Definition

Any parasite that lives in and derives nutrition from its host.

Clinical Signs

Depending upon the type of parasite the following sign or signs may be seen.

However, with some types of infestation signs may not always be apparent, or be apparent in an otherwise healthy rat.

Protozoa, Helminths, Acanthocephala

• Diarrhea, blood tinged stool
• Dysuria (bladder threadworm)
• Distended abdomen
• Weight loss
• Listlessness
• Pruritus(itching) in and around the rectum (e.g., pinworms)
• Rectal prolapse

Cuterebra botfly larvae

• Subcutaneous warbles can appear as a nodule or lump, and may look like boils with small opening.
• 0ozing, pus, or inflammation may be present at the site of the warble.

Etiology

Endoparasites, Helminths (worms), Arthropoda (class: insect, family: Oestridae), Entazoa, and Protozoa (one-celled or grouped cells), discussed here are living organisms that enter a host by being ingested through direct transmission as cysts (larval stage of parasite), via an intermediate (e.g., ingestion of fleas, cockroaches, or grain beetles), or by indirect methods through skin penetration (e.g., blood parasites: see also
Ectoparsites). Endoparasites derive their nutrition by feeding off the host’s blood.

There are many species of endoparasites, but not all of these parasites infest or produce pathological symptoms in the domestic pet rat. The actual number of parasites present in the rat will greatly affect whether pathological symptoms are seen, as well as if the rat is already immunologically impaired, ill, or malnourished.

Some species of parasites are termed species specific. This means that they can complete their life cycle in only one species of host. Should they enter the wrong species they are unable to complete their life cycle and die, all generally without the host requiring treatment. Some species of endoparasites are host specific meaning that they have adapted themselves to a group of hosts.
Though there are but a very limited number of endoparasites that are host specific to both rats and man, the best prevention is to maintain a clean environment for your rat, prevent your rat’s exposure to other animals’ excrement, treat any external parasites promptly, and always wash your hands before and after caring for your rat and his environment.

Because there are many different species of endoparasites, some with complex life cycles that take place in different organs of the body, no one drug can effectively treat all parasites. It is for this reason that the veterinarian may require a stool examination, urinalysis, or in some cases blood work, to treat appropriately.

Listed here are those endoparasites of most interest to the domestic rat owner for being either the most commonly seen, but not significant in producing pathological symptoms, or those which do produce pathological symptoms in rats. Also mentioned are those of zoonotic concern to man, although, if adequate preventive care is maintained, are not commonly seen in domestic pet rats. Diagnostics, along with treatment and care, are listed below in their respective sections.

• Protozoa (one celled or grouped celled organisms)
  • Spironucleus muris (flagellate), whose life cycle is direct, and transmitted via ingestion of cysts excreted in feces. It niches in the spaces of the small intestine. May not exhibit disease in an otherwise healthy adult rat, however, those with heavy infestation or who are immunocompromised are noted to have inflammation of the intestines, with diarrhea resulting. ¹
  • Giardia muris (flagellate), life cycle also direct and transmitted via ingestion of cysts in feces, niches in the small intestine. Symptoms are usually subclinical, but may see weight loss and abdominal enlargement. Changes noted in the small intestine are minimal. Hosts include mice, rats, humans, hamsters, and several other species.  ¹
  • Tritrichomonas muris (flagellate), transmission is by ingestion of cysts in feces, and inhabits the large intestine. It is considered commensal (living in harmony) and not pathogenic. It does not infect humans.
  • Entamoeba muris (genus ameba), transmission by ingestion of cysts in feces, and found in the large intestine. It is considered commensal, and nonpathogenic.

• Helminths
  • Nematodes (roundworms, this includes pinworms)
    • Syphacia muris (the Oxyurid known as rat pinworm), most common parasite seen in rats. The life cycle is direct and requires only a short time, 11 to 15 days for completion of the cycle. They are migratory moving after ingestion to the large intestine and then to the perianal area to deposit eggs. These eggs are then disseminated into the surroundings, surviving weeks, where they can be re-ingested or ingested by another rat. Age, sex, and immune status are factors. Weanling rats, particularly males, may be most affected. These numbers diminish with the age of the rat. This parasite is not considered pathogenic unless there is heavy infestation or in the immunocompromised, and then signs may include: constipation, fecal impaction, and rectal prolapse. Humans have their own species of pinworm, and are considered a wrong host for the rat pinworm because its life cycle can not be completed.
    • Aspicularis tetraptera (Oxyurid known as mouse pinworm), this parasite affects rats only rarely and is considered nonpathogenic. It niches in the large intestine with the exception of the cecum.
    • Trichosomoides crassicauda (bladder threadworm), found in the wall of the bladder. It can migrate to the lungs. Transmission is through eggs in the urine and can be passed from doe to offspring. It causes irritation to the bladder, with resulting dysuria.
    • Strongyloides ratti (dwarf threadworms; family Rhabditidae), a gastrointestinal parasite of the rat. These parasites have a complex life cycle. The adult parasites which are female only are approximately 2mm in length, and live in the mucosa of the small intestine. The adults produce eggs that pass out of the rat in its feces. Larvae infect rats through penetration of the skin.
    • Nippostrongylus muris (mouse hookworm), transmission by ingestion of eggs in feces, and found in the small intestine. It can produce pneumonia and pulmonary hemorrhaging on migration of larvae through the lungs.

• Cestodes (tapeworms)
  • Hymenolepis nana (Dwarf Tapeworm) transmission direct by ingestion of cysts, or by indirect transmission. Niches in the small intestine. These cyst can be found on fomites such as: in cages, water bottles, litter, carried on hands or spread through the air.
  • Hymenolepis diminuta (tapeworm) requires an intermediate host such as: the flea, cockroach, or grain beetle for transmission. It is found in the anterior ileum (part of intestine) of the rat.
  • Taenia taeniaeformis (tapeworm) known to be transmitted through cat feces, or feed contaminated with the parasite. Rats can also be an intermediary by harboring it in the liver, and Taenia seralis in the connective tissue. Keep pet rats from cats’ litter boxes when free ranging. Food, in particular meat if given, should be cooked well. Store dry food in tightly closed containers. These species of cestodes listed here can infect humans but the potential is low if care is taken to keep other domestic pets treated, and prevent pet rats having exposure to them and their waste. It is also important to prevent exposure to rats in the wild, and insects, known to spread the infection.

• Acanthocephala (entazoa)
  • Moniliformis moniliformis (thorny-headed worm), found in the small intestine of rodents, and may cause enteritis.

• Arthropoda
  • Cuterebra (rodent bot fly), a larval stage endoparasite, is of the family Oestridae. The species affecting rodents is tenebrosa. It is an obligate parasite of mammals, and tends to be host specific passing part of its stage in the subcutaneous tissue of its host.
    
    Cuterebriasis becomes apparent when the larvae create warbles (seen as a swelling) in the tissue. The depositing of ova by the female bot fly does not occur directly to the skin, but rather the ova are deposited on fomites, developing into small larvae, where they then transfer to the rat entering either through ingestion, through the nose, or penetrating through any small wound or abrasion in the skin. They then migrate through the body (roughly a week) burrowing under the skin to create a warble (lump seen under the skin of the rat). A warble pore (to the outside of the skin) is made by the larvae’s pointed mouth hooks through which respiration and excretion of fluid occurs. While within the warble the larvae derive their nutrition from the blood of their host.
    
    Larval development to maturity may last from two weeks to a month where they then exit through the warble pore falling to the ground where it burrows into soil to pupate and emerge as an adult fly. Adult flies do not feed and only survive a few days.
    
    Rats rarely die from myiasis (bot fly infestation & infection), but effects from the larvae may cause debilitation. Infected warbles, on occasion, may ooze blood or serum as a result of the larvae keeping the warble pore open. Pruritis (itching) may occur at the site of the warble or irritation may occur as the larvae migrate to the chosen site. Secondary bacterial infection may occur. Also it is a possibility that if larvae are not removed intact any remaining pieces may set up an anaphylactic response, or result in an abscess developing at the site of the warble.
    
    The rodent botfly is rarely encountered in the pet rat housed indoors, but may infest those that are housed outside in warm areas, or in those rodents living in the wild.

*Note: It is important to be aware that good handwashing, insect control, and the proper care and early treatment for any household animal is the key to prevention for many endoparasites!

Figures

• Fig. 1: Endoparasite slides and descriptions courtesy of
  DISEASES OF RESEARCH ANIMALS-DORA University of Missouri (IDEXX-RADIL)
• Fig. 2: Tapeworm cyst (Taenia taeniaformis)- Resulting in death of 17-month-old male rat
• Fig. 3: Cuterebriasis in adult male rattus (Hobo)

Diagnostics

Obtain history.

Fecal wet mount or flotation as appropriate.

Scotch tape test for presence of pin worms.

Urine specimen for presence of thread worm.

Eosinophil (EOS) count. In some endoparasite presence may see increase in this count.

The presence of dark-brown to black larva with spinules is definitive for cuterebriasis.

Treatment

Dosages for the following listed drugs can be obtained by reviewing the Rat Medication Guide

Nematodes (pinworms, roundworms)

Treat with one of the following drugs: fenbendazole, piperazine, or ivermectin.

Bladder Threadworm, Thorny-headed worm

Treat with one of the following drugs: fenbendazole, ivermectin.

Protozoa (exp. giardia, spironucleus muris)

Treat with metronidazole/Flagyl. *Note, should not be used in gravid females due to fetal deformities.*

Cestodes (tapeworms)

Treat with one of the following drugs: praziquantel, niclosamide, or thiabendazole.

Cuterebra (bot fly)

Treatment of the botfly involves removing the larvae intact.

The vet may use the following procedures for safe removal:

• Injection of 0.01-0.03 mL of Lidocaine into the cuterebra itself to numb it and prevent it from retracting into the warble, and then carefully extracting with forceps without crushing the larvae.
• Surgical removal of the larvae with forceps, while the rat is under anesthesia, by enlarging the warble pore.

Application of an antimicrobial cream may be applied following procedure until the wound heals. In the event of secondary wound infection an appropriate antimicrobial, or antiinfective agent may be prescribed.

Other inconsistent methods for cuterebra larvae removal are:

• Allowing the larva to develop and emerge naturally and placing the larvae in a small container of alcohol for disposal.
• Soaking the warble pore in an Epsom salt bath. Care should be taken not to let the rat lick at the bath solution.
• Application of a thick layer of Vaseline to the warble pore which blocks oxygen supply to the larvae causing it to eventually emerge where it can be extracted. However this can be difficult and time consuming and can result in an inability to extract the larvae intact, or in the rat consuming the Vaseline, or the larvae itself.

In the event larvae removal is incomplete the rat should be seen by a veterinarian so irrigation of the wound can be performed to ensure removal of any remaining particles, along with providing an antimicrobial cream for wound care, and an oral antibiotic if needed.

Nursing Care

• Treat as prescribed, or as instructed by veterinarian.
• Treat other indoor/outdoor pets as appropriate.
• Treat any additional illnesses if rat’s health is already compromised.
• Repeat cage and article disinfecting at least once a week.
• Remove and discard articles made of wood.
• Provide additional nutritional supplement to replace that absorbed by the parasite such as: soy baby formula, Ensure, Boost, or NutriCal paste (for dogs and cats found in pet store).
• Replace any fluid lost through diarrhea by offering and encouraging fluids, such as water, Jell-O water , or electrolyte replacement drinks such as Pedialyte or Gatorade (which can be found in local grocery stores).
  • Please note that Pedialyte is only good refrigerated for 24 hours after opened, but can be frozen as ice cubes and thawed as needed.
  • If necessary offer fluids by syringe every 2 hours. If dehydration is more severe, give warmed SQ fluids of Lactated Ringer’s solution per veterinarian assessment.

Outcome

• Free of parasite infestation
• Free of dermatitis
• No respiratory involvement, or infection controlled
• Weight within normal limits

Prevention

• Maintain rat’s overall general health.
• Use of prepackaged processed litter, and the freezing of litter where bags have been breached prior to purchase, may be of help.
  *Please note: that any bags of litter/bedding that have been noted to have a row of holes in the top of the bag or any bag that has been breached during storage in pet stores and feed/tack warehouses, where contamination through contact from residing infested animals, may be a potential risk. Freezing the litter before using in cages may be a helpful preventative measure.
• Refrain from using outdated mixed feeds or rat blocks. The freezing of prepackaged or mixed foods and rat blocks, prior to feeding, is recommended if bags have been breached at time of purchase.
• Be sure sipper water bottles are frequently washed and disinfected and filled with fresh clean water at all times.
• Provide a clean cage environment.
• Monitor humidity levels of the environment as cestodes favor very high humidity.
• Quarantine all new rats for a minimum of three weeks and treat for infestation or infections if present prior to introducing to existing colony.
• Prevent contact with excrement from other indoor/outdoor pets. Have other pets treated if fleas or ticks are present.
• Treat any ectoparasites promptly.