Fowlpox

WALTER J. HALL.

FOWLPOX, also called avianpox, birdpox, avian diphtheria (no relation to human diphtheria), sorehead, and canker, is a widespread disease.

The filterable virus that causes it occurs as inclusion bodies in the cells of the skin or mucous membranes. These bodies may be viewed by sectioning (slicing) the skin cells, staining them, and observing them under a powerful microscope.

One inclusion body may contain as many as 20 thousand elementary bodies, each of which can cause fowlpox. These bodies are thought to be the real virus carriers, and, when liberated from the inclusion body and stained, can be seen under the microscope.

The late J. R. Beach, after research at the University of California, in 1939 classified avianpox as fowlpox, turkeypox, pigeonpox, and canarypox.

Fowlpox and turkeypox cross-immunize; that is, fowlpox will immunize both chickens and turkeys, although some strains differ.

The lesions tissue changes produced by an attack of fowlpox occur on the unfeathered parts of the body, comb, wattles, face, and eyelids and sometimes on the feet and around the cloaca. Lesions in turkeys may occur on the breast.

Pock lesions begin as a small, round blister, which contains a serous--watery fluid. As the pock, or pimple, enlarges, the center becomes yellowish from an accumulation of virus and pus. Later the pock ulcerates, ruptures, and discharges a sticky fluid. The ulcer dries in a few days, and a dark-red or black scab forms over the raw lesion. The scab sticks for some time before it drops off. The pocks develop at different times. Usually it takes 3 to 4 weeks before all the scabs have been shed and the lesions are completely healed. Several pocks may coalesce to form a large lesion.

An internal form of pox usually is seen in the pharynx and sometimes in the nasal passages as a thick, Gaseous membrane, which sticks firmly to the mucous membranes. The diphtheritic membrane may restrict the air intake and cause suffocation if it extends down around the glottis. The false membrane may build up around the tongue and interfere with swallowing so as to cause emaciation or death.

Fowlpox may be diagnosed by the nature of the skin lesions; by microscopic examination of the lesions for inclusion bodies and elementary bodies; and by inoculation of immune and susceptible birds.

The incubation period of fowlpox is 1 to 2 weeks. The disease may appear at any time but is more common in fall and winter. The cutaneous form is more common in summer. The diphtheritic, or internal, form occurs oftener in winter, and mortality usually is higher.

If there are no complications, the disease usually lasts 30 days from exposure until the scabs drop off. The percentage of recovery is high if no complications ensue.

Fowlpox is transmitted by contact of infected or recently vaccinated birds with susceptible birds and by mosquitoes, which may carry the virus on the proboscis from infected to susceptible flocks. Mosquitoes are said to be able to infect susceptible fowls for 2 months after a single feeding on an infected bird.

A good preventive (there being no cure) is a prophylactic vaccine that gives lifelong immunity to chickens. The vaccines against pox most commonly used are fowlpox and pigeonpox and are propagated on chicken embryos. They are live-virus vaccines and must be handled with care to prevent spread of disease to unvaccinated birds.

To prepare the avianpox vaccine, the infected membranes of the embryo are collected, dried, ground to a fine powder, sealed under vacuum in ampules, and stored under refrigeration. A vial of diluent for reconstituting the vaccine is packaged with the dried virus. Only enough vaccine for a day's use should be taken out of the refrigerator at one time and reconstituted. Care should be taken to avoid spilling the vaccine on the poultry premises during this operation. Empty vials should be burned or disinfected.

FOWLPDX VACCINE is used to vaccinate chickens, turkeys, and pheasants.

It should not be used to vaccinate pigeons or hens that are laying.

Pigeonpox vaccine may be used to vaccinate chickens, pigeons, and pheasants, but not turkeys. It may be used on chickens in lay and on chickens that are in a debilitated condition from malnutrition or an intercurrent disease. In such circumstances, pigeonpox gives the protection of a temporary immunity for 2 or 3 months without a systemic reaction and without interfering with egg production.

Only canarypox vaccine should be used on canaries.

Chickens may be vaccinated against pox in two ways.

In the stick method, two needles (about one-fourth inch apart in a cork or other holder) are dipped into the vaccine container and then applied by puncturing the wing web. This gives four cutaneous inoculations simultaneously, two on each side of the wing web, since the needles pierce both layers of the skin. This method is fast, accurate, and economical of vaccine.

In the older follicle method, four or five feathers are plucked (usually from the upper lateral surface of the thigh) and the vaccine is applied to the feather follicles with a stiff bristle brush. The vaccinated birds should be checked for takes a week to 10 days: after vaccination. A take is indicated by swelling and redness of the vaccinated follicles or the stick wounds.

The flock should be revaccinated if most birds do not show takes.

The recommended age for vaccinating chickens is 6 to 12 weeks.

Vaccination of baby chicks has been tried but is not generally satisfactory because systemic reactions may slow growth rates. Mortality may be high.

Chickens should not be vaccinated within 4 weeks of the beginning of lay. Turkeys may be vaccinated at any age, except that they should not be vaccinated within 8 weeks of marketing in order to give time for the scabs and vaccination lesions to disappear.

Turkeys should be vaccinated only by the follicle method and on the upper thigh. When the stick method is used, turkeys put their heads under the wing and spread the disease to the head and other parts. Generalized cases of fowlpox are the result. Fowlpox vaccine gives turkeys an immunity that lasts about 6 months. Turkeys that are kept for breeders should be revaccinated before winter.

Chickens do not need to be vaccinated against fowlpox unless pox is prevalent in the neighborhood, the flock is likely to be exposed to the disease, or the flock had the disease the previous year.

After pox vaccine (a live virus) is used on a place, vaccination every year thereafter is necessary.

All susceptible fowls on the premises, including chickens, turkeys, guineas, and pheasants, should be vaccinated at the same time or else segregated.

WALTER J. HALL is a graduate of the New York State College of Veterinary Medicine. He joined the Department in 1924.