often attributed to the direct pathogenic effect of the parasite, such as through the loss of blood and the destruction of tissues or competition for resources. For example, many gut helminths act as so‐called kleptoparasites (literally, thieving parasites) and compete with their host for nutrients within the gut lumen. However, the situation is far more complicated than this. Although a functional immune system is crucial for an organism to protect itself against pathogens, immune systems are energetically costly and when nutrients are limiting, it must trade these costs against other physiological processes. Ilmonen et al. (2000) demonstrated this by injecting one group of breeding female pied flycatchers (Ficedula hypoleuca) with a diphtheria‐tetanus vaccine and a control group with a saline solution. The vaccine was not pathogenic and did not induce an infection, but it activated the birds’ immune system. They found that birds injected with the vaccine exhibited a lower feeding effort, invested less in self‐maintenance and had a lower reproductive output, as determined by fledgling quality and number. The authors therefore concluded that the energetic consequences of activating the immune system can be sufficient to reduce the host’s breeding success.
1.3 Parasite Hosts
A parasite host is an organism on or in which the parasite lives and from which it derives its nutrition. The host is usually not related taxonomically to the parasite although this is not always the case (see intra‐specific parasites). Most parasites are highly host specific and only infect one host species or a group of closely related species. This is because all hosts represent a unique challenge in terms of the complex adaptations the parasite requires to evolve to identify, invade, and survive within/upon them. Nevertheless, a few parasite species, exploit a wide range of hosts. For example, the protozoan parasite Toxoplasma gondii infects, grows, and asexually reproduces in virtually all warm‐blooded vertebrates although sexual reproduction only takes place within the small intestine of cats.
Hosts can be divided into classes, depending upon the role they play in the parasite’s life cycle. The ‘definitive’ (or final) host is the one in, or on, which the parasite reaches maturity and undergoes sexual reproduction, whilst the ‘intermediate’ host is the one in which the parasite undergoes its developmental stage(s). There may be just one or several intermediate hosts and the parasite may or may not undergo asexual reproduction during this time, but it cannot develop into an adult or reproduce sexually. In this way, some parasites exploit their hosts to maximum effect by combining the reproductive power of asexual reproduction in the larval stage with the advantages of sexual reproduction during the adult stage.
Parasites devote more of their energies to reproduction than free‐living animals because they do not have to worry about food, shelter, and fluctuations in environmental conditions. This is important because the chances of any offspring locating and establishing themselves within a suitable host are very low. The completion of a parasite’s life cycle sometimes depends upon the death of the intermediate host and the subsequent consumption of the larval form by the definitive host. In this situation, the parasite is often very pathogenic in its intermediate host but has relatively minor effects on the definitive host. The intermediate host is not always killed or consumed by the definitive host. For example, after undergoing asexual reproduction in the snail intermediate host, the cercariae of the liver fluke Fasciola hepatica physically and chemically bore their way out and swim off to transform into metacercariae attached to aquatic vegetation. The snail survives the damage to its tissues, and the lifecycle is completed when the metacercaria are consumed by the sheep definitive host (see Section 5.2.1.1.1 for more details).
Parasites of Parasites
Viruses infect several parasitic protozoa such as Leishmania spp. (Rossi and Fasel 2018) and Giardia lamblia (Janssen et al. 2015) but, at the time of writing, there was surprisingly little evidence of their presence in helminths – though this is probably because few scientists have looked for them. Some workers suggest that viruses could be used to combat parasite infections (Hyman et al. 2013), but there is increasing evidence that many of the viruses found in parasitic protozoa contribute to their pathogenicity (Gómez‐Arreaza et al. 2017).
Parasites are also infected by prokaryotic (e.g., bacteria) and eukaryotic (e.g., fungi and protozoa) parasites. Those parasites that infect other parasites are known as hyperparasites. For example, the microsporidian Nosema helminthorum is parasitic on the tapeworm Moniezia expansa that lives within the small intestine of sheep and goats (Canning and Gunn 1984). Sheep become infected by the tapeworm when they accidentally ingest oribatid mites containing the cysticercoids of M. expansa. Subsequently, the sheep must consume the infective cysts of N. helminthorum and these must then penetrate the tegument (tapeworms lack a gut of their own) of the tapeworm. Within the tapeworm, N. helminthorum reproduces and causes numerous raised opaque bleb‐like patches but is not especially pathogenic. Related microsporidia affect various other platyhelminth parasites (Canning 1975; Sokolova and Overstreet 2020), but there are remarkably few reports of them infecting parasitic nematodes (e.g., Kudo and Hetherington 1922). The discovery of microsporidia infecting the free‐living nematode Caenorhabditis elegans has opened the potential of developing a laboratory model for studying both nematode immunity and the biology of microsporidia (Zhang et al. 2016). This is because C. elegans is a commonly used model organism whose full genome is known. Several species of microsporidia cause pathogenic infections in humans and domestic animals and a simple laboratory model would prove extremely useful in the development of drug treatments etc.
A paratenic host, also sometimes referred to as a transport host, is one that a parasite enters but within which it cannot undergo further development. Paratenic hosts are not usually essential for a parasite to complete its life cycle although they may provide a useful bridge between the infective stage/intermediate host and definitive host. For example, the definitive hosts of the nematode Capillaria hepatica are primarily rodents although it infects several other species of mammals including dogs, cats, and pigs. Human infections are rare but potentially serious. The adult worms reside in the definitive host’s liver and their unembryonated eggs remain there until the host dies/ is killed and a scavenger/ predator consumes them (Figure 1.2). The unembryonated eggs pass through the gut of the scavenger/predator and then out with the faeces. This helps disperse the eggs in the environment. Development of the eggs to the infective stage occurs within the soil and takes several weeks or even months. If the definitive host’s body is not consumed, the eggs embryonate to the infective stage, but there will be little dispersal. Earthworms ingest infective embryonated eggs of C. hepatica whilst feeding on soil and detritus. Because many rodents consume earthworms, these probably facilitate the transfer of the nematode to its definitive host.
Figure 1.2 Life cycle of the nematode Capillaria hepatica illustrating the role of paratenic hosts in the transmission cycle. Drawings not to scale. 1 = A rodent becomes infected when it consumes embryonated eggs. These hatch in the small intestine, the larvae penetrate the gut, enter the circulation, and reach the liver. The larvae (L) moult become adult male (M) and female (F) worms and commence laying eggs. The unembryonated eggs remain in the liver. 2 = When the rodent dies its body decays and the unembryonated eggs enter the soil. If a scavenger eats the body, the unembryonated eggs pass through the gut and are dispersed. 3 = If a fox or other predator eats a live infected rodent, the unembryonated eggs are passed in its faeces. Scavengers and predators therefore act as dispersal hosts. 4 = The eggs embryonate to the infective stage in the soil. A rodent, human, or other susceptible mammal becomes infected when it consumes the infective eggs. 5 = Earthworms that consume infective eggs act as paratenic hosts if they are subsequently eaten by a rodent (or other susceptible
