This week we start a new series of articles about the diseases of birds. We will start with viruses and then progress to bacterial and fungal later in the month. In celebration of the next major holiday, we will cover turkeys and the threats to both our feathered friends and to their handlers. To begin our series will look at fowl pox, more specifically turkey pox. We begin with a short characterization of the virus, how it works in hosts, and the general routes of transmission. Then we progress to a short case study of turkey pox in Europe where it is becoming an endemic problem among breeders. Finally, we wrap up with a discussion of how our feathered friends help us in the wild (and on the plate.)
Pox viruses belong to two major families—Chordopoxvirinae, which infects vertebrates including mammals and birds, and Entomopoxvirinae, which infects invertebrates including beetles, butterflies, and flies. Both of these families share similar characteristics including large genomes, early RNA’s made in the virion core, and an internal envelope formed de novo, not during budding. Mature particles of Chordopoxvirinae attach to the target cell membrane glycosaminoglycans during the first uncoating stage and release enzymes ready to begin DNA replication (Acheson, 2011).
Pox viruses are unlike many other viruses because they replicate solely in the cytoplasm and therefore they must carry all of the genes coding for DNA replication proteins with them. These early genes code for proteins that break down the viral core and expose intermediate genes that code for DNA replication. As theses intermediate genes are activated by compound promoters they produce intermediate mRNA which code for intermediate proteins. These intermediate RNA’s are unique because they have 5’ terminal poly(A)heads added, facilitated by a TAAA sequence, which allows for a slippage mechanism adding the AAA head (Acheson, 2011). These proteins are created in viral factories and set the stage for late gene activation. The late genes code for structural proteins used in the encapsulation process. The process includes the packaging of completed DNA (incomplete viruses) and enzymes (mature viruses); the final step is dependent on the infection route of the virus.
If the host cell ruptures before the virus escapes then it is left with an extra protein layer and is called an extracellular virus, which can infect cells easier. If the virus is able to escape the cell then it sheds its protein shell during the budding process and is left with only its envelope. One difference between these viruses is their stability in the external host environment. These enveloped viruses are extremely stable in the environment and are found in the scabs and mucus of infected individuals (Acheson, 2011). Poxviruses have several means to evade host immune systems including TNF-binding proteins and soluble IFN-γ proteins which diminish inflammatory cytokine activity. Finally, the general routes of transmission include contact with abraded epithelium of mucosal membranes or skin and physical inoculation of epithelial tissues either by pecking or blood feeding arthropods (Kindt et al, 2007).
In 2010 a turkey farm in Austria experienced an outbreak of fowl pox in the cutaneous form (Hess et al, 2011). The farm had 11,680 birds spread over six flocks in stages ranging from polts to mature birds. The effect on the birds included “nodular red-brown wart-like growth” and encrusted lesions on the head and neck region. Samples were taken a plated on Columbia agar, McConkey agar, Schaedler agar, and Sabouraud-gentamicinchloramphenicol agar and included at 37 ͦC in aerobic conditions. Other tissue samples were used to isolate the virus using pathogen-free-Gallus gallus domesticus embryos. PCR DNA replication was performed by using tissue samples and fowl pox base pairs were isolated by gel electrophoresis. These isolates were reconstituted and sequenced with the original fragments and compared to the GenBank database using BlastN algorithm.
The results included antibiotic Staphylococcus aureus which is common on most skin surfaces but can become problematic when entered into the blood stream. The tissue isolates showed hyperplasia and hypertrophy as well as distended eosinophilic inclusion suggestive of excessive dissolved lipids. The GenBank search yielded 100% matches to avipox-AY530304 and turkeypox-DQ873808. Interestingly no lesions were reported on internal organs, however, no septicemia test was done so a blood infection could not be excluded.
In contrast to other outbreaks the morbidity rate was very low due to a lack of perceived aggressiveness among the birds. It was noted that a large number of flies were observed in heavy litter suggesting an initial route of exposure and a persistent route of infection to other individuals (Hess et al, 2011.) Both flies and mosquitoes are known transmitters of fowlpox (Larson et al, 2007).
In conclusion, there are many types of fowl pox ranging from pigeons and turkeys to ducks and chickens however it is generally assumed that the pox viruses that effect each are unique to that species. These pox viruses have commonalities among birds and generally affect the non-feathered regions of the neck, feet, and head. In many cases the disease is spread by pecking/scratching or by blood feeding arthropods and covers most of the southeastern part of the USA. In general, wild turkeys are less affected by the virus than domesticated turkeys, however that could be in part due to a lack of recorded data and the predation of sick individuals (Davidson and Doster).
No need to worry, though, as all farm raised stocks are vetted by the USDA and are disease free so the risk of contracting fowl pox by eating a farm raised turkey is very slim. So this Thanksgiving eat lots of turkey and remember the complex interactions that happened to get it to your plate.
Acheson, N. (2011). Poxviruses. In Fundamentals of Molecular Virology (2nd ed., pp. 312-323). Hoboken: John Wiley & Sons.
Davidson, W., & Doster, G. (n.d.). Avian Pox – A disease that can affect any bird. NWTF Wildlife Bulletin, 24:1-24:4.
Hess, C., Maegdefrau-Pollan, B., Bilic, I., Liebhart, D., Richter, S., Mitsch, P., & Hess, M. (2011). Outbreak of Cutaneous Form of Poxvirus on a Commercial Turkey Farm Caused by the Species Fowlpox. Avian Diseases, 714-718.
Kindt, T., Goldsby, R., & Osborne, B. (2007). Immune Effector Mechanisms. In Kuby Immunology (6th ed., p. 314). New York: W. H. Freeman and Company.
Larson, C., Beranger, J., Bender, M., & Schrider, D. (2007). Common Diseases and Ailments of Turkeys and Their Management. In How to Riase Hertigae Turkeys on Pasture (pp. 35-52). American Livestoock Breeds Conservancy.