Eimeria

Eimeria
Oocysts of various species
Scientific classification
Domain:	Eukaryota
Clade:	Diaphoretickes
Clade:	SAR
Clade:	Alveolata
Phylum:	Apicomplexa
Class:	Conoidasida
Order:	Eucoccidiorida
Family:	Eimeriidae
Genus:	Eimeria Schneider, 1875
Type species
Eimeria falciformis [1] (Eimer, 1870) Schneider, 1875
Species
See text

Eimeria is a genus of apicomplexan parasites that includes various species capable of causing the disease coccidiosis in animals such as cattle, poultry and smaller ruminants including sheep and goats.^[2] Eimeria species are considered to be monoxenous because the life cycle is completed within a single host, and stenoxenous because they tend to be host specific, although a number of exceptions have been identified. Species of this genus infect a wide variety of hosts. Thirty-one species are known to occur in bats (Chiroptera), two in turtles, and 130 named species infect fish. Two species (E. phocae and E. weddelli) infect seals. Five species infect llamas and alpacas: E. alpacae, E. ivitaensis, E. lamae, E. macusaniensis, and E. punonensis. A number of species infect rodents, including E. couesii, E. kinsellai, E. palustris, E. ojastii and E. oryzomysi. Others infect poultry (E. necatrix and E. tenella), rabbits (E. stiedai) and cattle (E. bovis, E. ellipsoidalis, and E. zuernii).^[3] For full species list, see below.

The most prevalent species of Eimeria that cause coccidiosis in cattle are E. bovis, E. zuernii, and E. auburnensis. In a young, susceptible calf it is estimated that as few as 50,000 infective oocysts can cause severe disease.^[4] Eimeria infections are particularly damaging to the poultry industry and costs the United States more than $1.5 billion in annual losses.^[5] The most economically important species among poultry are E. tenella, E. acervulina, and E. maxima.^[6] The oocysts of what was later called Eimeria stiedai were first seen by the pioneering Dutch microscopist Antonie van Leeuwenhoek (1632–1723) in the bile of a rabbit in 1674. The genus is named after the German zoologist Theodor Eimer (1843–1898).

Life cycle

The Eimeria life cycle has an

Oocysts

There are two forms of oocyst: sporulated or late oocyst, and unsporulated or early oocyst. An infected host releases oocysts into the environment in their unsporulated form. These contain a multi-layered cell wall making them highly resistant to environmental pressures.^[8] Once released, the unsporulated oocysts undergo meiosis upon contact with oxygen and moisture.^[9] This process is known as sporulation and the oocysts take approximately 2 to 7 days to become infectious.^[10] The sporulated oocyst is said to be tetrasporic meaning it contains four sporocysts, while each sporocyst is dizoic, i.e. it contains two sporozoites.^[3]

Once ingested, the oocysts undergo a process called excystation, whereby thousands of sporozoites are released into lumen of the intestine. In the case of E. tenella, this process is thought to occur due to the combination of enzymatic degradation and mechanical abrasion of the oocyst wall in the chicken's gizzard.^[11]

Sporozoites

The motile

Invasion requires the formation of a moving junction between parasite and host cell membranes. In E. tenella, this involves parasite micronemes and rhoptry proteins including RON2, RON5 and AMA-2.^[11] It is unlikely that the host cell is completely passive in the invasion process, although evidence of host physical forces that assist in mediating parasite entry remains controversial.

Merozoites

When schizonts rupture, merozoites are released, which either go on to re-infect more

Merozoite invasion also requires the formation of a moving junction, however the proteins involved in this process differs from those on sporozoites. Rhoptry proteins AMA-1 and RON4 are found exclusively on merozoites. There is also a greater diversity of variant surface antigens found on the surface of merozoites. It is hypothesised that this may be due to the fact that merozoites are short-lived and a greater antigen repertoire would permit faster binding and invasion.^[11]

Taxonomy

The Eimeria lie within the family Eimeriidae. Eimeria accounts for close to 75% of the species within this family, and it is the most specious of the genera of the Apicomplexa with 1,700 described species.^[13]

Attempts to subdivide this large taxonomic unit into separate genera have been made. The classification of eimeriid coccidian was largely based on morphological and life cycle details.^[14] More recently, classification has been done using rDNA and mitochondrial genes, which indicate Eimeria may be paraphyletic to Isospora and Cyclospora ^[15]

Eimeria: These species are tetrasporocystic with dizoic, nonbivalved sporocysts with or without Stieda bodies. This new genus retains the majority of the species.^[13]

Goussia (Labbe 1896): These species are tetrasporocystic, dizoic, lack Stieda bodies, and have sporocyst walls consisting of two valves joined by a longitudinal suture. This genus contains about 20 species.^[16]

Crystallospora (Thelohan 1893): The species in this genus is tetrasporocystic and dizoic, and have dodecahedral sporocysts composed of two hexagonal, pyrimidal valves joined at their bases by a suture. This genus contains a single species, Crystallospora cristalloides

Epieimeria: The species in this genus are tetrasporocystic, dizoic, possess Stieda bodies, and undergo merogony and gametogony on the lumenal surface of the intestinal tract. Three species are in this genus.^[17]

Species identification

Methods for species identification are varied and among others, include

A whole genome sequencing project is in progress with chosen species, Eimeria tenella. The genome is about 60 megabases in size and has a GC-content around 53%. The 14 chromosomes range in size from 1 to > 6 megabases. Since 2013, the sequencing and annotation of a further six avian Eimeria species genomes is in progress.^[25]

Pathology and symptoms

Coccidiosis typically results in diarrhoea, weight loss and dehydration. A combination of these factors may result in poor growth and death of the animal, particularly amongst young.^[10] Other clinical signs include lethargy, depression, and reduced normal grooming behaviour.^[26] Diarrhoea may be bloody due to intestinal epithelium dying off when a large number of oocysts and merozoites burst out of the cells.

The severity of the disease is directly dependent on the number of infective Eimeria oocysts that are ingested.^[27] The pathogenesis of infection varies from mild to severe, and is largely dependent on the magnitude of infection.^[28] In light infections, the damage to the gut might only be minimal and be rapidly repaired as cells are rapidly replaced by the body. However, in heavy infections, it may only take two weeks for many intestinal epithelial cells to be infected with either Eimeria meronts or gametocytes. These cause the epithelial cells to burst, which causes significant damage to the intestine epithelial layer, resulting in the release of blood, fluid, and electrolytes into the intestine.^[29]

Prevention and treatment

Good animal husbandry practices and prophylactic application of anticoccidial drugs that target different stages of the parasite lifecycle, such as sulfonamides, ionophores and toltrazuril, are the preferred methods of disease prevention, particularly in the poultry industry.^[7][2]

The following drugs can be used for treatment of coccidiosis in cattle: amprolium, sulfaquinoxaline, and sulfamethazine. However, it is often more effective to prevent this disease in cattle, which can be aided by the products lasalocid, decoquinate, and monensin.^[29]

There is a growing problem of

• Eimeria aurati - goldfish (Carassius auratus)
• Eimeria baueri - crucian carp (Carassius carassius)
• Eimeria lepidosirenis - South American lungfish (Lepidosiren paradoxa)
• Eimeria leucisci - common barbel (Barbus barbus bocagei)
• Eimeria rutili -
  European chub (Leuciscus cephalus cabeda), Iberian nase
  (Chondrostoma polylepis polylepis)
• Eimeria vanasi - blue tilapia (Oreochromis aureus)

Reptiles

• Eimeria amphisbaeniarum - Mann's
  Amphisbaena manni
  )
• Eimeria witcheri - Mann's worm lizard (A. manni)
• Eimeria yemenensae - rock agama (Agama yemenensis)

Birds

Mammals

List of species

References