Kathryn Stewart BVM BVS (Hons) MRCVS
Printed in Poultry Business December 2019 Edition
Cases of blackhead in turkeys started emerging in England unseasonably early this year, from August through to October. There appeared to be no clear correlation with age of onset, system, severity or geography and the higher than usual incidence has been worrying for the industry.
Blackhead is one of those diseases that, as vets, we can’t give you all the answers – perhaps due to its complex lifecycle and ability to defy past research.
The Disease – Background, Symptoms
The condition can affect many poultry species including chickens, turkeys and some game birds; however waterfowl appear “immune” to its affects. It is in turkeys that it is perhaps most feared, due its past ability to be virulent, cause severe clinical signs and high mortality; which in turn is of significant welfare and economic impact, with few known treatment options.
The disease is caused by Histamonas meleagridis, a ‘motile protozoa’ or ‘single celled parasite’. The text book lifecycle appears complex, with involvement of both the caecal worm Heterakis gallarium and the common earthworm. More recently, the involvement of the earthworm in cases is under scrutiny.
Blackhead primarily starts in and affects the blind ended part of the lower intestine, the caecum; burrowing into the wall and causing thickening, ulceration and commonly a yellow scour and often gathering of purulent material also known as caecal cores (pictured). It can then spread to the liver causing the classic ‘nail head’ lesions that we often associate with it. The term blackhead was derived from the rare symptom where the head turns a dark purple colour due to affected blood supply. Once recovered, there is often significant unevenness and secondary E.coli infection.
This year more so than ever, there appears to be two separate presentations of the disease in turkeys; one where there is rapid and sudden mortality; and one of a slower onset, lower mortality where birds appear dull and appetent. It is the second that as a practice, we feel appears to becoming more prevalent and can be seen with or without the liver lesions.
There does not appear to be a correlation between blackhead infections and prior health of the turkey and we see outbreaks on very well managed farms. Experience would suggest however, it is common for the clinical signs of blackhead to develop after periods of stress, particularly when coming into lay or following movement.
Cases have also been seen where the disease did not cross a basic wooden/wire partition in a house; with large numbers of birds affected in one half and apparently none on the other side, testing the boundaries of what we would expect with an infectious disease.
Where liver lesions are clear, post mortem examination is usually sufficient confirmation. Where caecal cores only are present, confirmation is best carried out with either histology or PCR testing. Histology involves looking at sections of the tissue, usually caecum, under a microscope for the presence of the protozoa and classic damage caused; however if the parasite is not found, it cannot 100% be ruled out. PCR testing looks for the DNA and has long been thought to be very non-specific, giving false positives; but newer, more specific, but equally sensitive versions of the PCR are now available, providing confirmation more quickly than histology, often within 24hrs of receiving the sample.
There are currently no licenced veterinary medicines for treatment of blackhead in the UK. For years, dimetronidazole (Emtryl), an antimicrobial with anti-protazoal properties was used with some success as a treatment and preventative; however this is now banned for use in the EU due to residue concerns. Parafor, another antimicrobial licenced in cattle has been trialled, but in our experience, once the disease is established in a group of turkeys, the product has limited or no effect. Where the disease is severe, depletion may also be an option.
More recent work in Ireland, Poland and other countries has found herbal oregano based products to potentially give us some hope. The practice has extensively used these products both in water and feed for broiler breeders and laying hens affected, with great success. Once used during an outbreak, mortality appears to reduce rapidly. We have now been able to apply this to turkeys under our care within the UK and early results suggest that herbal oregano based products may have a place in treatment and prevention for them too.
Prevention of blackhead involves an understanding of how the birds can be infected with Histamonas. There are ways of introducing blackhead into a house via poor biosecurity; particularly with brood and move systems. However in the majority of cases, it is likely the protazoa has remained in the environment for a number of years.
Where Heterakis eggs are present, they can host the protozoa and can survive dormant for years in the environment. If earthworms are involved, these can live for up to 12 years.
Regular worming of turkey flocks with flubendazole may be one way of reducing the risk; however Histamonas live inside the egg of the worm; so it is not necessary to have adult worms present for the turkeys to be exposed. Reducing earthworm exposure is impossible with free range birds; however in housed systems, making repairs to improve biosecurity, reduce water ingress, the use of lime around sheds and avoiding disturbing soil close to houses are ways of reducing this risk.
Historically it was thought that blackhead did not survive outside the host for more than a few hours. Cloacal drinking was the only other method felt to cause spread directly between birds in a house. More recent work has demonstrated the presence of the ‘naked’ Histamonas in areas around affected houses, in dust, on furniture, in water lines, even litter beetles and in high levels on catching modules. This is another advantage of the PCR test as it is has been shown to be useful in predicting houses which may still harbour the protozoa by taking environmental swabs. This is quite a novel approach and is used to identify areas which need particular cleaning and disinfection. The parasite itself is not difficult to kill so this can be a significant advantage when looking at control; but addressing litter beetle infestations and water hygiene are also important.
We hope going forwards that a move towards introducing herbal and essential oils in feed, as well as the ability to identify possible routes of infection on farms will lead to successful control of blackhead in the future.