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Oct 22, 2024

Wildlife Diseases: What is Poxvirus?

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What is poxvirus and why is it important to wildlife rehabilitators?

Poxvirus – what is this exactly, and why is it important to wildlife rehabilitators? The answer is complicated because it refers to 83 different species of viruses within the family Poxviridae that cause skin lesions. To make it easier, two subfamilies describe whether they affect vertebrates (Chordopoxvirinae) or invertebrates (Entomopoxvirinae), and the Poxviruses that only affect birds (Avipoxviruses) are further reduced to describe two common forms: cutaneous or diphtheritic pox, meaning “dry pox” and “wet pox.” Rehabilitators at DCHS more often see symptoms stemming from the cutaneous form versus the diphtheritic form, likely because wet pox leads to higher mortality rates. Dry pox can resolve on its own, but it can take up to two weeks to reduce.

Avipoxviruses are known to affect over 230 species of birds. In 2024, DCHS’s Wildlife Center treated over 50 individuals from 14 different species that had produced skin lesions suggestive of Poxvirus, which is higher than any other year to date (chart below). Poxviruses can spread quickly, and transmission is most likely to occur through direct contact with an infected host, shared resources like drinking water, or disease vectors such as mosquitos. The incubation period for avian pox can range from 4-10 days after contact, and cutaneous (skin) lesions are typically seen 5-8 days after that. Transmission rates are further amplified when the virus lives in suitable conditions (i.e., 55° to 75° F temperatures) and when distance between infected individuals is short.

Top Photo: Common Yellowthroat Warbler #24-3097 with a lesion on top of its bill. Above: Graph of Poxvirus cases at DCHS's Wildlife Center over 10 years.

How Does Disease Transmission Work in Practice?

Let’s work through a plausible example using a species that regularly breeds here in Wisconsin and was documented to have avian pox symptoms this year, both in the wild and from our own rehabilitation reports. Gray Catbirds come first to mind since they are one of the most abundant songbirds caught in mist nets during permitted banding activities at the Biocore Prairie Bird Observatory of the UW-Madison Lakeshore Nature Preserve. This area is a primary nesting location for catbirds after they migrate up from South America, and many parents have two separate broods during the summer. They average 1 to 5 eggs hatched per season, and the fledglings leave their nests between June and August. In 2024, dozens of catbirds were noted to have skin lesions suggestive of pox, many of which were first-year birds.

Young birds are more susceptible than adults when it comes to contracting and suffering from diseases like pox. Research suggests this is due to an underdeveloped immune system in youth that is not as robust until they have been exposed to and recovered from an illness. In the example using catbirds, with so many of them congregating together in one area through migration and nesting, then direct contact and transmission rates could theoretically increase. Additionally, mosquitos are known as the primary vector for transmitting avian pox, so a year where their population levels are highest may suggest further disease transfer between infected individuals.

Map of Gray Catbird prevalence from The Cornell Lab of Ornithology on July 5.

Just imagine all of these birds flying to a specific latitude at the same time when avian pox is prevalent in the environment. Then they end up occupying a small territory, sharing food and water sources, and having nests of babies – brooding (laying) on them, bringing them fruit, seeds, or insects, and fighting off other aggressive birds. Add other types of birds to the mix, such as ones who practice brood parasitism, or unexpected conditions like drought or habitat loss. All those factors together could be a recipe for disaster when we think about how many individuals may become infected and continue to spread it others, especially if that disease is zoonotic.

In rehabilitation, debilitated birds may either come in with symptoms of avian pox already present or they could have been exposed prior to admission. Sometimes, symptoms crop up well after a physical exam has occurred, or even by the time a bird is co-housed with other individuals. Once they’re co-housed, transmission rates can skyrocket, and it’s difficult to clean effectively, even with the right kinds of disinfectants and our best sanitation efforts. Viruses are also known to thrive better when an animal is experiencing stress, therefore lowering immune function, which is something we assume happens to all wildlife in rehabilitation. They suffer from chronic stress in care, despite going through recovery for that or something else, since they’re already not feeling well and are placed in an unnatural setting around scary predators (i.e., humans). This happened to many birds in our care this summer, particularly to our juvenile robins and doves, where identification, isolation, and treatment were key to getting them through their infectious periods.

Left: American Robin #24-1620 with early symptoms of cutaneous pox on digit #1 of the left foot (indicated by arrow). Right: House Finch #24-1925 with a suspected pox lesion on one toe (indicated by arrow).

Pox lesions are difficult to treat. For known cutaneous pox lesions (i.e., lesions that are actively shedding), we typically begin with 14-day quarantine period in an avian isolation cage. If possible, birds are individually housed in a quarantine avian cage, but you may consider housing a few birds together if they all display symptoms at the same time, were already housed together, and are moved to a quarantine cage together on the same day. We recheck lesions on weekly weigh days and apply betadine to help dry out the lesions if they are oozy.

For a bird with known exposure to an infected cage mate, but without active cutaneous pox lesions, we choose to remove the infected (shedding) individual and quarantine the remaining birds in their cage for 10 days. More simply, we start a quarantine procedure daily for 10 days listed as "Pox Watch" to ensure no other birds begin showing symptoms.

  • By day 10, if no lesions are visible, we discontinue the quarantine procedure.
  • Staff ensures that whatever cage the animal came from is thoroughly disinfected – with help from our wonderful volunteers and interns.
2024 Poxvirus cases admitted to DCHS's Wildlife Center, broken down by species and month.

The Long Road to Recovery

On May 22nd, 2024 DCHS’s Wildlife Center received a phone call and then photos of a songbird nestling reportedly in distress, as is typical for that time of year. A nest was knocked out of a tree during a recent storm, and while the original finders tried to put the nest back up, it kept falling down and babies kept falling out. After multiple attempts at re-uniting, our team of rehabilitators decided that this youngling would only survive if it was brought in for care, as there was an increasing risk that it could injure itself by repeatedly hitting the ground. In addition, the species was difficult for us to confirm without seeing it in person; it didn’t have many feathers or easily identifiable features, and it wouldn’t stay in the nest. Staff had a suspicion that the bird was a common species known to practice brood parasitism, such as a cowbird or house sparrow, since those birds may get “kicked out” from their nest on purpose for being… inconvenient, so to speak. At the time of admission, the bird was mistaken for a cardinal but later realized to be a Brown-headed Cowbird (#24-0818) since both birds look similar as hatchlings.

Brown-headed Cowbird #24-0818 recovering from symptoms of cutaneous pox around the beak and on its feet (indicated by the arrows). Photo credit: Wildlife Volunteer Lesley Haven.

Brood parasitism in birds is when an individual adopts the strategy to place their young (or lay their eggs) in a nest of completely unrelated individuals and/or species. For example, Brown-headed Cowbirds are known to parasitize over 220 species of North American birds, primarily targeted towards other Passerines (an order of birds), and yet their rate of post-fledgling success is less than 3 percent. Why? Because the babies don’t survive well once parents from the parasitized nest know that cowbird isn’t theirs. It might get pushed out and die from exposure or starvation, which probably occurs more often than people can document – hence why this little one may not have been successful in attempts to put him back in the nest. For others, they raise the cowbirds as one of their own.

Aside from being admitted for potential brood parasitism, this cowbird was also suffering from exposure to disease: avian pox. Lucky for him, he was given a safe space to grow up and recover until his successful release.

Jackie Sandberg is the Wildlife Program Manager at DCHS's Wildlife Center.

Brown-headed Cowbird #24-0818 molting formative feathers (left) and getting an assessment of a pox lesion on one toe (right).

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