HomeMy WebLinkAboutmosquito-borne-disease-fact-sheet-2019-2023-adaFigure 1. Mosquito-borne Disease Case Counts in Alameda County, 2019—2023
Mosquito-borne Diseases in Alameda
County, 2019—2023
Background
While most mosquitoes only cause bites that itch, some
types of mosquitoes can spread pathogens that cause
severe illness. In Alameda County, reported diseases
spread by mosquitoes include Dengue, Chikungunya,
Zika, Malaria, and West Nile Virus. No cases of
Oropouche Virus have been reported to date. Malaria is
caused by a parasite spread by the Anopheles mosquito.
While this mosquito can be found in the United States,
local transmission is rare. Dengue, Chikungunya, and
Zika are all viruses spread by Aedes aegypti and Aedes
albopictus mosquito species. These mosquitoes are not
endemic to Alameda County; although Ae. aegypti was
found in Alameda County in October 2024, it is not
considered established in the jurisdiction. Local
transmission of the above mosquito-borne diseases has
not been identified, and cases are often associated with
international travel. West Nile virus is spread by Culex
mosquitoes, and is found in local birds and mosquitoes
yearly. Recommended prevention measures include
using insect repellent when visiting disease-endemic
regions, and taking malaria prophylaxis when
appropriate. At home, avoid leaving out even a capful
standing water that can breed mosquitoes.
Provider Recommendations
Take comprehensive patient history
including travel in order to assess risk
Local transmission from viremic
individuals is possible due to the finding
of Ae. aegypti in Alameda County;
encourage diagnosed patients to use
insect repellant and long-sleeve
clothing during viremic or erythrocytic
phases
Ensure use of appropriate diagnostic
testing, not just serology; refer to CDC
test guidance
Provide patient education and guidance
on prevention of mosquito-borne illness
and advise patients to take appropriate
chemoprophylaxis and vaccinations
prior to travel
Report cases of mosquito-borne
illnesses to Alameda County Public
Health Department
For training and resources-refer to
CDC's Dengue Clinical Management
Page.
Year
Number of Cases
5
1 0
4 2
52
10 9 12
35
12
2
11
18
14
1 0 0 1 23111 0
Chikungunya Dengue Malaria West Nile Zika
2019 2020 2021 2022 2023
0
20
40
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Case Counts
Dengue is the most commonly reported mosquito-
borne disease in Alameda County, with a total of 118
confirmed, probable, or suspect cases reported from
2019-2023. Also reported during this time period were
a total of 57 Malaria, 12 Chikungunya, 6 Zika, and 4
West Nile virus confirmed, probable, or suspect*
cases. As many of these diseases are associated with
international travel, there was a sharp decrease in
2020 due to the COVID-19 pandemic.
Case Demographics
The majority of cases occurred in
residents ages 18-65 years old. Case
rates were highest among African
American or Black individuals, although
cases were most commonly reported in
Asian American residents. About half of
the cases occurred in females, and half
in males.
Table 1. Demographic Characteristics of Mosquito-Borne Disease Cases in Alameda
County, 2019—2023
Demographic Category Cases Percent Rate per
100,000
Age Group 0-5 yrs 3 1.5%2.7
6-17 yrs 19 9.6%8.2
18-35 yrs 54 27%14.4
36-50 yrs 70 36%21.8
51-65 yrs 42 21%14.4
>65 yrs 9 4.6%4.2
Race/
Ethnicity
African American or Black 38 19%25.1
Asian American 56 28%11.1
Hispanic or Latino/a/x/e 31 16%8.7
Pacific Islander < 5 — —
Multiracial < 5 — —
White 19 9.6%4.2
Other/Unknown 49 25% —
Gender Female 97 49%12.3
Male 99 50%13.1
Other/Unknown 1 0.5% —
*Suspect cases are not included in the CSTE case definition for these four diseases, and make up 5% of
cases for these diseases.
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Clinical Characteristics and Epidemiology
No reported mosquito-borne case-patients died during this period, and no Oropouche Virus cases
were reported.
Dengue
Thirty percent of Dengue cases were hospitalized, and 99% of those interviewed reported
international travel (Table 2). The most commonly reported countries of travel included India (61
cases), Mexico (16), and the Philippines (7).
During this 5-year period, results for 257 patients tested for Dengue were reported. Of those, 225
of 257 (87%) of Dengue reports were tested by IgM/IgG serology only. CDC recommends testing
for acute Dengue using a RT-PCR (i.e., a nucleic acid amplification test [NAAT]) or an NS1 antigen
test, as well as with an IgM enzyme-linked immunosorbent assay (ELISA) antibody test, regardless
of symptom onset date, although the sensitivity of RT-PCR and NS1 antigen tests decrease after
the first 7 days of symptoms.
There are four serotypes of Dengue, DENV1-4, that an individual may be exposed to over their
lifetime; exposure to one serotype does not provide immunity from other serotypes. Repeated
exposure and infection with different serotypes of DENV increases the risk for developing severe
outcomes, including Dengue Hemorrhagic Fever.
Chikungunya
Twenty-five percent of Chikungunya cases were hospitalized, and all travelled internationally
(Table 2). The most commonly reported destination was India, with 5 cases.
Malaria
Over half of the malaria cases were hospitalized. All had reported international travel (Table 2). The
most common destinations were Nigeria (18) and Sierra Leone (8). Only 7 (12%) reported taking
chemoprophylaxis, and of those, only 1/3 took the full course. Of the 57 Malaria cases, 44 (77%)
were caused by Plasmodium falciparum, 8 (14%) were caused by P. vivax or P. ovale, two (4%) were
caused by P. malariae, and three were unspeciated. The median time from symptom onset to test
date was 5 days, and the average time was 14 days.
Zika
None of the Zika cases were hospitalized, and 60% were pregnant (Table 2). All interviewed cases
travelled internationally, to different countries in the Caribbean and Central America, and India.
West Nile
Three of the four West Nile patients interviewed had travelled domestically in the United States;
75% of the reported cases were hospitalized.
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For questions or additional information, contact:
Alameda County Public Health Department
Acute Communicable Diseases Section
1100 San Leandro Blvd, San Leandro, CA 94577
Phone: 510-267-3250
https://health.alamedacountyca.gov/pro
gram/acute-communicable-diseases/
Last updated April 14th, 2026
Table 2. Epidemiologic and Clinical Characteristics for Mosquito-borne Disease
Cases*
Clinical or
Epidemiologic
Characteristic
Chikungunya
N = 12
Dengue
N = 118
Malaria
N = 57
West Nile
N = 4
Zika
N = 6
Pregnant 0 / 3 (0%)1 / 13 (7.7%)1 / 7 (14%)0 / 2 (0%)3 / 5 (60%)
Hospitalized 3 / 12 (25%)31 / 102 (30%)29 / 55 (53%)3 / 4 (75%)0 / 4 (0%)
Death 0/12 (0%)0/118 (0%)0/57 (0%)0/4 (0%)0/6 (0%)
International Travel 12 / 12 (100%)111 / 112 (99%)57 / 57 (100%)0 / 4 (0%)4 / 4 (100%)
Domestic Travel**2 / 12 (17%)0 / 104 (0%)0 / 57 (0%)3 / 4 (75%)2 / 4 (50%)
Recalled Mosquito
Bite
8 / 12 (67%)63 / 111 (57%)—2 / 4 (50%)0 / 4 (0%)
Did Not Recall
Mosquito Bite
2 / 12 (17%)25 / 111 (23%)—1 / 4 (25%)3 / 4 (75%)
Unsure if Recalled
Mosquito Bite
2 / 12 (17%)23 / 111 (21%)— 1 / 4 (25%)1 / 4 (25%)
*Percents are calculated based on the number of patients interviewed for each question.
**Cases reporting solely domestic travel were West Nile cases. All other cases reporting domestic travel
also travelled internationally.
Figure 2. Map of International Travel Locations and
Disease Reported for Returning Travelers with
Mosquito-borne Diseases, 2019-2023*
*Sizes of circles correspond to number of cases. Cases that visited multiple
countries are represented in each country visited. A data table is linked on
our data website page
Summary
From 2019-2023, there were
197 confirmed, probable, or
suspect mosquito-borne
illnesses reported for Alameda
County residents. Most cases
were associated with
international or domestic
travel, with India (70), Mexico
(18), and Nigeria (15) being the
most visited countries. There
were no deaths, but many
patients were hospitalized,
especially for malaria.
Measures to prevent mosquito
bites and chemoprophylaxis
are recommended for
individuals traveling to areas
with endemic disease.
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