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What should volunteers do with monarchs that have OE or other signs of illness?

Many people have asked us over the years what they should do with monarchs that either test positive for the protozoan parasite OE, or that have visible outward signs of OE infection. At Monarch Health, we recognize that the protozoan OE naturally occurs in all monarch populations examined to date and is often carried by wild monarchs. At the same time, we encourage volunteers to provide conditions that prevent unwanted transmission during rearing, and to avoid releasing large numbers of infected monarchs into the wild.

In a Nutshell:

  1. OE is a common and debilitating parasite of monarchs that can cause deformity and even death.

  2. Heavily infected monarchs with clearly visible signs of OE infection (e.g., deformity) should be euthanized by freezing.

  3. Monarchs with lower spore loads can be released, provided that the captive rearing conditions did not foster parasite transmission.

  4. Containers, cages, surfaces and nets that contact adult monarchs should be carefully sanitized with 20% chlorine bleach to kill OE spores and prevent transmission.

  5. Infected monarchs should not be kept as pets (as an alternative to euthanasia), as this will result in high rates of OE contamination to future generations of monarchs reared in the same household.

What should I do if a monarch appears healthy but tests positive for OE?

People who rear monarchs (or capture wild ones) will come across butterflies that appear outwardly healthy, but that test positive for OE upon examination of samples collected using Monarch Health kits. In these cases, a decision on whether to euthanize or release the monarch depends on several factors. Because OE infections in healthy looking butterflies can be difficult to visually identify, individuals must rely on OE testing score results to inform their decisions.

Case 1: Healthy looking Monarchs with few spores

Monarchs with only a few spores (less than 100 per entire tape sample) can generally be released into the wild. Research has shown that these monarchs probably emerged healthy, and later became contaminated with OE by contacting infected monarchs or substrates with spores.  Monarchs with low numbers of spores tend to lose their spores within 5-7 days. Volunteers should clean their nets and other surfaces carefully as a precaution.

Case 2: Healthy looking Monarchs with many spores

Yet other monarchs appear normal, but are found to be heavily infected (with many hundreds to many thousands of spores per tape sample) upon inspection. In these cases, we recommend that if a volunteer feels confident that the monarch was infected due to natural events, e.g. by eating spores on wild milkweed before being brought into captivity, they can release the monarch, as that infection was one that would naturally occur in the wild. In many parts of North America, by late summer, 10-20% of monarchs become infected with OE, although earlier in the breeding season, prevalence is much lower.

Case 3: When Rearing Causes OE Infection

If there is any indication that reared monarchs became infected due to conditions they experienced in captivity, we recommend those monarchs be euthanized. If adult monarchs are allowed to enclose in the same container where caterpillars are feeding, spores will be scattered over the milkweed that the caterpillars are feeding on, and on the inside of the container. This will expose caterpillars that would have been healthy to the parasite. Volunteers that handle infected adults might accidentally transfer OE spores to milkweed and rearing containers. A typical indicator of this is when over half of the caterpillars in the same cohort are heavily infected.

Healthy adult monarch.jpg

Pictured above an abdomen of a healthy monarch. Healthy monarchs have black and white scales that cover their abdomens and make the abdomen look bright and shiny.

Infected adults without scales.jpg

This picture shows a heavily infected monarch with normal looking wings. Heavily infected monarchs, in some cases, will lack most or of all of the white scales on their abdomens, giving the abdomen a sooty appearance. In severe cases, the abdomens will have bare patches where scales are lacking, giving them a green appearance.

What should I do when a monarch emerges deformed?

For badly infected monarchs that look like the photographs below, with crumpled wings, split proboscis, bare patches on the abdomen, or those that have difficulty emerging from their pupal cases, euthanasia is the most humane choice, as these monarchs would not survive for more than a few hours in the wild.

In these cases, we recommend placing the monarch in an envelope or small container and freezing them for 2 hours or longer prior to disposal.  Volunteers should be sure to use exam gloves (readily available in the first aid section of drug stores) when handling infected monarchs, and wash hands thoroughly to remove any stray spores. Also, sanitize rearing containers and surfaces that were contacted by infected monarchs using 20% Chlorine bleach.

It is important to note that OE spores are durable and can persist for many months or longer under favorable conditions, and that infected monarchs can be covered with millions of spores. Cleaning containers, cages, and other surfaces is a crucial part of rearing monarchs. Badly infected monarchs should not be kept as pets in cages or other enclosures (which is sometimes viewed as a more humane alternative to euthanasia), as this will contaminate the indoor environment with OE spores and result in widespread transmission to other monarchs in the same location.

split proboscis.jpg
Diseased Monarch
Monarch without scales

How can I ensure that I am not contributing to the spread of OE?


Community scientists cooperating with us will receive results of their OE samples by email. When we notice volunteers having an unusually high number of sick butterflies, we may contact individuals to recommend that they revise their rearing practices.


Here are some tips to limit disease transmission:

  • Raise small numbers at any given time, and put no more that 10 caterpillars (preferably only 1) in each container. High density rearing can cause stressed caterpillars that are more susceptible to disease. 

  • Collect monarchs at later stages (when they are resistant to OE) rather than as eggs.

  • Regularly clean rearing containers and surfaces with 20% chlorine bleach.

Are you rearing for the right reasons? Read our statement on responsible rearing here.

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