Anticoccidial Eimeria vaccines for poultry: where are we coming from and what is next? (Part I)

Eimeria vaccines have been widely used in poultry since the early 1950’s and their advantages have been clearly shown. In spite of this, there are some differences between them -attenuation, composition, administration route-, but they all consist of live parasites that need to undergo two and sometimes three entire life cycles inside the host gut in order to trigger the immune system and subsequently establish full protective immunity.

There are both live non-attenuated and attenuated Eimeria vaccines on the market. Live non-attenuated vaccines consist of parasites that still maintain their natural virulence. Control of the development of adverse reactions (coccidiosis disease) is achieved by using low numbers of oocysts in vaccine preparations and in some cases even by the use of anticoccidials to control the excessive spread of vaccine strains. This strategy of “controlled exposure” allows protective immunity to develop before the contamination of litter with non-attenuated oocysts becomes severe.

Live attenuated Eimeria vaccines are specifically designed to generate an immune response whilst limiting the threat of possible adverse events. The most widely used attenuation system is the selection of strains by precocious development (Jeffers 1975), although attenuation via passages on embryonated eggs has also been used in E. tenella for some commercial live attenuated vaccines. There can therefore be different grades of attenuation due to the use of different attenuation processes.

As regards the composition of the Eimeria vaccine, this depends mainly on the category of birds for which the vaccine has been developed: broilers or breeders and layers. In spite of this, the main core of any vaccine should contain at least Eimeria acervulina, E. maxima and E. tenella, as these are species affecting all the birds. E. mitis and E. praecox have very fast biological life cycles within the host and this is closely related to their effect in the first weeks of production cycles. Thus, the impact of these Eimeria species is relevant for broilers but very limited for breeders and layers. In fact, the growth rate and weight gain within the first weeks of life in breeders and layers is quite irrelevant and even later on (from 3-4 weeks) the weight is monitored and the feed administered is restricted.

On the other hand, in the case of long life cycle birds such as breeders and layers, it is necessary to include E. necatrix and E. brunetti in the composition of Eimeria vaccines.

In addition, two species of Eimeria – in the past considered as separate species – are no longer included by the scientific community in the list of Eimeria species affecting Gallus gallus:

However, both species are still present in a commercial vaccine registered on the American Continent.

Finally, regarding administration methods, a number of different ones have been developed for live Eimeria vaccines. The first ones included:

  • Drinking water: the best equipment to be used are bell-type drinkers. Usually one drinker is required for 100-150 chicks. In some cases, even pipeline systems have been used, but they are not suitable. In fact, when pipeline nipple drinkers are used, the risk is always that the vaccinal oocysts, due to their weight, become trapped in some angles of the line or in the biofilm that usually forms all along the lines.
  • On-feed spray: it should be pointed out that temperatures on farms are high on day 0, thus the time from vaccination to ingestion of feed becomes crucial. If this time is too long, vaccine oocysts may die from desiccation before the chicks can ingest them;
  • Eye drops: this is an individual method that can be very accurate, but due to the need for skilled labour it has fallen out of use, although it is still used in a few countries.

More recently:

  • Oral Gel/ Rain Gel: another delivery method is the incorporation of vaccine in a coloured gel that is placed in chick trays at the hatchery or on feed trays in the poultry house immediately after placement. However, this method can lead to non-uniform distribution of the oocysts inside the gel and consequently uneven intake of the vaccine.
  • In-ovo injection of sporulated oocysts into 18-day-old embryonated eggs. In-ovo administration has several distinct advantages, including the increased accuracy and repeatability of vaccine delivery. However, for this route of administration, a specific in-ovo machine adapted to deliver oocysts into the amniotic cavity of embryos is needed.
  • Coarse spray administration of 1-day-old chicks is probably the most common method and consists of spraying the vaccine over the chicks’ trays either at the hatchery or at farm level. The oocysts are suspended in a coloured dye (like the light purple used in HIPRACOX® & EVALON®) that has the dual advantage of allowing hatcheries to evaluate visually the success of the procedure and stimulating the chicks to take up the vaccine by preening themselves and pecking each other. In addition, HIPRACOX® & EVALON®, together with the colouring agent, contain an aroma –vanillin- that is able to enhance pecking and preening even under low light conditions. The devices most frequently used for the application of Eimeria vaccines are hatchery spraying cabinets or in-line devices that are able to combine coarse spray droplets -for the correct ingestion of the vaccine- together with uniformity of distribution of the vaccine within the chicks’ box. Hipraspray® is the first device that has been specially developed for the administration of the Eimeria vaccines EVALON® and HIPRACOX®.

References

Jeffers T.K. (1975). Attenuation of Eimeria tenella through selection for precociousness. J Parasitol. Dec; 61(6):1083-90.

Levine P.P. 1938. Eimeria hagani n.sp. (Protozoa: Eimeriidae) a new coccidium of the chicken. Cornell Veterinarian, 28: 263—266.

Shirley M. W., Jeffers T. K., Long P. L. (1983). Studies to determine the taxonomic status of Eimeria mitis, Tyzzer 1929 and E. mivati, Edgar and Seibold 1964. Parasitology 87(2), 185-198.

Vrba V., Poplstein M., Pakandl M. (2011). The discovery of the two types of small subunit ribosomal RNA gene in Eimeria mitis contests the existence of E. mivati as an independent species. Veterinary Parasitology 183, 47-53.