Arizona Wildlife Conservation Strategy

5. Disease, Pathogens, and Parasites

Wildlife populations are constantly changing. However, populations are under increasing pressure due to a number of factors, including climate change, habitat loss and modification, habitat fragmentation, and introduction of invasive species. These changes can contribute to the emergence of infectious diseases in wildlife populations.

Wildlife species are subject to diseases resulting from exposure to bacteria, viruses, parasites, toxins, and other biological and physical agents. Wildlife species can also be natural hosts for diseases that affect humans, some of which are transmitted directly while others are transmitted via vectors like insects. Climate also plays a role in disease dynamics in multiple ways, including altering the abundance and distribution of insect vectors and increasing the spread of pathogens in aquatic systems due to altered water temperature, quantity, or flow. Species forced to live outside of their preferred temperature range may be more susceptible to disease, parasites, and pathogens. 

To combat these emerging threats, AZGFD manages for the health of fish and wildlife by monitoring susceptible populations and responding to and investigating significant disease and mortality events. Engaging and educating the public to prevent further spread or introduction of wildlife diseases is also critical in our efforts to stem the tide of wildlife diseases. The public can report their observations of recently dead wildlife to AZGFD by calling the Wildlife Disease Hotline at (623) 236-7201.

Examples of diseases affecting Arizona’s terrestrial wildlife species include avian botulism, elaeophorosis, chytridiomycosis, epizootic hemorrhagic disease, plague, pneumonia (in bighorn sheep), rabies, rabbit hemorrhagic disease, ranavirus, snake fungal disease, trichomoniasis, and tularemia. Important pathogens affecting aquatic species include largemouth bass virus, koi herpes virus, spring viremia of carp virus, infectious pancreatic necrosis virus, infectious hematopoietic necrosis virus, viral hemorrhagic septicemia virus, bacterial coldwater disease, columnaris, bacterial kidney disease, enteric redmouth disease, furunculosis, and whirling disease.

Those diseases yet to be reported in Arizona, such as white-nosed syndrome (WNS) which affects bats, and chronic wasting disease (CWD) in deer and elk, are being closely monitored because confirmed cases have occurred in neighboring states. For example, AZGFD began conducting CWD surveillance in 1998 by acquiring samples annually through four different processes:

  • A sample submission process through a reimbursement program established with taxidermy and meat processor businesses

  • Collection of samples at check stations on the Kaibab Plateau

  • Collection of samples from hunter-harvested animals by regional personnel

  • Targeted surveillance of animals exhibiting signs consistent with CWD

Meanwhile, AZGFD is conducting surveillance for WNS by coordinating with governmental and non-governmental partners to collect environmental samples from bat habitats, as well as trapping and swabbing bats in the early spring to test for WNS.

Diseases Affecting Amphibians, Reptiles, and Fish

Compared with their terrestrial counterparts, wildlife that depend on aquatic habitats for part or all of their life stages tend to be more susceptible to environmental changes, especially the introduction of diseases and other pathogens. Due to their limited dispersal ability, specific habitat requirements, and the cumulative effects of multiple threats, amphibians are especially vulnerable to disease. Management strategies focused on understanding the effects and preventing introduction and spread of disease are a priority at local, regional, and global levels. Currently, AZGFD is working to monitor and manage the following diseases affecting aquatic wildlife in Arizona:

Chytridiomycosis affects amphibians and is caused by the chytrid fungus (Batrachochytrium dendrobatidis). The fungus spreads and persists in aquatic environments where it attacks the keratin in the skin of amphibians. Introduced species such as bullfrogs, African clawed frogs, and barred tiger salamanders (introduced for the bait trade) are known to harbor chytridiomycosis, yet they experience few symptoms of the disease (Bradley et al. 2002). While some native amphibian populations seem to be relatively unaffected by the disease, others are highly susceptible and the disease is contributing to population declines. In partciular, chytridiomycosis has caused declines in lowland leopard frogs, Chiricahua leopard frogs, and Tarahumara frogs. 

Ranaviruses can affect amphibians, reptiles, and fish. In Arizona, Ambystoma tigrinum virus can be particularly devastating to tiger salamanders, causing mass mortalities. Ranaviruses, in general, can cause tissue necrosis that leads to organ failure and death. Larval and juvenile amphibians are particularly vulnerable. Ranaviruses spread through food, water, human handling, and aquatic recreational equipment and gear (Miller et al. 2011; Allender 2019).

Upper Respiratory Tract Disease (URTD) affects desert tortoises and is spread through direct contact with an infected tortoise. If not treated, the infection can be deadly. Signs of the disease include a runny nose and watery eyes. To prevent the spread of URTD, it is important to never release a captive tortoise into the wild. 

The public can assist AZGFD by reporting sick or dead animals to the Wildlife Disease Hotline at (623) 236-7201, or by contacting their local AZGFD regional office.

Whirling Disease is a chronic inflammatory disease in salmonid fish caused by the myxozoan parasite Myxobolus cerebralis. The disease is characterized primarily by affected individuals displaying tight circular movements caused by spinal cord constriction and brain stem compression. The life cycle of M. cerebralis is temperature dependent and involves two hosts: salmonid fish and aquatic oligochaetes (Tubifex tubifex). Susceptibility to M. cerebralis varies with fish age, size, parasite dose, and environmental factors. Clinical signs of whirling disease may be evident three to eight weeks after infection. Survivors of long-term infections may only exhibit skeletal deformities (MacConnell and Vincent 2002).

Whirling disease is spread by infected fish and by fish feeding on worms that harbor the parasite. Asymptomatic carriers of the parasite are common and can act as reservoirs for the parasite. While the disease does not affect humans, it can also be spread as a result of human activity. Cleaning, draining and drying aquatic equipment after use and never transporting fish between water bodies can help prevent the introduction of the disease to new areas. Arizona has a “no tolerance” policy that bans the stocking or importation of fish infected with whirling disease. However, the potential for accidental introduction still exists.

Largemouth Bass Virus Disease (LMBVD) is caused by an iridovirus. As the name suggests, the most commonly-reported species with LMBVD is largemouth bass with some infections affecting smallmouth bass. Subclinical infections occur in other members of the Centrarchidae family, but LMBV in fish of other families appears to be rare. The first known isolation of LMBV was from largemouth bass collected from Lake Weir, Florida, in 1991 (Grizzle and Brunner, 2002). The virus is highly communicable by waterborne transmission (Grant et al. 2005), but most infected fish do not show signs of disease. Causes of subclinical infections converting to overt disease are unknown.

Water temperature and other variables are usually suitable for largemouth bass during LMBVD outbreaks. This disease occurs during summer in adult fish with subclinical infections occurring during all seasons. Occasionally, LMBVD causes a noticeable fish kill in wild populations, but there can be a decreased abundance of older age classes even when a fish kill is not observed (Maceina and Grizzle 2006). Diseased fish swim lethargically near the surface of the water and then lose equilibrium. Some moribund fish have an overinflated swim bladder and less commonly have exudate in the swim bladder.

Koi Herpesvirus Disease (KHVD) is caused by a recently recognized herpes-like virus also known as carp nephritis and gill necrosis virus. While clearly different from herpes viruses found in mammals, birds, and reptiles, the virus that causes KHVD is related to a group of herpes-like viruses found in fish and amphibians currently classified in the family Herpesviridae. Young koi and common carp are most susceptible although outbreaks and high mortality (90%) have been observed in larger and adult fish. Mortality begins approximately 7 to 14 days following contact with the virus most often via a recently introduced fish to the pond (koi) with one of two common outcomes: 1) the introduced fish dies shortly after introduction and spreads the virus to the naïve population, or 2) the introduced fish stays healthy yet the naïve fish succumb. The movement of the virus with suspected carriers has been the principal means by which the virus has spread so rapidly at both the regional and global scales.

Spring Viremia of Carp Virus (SVCV) is caused by the fish Rhabdovirus. The following names have historically been used for diseases that may have been caused by SVCV: swim-bladder inflammation (SBI), infectious dropsy, rubella, infectious ascites, hemorrhagic septicemia, and red contagious disease. In typical outbreaks, SVCV spreads horizontally during the winter when water temperatures are low and host immune systems are less active. When spring approaches and temperatures rise toward 50°F, fish develop clinical signs of SVCV and begin to die. Above 68°F, most fish are able to develop immunity to the virus and mortality ceases. Fish may also develop SVCV in the fall if temperatures are in the right range for virus transmission and disease. While SVCV has been isolated from fish lice (Argulus sp.) and leeches (Pisicola sp.) feeding on infected fish, there is no evidence that these mechanical vectors are important in the spread of SVCV.

Infectious Pancreatic Necrosis Virus (IPNV) and IPNV-like viruses were originally associated only with salmonid fish, several of which are found in Arizona, including brook trout, rainbow trout, and brown trout. These viruses have since been isolated from a variety of non-salmonid fish and invertebrates from freshwater, estuarine, and marine environments (Wolf 1988, Reno 1999).

Viral Hemorrhagic Septicemia Virus (VHS) is one of the most important viral diseases of finfish worldwide. In the past, VHS was thought to affect mainly rainbow trout reared at freshwater facilities in western Europe (Wolf 1988). Today, VHS is known as a cause of mortality in cultured and wild fish in freshwater and marine environments in several regions of the northern hemisphere.

Infectious Hematopoietic Necrosis Virus (IHNv) is caused by a Rhabdovirus. Observation from naturally-occurring disease and experimental infections indicate that fish up to two months of age are most susceptible. In recent years, epizootics have also been reported in yearling sockeye salmon and two-year-old kokanee salmon as well as adult rainbow trout. The disease is still less common and more chronic in larger fish than in smaller fish. Most IHNv occurs at temperatures of 59oF or colder in freshwater. Horizontal transmission has been demonstrated and waterborne transmission can be accomplished in the laboratory. Clinically infected juvenile salmonids and carrier adults are the reservoirs of virus for waterborne transmission. No other reservoirs of virus have been identified.

Bacterial Kidney Disease (BKD) is caused by Renibacterium salmoninarum. All fishes in the family Salmonidae are considered susceptible (Elliott et al. 2014). Among non-salmonids, clinical BKD has been reported in cultured ayu (family Plecoglossidae), and the bacterium also has been isolated from kidneys, but not blood, of sea lampreys (Eissa et al. 2006). R. salmoninarum can be transmitted both vertically and horizontally, and the bacterium has been detected in both wild fish and hatchery populations (see reviews in Evelyn 1993; Fryer and Lannan 1993; Pascho et al. 2002). Infections can occur at any life stage in a salmonid population, but clinical signs of disease are uncommon in fish less than six months old (Evelyn 1993). The disease occurs over a wide range of temperatures.

Bacterial Coldwater Disease, also referred to as rainbow trout fry syndrome, fry mortality syndrome, peduncle disease, or low temperature disease, is caused by the bacterium, Flavobacterium psychrophilum. Coldwater disease and its causative agent have been reviewed by Cipriano (2005) and Barnes and Brown (2011). All salmonids are probably affected, but juvenile coho salmon along with rainbow trout and steelhead are particularly susceptible. Juvenile fish are primarily affected but infections also occur in yearlings and smolts. Epizootics frequently occur in fish held in protected water supplies and there is substantial evidence for vertical transmission of F. psychrophilum even when eggs are disinfected with iodophor.

Columnaris Disease is caused by the bacterium Flavobacterium columnare (Bernardet et al. 1996). Most freshwater fishes (cultured and wild) are considered susceptible to F. columnare. Columnaris disease can affect fish of all ages but is more prevalent in young fish. The severity and occurrence of the disease is generally greater in water temperatures greater than 68°F but the disease can also occur in lower water temperatures. Columnaris disease can occur in fish without any predisposing conditions but outbreaks are commonly associated with stressful rearing conditions such as low dissolved oxygen, high ammonia and nitrite concentrations, and overcrowding. Additionally, handling and injuries to the skin/mucosa may predispose fish to columnaris disease.

Diseases Affecting Birds

Avian Botulism is caused by a naturally occurring toxin produced by a bacterium that is associated with aquatic environments. It affects mainly waterfowl and aquatic bird species. Botulism can be managed effectively if mortalities are detected/reported early. When detected, carcasses should be picked up and disposed of as soon as possible. Water bodies can be managed through draining or flooding to alter the environmental conditions sufficiently to stop the production of Type C botulism, one of the prominent strains. Lastly, hazing can move other birds away from areas where the toxin has been detected, or where there have been other bird mortalities.

Avian Trichomoniasis is caused by a single-celled protozoan, and is a common parasite of doves and pigeons. However other birds such as domestic and wild turkeys, chickens, and raptors may also become infected. Infected birds experience weight loss, appear listless, excessive salivation, difficulty eating and drinking, and difficulty closing their mouths. Yellowish lesions may be seen around the beak or eyes of mourning doves and the face may appear “puffy.” Doves and pigeons should be kept from congregating in large groups, particularly at food and water sources. Feeders, water dishes, platforms, and other surfaces should be decontaminated regularly with a 10% bleach solution (9 parts water: 1 part bleach). Risk of this disease can be reduced by avoiding overfeeding birds (e.g., only putting out a modest amount of seed out every other day so that all feed is gone before the next feeding).

Pigeon Paramyxovirus 1 is a viral infection of doves and pigeons that affects the nervous system. It is most commonly seen in Eurasian collared doves where it causes large mortality events in the summer. Dense populations around dairies and agricultural areas seem to contribute to the problem. Removing carcasses and limiting access to feed will reduce the prevalence.

Diseases Affecting Ungulates

Chronic Wasting Disease (CWD) is the single biggest threat to cervid (deer, elk, moose, reindeer, etc.) populations in North America and around the world. CWD is an untreatable and always-fatal neurologic disease affecting free-ranging and captive cervids and is characterized by progressive weight loss, abnormal behavior, and eventual death. Mule deer, white-tailed deer, and elk are all susceptible to the disease and once infected, populations can sustain severe declines. Since 1998, AZGFD has been conducting CWD surveillance and has tested more than 25,000 samples. CWD has not been detected in Arizona, despite bordering three CWD-positive states: New Mexico, Colorado, and Utah. Samples are collected by AZGFD staff at check stations and camps, from taxidermists and game meat processors, and directly from hunters. Hunters coming in from out of state or returning to Arizona should follow recommended guidelines to reduce the risk of introducing the disease. Such measures include cleaning equipment and vehicles including the undercarriage, not transporting whole carcasses (except to a meat processor), and removing all soft tissue from skulls and skull caps. Meat should be cut and wrapped or in quarters before crossing state lines.

Elaeophorosis is a parasitic disease caused by the filarid worm (Elaeophora schneideri) that is transmitted by horseflies. It occurs in mule deer, white-tailed deer, elk, bighorn sheep, pronghorn, bison, and domestic sheep and goats. Signs of the disease can include facial swelling due to food impactions, tooth loss, and degeneration or fracture of jaw, muzzle and ear necrosis, malformed antlers, blindness, lack of coordination, facial or lower limb dermatitis, and death. The disease is seen more frequently in younger animals. Animals reported to be showing signs of the disease can be targeted and tested on a case by case basis.

Hemorrhagic Disease (HD) and Bluetongue (BTV) are infectious viral diseases transmitted by tiny biting flies. It has been reported in white-tailed deer, mule deer, bighorn sheep, elk, and pronghorn. Signs of HD and BTV infection include animals exhibiting depressed behavior, fever, a swollen head, neck, tongue, or eyelids, or having difficulty breathing. In severe cases, animals may die within one to three days. More often, they survive longer and may become lame, lose their appetite, or reduce their activity. Animals reported to be showing signs of the disease can be targeted and tested on a case-by-case basis.

Pneumonia is a bacterial respiratory disease observed in bighorn sheep. Infected bighorn sheep may show signs including severe coughing, nasal discharge, head shaking, drooping ears, fever, lethargy, and death. State wildlife managers, federal officials, and bighorn sheep advocates emphasize the need to reduce contact between wild and domestic sheep species, citing evidence of disease transfer. Although keeping wild and domestic sheep separate is challenging due to the relatively large distance bighorn sheep are known to travel (up to 30 miles), this is the most viable current management option available. Several states have reported severe losses in bighorn sheep populations with the disease. AZGFD considers pneumonia to be a serious threat and closely monitors Arizona’s bighorn sheep herds for signs of the disease.

Diseases Affecting Other Mammals

Rabbit Hemorrhagic Disease Virus 2 (RHDV2) is a viral disease that only affects rabbits, causing sudden death. It can be spread through contact with infected rabbits, their meat or their fur, or materials coming in contact with them. In 2020 the disease was detected in jackrabbits and cottontails in Arizona and other parts of the southwestern United States. Wildlife managers are continuing to assess the impact of the introduction of this disease on rabbit populations.

To reduce the risk of spreading RHDV2, rabbit owners should wash their hands before and after working with rabbits and not share equipment with other owners. Avoiding contact with wild or feral rabbits is also important. Burying dead rabbits to reduce the risk of disease transmission is also recommended. If a case is suspected, veterinarians should contact USDA-APHIS or email to contact the State Veterinarian’s Office or call 623-236-7201 for wildlife issues.

Plague is caused by a bacterium, Yersinia pestis, that exists in the flea-rodent cycle. Fleas become infected by feeding on animals such as chipmunks, prairie dogs, rabbits, ground squirrels, tree squirrels, mice, and woodrats that are infected with the bacteria. Infected fleas then transmit the plague bacteria to other mammals and humans during the blood feeding process. Carnivores typically get plague by ingesting infected animals or from flea bites.  Large scale die-offs can result in the sudden loss of entire colonies of prairie dogs or other mammals, and should be reported immediately.

Plague is naturally occurring in Arizona, mostly in the northern part of the state and above 5,000 ft. elevation. Flea control techniques for wildlife species are limited and in the early stages of development, but include placing powdered insecticides in animal burrows, and offering food containing insecticide or vaccine. Plague can be a very severe disease in humans, and can be transmitted through contact with an infected mammal or flea. Symptoms in humans typically appear within 2 to 6 days after exposure and include sudden onset of fever, chills, headache, nausea, vomiting, stomach and/or back pain, and weakness. Care should be taken to avoid contact with mammals such as rabbits, rodents, and prairie dogs. Protect pets by keeping them indoors or administering flea control for outdoor pets or animals.

Rabies is a virus that is typically transmitted via bite by an infected animal. Rabies travels through the nervous system into the affected animal’s brain. Once symptoms begin, the disease is fatal. There are several behavioral symptoms associated with rabies. An infected animal may exhibit abnormal behavior such as being active during odd hours and appearing especially docile or extremely aggressive.

Once bitten, initial signs are flu-like and include general weakness, headache, or fever, but may progress to inflammation of the brain. Clinical symptoms include paralysis, anxiety, tingling in extremities, paranoia, delirium, agitation, insomnia, and hydrophobia (fear of drinking fluids due to difficulty swallowing). Once clinical symptoms begin, the disease is almost always fatal. People and pets that are exposed to rabid animals should receive post-exposure vaccinations to prevent the disease.

Humans and pets can contract rabies as well. Although exposure to rabid animals is rare, rabies is a serious risk in Arizona and AZGFD offers regular outlets to educate the general public about rabies and wildlife. To reduce the risk of contracting rabies, humans and pets should be kept away from wild animals. Minimize the available food sources around your property such as garbage and pet food. If handling dead carcasses, wear latex/nitrile gloves.

Tularemia is caused by the bacterium Francisella tularensis that can affect people, rabbits, hares, rodents, dogs, cats, pigs, sheep, and horses. It can be contracted through direct contact with ticks and biting flies. In Arizona, tularemia has been most commonly associated with mortalities in cottontails where the only sign of the disease is sudden death. In humans, the onset of symptoms typically occur within 3 to 5 days, but can take as long as 14 days to develop. People often have fever, headaches, diarrhea, muscle aches, joint pain, dry cough, and swollen lymph nodes. Fortunately, tularemia is treatable with antibiotics. To limit exposure, wear long pants and long-sleeved shirts and apply insect repellent.

Wildlife diseases are a growing global challenge with wide-ranging impacts beyond wildlife losses.  The increased transport of animals and animal parts by humans has contributed to the spread of wildlife diseases, while changing environmental conditions (e.g., drought) can simultaneously influence disease distribution and make wildlife more susceptible to diseases. These challenges include ecological disturbance, human health and safety, health of domestic animals, and the economic impacts and losses that result. Many animal diseases that are transmitted to humans originate in wildlife species (Sleeman et al. 2017). AZGFD has drafted management plans for some diseases. Management actions include distribution of oral vaccines, oral insecticides, topical insecticides, addressing human actions that increase disease prevalence and distribution, modifying population management objectives, and modifying hunt structure. Even with these rigorous management plans, fighting wildlife disease is an on-going and ever-changing threat to wildlife and humans. Disease surveillance, vaccines, research, and public education programs are key management actions that will improve our understanding of wildlife diseases, help slow their spread, and reduce the devastating impacts on wildlife populations.