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Fowl Pox Prevention

Fowl pox virus is a slow spreading viral disease of various avian species that causes skin lesions (dry pox) or diptheritic lesions (wet pox). Dry pox results in scabs and lesions around the comb, wattle, ear lobes, and eyes. Wet pox lesions are associated with the oral cavity and the upper respiratory tract, especially the larynx and trachea. Wet pox is more serious, results in higher mortality, and is a source of current industry problems. The course of the disease in individual birds is from 10 to 14 days and "on a flock basis" generally lasts 6 to 10 weeks.
 
There are many types of avian pox viruses and they tend to be specific to particular species of birds. All age groups are at risk and distribution of this disease is worldwide. The incidence of disease is variable depending on climate, management, hygiene, biosecurity, and use of a regular vaccination program.
 
History
Historically, outbreaks of wet pox have caused severe mortality losses in both vaccinated and non-vaccinated flocks. Field isolates from severe wet pox cases have been studied and some have been found to contain intact reticuloendotheliosis virus (REV) provirus or long terminal repeats of REV. Most of these field strains show a greater pathogenicity and induce an antibody response to both REV and fowl pox virus. Reticuloendotheliosis virus is associated with immunosuppression and, with integrated sequences in the genome of fowl pox virus, seems to play an important role in the pathogenesis and prolonged persistence of wet pox.
 
Wet pox alone can cause high mortality of up to 50 to 60% in unvaccinated chickens. This disease can start out as wet pox and spread to birds in the dry pox form and vice versa. It can be found causing both wet and dry pox at the same time. Infectious laryngotracheitis (ILT) can occur as a dual infection with wet pox.
 
Lesions
Dry pox
While dry pox generally does not increase mortality, a severe outbreak can cause drops in egg production up to 15%. A bird infected with dry pox will develop proliferative scabs on unfeathered areas of skin, and the flock will display a high level of morbidity. An infection in pullets will result in low mean body weight of the flock.

Dry pox lesions on comb, nostrils, beak, and wattles.

Wet Pox
Tracheas can appear reddened or hemorrhagic during certain stages of the disease, clinically similar to an ILT infection. The trachea wall is thickened with proliferative, inflamed, often patchy lesions on the interior surface. Mortality occurs when the lesion totally occludes the larynx or the upper part of the trachea.

Wet pox lesions can also be found in the mouth or conjunctiva of the eye. Other upper respiratory diseases such as ILT, viscerotropic velogenic Newcastle disease, avian influenza, Mycoplasma gallisepticum (MG), Mycoplasma synoviae (MS), coryza, and cholera may complicate an initial diagnosis of wet pox because of similar respiratory lesions.

Wet pox in trachea, patchy lesions.

 

Wet pox lesions in trachea, similar to infectious laryngotracheitis (ILT).

 


Wet pox in trachea, thickened wall with necrotic tissue.

 

Occlusion of the larynx; typical of wet pox or infectious laryngotracheitis (ILT).

Diagnosis
Dry pox
Dry pox can be identified visually by the characteristic scabs on the featherless areas of the bird. Histopathology may be required for conclusive diagnosis.
 
Wet pox
Diagnosis of wet pox can be complicated by similar appearing lesions of other respiratory diseases. The only conclusive way to confirm wet pox is by using histopathology on suspect lesion tissue fixed in formalin. The presence of eosinophilic intracytoplasmic inclusion bodies (Bollinger bodies) is diagnostic for pox virus infection.

Transmission
The virus contained in the scabs contaminates the environment and remains infective for many months. Mechanical transmission is considered the primary method for dissemination of the virus, and infection can occur through injured or lacerated skin. Mechanical vectors such as insects, can easily carry pox virus and may deposit the virus on susceptible birds. Flys may walk on the eyes of birds, leaving virus behind, and mosquito bites can result in rapid spread of pox throughout the farm. Airborne transmission is also suspected in many cases of wet pox.
 
The mucous membranes of both the trachea and mouth appear to be highly susceptible to the virus. The infection may occur in the absence of apparent trauma or injury. In a contaminated house, the infection can spread easily bird to bird, cage to cage, and by the standing water in drinking cups.
 
Prevention and control
Because there is no effective cure for fowl pox, prevention and control are vital for keeping flocks healthy. The following are some suggestions that will help decrease the impact of fowl pox on a flock:

  • Virus particles can be found in the environment and debris found in the poultry houses, thus dust control and disinfection of the environment are important.
  • An effective insect control program should be in place.
  • A biosecurity program to prevent the movement of equipment that could be contaminated with pox should be implemented.
  • Vaccination is practiced based on history of exposure—revaccination, if necessary, can be done in the face of an outbreak because pox infections are usually slow spreading.
  • In the event of an outbreak, liquid iodine disinfectant (used for disinfecting water lines) added to the water appears to aid in reducing mortality.
    • Create a stock solution by adding 30 to 45 mL/L (4 to 6 oz/gallon) iodine disinfectant to water.
    • Add the stock solution to the water line through a medicator at a concentration of 8 mL/L (1 oz/gallon) drinking water.
    • Spray or fog the house with a disinfectant to reduce exposure.

Vaccination
Vaccination should be completed prior to expected exposure to the fowl pox virus. Areas that have mosquitoes throughout the year often use 2 vaccinations, one early and one later for "permanent" protection.
 
Guidelines for pox vaccination

  • Chicks can be vaccinated as early as 1 day of age. Tissue culture origin (TCO) vaccines (at ¼ to ½ dose per chick) can be used at hatch, either alone by single needle wing-web, or in combination with Marek's disease vaccine. This type of vaccination does not protect for the life of the bird, but should protect adequately until the second vaccination is given.
  • For permanent protection, birds need to be vaccinated after 8 to 10 weeks of age with a chick embryo origin (CEO) vaccine.
  • Use the 2-prong needle applicators supplied with the vaccine. This allows the skin to be broken and exposed to vaccine virus 2 times and delivers a full dose of vaccine.
  • Check "takes" (i.e., a small swelling or scab at the inoculation site) 6 days post-vaccination. Vaccination "takes" should be seen in 99 to 100% of vaccinated pullets.
  • Check to make sure the correct quantity of vaccine is used and recorded for each flock.

Vaccine reaction or "take" about 6 days post-vaccination.

In addition to the above rules for pox, there are additional steps that may need to be taken during a wet pox outbreak.

  • In high challenge areas, birds may need 2 vaccinations in the pullet stage; an early vaccination at 3 to 6 weeks of age and a second at 8 to 14 weeks of age. Additional vaccinations can be added, depending on the degree and time of challenge.
  • Pigeon pox vaccine appears to provide better cross protection to some wet pox field strains. The combination of fowl pox and pigeon pox stimulates a broader spectrum immune response needed for optimum protection. Use the combination of pigeon pox and fowl pox vaccine at 1.25 doses of each per bird. These vaccines can be mixed together and given in one application by wing-web.
  • A better vaccination response occurs by breaking the skin at four places. The skin can be punctured four times with one application by gluing 2 of the vaccine applicators together. Additional vaccine diluent is required to provide 1.25 doses/bird. The quantity of vaccine used per flock should be checked and recorded.
  • In flocks that receive multiple pox vaccinations or a day of age vaccination, the percentage of takes or degree of vaccine reaction from subsequent vaccinations will be lower than 99 to 100% because some birds will still be previously protected and not respond to the vaccine. "Takes" should still be checked and recorded after each vaccination so a farm history can be developed.
  • There is no routine serological test for determining pox immunity, but one way to check immunity is to take 200 to 300 birds at 18 to 20 weeks of age that have been previously vaccinated and revaccinate them with a full dose of fowl pox. At 6 days post-vaccination, check for "takes." We should expect 99 to 100% of these birds to show no "takes." A "take" at this time means they were not previously protected and were susceptible to challenge. Flocks at this age (under severe challenge) that do not show at least 95% protection may need to be revaccinated.

Conclusion
Dry and wet pox challenges are always present and generally well controlled with a standard vaccination program, but you should monitor your pox control by constantly reviewing:

  • changes in the challenge for the farm or surrounding area,
  • understanding if the vaccines cover the current pox challenge,
  • checking on administration, technique, handling, and storage of vaccine,
  • recording "takes" and quantity of vaccine used on each flock,
  • timing the vaccine administration for early pullet protection and for permanent protection, and
  • implementing biosecurity practices to prevent introduction or spread of a pox challenge.

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