Coccidiosis: The Hidden Killer

Veterinarian Alan Pearson considers the best ways of treating Coccidiosis.

A case in 2009 prompted me to find out more about Coccidiosis in our game birds. I was looking at some young pheasants, about 14 days’ old and the keeper commented that they were eating a little less, were ‘food flicking’ and looking a little dull. The birds were housed in a poultry shed divided into six pens and they had not been outside at all; water was provided by a bell drinker mounted on a pallet to allow drainage, which had not been fenced off, so the birds could get to the space underneath; the litter was dry; and the droppings appeared fine. We sacrificed six birds for a post mortem examination. This and subsequent tests proved that the birds had Coccidiosis.

Recent years have seen the industry struggle with this enteric disease in young birds as they go onto the grass. Diseases such as Hexamita, Trichomonas and (allegedly) Brachyspira cause loose droppings or diarrhoea, weight loss, dehydration and death. Some birds never seem to recover and our findings caused us to wonder whether gut damage by other factors, such as Rotavirus in the first two weeks or Coccidiosis slightly later, was preventing the birds from mounting an adequate response to later parasite and possibly bacterial challenge.

cocci 001

The disease

Coccidiosis is caused by a protozoan parasite of the Eimeria family, each type of animal or bird will have one or more types of Eimeria which are specific to that species. Each type of the parasite may cause disease in different parts of the bowel. For instance, these species of Eimeria – E. colchici, E. duodenalis, E. phasiani, E. longeroni and E. megalostomata will be seen in pheasants, while E. legionensis, E. kofoidi and E. lyruri will be seen in Red-Legged Partridge.

Depending on environmental and climatic circumstances, the Eimeria parasite has a life cycle of about seven days. This spans from being picked up as the oocysts (eggs) from a contaminated environment, through several phases of reproduction in the cells of the gut wall, to finally reaching maturity and destroying the cell, which releases vast numbers of oocysts to start the next cycle. Successive waves of the disease cause greater contamination of the ground and increase the infective dose for the next flock placed there. The numbers are quite staggering: a single ingested oocyst results in the shedding of many thousands of oocysts, causing serious damage to the lining of the bowel in the process. The organism likes moist conditions and can be found around the bases of drinkers or other patches of wet ground.

The signs and disease control

If the infective dose is small there may be few signs besides the birds being dull and ruffled. In more serious cases there may be weight loss, lethargy, dehydration, diarrhoea and death. Partridge may suffer sudden death – and unlike in chicken there is rarely blood in the droppings. Some keepers rely on chalky droppings as a sign of the disease, but this is not always a good indicator.

Effective control is simply a numbers game. Keeping the numbers of ingested oocysts to a minimum is done mainly by attention to hygiene of drinkers and feeders and the ground around them, not re-using and certainly not overstocking pens. The inclusion of coccidiostat drugs in food, often from as early as one week will also control the numbers – but this must never be considered as the only plan. These drugs can only do so much. In the face of severe challenges, the use of additional medication in water or feed may become necessary, with drugs such as Toltrazuril (Baycox) or amprolium or sulphonamides.


Vaccines for Coccidiosis are available for poultry and are widely used in that industry as a socially responsible method of control. The basis of the vaccine is to give the bird a small, controlled dose of a mild form of the disease, allowing it to develop its immunity. The vaccine is very expensive to produce, requiring chickens kept in sterile environments to harvest the oocysts for the vaccine, so it is unlikely that there could ever be a sound economic case for it in the game bird industry. It is reasonably well documented that pheasants develop some immunity after a mild challenge but it has long been suggested that partridge do not, rendering them much more susceptible to further bouts of the disease.


In 2010 we persuaded one of our larger rearing clients to monitor bird droppings for Coccidiosis oocysts for several weeks from birth. Most of the birds on this unit are reared to three weeks indoors and then go into the traditional A-frame sun porch and outside run. Samples were collected each week and transported to a central reference laboratory. We were then able to see what the oocysts counts were doing in each bird species each week of life, and differences in the challenge between pens which were on previously used ground, and those on clean areas that had not previously been used.

We were then able to tabulate the results and analyse them in order to try and develop a programme of treatment for each species of bird. The results were quite remarkable and showed that Coccidiosis can be a hidden killer.

The results

We interpreted the results in the same way we would interpret chicken samples: Up to 5000 oocysts per gram of faeces (opg) and there is no need to treat; 5000 – 50,000 opg indicates no disease, but consult your veterinary surgeon; above 50,000 opg, and you’re usually seeing clinical disease. Consult your veterinary surgeon to start treatment.

The graph above shows the results for a batch of partridge. The first two samples were taken in a shed and had no counts at all, as one would expect. Within seven days of going out (period three) the counts had shot up to 100,000 opg. Treatment commenced on the Friday and the birds showed an even higher level the following Monday (period 4) but then dropped back in period five following treatment. These birds started to show higher levels again, with more than 50,000 opg in period seven.

In the graph above, the birds – which were the second batch of partridge sampled – were out to grass at the end of period two. They were treated at the end of period three, which helped keep the levels down. We thought that we had the answer, but the results in period six suprised us, as the birds again showed very high opg. We believe that this demonstrates that partridge do not develop any significant immunity to the disease.

The graph above shows the average of the results from all the pheasant pens. Here the birds were only sampled from going out to grass, so sample point one here is the same as point three in the previous graphs. All the birds had a five day course of coccidial treatment between periods one and two. No further treatments were given so the levels do rise again in period three, but although there is no treatment the pheasant has developed immunity by this point. This meant that the birds regained control over the parasite numbers as time progressed.


Neither the partridge nor the pheasant really showed any outward signs of the disease, despite the huge numbers of oocysts in some of the birds.

  • Coccidiosis can be amongst your birds and you may not know it
  • It should be possible to manage and treat birds in a specific way to reduce coccidial challenge and reduce overall drug usage
  • Improving gut health in this way should reduce the severity of other challenges, because the birds cope better
  • Clean ground seems to reduce initial challenges
  • Pheasant and partridge react very differently
  • Partridge do not appear to develop any immunity after exposure to the disease.

We recommend that you speak to your vet now, before the season begins, and seek their advice.

Tagged with: , , , , , , , , , , , , , , , , , , , , , ,
Posted in Game Bird Vet

Leave a Reply

Your email address will not be published. Required fields are marked *


Follow Us!

Newsletter Sign Up
Please take a few moments to register for our free e-mail newsletter to get all the latest news and views on the shooting world delivered straight to your inbox