Bluetongue

The Organism

Bluetongue virus (BTV) causes bluetongue and is transmitted primarily by biting midges. The virus occurs in multiple serotypes, or types distinguished by the immune response they provoke. At least 24 classical serotypes are recognized, with additional atypical serotypes reported.

Immunity to one serotype generally provides limited or no protection against another, making vaccine selection and disease control challenging [1].

In a summary dated September 2022, USDA APHIS listed seven serotypes as established in the United States: 3, 6, 10, 11, 12, 13, and 17. APHIS also listed additional serotypes that had been reported. Serotype 8, which spread across parts of northern Europe beginning in 2006, had not been reported in the United States in that summary [2]. These classifications describe the surveillance information available at the time and may change as additional serotypes are detected.

Bluetongue is closely related to epizootic hemorrhagic disease (EHD). The viruses are distinct but are transmitted by some of the same Culicoides species. They can cause similar clinical and pathological changes in deer and cattle, so laboratory testing is needed to distinguish them [1, 3].

Where It Lives

Bluetongue virus circulates between susceptible ruminants and competent insect vectors. It is not considered an environmentally persistent pathogen in the same way as organisms that survive for long periods in soil or water. In the western United States, Culicoides sonorensis is an important proven vector. Other Culicoides species can transmit BTV in different regions; only a small proportion of the more than 1,000 species worldwide are known to be competent vectors [4].

C. sonorensis breeds in nutrient-rich, wet environments. Potential breeding sites include wastewater lagoons, manure-contaminated pond margins, and muddy areas around water sources [3, 4]. Adult midges commonly feed around dawn and dusk, although activity varies with local conditions.

Midge activity is often seasonal in temperate regions, declining during cold weather. However, the timing and extent of seasonal transmission depend on local climate, vector survival, and the ecology of the virus. It is too simplistic to assume that a hard frost eliminates all risk or that every region experiences the same annual pattern [1].

An Arizona Culicoides survey was conducted in the low Sonoran Desert of Yuma and La Paz counties in 2000 and 2001. Researchers found C. sonorensis at only one of six sites. All 22 midge pools tested negative for BTV and EHD virus. The most abundant species was Culicoides mohave: 19 of 55 pools (35%) contained EHD viral RNA. None contained BTV RNA, and C. mohave has not been established as a competent vector for either virus [5]. These findings describe a limited survey in two counties and should not be generalized to the entire state. The distribution of C. sonorensis at higher Colorado Plateau elevations is not established by this survey.

How It Spreads

Bluetongue is transmitted primarily through the bite of an infected Culicoides midge. It does not ordinarily spread through direct contact between animals, and it is not considered to spread through contaminated soil or water [1].

A midge can acquire the virus when it feeds on an infected animal. After an incubation period within the insect, the midge can transmit virus during subsequent blood meals. Temperature affects how quickly the virus develops within the midge; warmer conditions generally shorten the time required for the insect to become infectious [3, 4].

Cattle can be important in the transmission cycle even when they show few or no clinical signs. They are frequently exposed to biting midges, are numerous in many livestock systems, and can have detectable virus in their blood for longer than sheep. Infectious viremia in cattle may persist for several weeks [1].

A key diagnostic distinction is that PCR can detect viral genetic material after infectious virus is no longer recoverable. A positive PCR result therefore does not, by itself, establish that an animal can infect a feeding midge. Studies of BTV viremia distinguish between detection of viral RNA and the presence of infectious virus [6].

BTV does not ordinarily establish a persistent infection in cattle. This is different from saying that viral RNA cannot be detected for an extended period. The duration of PCR positivity and the duration of infectious viremia are not interchangeable measures [1, 6, 7].

Infection Pattern

Sheep are the most clinically important domestic host. After infection, signs commonly develop following an incubation period of several days, often around a week. Affected sheep may develop fever, depression, facial and muzzle swelling, and nasal discharge that can become thick and crusted. Oral lesions may develop, and inflammation of the coronary band—the area where the skin meets the hoof—can cause lameness. Some animals develop respiratory or cardiovascular complications and may die [1].

The blue discoloration of the tongue that gives the disease its name is not a consistent finding. Its absence does not rule out bluetongue [8].

Disease severity varies with the virus serotype, the animal’s immune status, breed, age, and other factors. In regions where BTV is established, only a small proportion of a flock may show clinical disease. When a new serotype enters a susceptible population, or susceptible sheep are moved into an area where that serotype is circulating, a much larger proportion of the flock may become clinically affected. Published reports describe substantial variation in morbidity and case fatality [1]. These figures should not be treated as universal predictions for an individual flock.

Cattle and goats are often infected without obvious clinical signs. Clinical bluetongue can occur in cattle and may include fever, oral or nasal lesions, muzzle inflammation, skin lesions, and reproductive problems. Severe disease and death are less common in cattle than in susceptible sheep [1]. Goats are generally less likely to develop severe clinical disease, although susceptibility varies.

Wildlife: White-tailed deer can develop hemorrhagic disease that resembles EHD. Outbreaks can cause substantial mortality. Pronghorn may die with few warning signs, while bighorn sheep—including desert bighorn sheep—can develop disease resembling bluetongue in domestic sheep [1, 3].

What to Do If You Suspect Bluetongue

Call your veterinarian. Bluetongue can cause mouth, muzzle, and foot lesions that resemble other important diseases, including foot-and-mouth disease (FMD) and vesicular stomatitis. These diseases must be considered and ruled out as appropriate, particularly because FMD is a foreign animal disease and requires urgent notification [8].

In New Mexico, bluetongue is included in the state’s animal-disease reporting framework. Contact the New Mexico Livestock Board or State Veterinarian regarding the applicable reporting requirements [9].

While awaiting veterinary guidance, keep affected animals in a shaded, comfortable area with access to clean water and palatable feed. Oral lesions may make eating painful. Monitor animals for worsening respiratory signs, dehydration, and secondary complications.

Reduce midge exposure where practical. Consider housing or penning sheep away from wet areas during periods of peak midge activity. Manage standing water, wastewater, and muddy areas around tanks and troughs where feasible [3, 4]. These measures may reduce exposure but cannot guarantee prevention of transmission.

Report unusual wildlife deaths to the appropriate state wildlife agency. Wildlife surveillance can help identify circulation of BTV or EHD virus in an area.

Diagnostic Differential

Two differential diagnoses are particularly important.

Foot-and-mouth disease and vesicular stomatitis can cause oral and foot lesions that resemble those seen in bluetongue. FMD is a foreign animal disease, and both conditions have important reporting implications. Suspected cases should be handled through the appropriate veterinary and animal-health authorities [8].

Epizootic hemorrhagic disease is caused by a different virus but can produce clinical and pathological changes similar to bluetongue. EHD virus and BTV may be transmitted by some of the same midge species. Laboratory testing is needed to distinguish them [1, 3].

Other conditions that may cause overlapping signs include contagious ecthyma (sore mouth), malignant catarrhal fever, bovine viral diarrhea (BVD), infectious bovine rhinotracheitis (IBR), photosensitization, pneumonia, and foot rot or foot abscesses [8].

Diagnosis

RT-PCR on whole blood is commonly used to detect BTV in animals with suspected clinical disease. Molecular testing can also be used to identify the virus serotype, depending on the assay [1].

Antigen-capture ELISA and virus isolation are additional diagnostic methods. Antigen detection may be less sensitive than PCR in some circumstances. Virus isolation demonstrates the presence of viable virus, although it is not used as routinely as molecular testing [1].

Antibody tests—including competitive ELISA, virus neutralization, and agar gel immunodiffusion—are used for surveillance, exposure assessment, and certain animal-movement purposes. They are not generally the preferred tests for determining whether a sick animal currently has active BTV infection [1].

In areas where BTV is established, cattle may be antibody-positive because of previous exposure. A positive antibody test therefore indicates an immune response to exposure or vaccination; it does not, by itself, establish active infection or explain current clinical signs.

A positive PCR result also needs context. PCR detects viral genetic material, which may remain detectable after infectious virus is no longer present. A PCR-positive result does not automatically mean the animal is infectious to midges [1, 6].

Risk to Humans

Bluetongue virus is not known to cause disease in people and is not considered zoonotic [1].

The midges that transmit BTV may bite people, and their bites can be irritating, but they do not transmit bluetongue to humans.

The absence of a human bluetongue risk does not remove the need for ordinary food-safety practices when handling or consuming meat and dairy products.

Vaccination and Control

There is no specific antiviral treatment for bluetongue. Care is supportive and may include shade, rest, clean water, and palatable feed. A veterinarian can assess the need for additional treatment, including management of secondary infections.

Vaccine options in the United States are limited and serotype-specific. USDA materials describe a modified-live serotype 10 vaccine for sheep and additional modified-live products used in California, as well as the absence of a US-licensed cattle vaccine at the time of publication [8]. Product licensing, availability, and permitted use may change over time.

A vaccine directed against one serotype may not provide reliable protection against another. Vaccine selection therefore depends on the serotype involved and the applicable regulatory and veterinary guidance [1].

Modified-live vaccines require particular caution. Potential reproductive and fetal risks have been described, and product labels and veterinary guidance should be followed. Do not assume a vaccine is safe for pregnant ewes unless its current label specifically supports that use [8].

Midge control can reduce exposure but is unlikely to eliminate transmission during an outbreak. Practical measures include managing wet breeding sites, reducing manure-contaminated standing water where feasible, and limiting sheep exposure to midges during periods of peak activity [3, 4].

What Bluetongue Costs Producers

For sheep producers, losses can include sick and dead animals, reduced weight gain, lower productivity, wool damage, and reproductive losses. The impact can be greatest when a new serotype enters a susceptible flock or when animals without prior immunity are moved into an area where the virus is circulating [1].

For cattle producers, the effects may be less obvious because cattle frequently show few clinical signs. However, cattle can contribute to the transmission cycle by serving as hosts for feeding midges and by having detectable virus in their blood for an extended period [1, 7].

Bluetongue can also affect international trade. Importing countries may impose conditions on the movement of live animals, semen, embryos, or animal-derived products from affected regions. The requirements depend on the importing country’s rules, the product being shipped, the virus serotype, and the exporting region’s disease status [3, 7].

The introduction of a serotype not previously established in the United States could create additional animal-health and trade challenges. The scale of those effects would depend on its distribution, clinical impact, and the response of trading partners.

Reliable US-specific dollar estimates of bluetongue losses are not established by the available economic data.

Where to Report a Case in the Southwest

Bluetongue is not a human disease, so it is not reported through public-health channels. Animal-health reporting requirements differ among states. Veterinarians should follow applicable state rules and contact the State Veterinarian when a suspected case raises questions about reporting.

Federal

The 2026 National Animal Health Reporting System disease list identifies BTV serotype 8 as notifiable and other BTV serotypes as monitored, with EHD also monitored [12]. These classifications concern federal animal-health reporting and surveillance rather than human public-health reporting.

Suspected FMD or another foreign animal disease should be reported immediately through the appropriate state and federal animal-health authorities.

Documented Incidence in the Southwest

Bluetongue has a long history in the western United States. Sheep epizootics were reported in the region, including western Texas, in the late 1940s. The first US isolation of BTV, serotype 10, was reported from California sheep in 1952 [3]. In its September 2022 summary, APHIS listed seven serotypes as established nationally [2].

Publicly available livestock surveillance data for Arizona, New Mexico, and Texas are limited. Much of the regional information comes from wildlife studies. A lack of published livestock surveillance data should not be interpreted as evidence that the virus is absent.

Arizona

Researchers documented hemorrhagic disease in Arizona deer in 2002, isolating EHD virus serotype 2 from two of three deer examined [13]. That year, researchers also identified BTV serotype 17 in a desert bighorn sheep and EHD virus serotype 2 in a Rocky Mountain bighorn sheep [14].

In 2018, Arizona wildlife officials reported that EHD virus serotype 6 had been detected in Arizona wildlife [15].

In the 2000–2001 midge survey from Yuma and La Paz counties, researchers did not detect BTV, but 35% of C. mohave pools contained EHD viral RNA [5]. These findings do not establish the current prevalence of either virus in Arizona livestock. Published statewide cattle and sheep surveillance data are limited.

New Mexico

Published bluetongue case reports and statewide surveillance data for New Mexico are limited. The state’s reportable-disease rule establishes veterinary reporting obligations but does not establish how many cases have occurred [9].

Texas

Western Texas was among the areas where bluetongue epizootics in US sheep were reported in the late 1940s [3].

Bluetongue is not specifically included in the Texas reportable and actionable disease list [11]. This may contribute to the limited availability of publicly accessible state case counts, but it does not establish that infections are absent.

References

1. Bluetongue. Center for Food Security and Public Health, Iowa State University, October 2023. https://www.cfsph.iastate.edu/Factsheets/pdfs/bluetongue.pdf↑ a b c d e f g h i j k l m n o p q r s
2. Bluetongue. USDA APHIS, updated 30 July 2025. https://www.aphis.usda.gov/livestock-poultry-disease/cattle/bluetongue↑ a b
3. Rivera NA, Varga C, Ruder MG, et al. Bluetongue and epizootic hemorrhagic disease in the United States of America at the wildlife–livestock interface. Pathogens 2021;10(8):915. https://www.mdpi.com/2076-0817/10/8/915↑ a b c d e f g h i j
4. McGregor BL, Erram D, Alto BW, et al. Vector competence of North American Culicoides for bluetongue and epizootic hemorrhagic disease viruses. Journal of Medical Entomology 2022. https://doi.org/10.1093/jme/tjac021↑ a b c d e
5. Rosenstock SS, Ramberg F, Collins JK, Rabe MJ. Culicoides mohave: new occurrence records and potential role in transmission of hemorrhagic disease. Journal of Medical Entomology 2003;40(4):577–579. https://academic.oup.com/jme/article/40/4/577/1000594↑ a b
6. MacLachlan NJ. Bluetongue: pathogenesis and duration of viremia. Veterinaria Italiana 2004;40(4):462–467. https://www.izs.it/vet_italiana/2004/40_4/462.pdf↑ a b c
7. WOAH Terrestrial Animal Health Code, Chapter 8.3: Infection with bluetongue virus. World Organisation for Animal Health. https://www.woah.org/en/what-we-do/standards/codes-and-manuals/terrestrial-code-online-access/↑ a b c
8. FAD PReP standard operating procedures: Bluetongue etiology and ecology. USDA APHIS Veterinary Services, August 2016. https://www.aphis.usda.gov/sites/default/files/sop_btv_e-e.pdf↑ a b c d e f
9. 21.30.4.9 NMAC: New Mexico reportable disease list. New Mexico Livestock Board. https://www.srca.nm.gov/parts/title21/21.030.0004.pdf↑ a b c
10. Arizona Administrative Code § R3-2-402: Mandatory disease reporting by veterinarians and veterinary laboratories. https://www.law.cornell.edu/regulations/arizona/Ariz-Admin-Code-SS-R3-2-402↑
11. 4 TAC § 45.3: Reportable and actionable disease list. Texas Animal Health Commission. https://www.law.cornell.edu/regulations/texas/4-Tex-Admin-Code-SS-45-3↑ a b
12. Voluntary 2026 National Animal Health Reporting System reportable disease list. USDA APHIS, 2026. https://www.aphis.usda.gov/sites/default/files/nlrad-nahrs-disease-list.pdf↑
13. Noon TH, Wesche SL, Heffelfinger J, Fuller A, Bradley GA, Reggiardo C. Hemorrhagic disease in deer in Arizona. Journal of Wildlife Diseases 2002;38(1):177–181. https://experts.arizona.edu/en/publications/hemorrhagic-disease-in-deer-in-arizona/↑
14. Noon TH, Wesche SL, Cagle D, et al. Hemorrhagic disease in bighorn sheep in Arizona. Journal of Wildlife Diseases 2002;38(1):172–176. https://experts.arizona.edu/en/publications/hemorrhagic-disease-in-bighorn-sheep-in-arizona/↑
15. Deadly new insect-borne virus targeting Arizona wildlife. KNAU Arizona Public Radio, 6 August 2018. https://www.knau.org/knau-and-arizona-news/2018-08-06/deadly-new-insect-borne-virus-targeting-arizona-wildlife↑