Epidemiological News: Week 48, 2025
Infectious Diseases: Global Perspective
Respiratory and Droplet Transmission
Avian Influenza A(H5N5) – First Human Case (United States)
The confirmation of the first human infection with avian influenza A(H5N5) globally represents a sentinel event in zoonotic influenza surveillance. The patient, an older adult with underlying health conditions in Grays Harbor County, Washington, developed symptoms during week 43, was hospitalized in week 45, and died on 21 November 2025. Environmental samples from the patient’s backyard poultry flock tested positive for avian influenza, establishing the most likely exposure source as domestic poultry that had contact with wild birds.
Highly pathogenic avian influenza A(H5) viruses cause severe systemic disease in poultry with mortality rates approaching 100%, and human infections, while rare, can produce severe respiratory illness with high case fatality rates. The incubation period typically ranges from 2–8 days. Individuals with direct contact with infected birds, their secretions, or contaminated environments face the highest risk of infection.
The sequenced isolate (A/Washington/2148/2025) belongs to clade 2.3.4.4b, genotype A6, which has been detected in birds and mammals across North America and designated as EA-2021-I in European isolates. Critically, no mutations associated with mammalian adaptation were identified, and no human-to-human transmission has been detected among the patient’s contacts. This genetic profile aligns with strains circulating in wild birds across northern Europe during the current epidemiological year, where seven EU/EEA countries have reported detections. Additionally, A(H5N5) has been identified in wild mammals including Arctic fox, lynx, and American mink in Iceland and Norway since early 2024.
The US CDC assesses risk to the general population as low, and this assessment extends to EU/EEA populations. However, the global significance of this first human H5N5 infection necessitates enhanced surveillance, particularly for individuals with occupational or recreational exposure to poultry and wild birds.
Avian Influenza A(H5N2) – Second Human Case Confirmed (Mexico)
On 24 November 2025, WHO confirmed a second global human case of avian influenza A(H5N2), occurring in a young woman without underlying conditions in Mexico City. The patient was exposed to birds and a dog that tested positive for A(H5) in her residential area. Unlike the first human case reported in April 2024 (which involved low pathogenic avian influenza), this strain demonstrates a highly pathogenic avian influenza signature in the hemagglutinin segment, though it lacks mutations known to increase zoonotic potential. No human-to-human transmission has been identified, and the risk for EU/EEA populations remains low.
Vector-Borne Transmission
Marburg Virus Disease – Ethiopia
Ethiopia is experiencing its first-ever Marburg virus disease outbreak, confirmed on 14 November 2025 following detection of suspected cases in Jinka city in the Southern Nations, Nationalities, and Peoples’ Region. As of 27 November, 15 cases (12 laboratory-confirmed, 3 probable) have been reported with 11 deaths, yielding a case fatality rate of 66.7%.
Marburg virus causes a severe hemorrhagic fever with case fatality rates historically reaching 88% in some outbreaks. The virus is transmitted through direct contact with blood and body fluids of infected persons or animals, with an incubation period of 5–10 days (range 3–21 days). Fruit bats of the Pteropodidae family serve as the natural reservoir. Clinical presentation includes sudden fever, severe headache, muscle pain, fatigue, followed by gastrointestinal symptoms and, in later stages, unexplained bleeding. Healthcare workers face particular occupational risk, with two deaths already reported among this group.
A concerning development is the geographic expansion to Hawassa City, Sidama Region, following return travel from Jinka City, indicating potential for wider dissemination. Contact tracing has identified 349 individuals, with 119 having completed their 21-day monitoring period. Whole genome sequencing reveals similarities to previously identified East African strains. The WHO and Ethiopian health authorities are coordinating response efforts including enhanced surveillance, contact tracing, and community engagement.
ECDC assesses the risk for EU/EEA citizens visiting or living in Ethiopia as low, with the likelihood of importation and secondary transmission within the EU/EEA considered very low. Jinka serves as a tourist hub for the region, though it remains relatively remote from Addis Ababa.
Fecal-Oral Transmission
Cholera – Global Update
Global cholera transmission continues at elevated levels with 577,843 cases and 7,395 deaths reported since 1 January 2025—a 17% increase in cases compared to the same period in 2024. During the most recent reporting period (29 October–25 November), 15,394 new cases and 194 deaths were recorded, with Afghanistan (10,781 cases), Angola (2,493), Sudan (1,117), Burundi (567), and Ethiopia (413) contributing the highest case counts.
Sudan presents the most concerning mortality profile, reporting 114 deaths during this period—59% of global cholera deaths—reflecting the compound effects of armed conflict, population displacement, and healthcare system collapse. South Sudan has reported 78,034 cumulative cases in 2025 compared to only 114 during the same period in 2024, representing a catastrophic surge. Angola and Chad continue to experience ongoing transmission, while outbreaks persist across multiple regions in Asia (Afghanistan, Myanmar, Nepal) and East Africa (Burundi, Ethiopia, Kenya, Mozambique).
The risk of cholera infection for EU/EEA travelers remains low, though sporadic importation is possible. Vaccination should be considered for emergency and relief workers deployed to affected regions.
Contact and Sexual Transmission
Ebola Virus Disease – Democratic Republic of the Congo
The Ebola outbreak in Kasai Province, DRC, appears to be waning, with the 42-day countdown to declare the outbreak officially concluded initiated on 19 October 2025 following discharge of the last patient. Since outbreak declaration on 4 September, 64 cases (53 confirmed, 11 probable) and 45 deaths (case fatality rate 70.3%) have been recorded, all from Bulape health zone. No new cases have been reported since 26 September, and no contacts remain under active monitoring. A total of 44,453 individuals have received vaccination. This represents the sixteenth Ebola outbreak in DRC since 1976 and the eighth since 2018.
Infectious Diseases: European Union/European Economic Area
Respiratory and Droplet Transmission
Seasonal Respiratory Virus Activity – Early Influenza Season
The 2025–26 influenza season has begun three to four weeks earlier than in the previous two years, with over half of EU/EEA countries now reporting above-baseline activity. Influenza A dominates circulation (99% of typed specimens), with A(H3N2) subclade 2a.3a.1(K) accounting for 85% of genetically characterized A(H3) specimens and driving the current increasing trend. Children aged 5–14 years show the highest circulation rates, while early hospitalization increases are primarily affecting adults aged 65 years and above.
In week 48, primary care sentinel surveillance detected 256 influenza specimens with 16% pooled positivity (median 4.6%; IQR 2.9–8.3%), while hospital SARI surveillance identified 139 influenza specimens with 14% pooled positivity. Eight countries report baseline activity levels, eight report low intensity, and two report medium intensity. Geographic spread ranges from sporadic (5 countries) to widespread (5 countries).
ECDC published a Threat Assessment Brief on 20 November 2025 evaluating the risk associated with increasing A(H3N2) subclade K circulation. Respiratory syncytial virus activity is slowly increasing from low levels, approximately one week later than the previous season, predominantly affecting children under five years. SARS-CoV-2 continues to circulate but is decreasing across all age groups.
Pertussis – Post-Epidemic Surveillance Context
The 2024 pertussis epidemic that swept across Europe has receded substantially. While not directly captured in week 48 EU/EEA data, the Bulgarian surveillance reveals the epidemic’s trajectory: year-to-date 2025 cases (88) represent a 96.8% decrease from 2024 (2,709). This pattern likely reflects similar trends across EU/EEA Member States where pertussis surged in 2024 following pandemic-related disruptions to childhood vaccination programs.
Fecal-Oral Transmission
Hepatitis A – Multi-Country Outbreak
The ongoing hepatitis A virus outbreak affecting Austria, Czechia, Hungary, and Slovakia has now exceeded 6,000 cumulative cases in 2025 with 39 deaths, representing one of the largest HAV outbreaks in EU/EEA history. Sequencing analyses have identified two closely related HAV sub-genotype IB clusters, differing by a single nucleotide, with Cluster A comprising cases from Austria, Germany, Hungary, and Sweden, and Cluster B encompassing cases from Austria, Czechia, Slovakia, Sweden, and the United Kingdom.
Hepatitis A virus is highly transmissible through the fecal-oral route, with an incubation period averaging four weeks (range 2–6 weeks). The virus demonstrates remarkable environmental stability, resisting acidification and freezing. While often producing mild or asymptomatic infection in children, adults—particularly those with underlying liver disease or alcohol use disorders—may develop severe acute hepatitis with potential for fulminant hepatic failure.
The epidemiological profile varies considerably by country: Czechia has reported 2,310 cases and 27 deaths, with 16% of cases linked to identified outbreaks and 17% involving risk factors including intravenous drug use (178), homelessness (188), or incarceration (27). Slovakia reports 2,482 cases with ongoing transmission in Roma communities and those living in poor sanitary conditions. Hungary has recorded 1,548 cases with the highest incidence rates among children aged 3–9 years and in northeastern counties. Austria reports 216 cases with significant representation among people experiencing homelessness and people who use drugs, with Vienna as the outbreak epicenter.
No food source has been implicated; person-to-person transmission among groups with limited access to proper sanitation drives ongoing transmission. ECDC assesses risk as moderate-to-high for affected populations and low-to-moderate for the general population with access to sanitation and healthcare. Targeted vaccination campaigns are underway in affected countries, though vaccine availability remains challenging.
Vector-Borne Transmission
Chikungunya Virus Disease – Locally Acquired Cases
Local chikungunya transmission has continued in France and Italy during the 2025 season, with 795 and 385 locally acquired cases respectively reported as of 26 November. France reported 15 new locally acquired cases in week 48, distributed across 79 clusters with 9 currently active. Italy has not reported new cases this week.
Chikungunya virus is transmitted primarily by Aedes albopictus mosquitoes in Europe. The disease presents with acute onset of high fever and severe arthralgia, typically 4–7 days post-exposure. While mortality is low, debilitating joint pain may persist for months to years, particularly in older adults. The persistence of active clusters despite seasonal weather conditions becoming unfavourable suggests potential for late-season transmission and warrants continued vigilance for delayed diagnoses in the coming weeks.
West Nile Virus – Seasonal Summary
Fourteen countries have reported human West Nile virus cases during 2025, including Bulgaria (among EU/EEA countries also reporting: Croatia, France, Germany, Greece, Hungary, Italy, Romania, Spain) and neighboring countries (Albania, Kosovo, North Macedonia, Serbia, Türkiye). A total of 157 affected areas have been identified. The vector transmission season is concluding, though delayed case reporting may continue for several weeks.
Infectious Diseases: Bulgaria
Respiratory and Droplet Transmission
COVID-19
Bulgaria recorded 69 COVID-19 cases during week 48, representing a 40.5% decrease from the previous week (116 cases) and continuing the downward trajectory observed throughout autumn 2025. Year-to-date cases (2,959) represent an 80.3% reduction compared to 2024 (15,052), reflecting the transition to endemic circulation patterns and reduced testing intensity. Sofia city reported the highest number (13 cases), followed by Burgas (10) and Varna (4). All reported cases were laboratory confirmed.
Pertussis (Whooping Cough)
Three pertussis cases were confirmed during week 48, an increase of 2 cases from the previous week. Cases were reported from Plovdiv (1), Sofia city (1), and Stara Zagora (1). Year-to-date cases total 88, compared to 2,709 during the same period in 2024—representing a 96.8% decrease as the 2024 epidemic wave has resolved.
Pertussis, caused by Bordetella pertussis, is transmitted via respiratory droplets with an incubation period of 7–10 days. The disease presents classically with paroxysmal cough, inspiratory whoop, and post-tussive vomiting, though presentation varies by age and vaccination status. Infants too young for vaccination face the highest risk of complications including pneumonia, seizures, encephalopathy, and death. The 2024 European epidemic highlighted the importance of maintaining high vaccination coverage and implementing maternal immunization strategies.
Legionnaires’ Disease
Two cases of Legionnaires’ disease were confirmed during week 48, representing a doubling from the previous week. Year-to-date cases (15) exceed 2024 totals (10) by 50%, warranting enhanced environmental surveillance of water systems.
Legionella pneumophila, the causative agent, colonizes man-made water systems and is transmitted through inhalation of contaminated aerosols from sources such as cooling towers, hot water systems, and spa pools. The disease presents as severe pneumonia with an incubation period of 2–10 days, predominantly affecting older adults, immunocompromised individuals, and those with chronic lung disease. Case fatality rates can exceed 10% without appropriate antibiotic therapy.
Fecal-Oral Transmission
Gastroenteritis and Enterocolitis
A total of 169 cases of gastroenteritis and enterocolitis were recorded during week 48, a modest decrease of 4 cases from the previous week. Case classification included 76 possible, 88 probable, and 5 confirmed cases. The highest case counts were reported from Plovdiv (40), Sofia city (23), and Burgas (22). Year-to-date cases (8,848) represent a 6% decrease compared to 2024 (9,408), consistent with secular trends in enteric disease surveillance.
Salmonellosis
Twenty-three salmonellosis cases were confirmed during week 48, representing a 43.8% increase from the previous week (16 cases). Cases were geographically dispersed across multiple regions including Sofia city (5), Plovdiv (1), and Pleven (4). Year-to-date cases (661) are 25.7% lower than 2024 (890), though the week-to-week increase warrants attention to potential clustering.
Salmonellosis, typically caused by non-typhoidal Salmonella serovars, is transmitted through contaminated food products, particularly poultry, eggs, and unpasteurized dairy. The incubation period is 6–72 hours, with clinical presentation ranging from self-limited gastroenteritis to invasive disease in vulnerable populations. Antimicrobial resistance among Salmonella isolates remains a concern for public health surveillance.
Campylobacteriosis
Nineteen campylobacteriosis cases were confirmed during week 48, an increase of 3 cases from the previous week. Sofia city reported the highest count (9 cases), followed by Varna (5). Notably, year-to-date cases (456) exceed 2024 totals (316) by 44.3%, representing a sustained increase throughout 2025 that warrants investigation into potential sources and risk factors.
Campylobacter species, predominantly C. jejuni and C. coli, are the leading bacterial cause of gastroenteritis in developed countries. Transmission occurs through consumption of undercooked poultry, unpasteurized milk, and contaminated water, with an incubation period of 1–10 days. Post-infectious sequelae including Guillain-Barré syndrome and reactive arthritis occur in a small percentage of cases.
Rotavirus Gastroenteritis
Ten rotavirus gastroenteritis cases were confirmed during week 48, a decrease of 3 from the previous week. Cases were distributed across Sofia city (4), Pleven (2), and several other regions. Year-to-date cases (691) are 12.4% lower than 2024 (789), likely reflecting continued benefits of childhood rotavirus vaccination programs.
Vector-Borne Transmission
Lyme Borreliosis
Three Lyme borreliosis cases were confirmed during week 48, representing a decrease of 2 cases from the previous week. Cases were reported from Dobrich (1), Pleven (1), and Haskovo (1). Year-to-date cases (376) are virtually unchanged from 2024 (377). The end-of-season decline is expected given reduced tick activity during winter months.
Contact and Sexual Transmission
Sexually Transmitted Infections
Sexually transmitted infections in Bulgaria demonstrate concerning upward trends across multiple pathogens during 2025.
Syphilis cases numbered 10 during week 48, a slight increase from the previous week (9 cases). Year-to-date cases (342) represent an 11.4% increase compared to 2024 (307). Of particular concern, congenital syphilis cases have reached 34 year-to-date compared to 14 in 2024—a 143% increase—signaling potential gaps in prenatal screening and treatment programs.
Gonorrhea recorded 5 confirmed cases during week 48, an increase of 3 from the previous week. Year-to-date cases (114) represent a 28.1% increase compared to 2024 (89).
Urogenital chlamydia infection showed 5 cases during week 48, a substantial increase from the previous week (1 case). Year-to-date cases (175) demonstrate a 69.9% increase compared to 2024 (103), the most pronounced increase among bacterial STIs.
HIV infections numbered 3 during week 48, an increase of 1 from the previous week. Year-to-date cases (277) represent a 7.4% increase compared to 2024 (258).
The consistent upward trends across all STI categories suggest systemic factors potentially including reduced prevention services, changing sexual behaviors, or improved case detection. The dramatic increase in congenital syphilis cases—from 14 to 34 year-to-date—represents a particularly urgent public health concern requiring immediate attention to prenatal care services.
Childhood and Vaccine-Preventable Diseases
Varicella (Chickenpox)
Varicella demonstrates the most significant week-to-week increase among Bulgarian communicable diseases, with 526 cases representing an 84-case (19.0%) increase from the previous week (442 cases). Case classification included 47 possible, 411 probable, and 68 confirmed cases. The highest case counts were reported from Sofia city (82), Varna (69), Plovdiv (36), Veliko Tarnovo (31), and Stara Zagora (31).
Year-to-date cases (19,506) are 20.4% lower than 2024 (24,496), though the current weekly counts suggest seasonal acceleration consistent with expected winter transmission patterns. Varicella vaccination is not included in Bulgaria’s routine childhood immunization schedule, resulting in continued endemic circulation with seasonal winter-spring peaks.
Scarlet Fever
Ninety-nine scarlet fever cases were reported during week 48, an increase of 9 cases from the previous week. Case classification included 23 possible, 43 probable, and 33 confirmed cases. The highest case counts were reported from Plovdiv (26), Sofia city (17), and Varna (14). Despite the week-to-week increase, year-to-date cases (3,249) remain 59.7% below 2024 totals (8,071), reflecting resolution of the 2024 Group A Streptococcus surge that paralleled the pertussis epidemic.
Diseases Under Enhanced Surveillance
Invasive Meningococcal Disease
No cases of invasive meningococcal disease were reported during week 48. Year-to-date cases (14) represent a notable increase compared to 2024 (6), warranting continued surveillance attention despite the quiet week.
Crimean-Congo Hemorrhagic Fever
No cases of Crimean-Congo hemorrhagic fever were reported during week 48. Year-to-date, a single case has been recorded, unchanged from 2024. Bulgaria remains endemic for CCHF given the presence of Hyalomma tick vectors, though human cases have declined substantially from historical peaks.
High-Consequence Pathogens – Negative Findings
No cases were reported during week 48 for: anthrax, measles, rubella, congenital rubella, diphtheria, poliomyelitis, typhoid fever, cholera, tularemia, brucellosis, or Creutzfeldt-Jakob disease. Bulgaria reported 2 mpox cases year-to-date (compared to 1 in 2024) and 5 imported dengue cases (compared to 6 in 2024).