unusual pox

Unusual Fowl Pox Infections

A.A. Bickford, B.R. Charlton, G.L Cooper

California Animal Health and Food Safety Laboratory

Turlock, California

Introduction

Fowl pox is a fairly frequent and readily diagnosed disease in both commercial and backyard poultry in our area. The classical features include scab-covered warty proliferative lesions in the unfeathered skin of the head or neck (dry form) and proliferative plaques or diffuse diphtheritic lesions in the epithelium lining the upper respiratory or digestive tract (wet form) in unvaccinated birds of any age (but generally less then 6 months). The disease is seasonal to some degree correlating to time periods when mosquitos - the usual vector - are most active. Diagnostic challenges arise when there are variations in the classical expressions of the disease including signs, lesions, course, age of occurrence, etc. Other noteworthy exceptions to the norm involve occurrence of pox in vaccinated flocks or demonstration of unusual biological vectors or unique circumstances of mechanical transmission. This report documents several unusual pox infections observed in chickens and turkeys at the CAHFS Laboratory in Turlock, California.

Unusual Pox Cases

1) The Sticktight Flea as a Potential Vector.

This case involved adult mixed-breed chickens with extreme diffuse pox lesions over the head as well as a heavy infestation of sticktight fleas. Typical proliferative pox lesions were confirmed histologically and mouth parts of the fleas were noted along with the pox lesions. This case has been reported (Avian Diseases, 41:1006-1009).

2) Unusual Location of Pox Lesions in Mite - Infested Vaccinated Layers.

In four cases of leghorn chickens (aged 32,34,40 and 91 weeks) pox lesions occurred predominantly in abdominal skin especially ventral to the vent. These layer chickens had heavy northern fowl mite infestation and red mites were noted on the affected areas of skin in two cases. These flocks had been vaccinated for pox at the usual age range (8-12 wks).

3) Concomitant Infectious Laryngotracheitis (ILT) and Respiratory Pox in Leghorn Pullets.

We have had three cases of severe respiratory disease in Leghorn chickens (involving conjunctiva, sinuses and tracheas) which proved to be dual infections with ILT and respiratory pox. These flocks (aged 9 weeks, 11 weeks and 16 weeks) had been vaccinated for both ILT and pox and the respiratory problems likely represented severe concomitant reactions. Diagnosis was based on typical histopathologic features and inclusions of both infections. Dual infections with these two viruses have been reported (Fatunmbi et al, Avian Diseases 39: 925-930).

4) Tracheal Pox in Unvaccinated Backyard Chickens.

Disseminated hyperplastic plaques in the tracheal mucosa were noted in two 6-month-old Minorca chickens. Ten of 40 birds in this flock had died and no skin lesions were present in the examined birds. No basis was found for the preferential respiratory involvement in these birds.

5) Respiratory Wet Pox Without Skin Lesions in Vaccinated Layers.

Numerous cases in this category have been seen at our Laboratory (6 cases in birds aged 17 to 34 weeks in the past year). Mortality is variable but can exceed 10% over a period of several weeks. This type of pox infection tends to occur in closed (environmental) houses while open houses on the same farm are unaffected. Affected birds are also often infected with Mycoplasma (5 of the 6 flocks this year were strongly seropositive) or other respiratory infections.

6) Predominantly Necrotizing and Exudative Focal Alimentary Pox in Turkeys.

In the late 1960's poxvirus was isolated from 11-week-old tom turkeys with diphtheritic plaques in the mouth and pedunculated fibrinonecrotic lesions in the lower esophagus and crop. Over a 2-week period mortality in the flock was over 10% and the major necropsy findings were emaciation and dehydration. Histologically the dominant change in the alimentary lesions was necrosis and no significant epithelial hyperplasia or pox inclusions were seen. Inoculums prepared from the alimentary lesions reproduced necrotizing lesions in exposed feather follicles of pox-susceptible chickens and in the chorioallantoic membranes of inoculated embryonated eggs. This case was reported (Avian Diseases. 15: 614-625). An almost identical case was seen in 10-week-old tom turkeys submitted to the Turlock Laboratory in 1986.

Dr. Bruce Charlton recently had a case with diphtheritic lesions in the cloacal mucosa contiguous with scabby proliferative lesions affecting the Phallic ridges of the ventral vent. Some birds in the breeder flock had pox lesions affecting the skin of the head/neck and apparently pox had been mechanically spread during handling for semen collection.

1) Faulty Vaccination. Any occurrence of pox lesions in vaccinated poultry raises questions of adequacy of the vaccine or vaccination procedure. In many of our unusual cases it appeared that vaccine had been well handled and applied but this is difficult to ascertain after the fact.

2) Variant Field Pox Viruses. There is evidence of considerable antigen variation among avian pox viruses but the technology for antigenic typing of pox virus is not readily accessible. We have sent some isolates to Dr. Reed at Michigan State University and one of the 6 wet pox cases in vaccinated layers was definitely established as a variant strain. Other isolates have been submitted for typing.

3) Activation of Latent Poxvirus. This is a possible explanation for the occurrence of pox lesions in vaccinated or previously infected birds that have been subjected to extreme stress. It makes a great hypothesis but may be very tough to prove.

This has long been suspected in cases of respiratory pox in vaccinated birds that have no skin lesions. Infections with Mycoplasma, Ornithrobacterium, Hemophilus, etc. may in some way predispose the respiratory epithelium to poxvirus infection.