Altered Brood Pattern & Sacbrood Virus

A (Usually-Minor) Disease with a Poor Brood Pattern

Sacbrood virus causes the skin of infected larvae to form into a bulging “sac” of fluid, filled with viral particles (not to be confused with “sack” [English is difficult]). This is a minor brood disease that honey bees can clear on their own with good hygiene. The beekeeper assists by removing infected brood cells and minimizing stress from honey bee diseases and poor nutrition. If the outbreaks continue, consider providing the colony with a new queen from more disease-resistant stock.

This article will provide an overview and basic management techniques of sacbrood virus infections for Apis mellifera, and follow on with more technical information on diagnosis and subtypes of the virus. It then examines why sacbrood is more lethal to Asian honey bees, Apis cerana, and compare this disease with American Foulbrood.

Brood Nest Inspections

So what does the beekeeper look for when conducting a hive inspection? Infected larvae go from the creamy-white colour into a paler white, before darkening to a hard, brown scale¹. When conducting hive checks, inspect the developing larva- do they show signs of discolouration? Do any of the cells contain dead larvae with bloated appearances?

Treating Sacbrood

As with many other infections, 80% of the work is done by bees, and 20% by the beekeepers. Several management strategies are:

• Inspect the brood box regularly during the brood rearing season (spring and summer) and look for the following (the list is generalized and can be applied to other brood diseases):
  • Is it a spotty brood pattern or a good brood pattern?
  • Are the brood frames in good condition with a good ratio of drone and worker brood?
  • Does the brood area contain dead larvae?
• If there are a few infected larvae/cells, manually remove them by cutting or with tweezers. Be sure to clean and sanitize your beekeeping tools after use (a strong soapy solution will suffice).
• Check for other sources of stress: is there sufficient pollen and nectar forage for the bees? Do they require supplemental feeding? Are the small hive beetles, wax moths or Varroa mites at tolerable levels?
  • Sacbrood virus infections can make hives more susceptible to other pests and diseases and vice versa; poor colony health from pests and diseases can make them more susceptible to sacbrood infections.
• For recurring outbreaks, consider introducing a new queen bee from a reputable source and/or rehome the the bee colony with the “shook swarm” method – shaking the bees into a new hive box with new frames.

The basis of these strategies are early detection and early treatments. Antibiotics are not useful for sacbrood treatments as it caused by a virus, unlike in say, European foulbrood cases.

Diagnosing Sacbrood Disease

Laboratory tests available for confirming a sacbrood virus diagnosis (Grabensteiner et al., 2001), yet for many beekeepers in the field, it is a matter of brood pattern observation: the appearance of multiple uncapped cells amongst capped brood is a good indicator of something wrong (note that this is easy to confuse with other diseases like American or European foulbrood). The affected larvae will change in colour depending on the disease progression: initially, they appear off-white and later become glistening and watery, ending in a dark brown or black colour in a crescent-like shape.

If the beekeeper wishes to remove the larval remains by hand, they will note the skin is easily torn and can rupture to reveal the watery, viral-loaded fluid within. Otherwise, the dead larva has a tough, rubbery sack consistency. Unlike AFB, there is no foul odour associated with this disease, and the matchstick test will not show ropiness. It is more likely during the brood rearing seasons and the concerned beekeeper can opt for a confirmation via laboratory testing.

How do Bees get Infected with Sacbrood Virus?

Infected burse bees transmit sacbrood virus to larvae via during feeding. While adults get infected, they act as asymptomatic carriers, transmitting it through normal colony behaviours (e.g. trophallaxis) or through beekeeper activities like moving frames.

Larvae are the primary target of the sacbrood virus, and are infected when worker bees feed them contaminated food. The virus replicates in the larval gut, spreading to other tissues and preventing it from pupating (Wei et al., 2022). The larva dies in a fluid, sac-like mass, which triggers the hygienic behaviour of honey bees to clear out cells and thus spreading the infection. The dead larvae would have changed colour from white, into yellow and ending at a brown/black shade with dry & rubbery scales. No foul smell is associated with sacbrood disease, unlike AFB.

So, food-based transmission is a major driver of sacbrood cases in larvae. How about the adult bees? Studies have shown that sacbrood virus particles are present in drone semen and queen bee ovaries, and that an infected queen lays infected eggs (Phokasem et al., 2021). As a result, one indication of failing queen quality is persistent sacbrood virus infections, and why queen replacement is a suitable treatment (through beekeeper intervention, rather than colony-mediated re-queening via queen cells [as that newly-emerged queen is likely infected!]).

Infected adult bees do not show visible disease symptoms, however Anderson & Giacon (1992) found that sacbrood-infected adults had a high preference for nectar over pollen, resulting in overall nutritional deficiencies within the honey bee colony and a reduced overall lifespan.

Origins of Sacbrood Virus

As with other specific pathogens of bees, the origins of sacbrood virus are not documented; the virus is thought to have co-evolved alongside honey bees. The disease was first described in the U.S. in 1913, with its symptoms being specified in 1917. The sacbrood virus was not properly identified until 1964 (paraphrased from Grabensteiner et al., 2001). Now, sacbrood is found worldwide, with honey bees being the primary resivour. Over time, multiple strains have emerged, with the East Asian strains showing high colony lethality in Asian honey bees.

Strains of the Sacbrood Virus

Non technical readers are welcome to skip this section

The 2021 review of Wei and colleagues describe several genetic subtypes of sacbrood virus on the basis of geography: the Korean, Chinese and Thai sacbrood viruses, which belong to the AcSBV genotypes (i.e. A. cerana sacbrood virus). Previous work suggested that the individual SBV genotypes are specific to their honey bee host, however Sun et al (2017) showed the Chinese strain jumping species and infecting A. mellifera brood. The study reported a 85-90% A. mellifera larval mortality as a result of the AcCSBV infections, and further studies are required to determine the consequences of this host shift (after all, Varroa jumped from A. cerana to A. mellifera and look where that got us…).

AcSBVs are more infectious than AmSBVs, and the authors suggest that it is a only matter of time before the virus become prevalent in wild A. cerana populations. Many members in the Bombus species tested positive for sacbrood virus, including some solitary, native bees such as Megachile albisecta (leafcutter bee) and Hoplitis adunca (mason bee). Traces were found on small hive beetles and Varroa mites, however there was no evidence that the viral particles reproduced within these hosts, suggesting that they are physical carriers of the virus rather than disease vectors.

What Makes Honey Bees Susceptible to Sacbrood infections?

Unlike humans and other animals, insects lack an adaptive immune system (think B-& T-cells that “remember” an infection and thus provide us with an immunity to future infections). Sacbrood infections lead to an increase in molecules and proteins associated with the honey bee immune response. The virus disrupts normal biological processes such as metabolism, molecular cell structure and normal immune system functions (Han et al., 2013).

The social nature of honey bee colonies would mean that any pathogen can quickly spread inside the hive, whether through the food fed to larvae (particularly in the royal jelly-producing hypopharyngeal gland), or exchanged with other adult bees via trophallaxis. As mentioned above, if a queen bee is born infected, or inseminated with infected drones, this compromises the entire colony’s health, since that queen lays infected eggs. Most colonies can overcome the infection on their own, and it is those with a failing queen that require beekeeper intervention.

Of course, robber bees in a weak hive may pickup sacbrood virus particles from the infected stores, however they are more likely to contract something far worse, like a wandering Varroa mite or AFB spores. A more innocent introduction of the sacbrood virus into an otherwise-healthy colony is through bee drifting, in which a worker bee accidentally enters the wrong hive; introducing the virus when they deposit their contaminated forage into the hive stores.

Why is A. ceranae More Susceptible to Sacbrood Infections?

Sacbrood infected larvae exhibit different severities of the disease in A. cerana vs A. mellifera, as demonstrated by the differences in viral molecular structure. All Asian strains were found to cause widespread colony death in A. cerana (Huang et al., 2017, Ko et al., 2019). Temperature was excluded as a factor in increasing A. cerana susceptibility, as infection rates were lower during the warmer months (May to November), and this holds true for A. mellifera-infecting strains as well (Ko et al., 2019). When A. cerana sacbrood viruses infected A. mellifera, the symptoms became less severe. The authors suggest that if this same virus was able to cross back into A. cerana, it would make the European honey bee a reservoir for A. cerana viruses and further impacting A. cerana colonies.

One suggestion for why the viruses spread quickly in the Asian honey bees is due to their high absconding tendencies². In one review, the Thai strain of sacbrood virus killed 95% of the colonies in Eastern and Southern India, and was more destructive than the original “parent” sacbrood virus strain; there was a 5% survival rate for wild colonies infected with the Thai strain. Interestingly, the 2008 research of Rana & Rana showed that the Thai virus’ symptoms appeared on the ninth day of infection, two days after the brood cells were capped.

Another proposition has been on the basis of gut microbiota, which is involved a role in immune system function (Yongsawas et al., 2020). When A. cerana was infected with sacbrood, the population of Gilliamella Spp. decreased, whereas Snodgrassella and Fructobacillus increased. These bacterial groups were not significantly impacted in A. mellifera following sacbrood infection, supporting the hypothesis that the severity of sacbrood infections in A. cerana is due to microbiome dysfunction.

That’s not to say A. mellifera won’t be impacted – if an apiary contains both honey bee species and there is a dearth (or a stressful event), then cross infection between the two can occur, and suddenly all hives are infected by sacbrood. Attempts have been made to breed sacbrood-resistant A. cerana, however it was unknown whether this stock is resistant to all virus subtypes, as the researchers were focused on Thai sacbrood virus infections (Verma et al., 1990).

Comparing Sacbrood Virus and AFB

For more a comprehensive analysis into AFB, refer to the article on American Foulbrood. The table below summarizes the differences between the two:

Queen Health, Colony Health & Hygienic Behavior

As with many honey bee diseases, the overall colony health is determined by multiple factors, from the availability of diverse foraging sources, to a strong egg laying queen, to having bees with good hygienic behaviour. Sacbrood is a minor disease for the European honey bee Apis mellifera, perhaps due to their large colony size and relatively low tendency to swarm compared to the Asian honey bees (hence giving them more time to overcome infection), which have been decimated by outbreaks of sacbrood viruses.

So my dear beekeeper, we end this article with the following: to prevent sacbrood (or most diseases [most because AFB and Varroa are special]), keep strong colonies with a good laying queen, place them in an area with plenty of forage during the spring and summer and provide them with supplemental feeding if necessary. When inspecting, do it regularly and do it thoroughly; when one inspection can show good brood patterns, the next may be … not-as-good.

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Footnotes

1. The difference between a sacbrood scale and an American foulbrood scale is that the sacbrood scales are easy to remove, whereas the AFB-infected larval remains stick to the bottom of the brood cell wall. Also, larvae die during pupation in AFB, whereas in (most) sacbrood cases they die prior to cell capping. ↩︎
2. Indeed, A. cerana colonies can abscond as much as six times a year versus the one or two times of A. mellifera (covered briefly in on the scientific name of bees), which is thought to be an adaption to Varroa parasitism. ↩︎