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RODENT BIOLOGY, HEALTH AND DISEASES
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 and living in close
proximity with 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 their bodies.
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

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entire population. Only in rare cases is fumigation ever required. Fumigations should be
reserved for those rare cases that it is appropriate, and necessary.
There are two types of rats that typically tend to be the most common in the world.
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 Norway
rats 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. 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 plac ed
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 they 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.

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Rats and mice can squeeze through seemingly unbelievable spaces. Many believe this
is 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 trashcan with a lid is highly
recommended. Rodents are forced to sharpen and file 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.
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 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

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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 primar y
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

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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 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, fleas 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.

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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.