Medical Corner:
The Worms Crawl In, The Worms Crawl Out

Jan McArthur, RVT
From the September/October 1999 Rat & Mouse Gazette

Endoparasites are the internal parasites of living creatures. Infected animals are called the host for these parasites. These nasty creatures will feed from the blood of the host, or rob them of their needed nutrition. This is why it is vital to keep your pets free of these parasites or your pets will suffer with illness that eventually will lead to death if left untreated.

With ectoparasites such as mites and lice, the treatments are usually the same. Not so for the endoparasites. There are different classifications for endoparasites, and different life cycles, because of this the drugs for treatments are not the same. When your mice and rats are infected with these parasites you do need to consult a veterinarian and a fecal examination must be performed, first to identify the species of parasite and then to determine the method of treatment. Some endoparasites are not even worms; they are single-celled organisms called protozoa. Those require a completely different class of drugs for treatment.

Each species of worm or protozoa may be a parasite for one animal but not another. The worm that affects your dog may not affect your rodents and vise versa. If a parasitic worm enters the wrong host, we call that a dead host because the parasite will not complete its life cycle in a dead host so it just dies out naturally with no treatment required. What is important is to treat the pathogenic (disease causing) parasites in our pets.


  • Helminthes are parasitic worms. The classes of worms are:
  • Nematodes, the round worms.
  • Cestodes, the tapeworms.
  • Trematodes, the flukes or flatworms. Luckily there are none that infect rodents.
  • Acanthocephalans, the thorny headed worms. In a class by themselves, these are most common in avian and reptilian species rather than mammals.
  • Protozoa, the single celled organisms, not worms at all.


Nematodes are the roundworms. There are thousands of species of nematodes, but only a small percentage of those are parasitic to animals. Because there are many different species of nematodes, some have varied and complicated life cycles. Certain species of them can be found in almost any tissue of the body. Some of them migrate through the body in various stages of development. While in migration they pose different problems than they do when in the intestinal tract which can be more difficult to treat. The nematodes that infect mice and rats are the Oxyurids, commonly called Pinworms. Syphacia obvelata, Aspiculuris tetraptera, and Syphacia muris, are the species that infect them both, with the latter being the main pinworm of rats. Pinworms are basically commensal, which means they neither harm nor benefit the host. Mice and rats normally carry light to medium loads of pinworms with so signs of disease; however, it is important to keep those numbers of pinworms down because a heavy load can cause rectal prolapse, enteritis, intestinal impaction, sticky stools, and pruritus (itchy skin). Transmission of these happens when the ova (eggs) of the parasites are shed through the feces then accidentally ingested by a rat or mouse. Even gerbils and hamsters may be infected with these.

There is another nematode that is pathogenic in rats. This is Trichosomoides crassicauda, a threadworm found in the bladder. This one is species specific and normally causes no disease; however, it can cause granulomas in the lungs while in migration. It is transmitted via the urine /oral route. It produces few signs of disease, and none of these mentioned so far are considered a threat to humans.


Cestodes are the tapeworms. Mice, rats, gerbils, and hamsters are the definitive hosts for the tapeworms called Hymenolepis nana, Hymnenolepis diminuta. Mice and rats are the intermediate hosts for the feline tapeworms, Taenia taeniaeformis. It is important for those who own both rodents and cats to be aware that objects, feed, or bedding contaminated with cat feces may lead to infection of their pet rodents. If your cats are healthy and regularly dewormed, they will not pose a problem for your rodents. All tapeworms are spread via the fecal/ oral route or through infected insects. It's important to keep wild rodents and insects away from your pets. Keep your dogs and cats free of tapeworm infections and treated for fleas so that their fleas do not spread disease to your rodents.

H. nana is the most important of the tapeworms because it does not require an intermediate host to complete its life cycle. What that means is that the worm can live its entire life in one host, so it will cause disease and it can cause disease in humans. H. diminuta can also be infective to humans, but it requires an intermediate host such as a flea, cockroach, or grain beetle to complete its life cycle, so although it can infect humans its potential for doing that is low. In order to become infected, you must ingest the intermediate host.

Symptoms in mice and rats for tapeworm infection range from no signs at all to constipation, abdominal distension, lethargy, diarrhea, chronic weight loss, and emaciation and death.


The thorny headed worm, Moniliformos moniliformis, may live in the small intestine of rats, mice, and other rodents. Although not a common parasite in rodents, it can cause serious problems such as enteritis, ulceration, and intestinal perforation leading to peritonitis.


Coccidia is a broad term for a group of many similar one celled parasites. There are at least eight types of the Eimeria genus of coccidia that infect mice and four that infect rats. Transmission is as usual, the fecal/oral route. These may or may not be normal flora for the pet. Naming each individual Emeria species is not necessary because, if these are found in a fecal examination, that does not necessarily mean that they are a problem. Treatment depends on whether or not the mouse or rat is exhibiting symptoms of disease such as soft, watery feces with a foul odor, tinged with blood, or containing mucus, are greenish brown, and smear the fur of the mouse or rat. Other symptoms include enteritis, mild to severe dehydration, lethargy, rough hair coat, and failure to gain weight; in severe cases, hemorrhage which can lead to death.

The Cryptosproidium species of coccidia is worth mentioning specifically because humans can become infected. Special precautions for safety must be taken with animals suspected of carrying this one.

Other pathogenic protozoa that infect mice more so than rats are the flagellates Giardia muris, Spironucleus muris, Pneumocystis carinii, and Hepatozoon muris. P.carinii are found in the lungs of mice and rats so these may cause respiratory symptoms The symptoms of the others are enteritis particularly in newly weaned mice, diarrhea, weight loss and sporadic deaths. Coccidia infections are not all species specific, if you own other rodents you need to know that. There are species of Giardia known to infect humans, but it is believed that the Giardia of mice will not.

Endoparasites are so varied that there is no one treatment you can use that will rid your pets of them. Without identification of the parasite, you have no way of knowing what drug to use. It's always best to seek the treatment of a veterinarian. Using the wrong drug can be devastating to your pets because if you do that you may not have killed the parasite, leaving it free to continue feeding on your pets, which may lead to their death.

Prevention is, of course, the best way to handle these nasty parasites, but that's not always possible. Keeping your pets clean and away from wild animals or other potential carriers is the goal. Once they are infected, treatment by a veterinarian almost guarantees a healthy pet!


Host: An animal infected by a parasite
Definitive Host: The final host in which the parasite reaches sexual maturity and is able to reproduce itself
Intermediate Host: A host that the parasite passes through the larval or asexual stages of development.
Pathogenic Parasite: A parasite that causes disease in its host.
Dead Host: Host that a parasite infects, but is the incorrect host for the parasite, leaving the parasite unable to continue its development. The parasite then dies a natural death with no treatment.
Direct Life Cycle: Parasites that do not require an intermediate host to complete its life cycle. Those with a direct life cycle such as some tape worms are often easier to treat.
Indirect Life Cycle: Parasite that does require an intermediate host to complete its life cycle. It can pass from host to host infecting many during its life cycle.
Granuloma: A mass of granulated tissue that has formed in repsonse to a chronic infection, inflammation, foreign body, or of unknown cause.