Concerns over rise in rat-borne infections, deaths

• Rodent serves as carrier, reservoir for Lassa fever, monkeypox, plague, hepatitis E, typhoid fever, cowpox
• Biologists document secondary extinction of disease-carrying mosquito following pest eradication

It is no longer news that rats sparked the ongoing Lassa fever outbreak that has affected over 23 states in Nigeria. Latest reports from the Nigeria Centre Disease Control (NCDC) showed that from January 1 to December 9, 2018, a total of 3276 suspected cases have been reported with 149 deaths.

But what is more worrisome is that rats, which are found in every nook and cranny of this country, even in churches, are carriers and reservoirs of most deadly viral infections including Lassa fever, monkeypox, plague, typhoid fever, cowpox and hepatitis E.

“There is no magic wand. The virus is in a rat. As long as people are not taking precautions, they will continue to get infected. To stop Lassa fever, we have to stop rats carrying the virus getting in contact with our foods,” the Chief Executive Officer (CEO) of the Nigeria Centre for Disease Control (NCDC) told The Guardian.

Indeed scientists have warned that rat-borne diseases are on the rise. Researchers warned that infectious diseases carried by rats might be a human health time bomb.

Several studies have shown that rats carry nearly 70 diseases, but there were fears they could harbour many more.

With a nearly four million rats born every day, the much-loathed rodents could spread more disease through towns and cities in developing nations including Nigeria.

Although rats have plagued city-dwellers and farmers for centuries, researchers have yet to find an efficient way to stop them spreading disease or ruining crops. A staggering 3.6 million rats are born every day, although many do not live for longer than a few months.

Besides harbouring typhus and the bubonic plague, rats are also known to carry diseases such as leptospirosis, a potentially serious bacterial illness spread by their urine contaminating water or food.

6,000 cases of leptospirosis were diagnosed in Thailand alone in 2000, killing 350 people, but researchers say many more deaths should have been attributed to rats but were classified as unknown fever.

Recent studies published in the journal Nature and the New England Journal of Medicine calmed fears that the virus had mutated into a super-bug that could move more easily from person to person.

The study, published in the New England Journal of Medicine, eases fears that Lassa had mutated into a super-bug that was spreading swiftly between people. Instead, the viral genomes harvested from 220 patients were surprisingly diverse, indicating that most people had not acquired their infections from someone else.

The unprecedented speed of this analysis has helped officials to combat the spread of Lassa fever, and the virus’s genetic information will assist researchers as they develop vaccines against the illness. About 514 people fell ill with the disease between January through the end of September, and 134 of them have died.

People can contract Lassa virus from direct contact with the African soft-furred rat (Mastomys natalensis) — such as by eating them. And infected rodents can indirectly transmit the virus by salivating or urinating on rice, cassava and other crops stored in barrels or left to dry in the sun. The infection can also pass from person to person through bodily fluids — which is how many health workers contract the virus.

As with Ebola, Lassa causes fevers and can lead to death as a result of internal bleeding. Mortality rates range from 25 per cent to 69 per cent in West Africa.

Meanwhile, it was reported in September 2018 that a Hong Kong man developed the world’s first-ever human case of the rat version of the hepatitis E virus.

There had previously been no evidence the disease could jump from rats to humans, the University of Hong Kong said, warning the discovery had “major public health significance”.

“This study conclusively proves for the first time in the world that rat HEV can infect humans to cause clinical infection,” the university added.

Rat hepatitis E virus is very distantly related to human hepatitis E virus variants, HKU said.

The disease was found in a 56-year-old man who persistently produced abnormal liver function tests following a liver transplant.

He could have contracted the illness through food infected by rat droppings, researchers said, according to details of the findings reported in the South China Morning Post.

The man lived in a housing estate where there were signs of rat infestation outside his home. He is now recovering after being treated for the virus, the SCMP added.

The human version of hepatitis E is a liver disease that affects 20 million people globally each year, according to the World Health Organisation (WHO).

It is usually spread through contaminated drinking water. Symptoms include fever, vomiting and jaundice, and in rare cases liver failure.

Also, biologists document the secondary extinction of a disease-carrying mosquito following rat eradication on Palmyra Atoll.

Palmyra Atoll is one of the Northern Line Islands, located almost due south of the Hawaiian Islands, roughly one-third of the way between Hawaii and American Samoa.

The Asian tiger mosquito- carrier of such diseases as dengue, yellow fever, Rift Valley fever, Chikungunya and Zika- appears to have vanished from Palmyra.

Researchers began to wonder if the Asian tiger mosquito had disappeared along with the rats. Now, in the journal Biology Letters, a team of UC Santa Barbara scientists and colleagues at the United States Geological Survey (USGS) chronicles this unique example of local co-extinction.

“We believe that this is the first documented accidental secondary extinction of a mosquito,” said the paper’s co-author Hillary Young, an associate professor in UC Santa Barbara’s Department of Ecology, Evolution, and Marine Biology (EEMB). “We hypothesized that Aedes was eradicated from Palmyra primarily because its persistence depended on taking blood meals from rats.”

Also, the United States Centers for Disease Control and Prevention (CDC) had in 2003 confirmed that a shipment of infected rodents from Africa caused the first outbreak of monkey pox in the Western Hemisphere.

Tests have found that at least one Gambian giant pouched rat, three dormice and two rope squirrels imported from Ghana on April 9 by an exotic pet dealer in Texas were infected with the virus, the CDC said.

“This shipment is believed to be the source of the current U.S. outbreak of monkey pox,” the CDC said in a statement.

The monkey pox virus is closely related to the smallpox virus but is much less contagious and deadly. Previously, the disease had been reported only in Africa, where it has a mortality rate of about 10 percent and has been known to spread from person to person.

A study published in Cell Host Microbe has also demonstrated a mouse model for the human pathogen Salmonella typhi (causative agent for typhoid fever).

Scientists had in 2003 warned that rat-borne diseases on the rise. Researchers warned an international conference in Canberra, Australia that infectious diseases carried by rats might be a human health time bomb.

Scientists from 39 countries attending the second International Rodent Biology and Management Conference heard that rats were already known to carry nearly 70 diseases, but there were fears they could harbour many more.

Other studies have shown that rat bites and scratches can result in disease and rat-bite fever. Rat urine is responsible for the spread of leptospirosis, which can result in liver and kidney damage. It can also be contracted through handling or inhalation of scat. Complications include renal and liver failure, as well as cardiovascular problems.

Lymphocytic choriomeningitis (LCMV), a viral infectious disease, is transmitted through the saliva and urine of rats. Some individuals experience long-term effects of lymphocytic choriomeningitis, while others experience only temporary discomfort.

One of the most historically dangerous rat-borne diseases is the bubonic plague, also called “Black Plague,” and its variants. Transfer occurs when fleas from the rats bite human beings. Fleas transported on rats are considered responsible for this plague during the Middle Ages, which killed millions. From the transmission of bubonic plague to typhus and Hantavirus, rat infestations can prove harmful to human health.

Rats also are a potential source of allergens. Their droppings, dander and shed hair can cause people to sneeze and experience other allergic reactions.

Diseases transmitted by rats fall into one of two categories: diseases transmitted directly from exposure to rat-infected feces, urine or bites and diseases indirectly transmitted to people by an intermediate arthropod vector such as fleas, ticks or mites.

Also, researchers have established rat-to-human transmission of cowpox infection.

The researchers isolated Cowpox virus (CPXV) from the ulcerative eyelid lesions of a 14-year-old girl, who had cared for a clinically ill wild rat that later died. CPXV isolated from the rat (Rattus norvegicus) showed complete homology with the girl’s virus. “Our case is the first proven rat-to-human transmission of cowpox,” he said.