Rodents

Rat and mouse are rodents. The common species of commensal rodent in Hong Kong are:

1. Rattus norvegicus (Rn), sewer rat; also known as norway rat.
2. Rattus rattus (R2), house rat; also known as roof rat.
3. Mus musculus (Mm), mouse.

Special Features about Rodents

• social animal
• nocturnal
• good climber, especially for Rattus rattus.
• good jumper
• range of movement: 30-50m for rats, 5-10 m for mice
• good swimmer
• can dive
• rat takes 15-30 gm food and consumes about 30 gm water daily,
• mouse takes only 3 gm of food and small amount of water everyday.
• 5-8 litters per year, 5-14 per litter
• average life span is 1 yea

Rodent-borne Diseases

Rodents are carriers of viral, rickettsial and bacterial diseases. The causative agents could enter our body by four different ways:

• through the ectoparasites of rat like fleas, ticks and mites
• by food or water contaminated by rodent excreta
• through direct contact with rodent excreta
• by rat bite

Some of the common rodent-borne diseases are:

• plague
  • a disease of rodents caused by the bacterium, Yersinia pestis
  • rat fleas feeding on a bacteria-infected rodent would transmit the disease to the next host (rat or human) through biting
• Urban typhus
  • a rickettsial disease transmitted by rat fleas
  • flea faeces are rubbed or scratched into the wound or abraded skin
  • also possible by the inhalation of dust or ingestion of food contaminated with flea excreta
• Spotted fever
  • a rickettsial disease transmitted by rickettsia-infected ticks feeding on human through biting
  • larvae feed on rodents; adults feed on dog as their host
• Scrub typhus
  • a rickettsial disease transmitted by rickettsia-infected mites feeding on human through biting
  • transmitted by mites amongst rodents in scrubby areas
  • accidental infections in man: campers, hikers
  • Personal Preventive Measures for Scrub Typhus While Hiking/Camping
    • Wear long trousers and clothes with long sleeves
    • Avoid resting in scrubby area or on grasses
    • Apply insect repellents to exposed skin surfaces
    • Do not leave clothes in scrubby area or on grasses
• Hantavirus Infection
  • a disease of rodents caused by Hantaan virus
  • transmitted by air-borne droplets and faecal particles of infected rodent under poor ventilation
• Rat-bite fever
  • a disease of rodent caused by bacteria
  • usually transmitted to human through rat biting

Rodent Prevention and Control

Rodent Survey

1. Find out the rodent species, estimated rodent population, rodent harbourages, food attraction for the rodent and activity range of the rodent.
2. Assist in planning of rodent disinfestation operation and long-term rodent prevention measures.

   Rodents provide the essential link in the spread of rodent-borne diseases which are of public health importance. Our Department has carried out study on rat flea and rodent infestation survey to monitor the situation of flea and rodent infestation in 18 districts.

Rat Flea Survey

The number of rat flea collected from the rat examined per each rat examined gives Rat-flea Index (Number of rat flea collected from the rat examined/Total number of rat examined = Rat-flea Index).

Please click here for the latest information on rat-flea index

It has been reported from World Health Organization (WHO) that a Rat-flea Index of greater than 1 represents an increased potential plague risk for human. Although Hong Kong does not have human plague transmission since the middle of 1920s, our Department would carry out flea and rodent disinfestation operation in places with Rat-flea Index greater than 1.

Rodent Infestation Survey

Since 2000, FEHD has been conducting the Rodent Infestation Survey (RIS) regularly by setting baits in designated survey locations to monitor rodent infestation.  The ratio of the baits gnawed by rodents to the number of baits collected from the survey location generates the Rodent Infestation Rate (RIR).  When RIRs for all survey locations under a particular district are available, the District RIR could be compiled.  When RIRs of all survey locations are available, the Overall RIR could be compiled.  The RIS adopts a scientific and objective method in assessing the extensiveness of rodent infestation in the survey location.  Based on the RIR and its trend, the RIS can gauge the general situation of rodent infestation in individual survey locations and form the basis for devising anti-rodent measures.  The results also serve as reference for assessing the overall efficacy of rodent prevention and disinfestation work. 

The surveys are conducted in rodent-infested areas or places which are likely to have rodent problems, in particular areas bustling with human activities.   The survey in each survey location is conducted every six months.

At present, there are a total of 50 survey locations in 19 districts.  The survey in each survey location lasts for three days.  During the survey period, the Department will place around 50 census baits (a piece of uncooked sweet potato) within each survey location.  Baits are set at a distance of about 50 m apart to avoid the same rodent from gnawing on two different baits (average activity range of a rodent is less than 50 m).  Daily checking and replacement of baits will be performed.  On the third day, all the baits will be removed, and the number of baits gnawed by rodents will be counted against the number of baits collected from the survey location to derive the RIR.

A census bait (a piece of uncooked sweet potato)

A census bait with gnawing marks (indicated by the arrows)

The department has, over time, tried out different methods.  Having regard to a number of factors including our local climate, environmental conditions and the habits of rodents, the department has come to the conclusion that adopting the ratio of baits gnawed by rodents as the infestation rate is the most suitable method for Hong Kong.  FEHD has been using uncooked sweet potato as bait because it is generally attractive to rodents and clear gnawing marks could be identified once rodents bit on it.

The RIR is divided into three levels.  Details of each level and follow-up action to be taken are summarised in the table below.

Once the RIR results are available, respective District Environmental Hygiene Offices (DEHOs) and other relevant government departments will be notified for the specific follow-up actions.

For survey locations with more serious rodent infestations, the FEHD has strengthened follow-up actions, implemented mitigation measures for a month to strengthen rodent prevention and control work at the relevant locations, and conducted a new round of Rodent Infestation Survey at these locations afterwards to closely monitor the rodent infestation and evaluate the effectiveness of the mitigation measures.   

FEHD adopts an integrated approach in monitoring rodent infestation.  Apart from making reference to the RIR, the frontline staff of FEHD will also conduct on-site rodent assessment to evaluate the degree of rodent infestation during regular inspections.  Targeted rodent prevention and control actions will also be implemented taking into account complaints and views from the local community and residents.  Moreover, management of housing estates, hospitals, schools, construction sites and other venues or properties are advised to take specific measures to monitor and contain rodent problems in their own properties once rodent infestation is detected. The FEHD is working with a local university to review the current rodent infestation index and plans to introduce a new calculation method in 2023 to incorporate various data (e.g. complaint figures, number of live and dead rodent collected, etc.).

To ensure the accuracy of survey results, members of the public are advised not to disturb the census baits.

A warning notice (the white label) is displayed during the survey to remind members of the public not to disturb the census bait.

• Please click here for the information on RIR of the year 2013
• Please click here for the information on RIR of the year 2014
• Please click here for the information on RIR of the year 2015
• Please click here for the information on RIR of the year 2016
• Please click here for the information on RIR of the year 2017
• Please click here for the information on RIR of the year 2018
• Please click here for the information on RIR of the year 2019
• Please click here for the information on RIR of the year 2020
• Please click here for the information on RIR of the year 2021
• Please click here for the information on RIR of the year 2022

Rodent Infestation Rates for the 50 survey locations in 2023 are provided below:

Rodent Control

1. Direct disinfestations
   • Use poisonous bait or traps
   • Temporary effect
   1. Stomach poison
      • Mix rodenticide with bait for rodent consumption
      • The rodenticides used by our department are chronic anti-coagulants (vitamin K as the anti-dote)
      • The anti-coagulants take 4-5 days to give effects
   2. Traps
      • Cage trap
      • Snap trap/break back trap
2. Fundamental control

   Improve the sanitary condition of the environment and deprive rodents of :

   • Food
   • Harbourage
   • Passages

Rodent prevention

• Eliminate harbourages for rodents and confine the rodent activity area for enhancing the rodent disinfestations work
• Prevent re-infestation of rodent

Disposal of dead rodents

The following procedures can be used for handling dead rodents found:

• using tools such as tongs to put the dead rodents into a tough plastic bag (e.g. rubbish bag);
• spraying the carcasses with general household disinfectant or diluted bleaching solution until they are soaked thoroughly;
• sealing the bag tightly and placed it into another plastic bag. The second plastic bag should also be sealed;
• putting the bagged material properly in covered rubbish bin or disposing of it to the nearest refuse collection point.

When handling dead rodents, attention should be paid to personal and environmental hygiene. Wear gloves and face mask, if necessary, when handling dead rodents and avoid direct contact with them. All areas, clothes and items contaminated by the dead rodents should be disinfected thoroughly using general household disinfectant or diluted bleaching solution. Before taking off gloves, wash them with water and then cleanse with general household disinfectant or diluted bleaching solution. Hands should be washed thoroughly with soap and water after removing the gloves.

Figure

Rattus norvegicus, body length (head to tail) 34-46cm, body weight 150-600g

Rattus rattus, body length (head to tail) 35-46cm, body weight 80-300g

Mus musculus, body length (head to tail) 12-21cm, body weight 10-21g

Xenopsylla cheopis (Rat Flea), about 2-4mm long

Ctenocephalides felis, about 2-4mm long

R. sanguineus, a tick, is a rodent-borne disease vector for the transmission of Boutonneuse fever

L. deliense, a mite, is a rodent-borne disease vector for the transmission of scrub typhus