A menagerie of zoonoses

By Emily Margosian, Assistant Editor, December 1, 2022

From the Black Death to Avian Flu, throughout history, humans have been plagued by epidemics of infectious disease that originated from animals. Zoonoses represent a major public health problem around the world due to our close relationship with animals — those used in agriculture, as companions, or those found in the natural environment.

As evidenced by the recent COVID-19 pandemic, which itself is a suspected instance of zoonotic spillover, today’s highly globalized world allows for the rapid spread of infectious disease at unprecedented historical rates. Other factors, such as increased interactions with animals due to habitat loss, changed migration patterns from climate change, and the potential creation of animal reservoirs of disease outside their normal habitats, are also cause for concern.

This month, DermWorld discusses common or emerging zoonotic diseases with skin involvement, and what dermatologists should know regarding transmission, at-risk populations, identification, and treatment.

Monkeypox

Monkeypox has recently shot to the forefront of public health concern. In May 2022, the first United States case was documented in Massachusetts. As of reporting time, over 26,000 confirmed cases have been reported in all 50 states and the District of Columbia, with confirmed cases in 103 countries worldwide. The current majority of U.S. cases are among gay, bisexual, and other men who have sex with men, with women and children making up less than 2% of cases according to CDC estimates.

The primary mode of transmission in the current outbreak is person-to-person, through close contact of skin lesions carrying high viral loads, according to Misha Rosenbach, MD, FAAD, associate professor of dermatology at the Hospital of the University of Pennsylvania. “While there is some evidence of virus present on surfaces, close contact, including sexual contact, appears to be the greatest transmission risk.”

Animal carriers can also harbor monkeypox, including squirrels, rats, mice, prairie dogs, and small forest mammals. “Monkeypox as a name is a little bit of a misnomer. The reason it was called monkeypox is because the first identification of it was in a batch of lab monkeys, but it’s thought that endemic transmission is more likely from rodents,” explained Esther Freeman, MD, PhD, FAAD, director of global health dermatology at Massachusetts General Hospital and Harvard Medical School.

“The reason why we worry about zoonotic transmission is because it can lead to a massive human outbreak. That’s what we’re dealing with right now with monkeypox. It’s the end result of a chain that probably started with zoonotic transmission, and that’s why these zoonotic diseases matter.”

A prior outbreak of monkeypox in the United States was linked to exotic pets in 2003, raising concerns about potential animal reservoirs. “The previous U.S. outbreak was traced to pet prairie dogs that had been transported near other imported small mammals from Ghana. Notably, the current outbreak has included isolated reports of household pets getting infected — in this case, likely from their human owners,” said Dr. Rosenbach. “I want to emphasize that the current outbreak is being transmitted primarily human-to-human; any discussion of zoonotic transmission may be a distractor, but it is worth keeping in mind for the future because it touches on an important risk, namely, if humans transmit monkeypox to their pets, which then transmit it to native species, there is the opportunity for monkeypox to establish itself in animal reservoirs and continue circulating, which could lead to intermittent outbreaks.”

Dr. Freeman agrees that while the current outbreak is unlikely to be driven by any zoonotic transmission, monkeypox poses an important lesson about the consequences of zoonotic spillover events. “The reason why we worry about zoonotic transmission is because it can lead to a massive human outbreak. That’s what we’re dealing with right now with monkeypox. It’s the end result of a chain that probably started with zoonotic transmission, and that’s why these zoonotic diseases matter.”

Currently, the exact mode of zoonotic transmission of monkeypox to humans is unknown. However, prior outbreaks suggest that infections can occur from touching a sick animal, being bitten or scratched, and cleaning the cage or touching the bedding of a sick animal. “The current outbreak is human-to-human. But certainly, other times in the past where monkeypox has been transmitted, it’s been through either close handling of an animal or even animal bites,” said Dr. Freeman.

Cutaneous symptoms of monkeypox typically involve a rash that develops within three to 17 days, appearing as macules, developing into papules, and ultimately hard pustules that may result in scarring. Lesions often first appear in the mouth, on the genitals, or on the face, followed by lesions on the hands, feet, and trunk. “With the current outbreak, I think dermatologists need to be aware that it can look a little different from what we’ve historically seen in pictures of monkeypox that we might have been trained on,” said Dr. Freeman. “A significant portion of the monkeypox lesions we’re seeing are starting in the groin or the buttock region, which is not traditionally what we had understood about monkeypox outbreaks and is probably due to the mode of transmission being close contact of infected lesions.

Dermatologists should have a high index of suspicion for a new rash, particularly a pustular rash that might be in the groin but could potentially occur elsewhere in the body. I also think it’s helpful to take a sexual history to understand if there are any new partners or any other close sexual contact that could have potentially been a risk factor.”

Ebola

Considered to be among one of the most virulent human pathogens, with a fatality rate of reportedly 90% in some outbreaks, the Ebola virus was cause for global concern in 2014 during the West African Ebola epidemic, which was ultimately declared a Public Health Emergency of International Concern (PHEIC) by the World Health Organization (WHO). Although the outbreak was declared over in June 2016, Ebola has recently crept back into the public health spotlight due to recent activity in central and east Africa in addition to concerns about the high transmissibility of the disease.

While Ebola is mostly spread from person-to-person, fruit bats are suspected as natural Ebola virus hosts, and chimpanzees, gorillas, monkeys, forest antelope, and porcupines have also been associated (doi: 10.5070/D3213025964).

“A big issue with Ebola is that we don’t always know the vector of disease. We just know that it’s rapidly transmissible between people and animals,” said Colin Blattner, DO, FAAD. “Saliva and blood products are what we mostly worry about, although there are different hypotheses.”

At-risk populations are currently limited to Africa and global health workers due to ongoing outbreaks in Uganda and the Democratic Republic of the Congo. However, given lessons learned from the recent COVID-19 pandemic and the rapid spread of infectious disease via animal reservoirs, physicians and public health officials should remain cautious and alert. “It’s something to keep under consideration. You’d hope we’re not importing infected fruit bats or chimpanzees, but by the same token, now with the globalization of the world, it’s very easy for someone who is infected to get on a plane and come transmit it to someone in the United States, Europe, or Asia,” said Dr. Blattner. “It wouldn’t be unimaginable, and is similar to how the coronavirus spreads, given that Ebola is a very highly transmissible disease.”

Skin involvement typically includes a maculopapular rash that appears between day four and day six of disease. Other cutaneous manifestations of Ebola can include purpura, petechiae, and mucosal lesions that affect the eyes, mouth, and pharynx (doi: 10.5070/D3213025964). Treatment for Ebola mainly consists of supportive care. In late 2020, two monoclonal antibodies were approved by the FDA for the treatment of Ebola, and an FDA-approved, two-dose vaccine for the prevention of Ebola is also available. Public health officials recommend reducing contact with wild or imported animals and avoiding the consumption of their raw meat to avoid risk of infection.

Fungal infections

Tinea (ringworm)

A common fungal infection of the skin and nails, ringworm can be contracted after contact with dermatophytes from an infected person, environment, or animal. Dermatophytes are highly specialized keratinophilic and keratinolytic fungi that consist of seven genera (Infect Drug Resist. 2018; 11: 751–760). Zoonotic transmission of ringworm most commonly occurs between cats and humans. However, other potential carriers include dogs, guinea pigs, cows, goats, pigs, and horses according to the CDC.

People who have frequent close contact with animals, such as pet owners, pet breeders, or those working with livestock, may be more likely to encounter the fungi that causes ringworm. Patients may also want to exercise caution at petting zoos and similar attractions. Zoonotic transmission can occur through direct contact with an infected animal, or through contact with objects that have touched an infected animal (bedding, grooming articles, saddles, carpeting, etc.).

Skin symptoms can include hair loss, a scaly scalp, itching, and a ring-shaped rash, with potential to infect the feet, groin, scalp, beard, hands, nails, arms, or legs. A recent JAMA Dermatology report documented a case of a six-year-old boy who presented with a one-month history of itchy facial lesions that had not resolved after two weeks of treatment with topical corticosteroids (doi:10.1001/jamadermatol.2021.2566). A patient history revealed that the patient had been in contact with three pet guinea pigs with active tinea infection. The patient was prescribed oral terbinafine, 125 mg daily, and ketoconazole 2% shampoo, and saw complete resolution of the plaques after six weeks.

Ringworm is typically resolved with topical or oral antifungals, and in cases of suspected zoonotic infection, household pets should be taken to a veterinarian for treatment in addition to a thorough cleaning of the home.

Majocchi’s granuloma

Majocchi’s granuloma (MG) is a rare fungal infection of the skin that occurs when a dermatophyte invades into the dermis, because of a lack of host immune defenses, prolonged application of topical steroids to a tinea infection, or a penetrating injury.

MG commonly occurs in the presence of chronic dermatophytoses, such as tinea unguium and tinea corporis. However, documented cases suggest that animal exposure can also be a predisposing factor of MG. “Guinea pigs are a common cause of tinea infection; they can be cryptogenic carriers, meaning that they do not have any signs of infection themselves,” explained Camille Introcaso, MD, FAAD. “However, any mammal — dog, cat, rabbit, rat, hamster, or other small rodents — can carry a dermatophyte, which can be transmitted to humans and either stay as a tinea or progress to MG in the correct circumstances.”

Dr. Introcaso recently observed a case of zoonotic MG in which a 32-year-old woman presented with two months of tender, pruritic lesions at the vulva, and weeks of lesions on the shins that had not resolved after treatment with topical corticosteroids and intravenous vancomycin. A patient history revealed that the patient acquired a pet guinea pig with rash and hair loss shortly before her symptoms began. The patient improved after treatment with ketoconazole cream and terbinafine, 250 mg daily, and the guinea pig was successfully treated by a veterinarian. Dr. Introcaso’s paper has been accepted for publication in the journal SKINMed.

While MG is not typically caused by a zoonosis, “Dermatologists absolutely should ask about household pets or other animal contact when evaluating a patient with dermatitis,” Dr. Introcaso recommended.

Mites

Sarcoptic mange (canine scabies)

While most are familiar with the mental image of a “mangy dog,” the mites that cause mange in dogs can also have negative consequences for human hosts. Sarcoptic mange, also known as canine scabies, is most common in dogs, although cats can also be carriers. Wild canids such as foxes, coyotes, and wolves can also be afflicted.

Although sarcoptic mites are not able to complete their life cycle on humans, they can cause severe itching until they die, often accompanied by a papulovesicular rash. Left untreated, canine scabies can survive a few weeks on human hosts; however, the infected person will likely continue to experience a pattern of extreme itch until the pet is treated. Mites are transferred from animal to human through close contact, and signs of human infestation may manifest within 24 to 96 hours of contact with the affected animal (Cutis. 2003 Aug;72(2):107-9).

While the rate of transmission of canine scabies is unknown in humans, it’s relatively uncommon due to more diligent trends in pet care, said Dirk Elston, MD, FAAD, professor and chairman of the Department of Dermatology and Dermatologic Surgery at the Medical University of South Carolina. “There are other mite infestations that are more common in dogs, for example, Cheyletiella. While runt puppies can get demodectic mange, and other dogs can get scabietic mange, a ‘mangy’ dog — usually one that’s not well cared for — is how humans typically contract canine scabies. However, most people nowadays have their dogs treated for heartworm, and those treatments pretty much prevent scabies as well. So, we don’t see a lot of mange in pets that are well cared for.”

Patients can avoid contracting canine scabies by avoiding contact with wild animals or strays and treating any suspected cases in pets immediately. According to Dr. Elston, a dermatologist confronted with a case of sudden and severe itch should inquire about pet ownership and the appearance and health of household pets. While human infestations with canine scabies will resolve following treatment of the affected pet, patients may be uncomfortable waiting for the disease to run its course. The CDC indicates that patients can be treated with permethrin cream 5%, crotamiton lotion or cream 10%, lindane lotion 1%, or ivermectin.

Cheyletiella mites

More mites can cause problems for pets and their human caregivers. Cheyletiella are non-burrowing mites commonly found on rabbits, dogs, and cats. In animals, the mite often produces a subtle dermatitis, sometimes called ‘walking dandruff.’ “Cheyletiella mites can cause spots of dandruff that look like hot spots on the dog,” explained Dr. Elston. However, affected animals can also be asymptomatic, and up to 50% of rabbits in commercial colonies may harbor Cheyletiella or other mites (Cutis. 2017; 99:335-336, 355). Populations most at risk for contracting Cheyletiella mites include children, pet owners, farmers, and veterinarians.

“It can be something that’s really confusing to a dermatologist because you’ve got a blistering disease in a human, and the immunofluorescence is positive. That makes you think of immunobullous disease, but it can simply be mite infestation from a pet.”

Like sarcoptic mange, Cheyletiella mites cannot complete their life cycle on humans. However, they can still inflict a range of symptoms in human hosts, ranging from mild dermatitis to more severe illness with systemic symptoms. “Cheyletiella can be passed from cats, dogs, and rabbits, and is something we see more often than canine scabies,” said Dr. Elston. The human often gets blistering disease that can mimic autoimmune bullous disease, complete with positive immunofluorescence. It can be something that’s really confusing to a dermatologist because you’ve got a blistering disease in a human, and the immunofluorescence is positive. That makes you think of immunobullous disease, but it can simply be mite infestation from a pet.”

Taking a thorough patient history is key to avoiding a misdiagnosis, advised Dr. Elston. “You don’t want to misdiagnose as immunobullous where the patient gets put on immunosuppressive treatment. If diagnosed correctly, the dog gets treated by a veterinarian and the problem goes away.”

Additional DermWorld Resources