Coccidia and coccidiosis

(följ länken för full version inklusive tabell och figurer) 

Chapter 5 - Pathology

FAO Animal Production and Health Series No. 21

(new revised version)

by:

F. Lebas
Agricultural Engineer

P. Coudert
Veterinary Surgeon

H. de Rochambeau
Agricultural Engineer

R.G. Thébault
Engineer (INRA)

Coccidia and coccidiosis
Coccidia. 
Coccidia are protozoa, the most primitive phylum of the animal kingdom.
They are sporozoa, i.e. parasites with no cilia and no flagella, which reproduce both sexually and asexually. A large number of families are represented. The Eimeriidae family is typified by the independent development of male and female gametes.
Almost all coccidia belong to the genus Eimeria - they include four sporocysts containing two sporozoites. Typically they form oocysts, a parasite mechanism for dispersal and defence in an external environment.

The coccidia cycle. 
Eimeria are monoxenous and have very high host specificity. The rabbit therefore cannot be infested by the coccidia of other animal species, nor can they be infested by rabbit coccidia. Eimeria develop in the epithelial cells of the digestive apparatus (intestine, liver). Eggs (oocysts) are found in the intestine and faeces. After maturing (sporulation), the oocysts contain eight "embryos" (sporozoites).

The Eimeria cycle includes two distinct phases:

· an internal phase (schizogony + gamogony) in which the parasite multiplies and the oocysts are eliminated in the faeces; 

· an external phase (sporogony) during which the oocyst becomes able to infest if it finds favourable conditions of humidity, heat and oxygenation.

The internal part of the cycle begins with ingestion of the sporulated oocyst and the excretion of the sporozoites. The parasite then multiplies. This may entail one, two or more schizogonies (asexual reproduction) according to the species (E. media, two schizogonies; E. irresidua,three or four schizogonies). It can take place in different parts of the digestive system (E. stiedai in the liver; E. magna in the small intestine; E. flavescens in the caecum). The final schizogony leads to the formation of gametes.
The next step, gamogony (sexual reproduction), ends in the formation of oocysts that are excreted with the faeces; the total duration of the internal phase of the cycle also varies with the species (e.g. E. stiedai, 14 days; E. perforans, four days).
The external part of the cycle (sporogony) is typified by the extraordinary resistance of the oocysts to the outside environment. Their resistance to chemical agents is particularly striking. In the right conditions of humidity, heat and oxygenation the oocysts become able to infest. They sporulate. Hatching time varies: at 26°C, E. stiedai takes three days and E. perforans one day.
Coudert (1981) in France and many others have studied this part of the cycle, as the oocyst is the agent to be destroyed. In practice, oocyst resistance is hard to overcome, particularly in disinfecting rabbitries. Chemical disinfection is pointless as oocysts can only be destroyed by heating and drying.

Species. 
At least 11 coccidia species are rabbit parasites. One infests the liver, the other ten the intestine.

· Eimeria stiedai, the liver coccidiosis, causes no economic losses in Europe except at the slaughterhouse. It is relatively easy to eliminate this parasitosis by a few weeks of very strict health and hygiene measures and preventive medicine. A four- to six-week treatment with conventional anticoccidia products (Decox, Pancoxin, formosulfathia-zole) in the feed at preventive doses virtually eliminates the disease. In climates less clement than Europe's and in countries where it is not so easy to get the right medicines, liver coccidiosis can have more serious consequences. As the liver is an organ basic to every process involving homeostasis, chronic liver attacks cannot fail to diminish the animal's resistance capacity.

Intestinal coccidia can be classified in four categories (Table 48):

· Eimeria coecicola and E. exigua are apathogenic. No clinical sign is detectable even with an inoculum containing several million oocysts. 

· E. perforans is very slightly pathogenic. Alone, it never causes diarrhoea or mortality. Massive infestations (10⁶ oocysts) are needed before there is a slight and very brief decrease in growth.

· E. irresidua, E. magna, E. media and E. piriformis are pathogenic species that cause diarrhoea and growth retardation of as much as 15 to 20 percent of live weight for infestations with between 0.5 and 1 × 10⁵ oocysts. When they occur alone these coccidia are not usually lethal, even in a relatively heavy infestation.

· E. intestinalis and E. flavescens are the most pathogenic coccidia. They cause diarrhoea and mortality, even at very low dose rates (upwards of 10³ oocysts).

Observations
Pathogenic effect has been judged here solely on the basis of retarded growth and mortality. But it must not be forgotten that coccidiosis, like all diseases, can have certain after-effects on the kidneys or liver in particular, which in turn have repercussions on fattening status at slaughter or on the animal's future if it is to be kept as a breeding animal.
Often one disease is also complicated by other diseases. In fact, the above results were obtained with rabbits reared under especially favourable conditions, which means there were practically no bacterial side-infections. It is not known, for example, whether in an unfavourable environment coccidia of the second group (E. media, etc.) might not have a more severe impact.

Lesions. 
There are two kinds of lesions: macroscopic and histological.

· Macroscopic.  

Every coccidium has a preferential place to develop where it causes a reaction of the intestinal epithelium varying in visibility according to the bacterial species. The duodenum and the jejunum are parasitized by E. perforans, E. media and E.irresidua. The latter species is the only one which, at high concentrations, causes macroscopic lesions visible at autopsy. E.magna, E. vejdovskyi and E. intestinalis multiply in the ileum. E. intestinalis causes the most spectacular macroscopic lesions. The ileum becomes bruised and whitens; segmentation appears very clearly, especially in the part nearest the caecum. The appearance of the lesions is the same with high concentrations of E. magna. The caecum is the domain of E. flavescens, which at medium dose levels produces lesions on the colon. The caecum wall thickens and changes appearance according to whether there is microbial infection or not. It may look whitish in heavy infestations with no complications, but very frequently reddish striations, necrotic plaques or generalized congestion appear. The most constant factor is the emptiness of the caecum. Lesions can be caused in the colon by E. flavescens and above all by E. piriformis, the only rabbit coccidium capable of causing enterorrhagia in the Fusus coli at medium dose levels (30 000 to 50 000 oocysts). 

· Histological.

There are two points to stress here: lesions, both macroscopic and histological, are relatively short-lived. They appear towards the eighth or ninth day and disappear by the 12th or 13th day, despite their sometimes spectacular appearance (E.intestinalis, E. flavescens and E. piriformis). Histologically, hypertrophy can only be observed in the epithelial cells of the intestine. Cell structure remains intact. Moreover, the number of cells parasitized is extremely low in proportion to the number of cells of the epithelium, but all the cells, whether parasitized or not, look the same. Only a few cell clusters deep in the crypts of Lieberkühn will be destroyed.

Coccidiosis. 
Coccidia are specific pathogenic agents. When inoculated into rabbits pathogenic coccidia cause the same lesions and the same symptoms (diarrhoea, loss of weight, death) in all the animals tested.

Clinical signs. 
Most of these are not specific to intestinal coccidiosis. The main symptoms are: diarrhoea, weight loss, low intake of feed and water, contagion and death.
The clinical evolution of an intestinal coccidiosis is illustrated in Figure 22.
Depending on the coccidia species, diarrhoea appears between the fourth and the sixth day after infestation. The peak is from the eighth to the tenth day. It then declines in three or four days. Diarrhoea is the first visible symptom, together with cutaneous dehydration, clinically demonstrated by the persistence of skin folds.
Weight gain and feed intake evolve in a sequence that faithfully follows the evolution of the diarrhoea. For two or three days growth and feed intake are low. Between the seventh and tenth day after infestation there is weight loss, perhaps as much as 20 percent of live weight in two or three days. Recovery is equally rapid. Two weeks after inoculation the animals may resume their initial growth.
Mortality occurs during a relatively short period (three or four days), starting abruptly on the ninth day after infestation.
The intensity of these general symptoms naturally varies according to the Eimeria species involved (see above), the degree of infestation and the animal's general condition. Identical effects can be obtained by using different dose levels of different species of Eimeria.
Few data are available on simultaneous infestations, but there appears to be no synergistic action among the various species except with E.piriformis, which seems to augment considerably the pathogenicity of other species. This is rather easily explained by its locus of implantation and the fundamental role of the colon (see physio-pathology, below).
It is common for bacterial flora to develop at the same time as coccidiosis, complicating and aggravating the symptoms of the disease.
If there has been no contact with coccidia (non-immune rabbits), then age is not a major factor in susceptibility in rabbits. The disease is briefer in animals 10 to 11 weeks old and diarrhoea less severe, but weight loss and mortality are often more pronounced than in younger rabbits. However, the early contact does confer relative immunity.

Physiopathology of diarrhoea of coccidian origin. 
The main symptom of intestinal disease in young rabbits is diarrhoea. Rabbit enteritis following cocciodiosis has been studied with reference to calves and human infants, in whom episodes of diarrhoea are essentially linked with hydromineral perturbations. In calves and infants diarrhoea seems to be dominated by three main phenomena. There is, of course, considerable loss of faecal matter. The usual impact on the metabolism is extracellular dehydration and metabolic acidosis. Rabbits suffering from diarrhoea, like calves and infants, certainly have more watery faeces, but sick animals produce a smaller quantity of faeces than healthy ones. Calves and infants urinate little or not at all with diarrhoea and there is haemoconcentration associated with extracellular dehydration. In young rabbits, diuresis is not altered during diarrhoea and there is haemodilution. The distribution of water in the organism is unmodified except that the skin is heavily dehydrated. Blood pH is normal. The most marked modification of the blood plasma is severe hypokalemia.

FIGURE 22 The clinical evolution of coccidiosis

 

The pathogenesis of diarrhoea in the young rabbit thus appears to differ from the more conventional diarrhoea of the calf or infant, but the prime mover at the intestinal level seems to be common to all. In diarrhoea of calves due to E. coli, for example, the small intestine secretes water and minerals, especially sodium, which will be lost by the animals.
In the young rabbit there is also a lack of reabsorption - indeed an actual secretion -of sodium and water in the loci where the parasites multiply. Unlike calves, however, rabbits can compensate for these disturbances in the distal colon and, most important, they can initiate an Na-K exchange which limits sodium losses to a minimum. Potassium losses are replaced from body reserves. These parameters evolve at the same time as the symptoms described earlier. Peak intensity of symptoms occurs at about the tenth day after infestation. Certain elements are generally described as constants in rabbit enteritis: lengthening of the retention time of the ingesta in the intestine, high levels of colibacilli and intestinal pH tending towards basicity.
This suggests that the basic phenomena of the pathogenesis of diarrhoea are independent of the aetiology (infectious agents or non-specific causes) and that the diarrhoea syndrome is a complex process. It may lead to a single response but several elements are involved - digestion, flora, motility, absorption and secretion.
It could, likewise, be tempting to attribute the sometimes spectacular lesions to the pathogenicity of the coccidia. But this would overlook the fact that these modifications in hydromineral metabolism and pH are delayed manifestations of an attack that took place days before.

Coccidiosis and field conditions. 
All production units are parasitized, usually by several species of coccidia. Investigations show that the least pathogenic species are the most numerous (Eimeria perforans, E. media). E. magna is also very common and of ten found in great numbers.E. intestinalis, E. flavescens and E. Irresidua are less common. This is a good thing, because their mere presence is a real menace to the rabbitry. E. piriformis is rare in Europe and E. intestinalis has not been identified in Benin.

It must not be forgotten that a single pellet from a healthy rabbit raised in a sound, clean rabbitry usually contains enough coccidia to cause diarrhoea if the same number were inoculated into the animal. Yet not all rabbits contract clinical coccidiosis. It almost always depends on conditions in the rabbitry. If conditions are good only a few animals will die of diarrhoea. If conditions are bad there will be a chronic mortality rate of 10 to 15 percent. Indeed, this is the usual situation.

Whether the environment is good or bad, any stress can set off coccidiosis, whatever the animal's age. It is curious to note that diarrhoea strikes not only young, newly weaned rabbits, but also older animals which have been in contact with the parasites for several weeks. Naturally acquired specific immunity is always very weak.
The outbreak of coccidiosis, the progress of which is summarized in Figure 23, can therefore be attributed mostly to stress.

Non-specific stress occurring singly cannot cause diarrhoea in a rabbitry where sanitary standards and physiological comfort are good. In such an environment the animal is able to marshal fully its nonspecific defence potential. On the other hand, a simple change of feed in a rabbitry with a poor environment is enough to set off diarrhoea. The mere fact of raising five or six rabbits together in a cage one-third of a square metre in a room with 100 or 1 000 other cages acts as a sort of sounding board to amplify all these phenomena.
Finally, non-specific factors cannot be discussed without mentioning their intensity - five minutes of transport does not constitute the same amount of stress as does four hours. These upsets are the root cause of outbreaks and it is only later, in most cases, that specific disease agents intervene (viruses, bacteria, coccidia). Each agent, merely by its permanent presence at low or average level, can also help undermine the rabbit's defence mechanisms without there necessarily being any permanent clinical disease.

The same is true of the other specific chronic diseases such as respiratory ailments and myxomatosis which by the very process of sapping the organism's defence capacities will become the indirect agents of outbreaks of coccidiosis and diarrhoea. Cases of primary coccidiosis are therefore probably rare. They can nevertheless occur, in particular when animals which are carriers of pathogenic species are introduced to the rabbitry.

Diagnosis. 
Coccidiosis is often extremely difficult to diagnose. It can only be done in the laboratory, by counting coccidia per gram of excrement and examining the viscera. Counts must be made on several animals for several days running to diagnose coccidiosis properly. The specific coccidia species and their pathogenic potential also need to be identified.

FIGURE 23 Development of coccidiosis

For coproscopic investigations, examination of excreta several days old taken from under a cage where there are several animals is preferable to and far more reliable than a caecum contents examination. At any given moment (death or slaughter of the animal) there may be:

· no trace of coccidia and coccidiosis: this is the not uncommon case in animals that die before the completion of the coccidial cycle; 

· few coccidia and little coccidiosis: as above, with mortality occurring a little later. This happens mainly with very pathogenic coccidia which kill rapidly (E. intestinalis, E. flavescens) even at low concentrations;

· many coccidia and no clinical coccidiosis in the case of infestations with not very pathogenic coccidia (E. coecicola, E. perforans, E. media). The multiplication of the parasite will none the less be a negative factor.

Despite these difficulties it can be stated categorically that the presence of E. intestinalis, E. flavescens and even E. irresidua or E. piriformisis a serious circumstance and, for the first two, a definite menace. A post-mortem examination is often disappointing. The typical coccidiosis lesions appear only with massive infestations and persist for only two or three days. The presence of whitish spots on the intestine is an indication, but not a proof, of coccidiosis. In any case, it is recommended that an autopsy be carried out on all dead animals. A combination of observed factors, even if observed hastily, is far preferable to an isolated finding.

Liver coccidiosis, on the contrary, is very easy to diagnose. The presence of small whitish-yellow patches or small nodules on the surface or inside the liver is typical of this disease. But only massive coccidiosis, which sometimes provokes spectacular liver hypertrophy and considerable weight loss, can account for mortality.

Prognosis. 
A coccidiosis prognosis will not be of much use unless the expert also diagnoses why there has been an outbreak. All rabbits are coccidia carriers so it cannot be attributed to the parasites alone (they were already present). Conditions in the rabbitry and the animals' resistance being such as to produce a multiplication of Eimeria, the environment must also be examined and treated. This is why the prognosis is often quite bleak.

Control. 
Treatment is often disappointing and always expensive. There are basically two reasons for this:

· medical treatment is not really appropriate unless the cause of the disease is known. In the rabbit, the disease most often begins through a combination of several non-specific factors. The environment thus needs to be dealt with first; 

· anticoccidiosis treatment is feasible for animals that have been infected for only a few days (five or six), but it is not effective otherwise. Even after successful treatment it should be realized that mortality and diarrhoea will continue in the rabbitry for a few more days. The most disappointing thing is that an improvement lasting one or two weeks is often followed by a relapse. It must be understood that a few days of diarrhoea in a rabbitry breeds thousands of millions of coccidia, only a few hundred of the most pathogenic of which are enough to kill an individual rabbit.

Sulpha drug treatment. 
The most common drugs are nitrofurans and sulpha drugs. The former have been used non-stop for nearly 30 years in feed. This may be one reason why present-day coccidia control is so ineffective. Nonetheless, the bacterio-static activity of these drugs probably favours recovery or avoids problems. Bifuran (50 percent furazolidane, 50 percent furoxone) at rates of 200 mg/kg of feed is now used only as a preventive measure.

Sulpha drugs are most effective in treatment, not in prevention. Sulfadimethoxine is the most effective sulpha drug and the one best tolerated by nursing or pregnant does:

· curative dose: 0.5 to 0.7 g/litre drinking-water

          · preventive dose: 0.25 g/litre drinking-water.

The bacteriostatic activity of this drug, especially on pasteurellosis, makes it one of the best rabbit medicines. It should not be overused.
Sulfaquinoxaline is commonly used, but at higher doses:

· curative dose: 1 g/litre drinking-water;

· preventive dose: 0.50 g/litre drinking-water.

Sulfadimerazine, at 2 g/litre, is less effective.
These sulpha drugs can be boosted by antifolics, such as pyrimethamine or diaveridine, which allows the dose to be reduced considerably, but this increases their toxicity, especially for pregnant does. The use of sulpha drugs for pregnant does must be systematically avoided.
Formosulfathiazole is another excellent coccidiostatic drug at rates of 0.5 to 0.8 g/kg of feed as a curative measure, or 0.3 to 0.5 g/kg as a preventive measure. Unfortunately it is not water-soluble.

Curative treatments should always be applied to all growing animals for four or five consecutive days followed by a therapeutic rest. Treatment is then resumed for a further four or five days. If the medicine is given in the drinking-water care must be taken that the water is constantly clear. Where animals are fed watery forage such as roots and greens, these should be replaced by dry feeds or the animals will not drink enough water. Drug concentrations as generally indicated correspond roughly to a water intake of 100 to 150 g water per kg of live weight. When water intake exceeds this normal quantity (nursing does, very hot weather) the drug should be further diluted. Stepping up the concentration is not really possible - the rabbit would then probably refuse to drink the water.

Treatment with antibiotics. 
Antibiotics do not cure coccidiosis. They may, however, be used in cases of persistent diarrhoea or to prevent secondary bacterial complications. The most common antibiotics used for rabbits are neomycin (0.1 to 0.4 kg/litre of drinking-water), colimycin (3 to 4.10⁵ IU/litre) and the tetracycline group (0.2 to 0.3 g/litre). Once treatment with antibiotics is started it must be continued for three or four days at steady doses if it is to have any chance of being effective.

Antibiotics must be used cautiously in treating rabbits. Some that basically act on gram-positive flora are toxic to rabbits (ampicillin, lyncomycin, clyndamycin), while others should not be administered orally (chloramphenicol, penicillin, erythromycin, tylosin). With the possible exception of neomycin and the tetracyclines, antibiotics always entail the risk of digestive troubles.

In treating undiagnosed diarrhoea the proper treatment of coccidia alone is often enough to reverse the situation. Many French and other authors stress the importance of intestinal coccidiosis as a factor in the outbreak of enteritis and the benefit of treating coccidia. It should be remembered that giving medicine is not in itself sufficient treatment.

Prevention. 
Non-specific attacks and coccidiosis are the basic causes of diarrhoea. Diarrhoea prevention therefore consists of controlling these two factors. Good hygiene is the proper way to prevent the first. Preventive medicine should be added to combat coccidiosis.

There are two kinds of preventive medicine: vaccination and chemopreventive treatment. There is no anticoccidiosis vaccine (as of 1996). Active research is ongoing and hopefully short-cycle attenuated strains may soon be seen (early strains). Doses of sulpha drugs (see preceding pages) given to the young rabbits at weaning for eight to ten days are a good preventive measure in problem rabbitries.

Anticoccidial drugs administered as a preventive measure in balanced pelleted feeds are without doubt the most popular control method. A certain number of products can be used for rabbits (Figure 20). Robenidine has been used as a food additive in Europe since 1982 (66 mg/kg) and is very effective and well tolerated by rabbits. But ten years of use in the region have produced chemoresistance (E. media and E.magna). Others are effective (Lerbek) or highly effective (Salinomycin, Diclazuril, Toltrazuril-hydrosoluble), but had not been used for rabbits as of 1993. Anticoccidial products of the ionophore family used in poultry husbandry are usually very toxic to rabbits: Narasin, Monensin, Maduramycin. Some are well tolerated (Salinomycin 20 ppm; Lasalocid 50 ppm), but overdosage must be avoided. Anticoccidial products much in use in poultry husbandry, such as Amprolium and Coyden (methylchlorpindol), have little, if any, effect on rabbits. Coudert (1981) has made an exhaustive bibliographic review of these products. The disadvantage of such drugs is that they are not water-soluble, which means they can only be administered in balanced pelleted feeds. Antibiotics added to feed in constant low doses are strongly warned against as ineffective and dangerous.

Preventive hygiene is the keystone of coccidiosis control and successful rabbit production. It is far more important than any other anticoccidiosis measures and for this reason the last section of this chapter deals solely with preventive hygiene.

Acquired immunity to coccidia is species-specific. Coccidia cannot develop in young rabbits before 21 to 25 days, i.e. while lactation is the principal source of nourishment. The presence of coccidia before the age of 28 days is a sign of insufficient milk or poor hygiene. After weaning, in the presence of contamination, immunity is acquired in 10 to 12 days and lasts to adulthood. Acquired resistance is weakened, however, by the immunodepressive effect of major stress.