Chapter 4: Deforestation and Human Health

The settling of a group of colonists, road builders, cattle ranchers, or loggers, brings about many changes in the forest. Large tracts are cut by newly arrived  colonists, often more than is necessary for the maintenance of their families. Hunting and destruction of habitats by deforesta­tion decreases the numbers of forestdependent animals and causes beasts of prey to retreat. The only animals favoured are rodents which proliferate unhindered, caus­ing damage to crops and property, while providing more food for hematophagous insects. Shrinking bird and bat populations enhance the expansion of insect populations.

Deep tracks and ruts cut by heavy vehicles and the puddles which arise everywhere  where there is stagnation in drainage provide excellent breeding sites for insects. Even the water left behind  by the rain in a discarded  tyre or car can sustain a whole population of malaria mos­quitoes. This disturbed environment, the introduction of new diseases by travellers and colonists, and the increas­ing numbers of rodents and insects can change a relatively healthy forest area within a few years into a diseaseridden hell.

Although quite a number of diseases are in fact forest borne, they normally do not afflict human populations in a serious way unless ecological disturbance takes place.


Disturbed Environment and Disease

Diseases present in the undisturbed forest may not consti­tute a threat as man may not be the prime host, or the vector may live in the higher strata of the forest so that it would not normally meet a potential human victim. But, as soon as the preferred host animals disappear or as the stratification of the forest is altered, a disease may suddenly break out.



The parasitical disease American leishmaniasis, a disease normally only acquired  by  those  penetrating  natural forests, occurs in different forms which cause a range of severe and incurable skin lesions in  humans  and  can  be fatal. The infection is caused by a protozoal parasite and is transferred by sandflies. The normal natural hosts of the disease are several  species  of  forest  animals,  such  as sloths, anteaters and rats, between which parasites and sandflies circulate. When the forest  is entered  or  disturbed by man the sandfly can turn itself on this new  host.  In  Central America the disease is known  as  chic/era’s  ulcer. The chicleros, as their name implies, are the men who col­lect the ‘chicleor  chewing  gum from the sapodilla tree, for which purpose they spend up to six months of the year living in the forest. As this period coincides with the rainy season, and therefore with maximum sandfly  density, it is not surprising that there has always been  a high  infection rate among the chicleros. The reservoir of infection is found among forest rodents in which the parasite also causes skin lesions.

Nowadays, the disease is more and mo re common, throughout Latin America, in people who are actively in­volved with deforestation: miners, colonists, loggers, road construction teams. Since forest destruction actually favours some of the animal hosts such as rats and opos­sums, the disease can easily spread. This happened, for example, near Manaus (Brazil) in a newly developed housing area, surrounded by high forest, where a n in­ creasing number of leishmaniasis cases in human beings were being reported. It appeared that 60-70% of the opossums caught in this area were infected. It was suggested that this very high infection rate resulted from the with­drawal of the primary hosts (such as sloths and anteaters) due to man’s disturbance and an increase in the opossum population because these animals are scavengers by nature and are attracted by man’s presence.  In undis­turbed natural forests, opossums are rarely found infected since they do not concentrate and therefore are only rarely exposed to the sandfly vector (Lainson, 1983).

The habits of the different sandfly species are a limiting factor in the spread of the disease. Scientific research has proved that there exists a much greater number of differ­  ent leishmaniasis parasites in sylvatic animals, which until now do not affect man, only because the transmitting sandfly species does not feed on humans. Some cases of new forms of leishmaniasis in the Brazilian Amazon are attributed to ‘errors’ of the sandfly vector. Introduction of a better-adapted vector along the newly constructed high­ ways or a change of taste in the actual vector might cause new and unexpected outbreaks of leishmaniasis.


Chagas’ Disease

A forest-borne disease which is often freely carried into houses is the sleeping-sickness related Chagas’ disease. In Venezuela it was discovered that the traditional  use  of palm thatch for roofs often carries the eggs of the Chagas’ disease vector ,a bug, into the houses. The eggs, which stick to the palm fronds, appeared to be a major source of infection of this illness. Replacement of palm thatch with corrugated metal roofs has been a major factor in control­ ling the Chagas vector bug in Venezuela (Schofield  and White, 1983).

As with leishmaniasis, there are a number of vector bug species which can be a potential threat to people penetrat­ing the forest. Since 1969, seven cases of an unknown variety of Chagas’ disease have been reported, transmit­ ted by a non-domiciliated bug. Introduction of an already domiciliated vector bug might cause an accelerated spread of this new disease (Lainson, 1979).

The house sparrow, by carrying the vector in its feathers, has been implicated in spreading Chagas’ disease northward in Brazil it spread 1,500 km up the Brasilia­ Belem highway in six years and has already advanced beyond Maraba (Goodland, 1975).


Possibly the most serious disease of the tropics, malaria, is present in every area being deforested. Globally there are some 5 million new cases per year of this mosquito-spread disease, with one million African children dying of it annually. A total of 1.2 billion people live in  risk areas. The disease is spreading after recent major resurgence, although WHO has carried out intensive eradication cam­paigns which have been successful in some areas in Asia and Latin America. The disease is most often carried and spread by travellers or migrants; this has contributed to its resurgence in Northern Brazil and in Nepal. Infected agri­ cultural colonists are major carriers. With their forestry activities they clearly extend the geographic limits of malaria.

In Paraguay it was found that the productivity of farmers engaged in slash-and-burn agriculture can be severely curbed by malaria, and, most strikingly, that the most affected families were always relative newcomers to the region. The extra effort which they had to spend on clearing forests for crops was high, because when they took up residence there were only very small portions, if any, cleared for crops. It is possible that this extra work­ load made these farmers more vulnerable to malaria. Har­vesting normally coincides with the malaria season, but due to illness from malaria, productivity could decrease by 25%, affecting their economic as well as their nutritional position (Conly, 1975).

Although the ecology of the different malaria transmit­ters, the Anopheles mosquitoes, can differ significantly, there are a number of them which only live in a forest environment. One of the malaria vectors in South East Asia, anopheles balbacensis, proliferates especially in half-shaded pools, a habitat which is found particularly in selectively logged forests and along forest streams and rivers. Research has proved that elimination of the mos­quito is best achieved by clearing forest along suspected streams and pools. Clear felling, however, normally conflicts with sound forest management in the tropics and can, especially on riverbanks, accelerate erosion. More­over the wheels of extraction equipment may inadvertently create additional breeding sites. Anopheles balbacensis only starts feeding after midnight and its flight range is normally not more than 500 m. Dr G.B. White of the Lon­don School of Hygiene and Tropical Medicine therefore suggests that dwellings be situated at least 1km away from the forest fringe, while people such as loggers and hunters should use bed nets when staying overnight in the forests. Focal spraying is especially recommended in situations such as in Thailand where development programmes re­ quire villages to be surrounded  by forests and  plantations in which these vectors are likely to thrive.

Two diseases, malaria and scrub typhus, also have a relationship with the establishment of plantations, where they represent a major health hazard to the human population in modern development schemes. In 1911, during the establishment of the first rubber plantations in Malaysia, it was reported that 9,000 out of the total estate labour force of 143,000 died from malaria. Some estates had a mortality rate of 20% every year in the pioneering phase. In 1958, eradication campaigns successfully reduced malaria admission to hospitals to 0.095% of the population. But this considerable improvement has  not been sustained in rural areas. In 1974, it was observed that in new settlements the incidence of malaria yielded 100 cases a month in 2,000 households, with almost every household having at least one sufferer. The vector, anopheles maculatus, is particularly associated with the slower flowing sections of sunlit streams, a habitat which is easily created during felling for a plantation. Conditions become unfavourable for the insect as soon as the planta­tion shades out the stream again (Edington and Edington, unpublished).


Scrub Typhus

Scrub typhus is propagated in the imperata grass which appears in the process of savannization after  the clearing  of a tropical forest. The picture  which emerges  is similar to the transmission of malaria. From observations made in Malaysia in 1975 at a rural health centre serving an oil plantation scheme, it was estimated that there were about

400 cases of scrub typhus annually among the 10,000 settlers. Extrapolated to West Malaysia as a whole, this would give a figure of 500,000 people affected. Since the complicated ecological background of the disease has not been fully appreciated by every leader of development project s, the disease is still likely to thrive. The main point of attack are chiggers which normally feed on the Malayan wood rat , a rodent living high in the palm canopy. A young, recently established oil  palm  plantation brings host, vector and man in a dangerous juxtaposition for the transmission of scrub typhus. The risks appear to diminish towards the end of the first decade of the plantation when the canopy of the palm trees is high and closer.

These findings have direct relevance to human health and to Malaysia’s land development programme, in which the establishment of new oil plantations is playing such an essential role. Both malaria and scrub typhus are an extra burden on the independent farmers who are obliged to make regular loan payments once their crop has started to yield. The loss of working time due to illness only adds to this pressure.



Some tropical diseases are directly related to a changing use of the environment. One case is the spreading of schistosomiasis in Amazonia. Since the natural conditions of the waters in Amazonia are inimical to the spread of the disease, nearly all water being acid and oligotrophic, there was no great danger of the disease spreading . This situa­tion, however, is now coming to an end, as colonists throughout Amazonia are encouraged by agricultural agencies to use lime and fertilizer on their fields. The poor quality of the soil combined with heavy rains soon leaches all these nutrients into ditches and, subsequently, rivers. The resulting autrophication and rise of alkalinity, together with the general increase in agricultural settle­ments and colonies, causes very favourable conditions for snail vectors to thrive.

The continuous influx of settlers out of the already infected region of north-eastern Brazil poses an increasing threat to the entire area. Control of the snail and of the infection is possible, but requires measures combining strict sanitation with application of chemicals. Worldwide, throughout the tropics, 180 million people carry the disease, with these numbers still increasing.


Deforestation and Trypanosomiasis

The tse-tse fly is the vector of the greatly feared African trypanosomiasis, which affects both humans and cattle (the human form is also known as sleeping sickness). It is practically impossible to raise cattle in an area where try­ panosomiasis is endemic. An indication of how much the disease is feared is shown by the observation that a single tse-tse inside a car is a common cause of road accidents due to panic among the passengers and distraction of the driver (World Bank, 1980). It is claimed that if the tse-tse fly were not present, the whole of the African tropical forest area could be converted to cattle ranching for export meat production. The factors inhibiting this development are discussed below.

The vector has a complicated ecology. The tsetse fly transmits the trypanosomes to wild animals, cattle and man. For its survival the fly depends on bush or humid areas where it breeds and rests. The flies are most dangerous near the hot dry limit of their range, because they tend to gather along springs and pools, where the chance of contact with humans is very high. Even a small plot of forest left in an otherwise tree-less area, for example a sacred grove, can provide a habitat for the fly. It thrives at its best in the fringes between human settlements and the undisturbed forests, waiting to re-colonize the rest whenever suitable bush-cover offers a new habitat and animals to feed on. This implies that in areas with active deforestation the chance of contracting the disease is very high .

The vulnerability  of cattle and  man to the disease makes it unattractive to encroach upon this environment. Until now there are only a few breeds of cattle, like the dwarf Dahomey cows, which are trypano-tolerant. This has evolved over thousands of years by natural selection and is almost equal to the tolerance in indigenous wild animals, but Dahomey cattle are much less productive than other breeds. FAO plans to improve matters by breeding a new hybrid cow which combines tolerance with higher meat production. Success in this would make new human expansion into the African rainforests possible. Control of the disease is being pursued in large-scale programmes throughout the African continent. Initially this focused on destroying the flies’ habitat by ruthless forest clearing, which proved very successful, particularly in Nigeria. The effects on the vegetation and its dependent fauna, how­ ever, were disastrous and can well have serious side­  effects on climate and water-flows. Other rigorous measures consisted of killing all wild animal hosts, a solution as radical as clearing the vegetation since these hosts range from rats to rhinoceroses.

Research in Kenya proved that tse-tse flies obtain 50% of their meals from bush-buck, an antelope species which prefers thickets near streams. Infected flies transmit the parasite to the animals during a blood meal. Later on, healthy flies pick up trypanosomes from sick animals and become infected. Man may become part of this cycle by exposure to the fly bite. Normally infection can also be transmitted by a fly which has fed on  an  infected  animal and not an infected person. Control of the animal host can therefore be an important means of control.

Both vegetation clearing and animal killing methods are virtually abandoned today, due to the high costs and the need for follow-up every few years when the vegetation has regrown or the animal population has returned. Mod­ern vector control methods consist mainly of spraying in­secticides, focusing on highly suspected resting places. Effective control can be done at high cost (e.g. Zim­babwe) but has proved almost impossible in dense forest areas.

Sprayed areas need periodic treatment since the fly returns as soon as the  toxic effect  has  washed  away.  Most of the insecticides used are organochlorines, which can well build up in the food chain, eventually affecting man. The most  reasonable  way  to cope with  trypanosomiasis is to accept it as part of the ecosystem, in fact as one of its in-built safeguards. The introduction of cattle ranching into the tropical forests of Africa will have serious effects on the soil and on human societies, as can be observed in Latin America. The forest might instead be used for the extraction of bushmeat and other forest products. When this is carried out in a sustainable manner, it avoids risks to cattle and man and can offer natural and long-lasting services to local economies.


Difficulties in Disease Control

There is a strong relationship between the disturbance of tropical forests and the emergence of diseases as described above. The paradoxical fact that some diseases, such as Ieishmaniasis, are actually best remedied by cutting down the forest completely while other diseases such as scrub typhus only begin to thrive after the forest has been cut, does not make for easy solutions. The highly ingenious mechanisms of disease transmission can cause unpleasant surprises during the development of a  forested  area.  It may suddenly appear that a certain unexpected disease vector, such as the chiggers in the scrub typhus example, poses a serious threat to a development project and the people working on it.

It is very difficult to foresee which specific action in a forest might cause a disease outbreak, firstly because the ecology and epidemiology of many diseases are not completely known and secondly because not all forest-borne diseases are known. Goodland (1975) gives an example of the latter possibility: during work on the Trans ­Amazonian Highway, a ‘black fever’ broke out at Labrea. This highly lethal disease appeared to be a kind of hepati­tis, but of a completely unknown ethology. An unknown haemorrhagic fever, allegedly transmitted by the black fly which is also the onchocerciasis vector, has caused two deaths and 92 cases in Altamira.

Presumably there are many more of these undescribed or unknown diseases, particularly those caused by viruses and mycoses. Whether their discovery will result in im­mediate remedial action depends on the alertness of the responsible health officers rather than on a well-designed strategy for the prevention of diseases.


The Green Frontier

Many of the diseases among people living in areas of de­ forestation are associated with low standards of living. Settlers and colonists do not normally tend to come and live in tropical forest areas because of their  affluence. They are anxious to get started as soon as possible, build their homestead and develop their land, and often do not bother too much about hygiene and sanitation. Household refuse, water left in cans, cattle, poultry, dogs and very provisional sanitary supplies, combined with a disturbed ecological balance, cause many other (not directly forest­ borne) diseases to thrive.

Intestinal parasites are common everywhere throughout tropical forest areas. The primitive living conditions and lack of hygiene can cause whole populations to carry cer­tain infections. In Amazonia over 90% of a test group examined carried Ascaris parasites and over 50% Trichur­is. For Amazonia Goodland (1975) also mentions the fol­lowing diseases which are carried into the area by migrants: diphtheria, poliomyelitis, tetanus and typhoid. All of these are controllable by routine immunization of newcomers. Good health education should cause the awareness necessary to prevent them from  becoming more serious.


Tribal People, Health and Deforestation

All large areas with rain forests have their own groups of tribal people, who may have lived there for centuries. As hunter-gatherers or as true shifting cultivators, they roam vast territories in small numbers. By trial and error they have learnt to use the ecosystem of the tropical forest on a truly sustained basis.

Tribal people have always had their own medical treatments to control endemic diseases. But, because of their independent development, out of contact with society at large, they are susceptible to any infectious disease to which they have not previously been exposed, often with fatal consequences. Examples of this are to be found throughout the world, but again the most striking are in

Brazil. In A.O. 1500 there were an estimated six to nine million Amerindians in Brazil. Today barely 200,000 survive a decline of two million people per century. The total number of tribes dropped from 230 in 1900 to about half that in 1980. This was caused more by disease and starvation than by conflict (Goodland, 1982). Even common and curable infections such as measles or the common cold can have a disastrous effect on tribal people, primarily attacking the children. Protection or isolation is essential until a vaccination campaign can be carried out, both for tribal peoples and settlers.


Deforestation Without Disease?

Some 140 million forest farmers all over the world depend on tropical forests to provide them with new land every two to three years, in remote areas and conditions. They live on the real frontiers of human society, facing nature and urged to open up the forest in order to survive. These frontier conditions do not encourage them to create healthier surroundings as their homestead will always be a temporary one.

Moreover, it is clear that their numbers will not decline in the foreseeable future. The influx of new forest farmers, together with the natural population increase, could well double the number of forest-dependent people by the year 2000. Spears (1983), assuming 100 million peo­ple directly dependent on tropical forest for their farm lands, calculates that about 0.6 to 1.0 billion hectares are needed to sustain that population under a shifting cultiva­tion system, i.e. all the remaining area of tropical forest in the entire world. This population will not stabilize at 100 million, so in fact their prospects become grimmer every day. This outlook poses a threat to the forest, but also to the health and welfare of more and more people.

Most of the forest-related diseases described above can be controlled or prevented, provided the government and the settlers are prepared for them. A sensible planning programme for tribal people must give them freedom to adapt at their own speed  and  also allow time to carry out the necessary vaccination programmes. Many of the diseases are associated with the low standards of living and will disappear if these are improved. However, this is not a simple task with a steadily rising population of rural poor living in the forest fringe.

A settlement programme projected for a forest area should be preceded by a thorough research of the possible diseases present in the area. When these are difficult to control and dangerous to man or his lives tock, or when the area contains diseases which will be aggravated by the disturbance of the environment (e.g. leishmaniasis), a change of plan or project area should seriously be considered. This can particularly be the case with a cattle­ raising project in tse-tse infested areas. When the soil is too poor to secure the long-term success of the project on the same site, it may well be that the financial and environmental costs of regular residual spraying will not be outweighed by the short-term benefits of the project. In that case the project should be planned in a different area or involve different ways of using the soil.

Encouragement to keep house surroundings clean and a basic education in sanitation, hygiene and nutrition will go far in checking diseases which are directly related to un­ hygienic and poor living standards. Sound planning should anticipate the possible constraints of the project site and their consequences for new settlers. The central planning authority, the conservation agency, the  ministries of health, resources and agriculture and the  implementing local authorities must all recognize their responsibility in this.

Some necessary components of an effective assessment of the project site in a tropical forest and its capacities for development should include: intensive survey for disease vectors and hosts; inventory of drainage patterns; clinical survey of the population in the project area for contagious diseases; direct treatment of cases of vector-borne diseases; spraying of aircraft and vehicles arriving from areas where vector-borne diseases are endemic (e.g. malaria).

A stringent screening of all people entering the area will help prevent the spread of disease. Free and obligatory treatment will prevent uncontrolled spreading of disease and will then protect the loc al, uninfected population. Ex­ tensive immunization programmes for contagious diseases such as diphtheria, tuberculosis, poliomyelitis, tetanus and the typhoids are indispensable.

A project which also involves the provision of housing should comply with the  following  conditions  (World Bank,1980): adequate sanitary facilities; regular sanitary inspection and enforcement ; safe water system; elimination of hiding places for vermin; monitoring of water and food supplies; control of the number of inhabitants.

Those diseases which are forest borne and increased by ecological disturbance, such as leishmaniasis, malaria and onchocerciasis, should be controlled by environmen­tal management as far as possible. However, if over ­ population or infertile soils lead people to move to a new tract of forest every few years, there is then little chance that these measures will be continuously applied. This demands great effort on the part of the responsible authorities and great awareness on the part of the inhabitants.

A high priority should therefore go to developing safe and sustainable agricultural  methods which are adapted to the soil. Planning which involves both social and ecological constraints can provide a safeguard against both diseases and project failure.