Honey bee viruses: why finding one is not finding disease

Infection is not disease

Most of what is written about bee viruses goes wrong at the first step, by treating the presence of a virus as an explanation. Colonies commonly carry deformed wing virus, black queen cell virus, sacbrood virus and chronic bee paralysis virus at low levels with no sign of illness. The same virus can be harmless in one colony and lethal in the next, and what usually decides which is something other than the virus: how many varroa mites are injecting it, how crowded the colony is, what else is stressing it.

Deformed wing virus and the mite

Varroa makes deformed wing virus dangerous by changing how it reaches the bee. A mite feeding on a developing bee injects the virus directly, bypassing the gut, and the virus multiplies inside the mite as well as in the bee; the WOAH standard says DWV is the only bee virus for which that replication in the mite has been shown. Mite feeding also raises the amount of virus in the bee and can set off infections that were already present at low levels. The outcome follows the mite count.

Acute bee paralysis virus, Kashmir bee virus and Israeli acute paralysis virus are carried by varroa too, and are found in collapsing colonies carrying many mites. They have no reliable field sign of their own. For all four, the practical measurement is the mite count rather than a virus test, and the practical response is managing the mite.

Chronic bee paralysis

Chronic bee paralysis is the exception to the varroa story: the National Bee Unit says the virus is not thought to be transmitted by the mite. It replicates in nerve cells. Affected bees tremble, cannot fly, and some lose their hair and look black and shiny; they die within about a week, and the NBU says roughly 40 to 50% of colonies with the disease are lost. It is most often seen in early summer.

Inspection records for England and Wales show cases rising exponentially between 2007 and 2017. The apiaries of commercial beekeepers had about one and a half times the risk, and importing bees in the previous two years raised it; cases clustered within a year but not from one year to the next. The study describes England and Wales and is not evidence of a trend anywhere else.

Black queen cell virus and sacbrood

Black queen cell virus kills developing queens: the larva or pupa dies in its cell and the cell wall turns dark brown to black. It passes in brood food and from queen to offspring, adult bees usually carry it without signs, and it is associated with the gut parasite Nosema apis. It is found at high levels in collapsing colonies — an association, not a cause. Sacbrood virus kills worker larvae after capping and is covered with the brood diseases; it rarely kills a colony of the western honey bee, but has caused losses of 95 to 100% in the Asian honey bee.

Beyond the honey bee

Honey bee viruses are not confined to honey bees, and the evidence for that shows the infection-and-disease problem in its sharpest form. A 2022 review of sacbrood virus collects reports of it in the dwarf and giant honey bees and in wild pollinators including bumblebees and hoverflies, with detection rates averaging above 37% in common wild bee species and up to 91.7% in some bumblebees. The same review is careful about what those figures mean: most of the work tested whole insects by RT-PCR, and finding the virus in an insect does not show that it infects that insect, let alone harms it. It also records that the virus was found less often in richer pollinator communities.

Predators carry them too. A study of one yellow-legged hornet nest near Bologna found deformed wing virus in about 83% of samples across the hornet's life stages, and the gut parasite Nosema ceranae in about 67%, with evidence that the viruses were replicating. That is a finding about one nest, and it does not show that hornets pass the viruses back to bees; it shows that the virus story does not stop at the hive entrance.

The viruses at a glance

What a virus test can and cannot tell you

A laboratory test — usually RT-PCR — shows that a virus is present and, if quantified, how much of it. It does not show that the virus caused what the beekeeper saw: a sacbrood review is explicit that a positive result alone does not demonstrate disease. Virus tests answer research and surveillance questions well. For the mite-borne viruses, the decision a beekeeper faces rests on a mite count, and for chronic bee paralysis on the pattern of dead and dying bees, with a sample to the authority where it matters.