Bed bugs (Cimex hemipterus) feed on blood, are arguably the most irritating household pests, and are indiscriminate lodgers. They can be found in all types of homes, clean or dirty, big or small.

Pazos next takes us to a luxurious, 3,700 sq ft Mid-Levels flat. One of two domestic helpers suspects the critters hitchhiked to Hong Kong with her employer when he returned from a trip to the mainland. The parasites have since spread to all of the bedrooms.

Hong Kong is on the verge of a major bed bug epidemic, Pazos warns. “There are only two types of households in Hong Kong: those who have bed bugs and those who will have bed bugs.”

The Spanish expert, who has dealt with the pests for more than 10 years, says the city is a “Disneyland for bed bugs” because the high population density and small, cluttered homes are perfect breeding conditions. Seams of mattresses, crevices in furniture and cracks between floorboards are all ideal hiding places for bed bugs.

Pazos says his team receives more than 500 phone enquiries a month. They visit some districts more frequently, such as Tung Chung and Ho Man Tin, but Pazos says this is not a reflection of how severe the problem is in these areas. Rather, middle-class families in these neighbourhoods are more likely to have the financial means to pay for treatment. The cost ranges from HK$1,500 to HK$18,000, depending on the type of treatment, size of the home and the level of infestation.

The resurgence of bed bugs is a worldwide problem. They were almost wiped out in the 1960s through use of the pesticide DDT. The pesticide was later found to have wide-ranging harmful effects, and was banned worldwide in 2001 under the Stockholm Convention. Since then, the critters have made a comeback.

“We experienced a sudden reappearance around 2000, followed by rapid population growth, mirrored across most of Europe, the US, Canada and Australia,” says Dr Richard Naylor, an entomologist from University of Sheffield, in northern England, who studies the bloodsuckers’ behaviour.

One theory about the spread of bed bugs in Hong Kong is that they are “imported” from the mainland, where migrant workers’ dormitories and trains have spread the pest rapidly across the country. “Shenzhen has a large immigrant population and many factory dorms, where bed bug infestations are often reported. Considering its vicinity to Hong Kong, the risk of bed bugs spreading from Shenzhen to Hong Kong is high,” says Dr Changlu Wang, an entomologist from Rutgers University in New Jersey.

New York, with a population density comparable to Hong Kong’s, suffered a mass infestation in 2010, when bedbugs were reported in hotels, shops and cinemas. City authorities had received more than 8,000 complaints in 2008 alone.

Statistics on the number of homes affected are difficult to obtain because many sufferers are too embarrassed to report the problem. “There is a stigma surrounding bed bugs. People believe that they are associated with dirty places, which isn’t true. But because of this, people tend not to talk about the problem,” Naylor says.

Some people may be aware of the problem because not everyone reacts to their bite.

Cheung (who didn’t want her full name revealed) took her eight-year-old son to two doctors when he developed rashes on his skin. One doctor laughed off the suggestion the rash could have been caused by insect bites.

“Our youngest son was being eaten alive and was misdiagnosed by two different doctors, who put him on antihistamines for several weeks,” Cheung says. The drugs made him drowsy but his rashes just got worse. The cause was only discovered after another of Cheung’s sons got rashes from bites, and they did some online research.

The easiest way to tell if you have a bed bug problem is by looking for droppings – pin head sized black dots normally found on bedsheets, Pazos says. Another way is to blow hot air from a hairdryer into the corners of a wooden bed, which will flush them out. They are typically brown and flat, and up to 4.5mm long.

Bed bugs possess many qualities that make them hard to get rid of. Containing an infestation is notoriously difficult due to how quickly they grow and reproduce. “An adult female bed bug can lay about 25 eggs a week. The life cycle from egg to adult takes about six weeks, which means if a single female bug finds its way into your bed, it can keep on laying fertile eggs, and by the time it’s ready to mate again, its own offspring will be reaching maturity,” Naylor says.

This ability to inbreed means they can mutate, and produce stronger detoxifying enzymes that can break down insecticides. They can grow a thicker protective exterior that prevents insecticides from harming them.

“Modern populations of bed bugs now have widespread resistance to every major class of insecticide,” Naylor says. “So it’s not surprising that people are struggling to control the insects using conventional insecticides.”

Most of Pazos’ clients have already tried terminating bedbugs with flea bombs or by employing pest control companies, to no avail. “[Flea bombing] is the worst thing you can do,” Pazos says. “They feel uncomfortable and will stop feeding for a night or two, but they will come back when they are hungry. It only makes them more difficult to kill because they hide deeper inside the walls.”

Pazos has found another way of combating bed bugs that he says has been particularly effective. He uses an amorphous silicon gel powder, which he dusts on every surfaces of an infected apartment, and kills the insects by dehydrating them. “It is completely harmless,” he says. To prove his point, a colleague dips his finger in the powder and slips it in his mouth. (Not all types of silicon gel are non-toxic, Pazos stresses.)

Another, more expensive, method, is to raise the temperature of an apartment up to 60 degrees, which Pazos can do using special equipment. “It is almost like a sauna,” he says. Bedbugs die when the temperature reaches about 48 degrees, but Pazos maintains the heat for three hours so it penetrates walls, furniture and mattresses.

Pazos shows how the heat treatment works, on a visit the cluttered flat of an elderly couple in Shau Kei Wan. It was one of the worst cases he had seen. When he first visited, the bed bugs, which are normally inactive in daylight, were crawling on the floor. The elderly man, who suffered the most, had thrown out the mattress and was sleeping on plastic sheets to prevent bites.

“I wanted to cry seeing how my old man suffered,” his wife says.

It’s been five weeks since Pazos and his team first visited the flat. They have since done one heat treatment and applied the silicon dust three times. Like a detective on the hunt for evidence, Pazos lifts up the wooden bed panel and, bingo, he finds fresh bedbug droppings.

Despite rounds of chemical warfare and turning up the heat, Pazos’ job is not yet done.

Tips to deal with bed bugs

• Wash infested blankets and bedclothes in hot water (60 degrees Celsius) and dry in the sun or with a hot iron
• Treat the bedding of infants, including cribs by physical means rather than with residual insecticide
• Seal items that cannot be heat-treated in plastic bags and placed in a freezer (-18 degrees) for 24 hours to kill the bedbugs
• Apply hot air or stream directly to the cracks and crevices where the insects are in order to kill them and their eggs
• Clean and vacuum premises regularly
• Inspect bedding and clothing regularly
• Maintain a non-humid environment
• Avoid using second-hand furniture
• Promptly replace loose wallpaper, and seal cracks and crevices
• Stay alert to the presence of the insect in hotel rooms when travelling to prevent bringing the insect back home
• Source: Food and Environmental Hygiene Department