Animal and Plant Health Inspection Service | Veterinary Services | 1
1. Disease Information
1.1 General Disease and Pathogen Information: Varroosis is infestation by the
ectoparasitic mite, Varroa destructor, of brood and adult honey bees, Apis mellifera.
The mites are found throughout the world due to the global scale of honey bee trade
and difficulty in detection. Nearly all honey bee colonies around the world are infested
with Varroa to some degree. Varroa infests late-stage larval honey bees just before the
brood cell is sealed for pupation. In the pupal cell, the foundress Varroa lays four eggs,
the first of which is a male and the other three are female. Mating occurs between
these siblings resulting in highly inbred populations. These Varroa will feed on the fat
body of the pupa and cause physical damage, induce physiological changes, and
transmit viruses. The male mite offspring dies before the bee emerges as an adult and
the foundress with two or three daughter mites escape the brood cell with the
emerging bee. In adult bees, the female Varroa will most commonly insert herself
between the abdominal sclera and puncture the intersegmental membranes to feed on
the host’s fat body. Although they are mostly present on the honey bee’s abdomen, the
mites can also be found between a bee’s head and thorax. Mites move freely between
bees in the hive and can be spread between colonies by bee drift, robbing of failing
colonies by other bees, or movement of infested bees and equipment by beekeepers.
1.2 Clinical Signs: A single mite can cause changes in a bee’s behavior, increase its
susceptibility to other diseases, and ultimately shorten its lifespan. If more than one
mite enters a brood cell to complete their reproductive life cycle, emerging bees are
weakened from feeding activity and virus transmission by Varroa. Emerging bees
show severe developmental defects, such as absence of wings, shrunken wings, loss
of hairs on the body, or shortened abdomens. Often, these highly impaired bees die in
a few days of emergence. High rates of Varroa-infested brood may result in spotty
brood patterns or large numbers of uncapped pupae that may have their heads
chewed down by worker bees. Parasitic infestation increases with increasing brood
activity and bee population growth, particularly in August and September. Early signs
of infestation regularly go undetected and are often only noticed at late stages when
damage at the colony level is apparent, characterized by clinical signs such as
scattered brood, crawling or crippled bees, supersedure of queens, and substantial
reduction of the bee population. Infestation with Varroa mites poses the risk of
additional exposure to various honey bee viruses that can be vectored by the mites,
especially variants of Deformed wing virus (DWV) and Chronic bee paralysis virus
(CBPV). Honey bees from healthy colonies may rob failing honey bee colonies of their
nectar and pollen resources. This as noted by a large amount of bee activity near the
colony that is failing due to high Varroa infestation.
2. Laboratory Criteria
2.1 Agent Isolation and Identification: Diagnosis of varroosis is based upon examination
of the hive debris, the brood, or the adult bees. Visual observation of Varroa on adult
Case Definition
Varroosis of Honey Bees
(Monitored)
December 2023
Animal and Plant Health Inspection Service | Veterinary Services | 2
bees is usually a qualitative sign of high infestation as most Varroa in the colony are in
the sealed brood cells. Varroa on adult bees can be measured quantitatively by
washing a sample of approximately 300 bees with either powdered sugar, soapy
water, or alcohol (usually 30-50% ethanol or isopropanol) and counting the number of
Varroa that are dislodged. Generally, greater than 10 Varroa in the sample is
considered a high infestation rate. However, this threshold varies throughout the
season with lower thresholds occurring earlier in the season (i.e., 3 Varroa in April)
and among different stocks of honey bees. Examination of the brood requires a
microscope and forceps for the removal of cappings and pupae to observe signs of
Varroa infestation such as the foundress Varroa, eggs, offspring, and the presence of
small white spots of fecal matter on the cell walls. Quantitative examination of Varroa
infestation of honey bee brood requires inspecting 100 pupal cells or observing 30
cells infested with Varroa to calculate the percentage of brood cells infested with
Varroa. Bees with crumpled or malformed wings due to DWV is a reliable indicator of
high Varroa infestation since this virus becomes apparent in colonies with high levels
of Varroa infestation.
2.2 Agent Characterization: Partial and whole genome sequencing are recommended for
characterization. Taxonomic expertise is required for morphological validation.
2.3 Serology: NA.
3. Case Definition
3.1 Suspect Case: A hive experiencing colony decline and mortality or overt signs of viral
infection while waiting for positive identification of the mite.
3.2 Presumptive Positive Case: Positive identification of the mite within the bee colony.
Varroa are commonly found and morphologically distinct which makes them easy to
recognize with minimal expertise.
3.3 Confirmed Positive Case: Suspect case with the taxonomic identification of Varroa
samples collected from the affected hive.
4. Reporting Criteria: Varroosis is a U.S. monitored condition that is reportable monthly under
the APHIS National List of Reportable Animal Diseases (NLRAD).
4.1 NLRAD reporting in accordance with the NLRAD Standards for monitored diseases;
and by APHIS to the World Organisation for Animal Health (WOAH).
