Surface detail of mature SCP-4167 specimen.

Item #: SCP-4167
Object Class: Keter
Special Containment Procedures: Living hosts of SCP-4167 are to be kept in a secure humanoid quarantine ward at Site 66. Social interaction between hosts is permitted, but physical contact should not be allowed - see Incident Report 4167-2. Hosts are likely to be compliant, but careful surveillance is required in case of attempted self-mutilation of the cranium. All equipment, personnel, and waste leaving the containment ward must be inspected and sanitised in accordance with Bio-Safety Level 4 standards.
In case of host death, an attempt should be made to retrieve the specimen of SCP-4167 alive and intact. When surgery is successful, mature specimens are to be contained in standard aquatic specimen tanks. Tanks should be filled with cerebrospinal fluid, or synthetic substitute, and nutrient solution NS-4167 circulated regularly.
A sample cluster of viable SCP-4167 eggs is to be kept in secure cold storage. All other eggs must be destroyed immediately upon discovery.
Modern standards of sanitation have greatly decreased the prevalence of SCP-4167 infection, and the Foundation's ongoing efforts in this matter are critical. However, at the present time SCP-4167 must be considered uncontained. Effective immunisation methods are an urgent research priority. A feasibility study on the eradication of wildlife reservoirs is being conducted by Dr. Nazario.
Description: SCP-4167 is Heterophyes neurensis, a species of parasitic flatworm. Specimens range in length from 1 mm to 20 cm, depending on maturity. SCP-4167 exhibits behaviour somewhat comparable to Cymothoa exigua1. Juvenile specimens of SCP-4167 may be found dormant in freshwater fish and birds, but human hosts are required for reproduction.
Infected humans may experience a variety of neurological symptoms including dizziness, hallucinations, hemiparesis2, amnesia, aphasia3, and personality changes. Symptoms are usually mild, and seldom last more than a few days. Personality changes are the exception, with many hosts displaying two persistent effects: a significant reduction in self-reported stress and anxiety, and tendency towards reclusive behaviours. Researchers have noted that these traits reduce the likelihood of other symptoms resulting in medical attention. Serious complications are rare, and researchers estimate that only ██% of infections are discovered prior to the death of the host.
Upon entering a human body, a juvenile specimen of SCP-4167 will migrate through blood vessels to the spinal cord. Once it has breached the central canal, it will swim upward to reach the subarachnoid space within the cranial cavity. The specimen will then clamp itself to the surface of the brain, before beginning to consume neural tissue. A complex network of nervous fibres develops on the specimen's lower surface, penetrating the brain. As tissue is removed from the brain, the specimen will grow to fill all available space. It typically takes 10-15 years for this stage of SCP-4167's life cycle to run its course.
Despite severe trauma to brain tissues, hosts of SCP-4167 tend to manifest only mild neurological effects, generally maintaining most cognitive abilities. Post-mortem dissection reveals that, below a tough yet flexible outer skin, the majority of a specimen's body is composed of non-human neural tissue. Testing has confirmed the replication of some human brain structures within this tissue. The nervous fibres connecting specimen to host display a high level of synaptic activity. All evidence suggests that SCP-4167 contributes its own cognitive capacity in order to maintain normal functioning of the host. The level of consciousness attributable to SCP-4167 is a matter of some debate - see Interview Log 4167-1.
Specimens of SCP-4167 display a reluctance to destroy the corpus callosum, the structure which connects the two halves of the brain. They are therefore considered to have reached full maturity once one entire hemisphere of the brain has been replaced. At this point their growth will cease, and production of eggs will begin. If unsupervised, the host of a fully mature parasite will attempt to create an opening in their skull, by any means available. Following this, they will seek out fresh water and immerse their head, whereupon clusters of SCP-4167 eggs will be released. This process is often fatal for both organisms. Sexual reproduction of SCP-4167 is as of yet unobserved.
Interview Log 4167-1:

Interviewed: Subject 4167-K. Male, 35 years old. SCP-4167 infection progress: 70%.
Interviewer: Dr. Nazario.
Foreword: Entry interview shortly following retrieval of subject. Subject spent the previous night in the ward, and remains in bed. Security Officer J. Ballard is supervising.
<Begin Log>
Dr. Nazario: Good morning, Mr. Yang. How are you finding our facilities?
4167-K: Fine, I suppose. It's not so bad, being away from… everyone out there, for a while.
Dr. Nazario: I imagine you'd like me to explain why you were sent here.
4167-K: I guess so.
[Dr. Nazario describes SCP-4167, and shows the subject where it can be seen on his scans. Subject momentarily displays signs of distress, and then is seen to relax. After a pause, he speaks.]
4167-K: So, it's real, then. I suppose I knew that.
Dr. Nazario: You were aware of the worm?
4167-K: Not exactly. But… it knows it's there. It remembers, so I do too. It remembers for me. We remembered we weren't alone.
Dr. Nazario: I see. I've heard similar things from other patients. Please, describe some of these memories for me.
4167-K: Ah. Hmm. I'm sorry, it's like trying to remember a dream. A good dream, though. Warm. Wet. Safe. Red… Hey, doctor, can I just ask, how long do you think I'll be in here for? The last hospital, they had some trouble talking to my insurance company, and -
Dr. Nazario: That won't be a problem.
4167-K: Okay, it's just, this place looks expensive.
Dr. Nazario: We provide our services free of charge. You'll be able to stay here, secure and protected, until we can get you fixed up.
4167-K: Fixed up? I'm sorry, you don't mean…
[Subject's body language indicates high level of fear.]
4167-K: There's no way you can take it out of me. I, I need it now. I can't think without it, I can't be me without it. That just can't happen.
[Subject makes a move as if to climb out of bed. Security Officer prepares to enter the room if necessary. ]
Dr. Nazario: Hey, hey, easy. Nobody's suggesting that right now. We'll only do what's best.
4167-K: Not right now and not ever, got it?
Dr. Nazario: I understand, sir. You seem very certain, and I hear you. I believe you. It's a bad idea. I have to say, though, you formed this opinion very quickly. Do you think… Did the creature maybe speak to you?
4167-K: I don't know. Maybe. It doesn't matter.
Dr. Nazario: I wonder, perhaps, if I could speak with it. Directly. Can you help me?
[Subject sits silently for some time, then slowly shakes their head.]
4167-K: No. No, that's not it at all. There's no just talking to it, or talking to me. There's only talking to us, and you've been doing that since you got here.
<End Log>

Incident Report 4167-2:
11/07/██: Subject 4167-C (Female, 27 years old. Infection progress: 100%) was apprehended following fatal assault on Subject 4167-F (Female, 53 years old. Infection progress: 100%). 4167-C forcibly destroyed 4167-F's skull via repeated bludgeoning against a door frame. Surveillance footage shows that 4167-F made no attempt to resist. 4167-C was observed attempting to bite and swallow human and non-human neural tissue before being restrained. When questioned, 4167-C said the following:

4167-C: Please, I'm sorry. It's just… I can't… I'm not finished, I'm not whole. We can't stand it. Helen and me, we agreed… her child would complete me. I need it. Please, let me make sure it's done.

Scans have confirmed a secondary infection of Subject 4167-C.

Footnotes
1. A parasitic isopod which consumes and replaces the tongue of a fish.
2. Weakness of one side of the body.
3. Difficulty in using or understanding language.

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