Pigeon pox is a common, contagious, and highly infectious viral disease in pigeons.

1. Pathogen

Pigeon pox is caused by the Pigeon pox virus (PPV), a member of the Avipoxvirus genus within the Poxviridae family. It is one of the largest known animal viruses, measuring 250-350 nm. Many avipoxviruses exhibit strong host specificity, typically causing disease in their natural hosts but not, or only mildly, in others. However, some can infect heterologous hosts under experimental conditions. Fowlpox virus (FPV), the type species of the genus, is common, highly pathogenic, and can infect various birds, including pigeons, quails, turkeys, and sparrows. PPV is highly pathogenic for pigeons but causes only mild infection in chickens and turkeys, and does not infect other poultry. Due to significant antigenic similarity between PPV and FPV, and its strong immunogenicity in chickens and turkeys, PPV can be used as a seed virus to develop vaccines for fowlpox and turkeypox.

The virus is highly resistant to environmental factors, surviving for weeks under direct sunlight and showing notable resistance to desiccation, remaining viable in dried scabs for months or even years. It can be preserved for several years by lyophilization or in 50% glycerol-saline, and retains pathogenicity after storage at -15℃ for many years. It is resistant to ether and can withstand 1% phenol and 0.1% formaldehyde for up to 9 days. However, it is sensitive to chloroform. The virus typically survives for no more than a few weeks in pigeon droppings and soil and is quickly inactivated in decaying organic matter. Common disinfectants like 1% sodium hydroxide, 1% acetic acid, or 0.1% mercuric chloride kill the virus within 5-10 minutes. Formalin fumigation inactivates it within 1.5 hours, as does heating at 50°C for 30 minutes or 60°C for 8 minutes.

2. Epidemiology

Pigeons of all breeds, ages, and genders are susceptible. Nestlings with little feather cover and low resistance, if never exposed, are the most susceptible, with morbidity rates exceeding 95% and mortality reaching 10%-40%. Young pigeons are moderately susceptible, with morbidity around 50%. Adult birds have lower morbidity, and those previously exposed or recovered often acquire lifelong immunity. Once introduced, the disease can persist on a farm for years, easily causing outbreaks and posing a serious threat to unvaccinated pigeons.

Transmission primarily occurs through direct contact with infected birds. Shed scabs are a major source of virus spread. The virus enters through damaged skin or mucous membranes, commonly around the head and cere, often facilitated by abrasions from cages. PPV is a mosquito-borne arbovirus. Blood-sucking insects like Culex and Anopheles mosquitoes, and ectoparasites like lice and mites, are vectors. Mosquitoes are particularly important, remaining infective for 10-30 days after feeding on lesions and easily transmitting the virus during bites, which is the main route of spread in summer and autumn. Due to its resistance to drying and prolonged environmental survival (viability for months), human activity, fomites, and vehicles can also spread the virus. Besides direct contact, secretions from the digestive tract of pigeons with the diphtheritic form contain large amounts of virus, allowing indirect transmission via contaminated water, feed, and equipment.

Disease occurrence is closely linked to mosquito bites, which correlate strongly with temperature. Mosquitoes rarely bite below 17°C, are active biters above 27°C, and bite rapidly above 37°C, giving the disease a distinct seasonal pattern. Specifically, the potential risk period is from March to October in southern China and May to August in northern China. However, cases can occur year-round: the cutaneous form is more common in summer and autumn, while the diphtheritic form is more frequent in other seasons.

3. Clinical Signs and Pathological Changes

Pigeon pox manifests in three forms: cutaneous (skin), diphtheritic (mucous membrane), and mixed. The cutaneous form is most frequent. The natural incubation period is generally 4-10 days, sometimes up to 2 weeks. The course typically lasts 3-4 weeks but can be prolonged with co-infections. Recovery is usually gradual, with the cutaneous form having a better prognosis than the diphtheritic form.

(1)  Cutaneous Form: Systemic signs are often absent unless the infection is severe or the bird is debilitated, in which case lethargy, anorexia, weight loss, and stunted growth may occur. Lesions on the eyelids are particularly impactful, causing photophobia, lacrimation, conjunctivitis, glued eyelids, even blindness, leading to starvation and death. Laying hens experience reduced or ceased egg production.

    Lesions: Appear on unfeathered skin – eyelids, cere, beak, legs, feet, and around the vent. Small, greyish-white pockmarks develop, rapidly enlarging to form pea-sized, grey or grey-yellow nodules. These nodules are hard, dry, and have a roughened surface. They can be numerous, often coalescing into large crusts. Scabs fall off after 3-4 weeks, leaving grey-white scars.

(2)  Diphtheritic Form (Pigeon 'Diphtheria'): Lesions affect the mucous membranes of the nares, mouth corners, oral cavity, pharynx, and esophagus. Initial signs are subtle. As it progresses, birds become depressed, anorexic, and discharge serous mucus from eyes and nostrils, later turning yellowish and purulent. If the infraorbital sinuses and conjunctiva are involved, eyelids swell with purulent or fibrinous exudate. A few days after rhinitis, pockmarks appear on the beak, mouth, and pharynx membranes – initially small, round, yellowish nodules that coalesce into a yellowish-white, caseous, fibrino-necrotic pseudomembrane covering the mucosa. This membrane, composed of necrotic tissue and inflammatory exudate, is difficult to remove, has a foul odor, and reveals a hemorrhagic ulcer if ripped off.

(3)  Mixed Form: A combination of cutaneous and diphtheritic forms. This is common clinically, often more severe than either form alone, and carries a poorer prognosis. Birds show pronounced clinical signs and exhibit lesions characteristic of both forms.

4. Diagnosis

A preliminary diagnosis is based on the presence of typical crusty skin lesions on unfeathered areas (eyelids, beak, cere, legs, feet) or pockmarks/pseudomembranes on oral/esophageal mucosa, combined with seasonal patterns (cutaneous in summer/autumn, diphtheritic in winter).

Confirmatory diagnosis requires:

Virus isolation and identification.

Histopathological examination of tissue sections revealing large eosinophilic intracytoplasmic inclusion bodies (Bollinger bodies) and elementary bodies (Borrel bodies) in infected epithelial cells.

Serological/immunological methods like immunodiffusion, indirect fluorescent antibody test, ELISA, or PCR.

Differential diagnoses include trombiculosis (mite infestation), candidiasis, trichomoniasis, and Vitamin A deficiency.

5. Prevention and Control Measures

(1)  Biosecurity and Hygiene: Improve management and environment. Maintain strict sanitation and disinfection in the loft. Avoid overcrowding. House different ages and breeds separately. Provide balanced feed and fresh grit to enhance natural resistance.

(2)  Prevent Injuries: Avoid cannibalism and mechanical injuries. Do not mix chickens and pigeons. Quarantine new arrivals for at least 30 days before introducing them to the flock.

(3)  Mosquito Control: Eradicate mosquitoes and other biting insects in and around the loft during summer and autumn using insecticides like 0.03% pyrethrins, 0.01% deltamethrin, 0.01% fenvalerate, or 0.06% coumaphos. Electronic mosquito zappers in the loft can be effective. Traditional Chinese herbal remedies are also used:

    Formula 1: Helicteres angustifolia (500g), Houttuynia cordata (500g), Emilia sonchifolia (500g) boiled in 2.5L water until reduced by half, then diluted 1:5 for drinking.

    Formula 2: Lonicera japonica (50g), Chrysanthemum indicum (100g), Taraxacum mongolicum (50g), Viola philippica (100g), Gynura bicolor (30g), Scutellaria baicalensis (30g), Commiphora myrrha (10g), Boswellia carterii (10g), Forsythia suspensa (50g). Decoction boiled three times; mixtures combined and diluted 1:10 for 1000 pigeons.

(4)  Vaccination: The most effective prevention is vaccination. As no commercial pigeon pox vaccine exists in China, Fowlpox live vaccine (quail-adapted attenuated strain) is commonly used, providing cross-protection. Vaccinate before the high-risk season (or anytime in tropical areas). On breeder farms or farms with a history of pox, vaccinate pigeons of all ages, including squabs, before the expected age of susceptibility.

Method: Wing-web stabbing is standard; intramuscular or feather follicle inoculation also works well; oral administration is unreliable.

Procedure: Disinfect the stab site with 75% ethanol (not iodine). A successful 'take' shows as swelling and scab formation at the site 7-10 days post-vaccination, with the scab falling off in 2-3 weeks. Check all birds 7-10 days post-vaccination; re-vaccinate non-reactors.

Immunity: Vaccination at >3 weeks of age protects for over 9 months. Breeders should be re-vaccinated before laying.

Treatment of Affected Birds: Isolate sick birds promptly. Treat if possible; cull and dispose of severely affected birds (burying or burning). Implement emergency vaccination for healthy birds. Increase disinfection frequency in and around the loft.

   Cutaneous Form: Carefully remove scabs and apply topical disinfectants (e.g., 1% iodine tincture, 0.5% mercurochrome, 0.02% KMnO4 solution, 1% PVP-I solution). Provide 0.1% crystal violet in drinking water. Add antibiotics to feed/water to prevent secondary bacterial infections. Due to high environmental resistance of the virus in scabs, recovered birds should remain isolated for 2 months after full recovery before reintegrating.

   Diphtheritic Form: In early stages, use gentamicin eye drops and provide 0.4% Moroxydine HCl in drinking water. For oral lesions, carefully remove pseudomembranes with forceps, rinse with 0.01% KMnO4 or 0.2% PVP-I solution, and apply iodine glycerin or Bing Peng San powder. Add antibiotics (e.g., 0.2% Amoxicillin) to feed to prevent secondary infection. Supplement feed with 3-5 times the standard dose of multivitamins to reduce stress, improve tolerance, and lower mortality.