red blood cells also gives rise to severe haemolytic anaemia. The infected animal develops a fever and cerebral involvement is possible. The precise mechanism by which Babesia induces brain pathology is uncertain although Schetters and Eling (1999) suggest that it might provide a useful animal model for human cerebral malaria.
Human cases of babesiosis are uncommon, and severe disease is usually associated with splenectomised patients or those already suffering from immunodeficiency or AIDS. The importance of retaining the spleen wherever possible means that far fewer splenectomies take place than was once the case. Surgeons perform the operation for a variety of reasons ranging from injury following a car crash to cancer.
3.5 Subclass Coccidiasina
Commonly known as the Coccidia, this is the largest group of the Apicomplexa. All members are intracellular parasites of vertebrates and invertebrates. They usually parasitize the intestinal cells although other cell types may also be infected. Some species have only a single host, whilst others employ two – commonly a vertebrate and an invertebrate, although it may be two vertebrates, one of which feeds on the other. The life cycle usually begins with the invasion of a host cell by a sporozoite stage followed by a cycle of merogony, gametogony, and sporogony. The group used to contain just the Eimeria, the Isospora group, and the haemogregarines (mainly parasites of red blood cells of amphibians and reptiles) but currently includes Cryptosporidium, Sarcocystis, and Toxoplasma.
3.5.1 Genus Eimeria
There are probably tens of thousands of Eimeria species and new ones are described on a regular basis. Unfortunately, there are the usual problems with taxonomy and species identification, so the literature is a bit confusing. The host range encompasses fish, lizards, and mammals, and most Eimeria species are host specific or infect a few closely related host species. Several species are of economic importance. For example, estimates for the annual worldwide losses owing to avian coccidiosis in commercially reared chickens and other birds are probably in the region of £500 million (Shirley et al. 2007). In addition, some Eimeria species play an important role in wildlife ecology although they are not easy to detect (Jarquín‐Díaz et al. 2019). By contrast, there are few reports of Eimeria infections in primates and human infections either do not occur or are exceedingly rare. We will only consider one species, Eimeria tenella as a representative example.
3.5.1.1 Eimeria tenella
This is the commonest and most pathogenic of the seven species of Eimeria that infect domestic poultry. Each Eimeria species develops in a different region of the bird’s digestive tract, and co‐infections with two or more species are common. Eimeria tenella occurs throughout the world and is responsible for a great deal of economic loss. Although it can cause high flock mortality, the availability of vaccines and anticoccidial drugs coupled with effective hygiene means that most losses result from chronic and subclinical infections causing reduced growth and egg production.
The life cycle is monoxenous (i.e., involves a single host) and begins when a bird ingests an infective oocyst with its food or in its water. The oocyst releases infective sporozoites when it reaches the small intestine. Gut peristalsis moves sporozoites down the intestinal tract with the digesta and once they reach the caecum, they invade the intestinal epithelial cells. In common with the other species of Eimeria, a membrane bound parasitophorous vacuole of host cell origin surrounds all the intracellular stages. The incorporation of parasite proteins into the vacuole membrane prevents the fusion of lysosomes or other vesicles. The sporozoites travel through the epithelial cells and emerge into the lamina propria where macrophages promptly ingest them. The macrophages act as a sort of transport host and move the parasites to the glands of Lieberkuhn where they escape and invade the glandular epithelial cells. Within the epithelial cells, the parasites transform into meronts and undergo a form of asexual reproduction called merogony to produce numerous merozoites. This kills the host cell and releases the merozoites to invade other caecal epithelial cells within which they produce another generation of merozoites. The parasites kill these host cells and following their release, these second‐generation merozoites invade new epithelial cells. However, at this point, for some reason, the subsequent development can follow one of two paths. Some of the merozoites will give rise to a third generation of merozoites, whilst others undergo gametogony to produce become either macrogametocytes (female) or microgametocytes (male). The microgametocytes leave their host cell and invade those containing a macrogametocyte and fuse with it to effect fertilization. After fertilization, the macrogamete transforms into a zygote and then into an oocyst. The oocyst contains only a single cell – referred to as the sporont. The oocyst leaves the host bird in its faeces but is not infectious at this stage. It now undergoes sporogony in which four sporocysts each of which contains two sporozoites develop. This takes two or more days depending upon the environmental temperature. Therefore, prompt removal of faeces and good farm hygiene can effectively prevent the transmission of disease both within and between rearing sheds.
Serious disease primarily affects young poultry particularly those between 3 and 8 weeks of age but older birds infected for the first time later in life also suffer badly. Infected birds become listless, cease to feed, and huddle together to keep warm. Damage to the caecum results in bleeding into the gut and stains the birds’ faeces with blood. The damage allows secondary invasion by bacteria present naturally in the gut, and this extends the lesions and causes further pathology. Acutely infected birds often die from blood loss 5–6 days after infection. In addition to haemorrhages, the gut swells and thickens, so it appears ‘sausage‐like’. Birds that are still alive 9 days after infection will usually recover: a caseous (cheese‐like) plug may form in the lumen of their caecum, which is voided with the faeces.
Recovering birds develop protective, species‐specific, cell‐mediated immunity to re‐infection based on CD4+ and CD8+ T cells found in the lymphoid tissues associated with the gut (Shirley et al. 2007). Co‐infections with two or more species of Eimeria do not necessarily compromise the development of immunity (Jenkins et al. 2009).
3.5.2 Genus Isospora
Taxonomic revisions have split the genus Isospora into two: those that do not express a tissue cyst stage in their life cycle remain in the genus Isospora, whilst those that do have a tissue cyst stage form the genus Cystoisospora. Most of the several hundred species belonging to the genus Isospora are parasites of birds. They are all monoxenous and therefore complete their life cycle in a single host – although some may exploit paratenic hosts to effect transmission. Berto et al. (2011) provide a review of both Isospora and Eimeria that infect birds. Isospora do not appear to affect chickens and other poultry, but they can be pathogenic in finches, sparrows, and other passerine birds, as well as mynahs and starlings.
3.5.3 Genus Cystoisospora
Members of this genus formerly belonged to the genus Isospora. The presence of a tissue cyst stage within their life cycle distinguishes them from parasites of the genus Isospora. They are monoxenous although some species (e.g., those infecting cats) exploit paratenic hosts. The genus includes several species that infect mammals and us. For example, Cystoisospora canis causes diarrhoea in puppies, Cystoisospora suis causes neonatal piglet diarrhoea, and Cystoisospora belli is an important human pathogen.
3.5.3.1 Cystoisospora (Isospora) belli
This species occurs throughout the world but is particularly common in tropical and subtropical countries. The first cases were recognised in people returning from the battlefields during the First World War, hence the species name (Latin bellum = war). It only infects humans and there do not appear to be any paratenic hosts (Dubey and Almeria 2019). It causes diarrhoea and is frequently associated with persons suffering from AIDS or immunosuppressive illnesses.
The life cycle begins when we ingest