getachew et al. 2010; Vickery and Poulin, 1998).


Figure : Digestive tract of horse

Subsequently, in caecum S. vulgaris is one of the most prevalent species recorded, with the highest mean intensity and mean abundance (Yanzhen et al. 2009). Besides that, the concentration of strongylid nematodes is generally high with at least three species recorded for a single animal where the great majority are cyathostomins. Further, studies of dorsal and ventral colons reveal that internal communities represent a stable although complex structure, indicating independence and lack of competition for space between species within a single colon (Anjos and Rodrigues, 2003; Anjos and Rodrigues, 2006). In addition, once again, cyathostomins namely C. longibursatus and C. tetracanthus were found to be the most abundant and prevalent (Anjos and Ridrigues, 2003; Anjos and Rodrigues, 2006).

Other economically and ecologically important parasitic nematodes are roundworms from Ascaridae family of which Parascaris equorum is considered as the biggest threat. Its life cycle is very similar to already discussed ones except that after burrowing into the wall of intestine, larvae are carried with the bloodstream into the liver and lungs, where they can cause bleeding of lungs, respiratory infections, or pneumonia. When the larvae are coughed up and swallowed again, they travel back to the intestine where they mature (Fig. 5).

Figure : Life Cycle of Parascaris equorum (Ascanididae); downloaded from

This family in contrast to Strongylidae is most dangerous to young individuals (KornaÅ› et al. 2006) where recovery by heavily infected foal is very unlike (Southwood et al. 1998; Cribb et al. 2006) and if untreated surgically may have fatal consequences. On the other hand, adult animals are generally immune to the infection (Roberts and Janovy, 2000) which was also proven in 12-year research performed by Pereira and Vianna (2006) on equines, who found out that the prevalence of P. equorum was only 5% of total intestine population studied.

Regarding all examples given, it can be suggested, that populations of helminths as a whole have a distinct fauna and consequently a characteristic structure in each intestine compartment (Anjos and Rodrigues, 2003). As a result, mono-infections would be more likely to occur (Kennedy and Bush, 1992) which could be due to the way helminths manipulate abundance of other nematode species (Anjos and Rodrigues, 2006). Removal of some species from the community may affect the structure of the whole population, which may turn into more or less pathogenic. Moreover, other authors argue that there may also be a competition between species or greater adaptive capacity, which is why nematodes are abundant in more than one intestine compartment (Anjos and Rodrigues, 2006). Because worm diversity may influence endoparasitic community and intensity of infection, the number of hosts with multiple infections decreases when parasitic richness increases (Kennedy and Bush, 1992; Slivinska, 2009). In the contrary, Anjos and colleagues (2006) suggested that multiple infections are equally frequent as mono-infections showing that nematode communities are similar in different compartments of intestine.

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Further, as research imply the diversity and relationship between the parasites within a community may be a consequence of heterogeneous distribution of infection stages, differences in exposure to parasitic infections, differences in the hosts (e.g. age, sex, immunity response, breed, susceptibility), interactions between parasitic richness and abundance within infracommunities and climatic conditions (Anjos and Rodrigues, 2006). This is why, once again, the study of each compartment and the accuracy in counting species (Chapman, 2003) may be very beneficial and helpful in maintaining animals welfare as well as drug resistance.

One of these studies show that the internal communities across Equidae may not necessary be stable and express a common pattern even though studied horses, donkeys, and zebras inhabited the same region (Kuzmina et al. 2009). Although the majority of recorded nematodes belonged to Cyathostominae the pattern varied across host species suggesting that parasite communities depend on the host. This idea can be further supported by Matthee (2004) research. He concentrated on relationship between the host and its parasite proposing that related host species are more likely to be infected by the same or similar parasitic communities because helminthes are dependent on host’s phylogeny (Matthee, 2004). Considering it, related host species that share the same helminth communities may be helpful in establishing treatments and parasite control programs.

In overall, all Equidae species are usually infected by more than one species of parasites, which suggests a complex and diverse community of helminths (Silva et al. 1999).

Designed control programs concentrate on good sanitation, changing the pastures annually, avoiding overgrazing pastures, environmental management, and regular deworming. Current research on the use of efficient antiparasitic treatments has decreased the prevalence of nematodes, which, in the past, had been a reason of the most damage to Equidae. Moreover, from numerous anthelmintic products available nowadays, almost all are efficient against adult nematodes as well as developing larvae, both found in the gastrointestinal track. The most common anthelmintics include ivermectin, moxidectin, fenbendazole, oxfendazole, oxibendazole, pyrantel, and univerm (Heile and Schein, 2004; Lindgren et al. 2008; Lyons et al. 2007, 2008; Yatusevich and Sinyakov, 2005). As it occurs from current study, successful suppressive antiparasitic treatment strategy against S. vulgarus, causes resistance across Cyathostominae to benzimidazole and pyrantel drugs (Kaplan, 2002). Although, there are still no reports of ivermecin resistance in Strongylidae (Kaplan, 2002), several authors argue that ivermectin is not an efficient drug in controlling P. equorum anymore (Lindgren et al. 2008; Reinemeyer, 2009). Its resistance probably occurred due to exclusive and frequent use of anthelmintics in youngsters in the first year of life. Their studies suggest, however, that fenbendazole or pyrantel were much more effective showing lower infection rate across studied animals. Hence, increasing number of research brings improvements to parasite control and treatments as well as allows delay the appearance of resistance to the antiparasitic compounds (Pereira and Vianna, 2006).

According to numerous authors the most common and used worldwide methods to diagnose nematode worms include clinical signs, faecal flotation (egg count), and necropsy (Getachew et al. 2010; Kuzmina et al. 2007; Silva et al. 1999; Slivinska, 2006, 2009; Chapman, 2003). These methods as so far are considered the most efficient in establishment of parasite community, prevalence, intensity, parasite control programs, and drug resistance.

In the conclusion, Equidae family serve as a shelter and source of food to enormous quantity of parasites, both in terms of number of individuals and number of species represented (Pereira and Vianna, 2006), from which the most prevalent and abundant are nematodes. They are also considered as the biggest danger across these noble animals which are able to wreak havoc if untreated. For those reasons, it is crucial to perform regular surveys, which improve the welfare of animals, and at the same time improve human life since Equidae family is one of the most important animals in terms of commercial use.