Answer to the Parasite Case of the Week 706: Haycocknema perplexum infection
Wow, I am so impressed by the responses on this case! There are many helpful comments and links in the comment section of this blog which you may want to check out.
Haycocknema perplexum infection (haycocknematosis) is an extremely rare parasitic infection. There have only been 13 humans cases (including this one) reported to date. As Florida Fan noted, Blaine and I previously presented this case as a poster, and it has now been published in Emerging Infectious Diseases. You can read the article HERE. The article describes the diagnostic features of this nematode in histologic sections, including the presence of adult females containing fertilized eggs. I included a photo of one such female in this case as it played a key component in the diagnosis.
Note the presence of numerous (8-12) eggs (arrows) within the body of the female in this image. These eggs will hatch within the female, develop to 3rd stage larvae, and then burst through the head of the female, killing her in the process (yikes!) The larvae will mature into male or female worms within the muscle, and these will then mate and propagate the infection. This allows the infection to last for years (and potentially for the life of the host).
As noted by Nema, it is very important to differentiate this infection with trichinellosis (a.k.a. trichinosis), as the prognosis and treatment are very different. While both nematodes infect striated muscle throughout the body, the following are important features that allow for differentiation of haycocknematosis and trichinellosis:
- Trichinellosis is acquired through ingestion of undercooked meat (e.g., pork, wild boar, bear, moose) containing viable Trichinella sp. larvae, and it is common to see outbreaks of disease among individuals with the same exposure. In contrast, we don't currently know how haycocknematosis is acquired, but there have not yet been any reported outbreaks.
- Trichinellosis is seen in many parts of the world, whereas human cases of haycocknematosis have only been reported from patients with exposure to Australia, specifically Tasmania or northern regions of Queensland. Exposure to the Australian bush and marsupial wildlife may be associated with infection.
- Trichinellosis is self limited in the host. After ingestion of affected meat, the larvae are released in the stomach of the host and mature into adults in the small intestine. Male and female adult worms mate and the female releases larvae into the bloodstream. Larvae then migrate throughout the body and encyst within muscle cells. If they are ingested by the next predator, the life cycle is continued (https://www.cdc.gov/dpdx/trichinellosis/index.html). If they aren't ingested within months to years, they will die and eventually calcify. Unless more infected meat is consumed, the numbers of larvae in muscle will not increase over time. This is very very different from Haycocknema perplexum in which the life cycle continues indefinitely, with more and more parasites produced over time!
- Given the life cycle of Trichinella spp., the symptoms of trichinellosis are limited to the acute stages when the adults are in the intestine and larvae are migrating through the tissues and encysting in muscle. Once the larvae are done encysting, the symptoms will end. This is in sharp contrast to haycocknematosis in which symptoms increase over time as more and parasites accumulate.
- Lastly, only larvae are found within muscle in cases of trichinellosis. The presence of adult females allows us to definitely rule out trichinellosis in this case. Also, unlike most cases of trichinellosis, the worms of H. perplexum are not encysted within muscle and nurse cells are not formed.