Tick-Borne Diseases Could Increase with Global Warming

Some scientists believe that global warming could lead to increases in tick-borne diseases due to ticks spreading into previously unoccupied habitats.

There are about 15 tick-borne diseases caused by infection with Rickettsia bacteria. Related bacteria that cause Mediterranean spotted fever and Rocky Mountain spotted fever are transmitted by the brown dog tick, Rhipicephalus sanguineus, distributed widely around the world. The tick is well adapted to human environments and attaches to domestic dogs, but bites humans rarely.

One group of European scientists believe the brown dog tick may bring about the emergence of new human infections as average global temperatures increase, although they acknowledge that the risk of being bitten is small when another host is available to the ticks. But their research showed the ticks became aggressive to people before a domestic dog infested with them dies. This suggests that a lack of another host wasn’t the reason behind their behavior, and additional evidence points to unusually warm temperatures as the likely explanation.

Ticks need a humid environment to survive and are by nature aggressive, always looking for a host on which to attach. In warmer weather, the ticks could complete their life cycle faster, and higher temperatures could also mean that the air has more drying power. This kind of humidity-related stress can make ticks more eager to attach to a readily available source of blood.

Cholera Epidemic in Zimbabwe

The Zimbabwean government has declared a national emergency in the face of a cholera outbreak that has killed more than 560 people, to date. As of 1 Dec 2008, the Ministry of Health in Zimbabwe has reported a total of 11,735 cholera cases, affecting all provinces in the country. The overall case fatality rate is 4 percent but has reached up to 20-30 percent in remote areas.

Cholera is a severe diarrheal illness that spreads through contaminated water. Left untreated, it can cause death in a matter of hours.

The Limpopo River, which delineates the border between Zimbabwe and South Africa before flowing through Mozambique into the Indian Ocean, has tested positive for cholera, and appears to be the source of the epidemic.

The Vibrio cholerae bacteria occur naturally in coastal waters where they attach to tiny crustaceans called copepods. As many as 10,000 bacteria may adhere to a single crustacean. The bacteria travel with their hosts, spreading worldwide as the crustaceans follow their food source — certain types of algae and plankton that grow explosively when water temperatures rise. The rampant algae growth is further fueled by the urea found in sewage and in agricultural runoff.

Modern sewage and water treatment facilities have virtually eliminated cholera in industrialized nations; the last major cholera outbreak in the United States occurred in 1911. But the illness is still present in much of the world, particularly India and sub-Saharan Africa. The risk of epidemics is highest when poverty, war or natural disasters force people to live in crowded conditions without adequate sanitation.

The great irony is that unlike many infectious diseases, cholera is easily treated. Death results from severe dehydration, which can be prevented with a simple and inexpensive rehydration solution.

Although V. cholerae is the source of cholera infection, the deadly effects of the disease are the result of a potent toxin, CTX, that the bacteria produce in the small intestine. CTX binds to the intestinal walls, where it interferes with the normal flow of sodium and chloride. This causes the body to secrete enormous amounts of water, leading to diarrhea and a rapid loss of fluids and salts (electrolytes).

The UN children's agency, UNICEF, said in a statement: "The onset of the rainy season threatens to make the waterborne disease endemic, as the spread of cholera becomes increasingly unpredictable and the response is outpaced by the outbreaks. Harare (the capital of Zimbabwe), where the highest numbers of cholera cases have been reported, continues to experience serious water outages."

The cholera epidemic is now affecting 42 of Zimbabwe's 62 districts.