1. Commensal Rodents
Part 1. Types of commensal rodents
Rodents are a diverse type of animal and many types of rodents are considered pest species. The most
particular of these rodent species are rats and mice. While gophers and other non-commensal rodents can be
invasive and of large concern, some of the most problematic structural pests include rats and mice. Several
species of rats and mice are considered commensal pests because of their dependence on humans.
Between rats and mice, rats are the larger of the two species. Obvious differences can be noted between them
with regards to their appearance as well. A rat pup can be about the same size as an adult mouse, making
identification difficult, but mice have much smaller ears in relation to their heads. This could help to identify a
caught rodent, but if there is still doubt identification can be made by looking at the tail. Rats have a tail that is
longer than their bodies, especially in the young, but mice have a tail that is almost always equal in length to
The more common species of mouse encountered in homes is the house mouse (Mus domestics). While mice
usually don’t stray more than 10 – 30 feet from the nest on average, they can in some cases go as far as 50
feet for food. 50 feet is not however typical, and when mice are entering a structure it can usually be traced
back with the entry point very close to the areas of activity. Mice living within a structure are more complicated
however, and careful distinctions should be made as to the current level, and type of activity. Mice, unlike rats,
do not require a water source. Mice receive water from the food they eat and are sustained even from the
driest foods. This is why liquid rodent baits for mice are ill-advised solutions. Rats on the other hand require
water constantly and liquid baits can be a beneficial tool for their control.Interior treatments for the house
mouse should focus on trapping the majority of the population, removing food sources, and excluding areas of
entry into the structure. Outside, smaller mouse specific bait stations can be used and harborage areas
corrected. This means that bushy plants close to the structure and more specifically close to entry points
should be trimmed or removed to prevent nesting close to the structure.
House mice have 4-7 pups per litter on average according to studies. This number is significant because mice
are able to mate in as little as 5 weeks and up to 8 times per year. They reproduce quickly and if a large
majority of the population is living and nesting within a structure fumigation may be required to control them, or
a relatively large number of traps combined with a diligent integrated pest management plan. In the case of
mice living inside of a structure the most reasonable and least intrusive method for control is trapping.
Properly placed traps should be able to catch and eliminate the entire population. Only in rare cases is
fumigation ever required. Fumigations should be reserved for those rare cases that it is appropriate, and
Norway rats are relatively large blunt nosed rodents that most often make burrows outside, with food and
harborage close by. Roof rats on the other hand are slightly smaller, forage much larger areas, and have
pointed noses. The forage area for Norwayrats is roughly 100 – 300 feet from their nest with roof rats foraging
up to 450 ft. Norway rats are often seen or found closer to the ground where they make their burrows but roof
rats can be seen in all areas from the ground to the roofline, and in trees. Bait station and trap placement may
differ such as is the case when bait stations are placed close to Norway rat burrows. Stations are most
effective however when placed between food and harborage. Therefore, placement should be made only after
a thorough assessment of the property is made. While bait stations can be effective alone for Norway rats,
baiting the individual burrows is sometimes helpful. The burrows are essentially where the rats live, and filling
their burrows with tracking powder eliminates them at their source. The product works very simplistically. As
rats lick the product off of themselves, they ingest it, and it contains a poison. They can’t avoid it because it’s in
their burrow. Norway rats have much larger litters than roof rats and house mice. Their litters are on average 8-
12 pups, but house mice average 4-7 and roof rats average 4-8. Rats in general have 4-7 litters per year for
roof rats and 4-8 litters per year for Norway rats. Treating the burrows helps with the younger rats since they
tend to stay with the mother until they reach maturity.
2. 2. Rodent specific biology.
All 3 species of commensal rodents are thigmotactic. Thigmotaxis is when an organism uses touch to identify
its path or direction. As a result of rodents being thigmotactic they can be found moving along vertical surfaces
such as walls. Traps should therefore most often be placed along these vertical surfaces inside, and bait
stations can be placed along exterior walls and fences for enhanced levels of control. This small but significant
bit of information can be used to identify failed control methods. In many cases traps are used unsuccessfully
because they are placed in the open center of an area. Rats rarely forage away from the areas near walls and
other areas that provide the touch stimulus. When they forage away from walls and other areas where they
can touch, it is because they have developed a familiarity with the area. In this case however setting traps in
an area away from vertical surfaces such as the center of a room is still ill-advised. Those traps placed openly
in the center of an open area are less likely to catch anything.
Rats and mice can squeeze through seemingly unbelievable spaces. Many believe this the result of bendable
bones, cartilage in place of bones, or issues to do with the bone structure of their bodies, but rats actually have
normal bones that don't flex or bend. Rats and mice instead use their whiskers to identify which holes their
body can fit through. In general, if their heads fit, their bodies fit. Rats can generally get through a hole as
small as ½ inch, and mice holes as small as ¼ inch. If rats and mice can fit their nose into a hole and gnaw or
chew at that hole they will do so until their entire head fits. With their entire head through and whiskers free
they know that the rest of their body can fit. In rare cases rats can get stuck however. This is the result of a
larger than normal body that occurs as a result of overeating.
Rats also chew regularly on items of little consequence to them such as electrical wires, and prefer food that is
crunchy such as dog and cat food. This creates an obvious problem for those customers with pets, but
likewise an obvious IPM minded control solution. Customers should control the access that rodents may have
to any kind of animal feed. This is often accomplished by emptying and putting away animal feeding bowls,
and storing the inventory in rat proof containers. Dog and cat food usually comes in paper bags that rodents
can easily eat their way into, therefore storage inside of a metal cabinet or transferring the food to a metal trash
can with a lid is highly recommended. Rodents are forced to sharpen and fine down their teeth as their teeth
continue to grow throughout their life cycle. This makes crunchy items such as dog and cat food highly
desirable to rodents. This small piece of pertinent information provides a level of insight that can add
tremendously to any rodent control plan.
Rodent droppings are a key sign of infestation and while they may be confused with other types of pest
droppings, they can also be easily distinguished from other types if careful attention is given. Norway rats
have the largest droppings of the three types of commensal rodents. The individual droppings can be as long
as ¾ of in. The key factor is shape is their blunt ends. While cockroach droppings may be a similar shape (not
exactly however) they are not as large. Roof rats on the other hand have droppings that are pointed on the
ends, and can often have a slight curve giving them a sausage shape. They are not as large as Norway rat
droppings, but can often be found scattered, unlike the Norway rat droppings that are found in groups, or
clusters. The droppings of a house mouse are about the size of a grain of rice, and pointed at both ends.
Mouse droppings may appear to be long in regards to their shape as compared to the rat droppings. As rodent
droppings get older the color changes, and in many cases the older droppings will appear to be grayish. When
fresh however, they are dark and soft. As time passes they change color as they dry out, and also lose the
initial glisten of moisture. But the lighter the color, the less fresh they are, and the darker the color, the fresher
Part 3. Health Concerns
Rodents also pose severe health concerns. The first of which involves allergens. Some studies have shown
that the urine of mice can cause asthma and other similar conditions. Similar theories exist in regards to rodent
droppings, and hairs. No specific study is cited here however in regards to hairs and droppings. Many people
however do have allergies to different types of animal hairs including dog, cat, rabbit, and others. It seems
reasonable to assume then that allergies to hairs found on rodents, and left behind may cause some type of
3. allergy in some people. Cleaning, and sanitation is considered the most viable solution in cases of believed
allergen presence in addition to removing the rodent problem.
Rodents can also carry many types of diseases, bacteria, and viruses. The first of which is Hantavirus. There
are several hantavirus strains, but the one of most importance is referred to as HPS, hantavirus pulmonary
syndrome. HPS has been implicated in several high profile instances, such as a string of cases in 1993 near
the four corners states where it was referred to as the "sin nombre" virus, or in 2012 where a campsite at
Yosemite National Park saw several campers become ill as a result of the virus. Before the incident in 1993
hantavirus hadn't been found in the United States, but upon further review it was determined that earlier cases
did exist, but were simply unidentified or misidentified. Hantavirus strains had been previously found in
Europe, but the American strains affect the lungs, and the respiratory system while European strains cause
hemorrhagic fever with renal syndrome (HFRS).
Hantavirus is zoonotic, which means that it is a pathogen spread from animals to people. It has not been
shown capable of transferring from human to human, and the HIPS causing strain is very limited in which
species of rodents can be carriers. The primary host of the HIPS pathogen is the deer mouse while other
species have been implicated, but either carried different strains, or were unconfirmed carriers. The
commensal rodent species such as the house mouse, roof rat, and Norway rat have not been shown to carry
the strain that causes HPS, but the Norway rat has been shown to carry other strains that can cause
hemorrhagic fevers and renal failure. Therefore, limiting exposure of these commensal rodents is important.
Exposure to rodent feces, urine, blood and saliva are the most common means of transfer from rodent to
human for hantavirus. The best method of controlling exposure therefore comes from controlling the rodent
problem and decontaminating suspected areas. People and animals should stay away from suspected areas of
infestation until current activity can be ruled out and the entire area properly sanitized.Hantavirus generally
dies outside of the body of rodents within 48 hours, therefore removing active rodent populations becomes
important, and sustained sanitation adds a second level of control in the case of unknown recurring
infestations. Sanitizing should always be done with all possible vectors of disease included however, not just
the suspected issue. Cleaning and exclusion should be done in a manner that allows for control of all possible
rodent caused health issues.
Leptospirosis is another condition that is caused by exposure to a specific pathogen. In this case, bacteria
from the genus leptospira are what cause the illness. Leptospirosis has been shown to cause meningitis which
is a type of swelling around the brain stem. It can also cause liver failure, kidney damage, and respiratory
distress. These conditions are all serious and can lead to death. The bacteria is usually spread by contact
with rodent urine, but if the contamination spreads to pets, or livestock infection can occur as a result of
exposure to the urine of otherwise domesticated animals. Some pets can be vaccinated, and in cases where
this is appropriate it is recommended. Removal of rodents, and rodent evidence is suggested, but care should
be taken with water supplies that may be contaminated, and surfaces that can have rodent urine on them. The
methods of cleaning, sanitation, and removal are important since the primary route of infection is through the
skin, membranes, or into open cuts, and sores on the body. Water sources however do become
contaminated, as well as food preparation surfaces, and dinnerware as a result of rodents crossing them. This
means that infection is also common from ingestion. Proper disinfection techniques include the use of
sanitizers, and may require a lot of scrubbing and ‘elbow grease'. Contaminated water and food or feed should
be discarded and not used. Larger water sources such as lakes, rivers and ponds are beyond the scope of
pest control technicians and not mentioned here.
Rat bite fever is another condition associated with rodent infestations. In spite of the name it is not always the
result of a bite from a rodent. It can also occur as a result of consuming contaminated food and water. As a
result of ingestion the normal symptoms of vomiting and sore throat may be more severe. The other
symptoms include fever, swollen lymph nodes, joint pain, muscle pain, headaches and rash, as well as
localized swelling and even ulcers in the areas of rodent bites or scratches.
Streptobacillary RBF is one of the two types of rat bite fever and has an onset at about 3-10 days, but in some
cases won’t show for up to 3 weeks. This makes a diagnosis more difficult, since the original exposure may
not be remembered. Streptobacillary RBF is the type most associated with aches and pains.The second type is
Spirillary RBF which has symptoms appearing within 7-21 days. Spirillary RBF is most associated with
4. swelling around the bite or wound, ulcers in the infected area, and a fever that may come and go. Rat bite
fever however is not a major medical concern and is rarely diagnosed in the United States.
Part 4. Insect disease vectors and other secondary pests
While many insects are considered vectors of disease, they are of particular importance with regards to rats.
Rats, and several other urban animal species are known carriers of the bacteria that causes endemic typhus,
sometimes referred to as murine typhus (Rickettsia typhus). As rats enter a structure or sometimes nest close
to it, they bring fleas with them. The fleas then enter the structure, or get onto those that come outside.These
fleas that get onto unsuspecting people have often contracted the typhus causing bacteria and can spread it to
those unsuspecting people. The infection generally occurs when fecal matter from fleas gets into a flea bite, or
through the skin in some other way. Many people scratching the bites don't realize they are infecting
themselves by forcing the fecal matter into these areas. Symptoms include headaches, fevers, muscle pain,
joint pain, nausea and vomiting. Some people will develop rashes and neurological signs such as confusion.
These symptoms can cause a misdiagnosis since they resemble other similar, and dissimilar conditions.
Plague is another bacterial condition often contracted from fleas that are brought onto a property or into a
structure by infected animals such as rats. This can however be more troublesome because if pneumonia
develops in a human host the condition can then be spread from person to person. Likewise, flea bites are not
the only way of contracting it. Contact with any bodily fluid from infected animals can lead to contracting the
serious condition that is identified by fever, chills, muscle aches and swollen lymph nodes that are referred to
as buboes. These buboes are the reason that the condition is referred to as the bubonic plague.
Rat and bird mites are secondary pests that can often cause grief for a home owner and a pest control
technician.When rodents and birds leave a structure these insects are sometimes left behind. They then move
into the human occupied portion of the structure and attack unsuspecting people and pets. In many cases
rodents continue to bring more of these insects, and a less than complete rodent control plan may provide for
an ongoing infestation of secondary pests. Therefore, thorough exclusion is recommended, removal of all
nesting material, and at least one application of a proper miticide. If mite infestations continue it is more likely
that an exclusion plan has failed, or nesting material was left behind. The application of the miticide is to give
clients immediate relief, and is not a complete solution by itself.